Multiphoton excitation and mass-selective ion detection for neutral and

J . Phys. Chem. 1991, 95, 2949-2962

2949

FEATURE ARTICLE Multiphoton Excitation and Mass-Selective Ion Detection for Neutral and Ion Spectroscopy Ulrich Bowl Institut fur Physikalische und Theoretische Chemie, Technische Universitat Miinchen, Lichtenbergstrasse 4, 8046 Garching, FRG (Received: June 5, 1990; In Final Form: December IO, 1990)

The combination of resonant laser excitation, supersonicbeams, and mass-selective detectors is one of the outstanding techniques for neutral and ion spectroscopy. It offers a large variety of configurationsand therefore may encourage many new experiments. Resonant laser excitation facilitates multiphoton ionization spectroscopyof neutrals and dissociation spectroscopy of molecular ions and also forms a very special ion source. Furthermore, secondary excitation with further lasers is possible. Concerning the mass-selective detectors, the combination of time-of-flight analyzers with pulsed lasers turned out to be very favorable. This is due to a high sensitivity, a low signal-to-noise ratio, and the experimental possibilities available when working with small ion clouds. In this article, important features of multiphoton ionization and fragmentation for laser spectroscopy are discussed as well as some modern arrangements of time-of-flight analyzers. The application of mass-selective analyzers to a special field of neutral spectroscopy, namely, to isotope selective spectroscopy,is presented. As examples of mass-selective ion spectroscopy, resonance-enhanced multiphoton dissociation spectra of some molecular cations are shown. Finally, the application of mass-selective laser spectroscopy to several problems, Le., Rydberg states, isomers, large molecules, or clusters, is discussed.

I. Introduction The combination of resonant laser excitation and mass-selective detectors has led to a whole variety of experiments and techniques not only in the field of mass spectrometry but also in that of laser spectroscopy of neutral and ionized molecules. In particular, pulsed lasers allow efficient multiphoton ionization and dissociation on the one hand; time-of-flight analyzers permit high sensitivity and a large variety of experimental techniques on the other hand. Pulsed lasers and time-of-flight analyzers are therefore a very favorable combination for laser mass spectrometry. Furthermore, both techniques are ideally compatible due to the time and space behavior of pulsed laser ionization. In 1970 a molecule, namely molecular hydrogen, was ionized for the first time by a laser and detected by a mass ana1yzer.I One year later, laser ionization and mass-selective detection were successfully applied to molecular iodine, heavy water, and carbon tetrachloride.* These experiments were carried out with fixedfrequency lasers. Tunable lasers have been used since 1977 to measure optical spectra of mass-selected molecular sodium, potassium m~lecules,~ iodine: and lithiumnS Our group succeeded in measuring the first mass-selected laser spectra of polyatomic molecules.6 The first resonance-enhanced multiphoton mass spectra of polyatomic molecular systems were published in 1977' and 1978.6 Since 1980, the field of laser mass spectrometry, as a new technique for mass spectrometry as well as for laser spectroscopy, ~

( I ) Berezhetskaya, N. K.;Varonov, G.V.; Delone, G.A.; Delone, N. B.; Pisskova, G. K. JETP 1970, 31, 403. (2) Chin, S.L. Phys. Reo. 1971, A4, 992. (3) Herrmann, A.; Leutwyler, S.;Schuhmacher, E.; Wbte, L. Chem. Phys.

Lett. 1977, 52, 418. (4) Zandee, L.; Bernstein, R.; Lichtin, D. A. J. Chem. Phys. 1978, 69, 3427. ( 5 ) Rothe, E. W.; Mathur, B. P.; Reck, G. P. Chem. Phys. Lett. 1978,53, 74. ( 6 ) Boesl, U.; Neusser, H. J.; Schlag, E. W. Z . Narurforsch. 1978, 33a, 1546. B a d , U.; Neusser, H. J.; Schlag, E. W. In Loser Spectroscopy; Springer Series in Optical Science Vol. 21; Walther, H.,Rothe, K. W., Springer Verlag: Berlin, 1979; p 164. (7) Antonov, V. S.; Knyazev, 1. N.; Letokhov, V. S.;Matiuk, V. M.; Movshev, V. G.; Potapov, V. K.Opt. Lett. 1978,3,37; Pis'ma Zh. Tekh. Fiz. 1977. 3, 1287.

as.;

0022-365419 112095-2949$02.50/0

experienced a dramatic increase. The early spectroscopic work was mainly concentrated on neutral molecules.* Since the first high-resolution photodissociation spectroscopy experiments on ionized molecules were successfully p e r f ~ r m e d ,laser ~ mass spectrometry also gained more and more importance for the spectroscopy of molecular ions. Further progress in mass-selective laser spectroscopy of molecular ions was achieved by the introduction of resonance-enhanced multiphoton ionization as a special ion source for ion spectroscopy.lOJ1 Multiphoton Ionization and Neutral Spectroscopy. In multiphoton ionization spectroscopy, the resonant neutral intermediate state is investigated while the ionization step serves as a detection process for its excitation. One measures the ion current as a function of the laser wavelength. This type of spectroscopy is ideally compatible with pulsed laser mass spectrometry. In general (exceptions will be discussed later), it is not restricted to fluorescing or predissociating states as in fluorescence and predissociation spectroscopy. Multiphoton ionization spectroscopy is also very sensitive and can easily be combined with supersonic molecular beams allowing considerable simplification of molecular spectra due to cooling of internal degrees of freedom. Multiphoton ionization offers the choice between soft ionization and ionization followed by fragmentation. In many cases, the detection of fragment ions rather than molecular ions may be preferred, e.g., for the spectroscopy of a mixture of isomers. This advantage, however, can only be exploited when mass-selective detectors are involved. The special features of pulsed multiphoton ion sources,such as well-defined ionization volumes and ionization times, are particularly advantageous to time-of-flight analyzers, but other mass analyzers have also been used successfully. (8) For a review see: Johnson, P. M. Appl. Opt. 1980, 19.3920. Lin, S. H.; Fujimura, Y.; Neusser, H. J.; Schlag, E. W. Multiphoton Spectroscopy of Molecules; Academic Press: Orlando, FL, 1984; Letokhov, V. S.Loser Photoionization Spectroscopy; Academic Press: Orlando, FL, 1987. (9) Moseley, J. T. Ado. Chem. Phys. 1985,10,245. Syage, J. A.; Wessel, J. E. J. Appl. Spectrosc. Reo. 1988, 24, 1. (IO) Chupka, W. A.; Colson, S. D.; Seaver, M. S.;Woodward, A. M. Chem. Phys. Lett. 1983,95,1237. Woodward, A. M.; Colson, S.D.; Chupka, W. A.; White, M. G. J . Phys. Chem. 1986, 90,274. ( I 1) In the following sections, "multiphoton ionization" always means resonance-enhanced multiphoton ionization if not otherwise specified.

0 1991 American Chemical Society

2950

The Journal of Physical Chemistry, Vol. 95, No. 8,1991

Mass-selective detectors in general allow excellent discrimination against spectral contributions from unwanted molecules; these may be contaminants in the sample inducing a high chemical background, or the most abundant species in a mixture covering the spectral feature of the molecule of interest, or fragments of the target molecule itself inducing misleading spectral structures. Thus with mass-selective detectors, even spectroscopy of one out of several neutral fragments and radicals (e.g., CHI2) formed by multiphoton absorption or other processes is possible. Multiphoton Ionization and Ion Spectroscopy. In contrast to neutral spectroscopy, in ion spectroscopy multiphoton ionization may form the source of the probe species, namely, the molecular ions themselves. The main features of multiphoton ionization as an ion source are species selectivity, state selectivity, soft ionization, and high ion concentration. Due to the resonance with a neutral intermediate state of the molecular species of interest, selective ionization is possible, making primary mass selection unnecessary; by choosing the right neutral rovibronic intermediate state, preferential population of one ionic state (often vibrationless ionic ground states with little rotational excitation) can also be reached for many molecules because of large Franck-Condon factors; high ion concentrations in a small volume allow efficient excitation of the molecular ions by a second laser. The ion spectroscopic method best adapted to laser mass spectrometry is dissociation spectroscopy of ions with one-photon excitation (for predissociating states) or resonance-enhanced multiphoton dissociation (for nondissociating states). The photodissociation itself is only used as a detection mechanism for the observation of the excitation of these states. For nondissociating states, the absorption of more than one photon is necessary, with the state of interest acting as resonant intermediate state; this is quite similar to multiphoton ionization spectroscopy. For registering ion spectra, the fragment ion current produced by the photodissociation of the molecular ion is measured as a function of the laser wavelength. Resonance-enhanced dissociation spectrosoopy of ions is not restricted to fluorescing or dissociating states, quite similar to multiphoton ionization spectroscopy of neutral molecules. A very interesting feature of the combination of a multiphoton ion source and ion spectroscopy is that due to the production of ions in their electronic ground state with different vibrational populations it is possible to obtain a series of ion spectra, each with different vibrational information about the same electronic states. Mass-Selective Detectors. For spectroscopy with pulsed lasers the preferable mass-selective detectors are time-of-flight analyzers. This type of mass selector is distinguished by its high transmission and thus sensitivity: furthermore, all ions produced during one laser shot are registered, which allows the synchronous recording of spectra of different species (e.g., spectra of different isotopomers). Since energy-correcting time-of-flight instruments (so-called reflectrons) were invented in 1966,I3J4the advantages of high mass resolution are available for laser spectroscopy with time-of-flight analyzers. The major advantage is the increased signal-tenoise ratio, because signals of specific ions can be detected in a very narrow time window while the total ion current is spread over a wide time-of-flight range. In conventional time-of-flight instruments, the kinetic energy distribution of the molecular ions is mainly responsible for the time-of-flight broadening and therefore limited mass resolution. In a reflectron time-of-flight analyzer, corrections can be made for this energy spread by a special ion reflector. In addition, reflectrons offer a variety of options which allow supporting experiments parallel to pure spectroscopy (e.& measurements of decay times, kinetic energy release). Other Ionization Processes. Of course, laser spectroscopy including mass-selective detection is also possible with ionization techniques other than multiphoton ionization, e.g., an electron ~~~~~

(12)

Chen, P.; Pallix. J. B.; Chupka, W. A.; Colson, S . D.J. Chem. Phys.

1W7.86. 5 16. ~

(13) (14)

Mamyrin, B. A. Aut. wid. (UDSSR Parent), No. 198034 (1966). Mamyrin, B. A.; Karataev, V. I.; Shmikk, D. V.;Zagulin, V . A. Sou.

Phys. JETP 1973,37,45.

Boesl

Ion

Figure 1. Excitation scheme for resonance-enhanced multiphoton ioni-

zation. impact or a plasma ion source. For instance, predissociation spectroscopy of neutrals (sym-tetrazine and its isotopomers) has been measured by our group analyzing the degree of photodissociation as a function of the laser wavelength; in this experiment, a quadrupole mass spectrometer with an electron impact ion source was used.Is As far as ion spectroscopy is concerned, mass spectrometers with other than multiphoton ion sources have been used for one-photon dissociation ~pectroscopy.~In our group, a plasma ion source has been combined with a magnetic sector analyzer for forming a pure beam of O C S cations to perform predissociation spectroscopy.'6 The fragment ions were detected in a special ion mirror arrangement. For SOz, a considerable degree of ionic state selectivity has been found for resonant charge exchange, which resulted in interpretable ion spectra of SO2+." Rotationally cold molecular ions were found for electron impact ionization of CH31 in a supersonic molecular beam; the observed rovibronic bands of CH3I+I8did not differ significantly from those of multiphoton formed CH3I+.I0 In the present report, however, only the multiphoton ion source and its unique features for neutral and ion spectroscopy combined with time-of-flight analyzers will be considered.

11. Multiphoton Ionization Most multiphoton processes can be described by time-dependent perturbation theory, with the electric dipole approximation for the light field (the field within the laser focus being much smaller than internal atomic fields). If coherent effects can be neglected, very simple rate equations are applicable to multiphoton absorption. In Figure 1 an ionization scheme with (1 1)-photon absorption is displayed (generally, (n + m) photon absorption). The general stimulated emission or absorption rate constant is a = u,,I" and the ionizing absorption rate 0 = u,im; ai is an i-photon absorption cross section, I being the light intensity. The rate equations for the depopulation of the neutral ground state A, the population of the intermediate state B, and the yield of ions C are dA/dt = -aA + (a + k,)B (la)

+

dB/dt = CYA- (CY

+ k, + k,, + P)B

dC/dt = BB

(1b) (IC)

with the rate constant k, for spontaneous emission and k,, for nonradiative decay processes of the intermediate B state, Le., (15) Boesl, U.; Neusser, H. J.; Schlag, E. W. Chem. Phys. Lett. 1979.61, 57; Chem. Phys. Lett. 1979,61, 62. (16) Kakoschke, R.;Boesl, U.; Hermann, J.; Schlag, E. W. Chem. Phys. Lett. 1985,119, 6. (17) Gas, S . P.; Morrison, J. D. J . Chem. Phys. 1987,84, 2423. (18) Syage, J. A.; Pollard, J. E.; Steadman, J. Chem. Phys. Leu. 1989,162, 103.

The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 2951

Feature Article

TABLE I: Cross Sections and Ion Yields of Multiphoton Ionization for Some Organic Molecules ~

laser wavelength, h,P nm

benzene toluene aniline naphthalene thiophene

~~

cross sections, cmz 9

decay rate k,, + k,,b s-I

fJQ

1.27 X lo6 > 108 k,. k,, s-I

258.8 266.8 293.8 299.0 239.9 A, nm

+

furan

+

( 2 I)-MPI 2-MPI

-1.4

370.0 260.0

2.7 x 10-17 6.0 X 4.5 x 10-17 4.8 X 1.8 X IO-'' a,.

cm4/s

- 3 x 10-54 2 x 10-48

X

3.4 x IO-'* 2.0 x 10-1' 2.0 x 1 0 4 7 1.6 X

cm2

-10-18

'Laser wavelength. bDecay rate. c - l O O % ion yield. d l = IO MW/cm2. CLaserintensity for ( 2 W/cm2. 'Value estimated from spectral width of the vibronic band at 370 nm. internal conversion (IC), intersystem crossing (isc), or dissociation. In most cases, the rate constant k , for spontaneous emissions is small in comparison with the other rate constants and can be neglected. A general solution for these equations has been pub1i~hed.l~ Here, I will concentrate on some important cases: (i) low laser intensity with CY,@, but also k, and k,, > l / t l the following equation is valid: c = .8/kpA0tl (2b) (ii) high laser intensity with a,/3 >> I / t , C=

P / ( P + knr)&

+ 1) photon ionization):

(iii) small a, but large /3 (e.g., (2 C = a/3/(8+ k,,)Aotl C = aAotl

(2c)

for k,,

for k,,


I / t l

I/tl

(2d) (2e)

Experimental cross sections for multiphoton ionization of some organic molecules measured in our laboratory are in good agreement with these formulas. A list of these cross sections together with excitation wavelengths and k, + k,, rate constants is given in Table 1. The ion yields in Table I include only the molecules that have the correct rotational population to absorb a t the excitation wavelength. As the rotational envelope of an absorption band is usually spread over several cm-I and conventional pulsed dye lasers have a bandwidth of 1 cm-I (as in Table I) or less, only part of all neutral molecules are able to absorb and to contribute to the ion yield. For molecules of the size of benzene, this ratio is typically 30% (with a laser bandwidth of 1 cm-I) at room temperature. By cooling in a supersonic molecular beam this rotational envelope can be narrowed considerably, thus inducing higher ion yields. The ion yields given in Table I have been obtained with a relatively small laser intensity of IO7 W/cmZ (laser bandwidth 1 cm-I) and a pulse length of about 5 ns. The intensities I,,,, where process a or 8 is saturated, are also given in Table 1. Using much higher laser intensities than ,Z will not increase the ion yield efficiently but may lead to an increase in other unwanted multiphoton processes (see section IV). In some special cases, higher laser intensities may, however, be important to overcome fast nonradiative processes k,, which depopulate the intermediate neutral states (B level; see eqs 2). In Table I two examples of rather ineffective ( 1 + I)-photon ionization are also included, namely, naphthalene and thiophene. An explanation is given below. As examples of typical ion yields for other than ( 1 + 1) photon ionization processes, some valves for (2 + 1)photon as well as two-photon nonresonant ionization of furan are given in Table 1. In the following, some problems that may arise with multiphoton ionization will be discussed. Please note that the (1 + 1)-photon (19) Zakheim, D.S.;Johnson, P. M. Chem. Phys. 1980, 46, 263.

ion yield! %

1.5 X 108/3 4.2 x 1 0 7 ~ 2.3 x 1078 2.8 x 1098

2.7 6.2 25.0 0.06 0.01

?

? 5,.

I,,,: W/cm2

I..,. W /cm2

ion yieldC

-2.5 X 10I2 1.3 X IO'"

10-8 10-5

+ I)-MPI, 6 X

IO9 W/cm2; for 2-MP1, IO8

ionization of naphthalene and thiophene (Table I) as well as (2 1)-photon ionization of furan (Table I) is very unfavorable. For thiophene and furan very fast nonradiative processes k,, are active. In the case of thiophene, a two-wavelength excitation can overcome the problem: while the intensity of laser 1 for process a can be kept small, a high-intensity laser 2 is used for process 8; if the wavelength of laser 2 is chosen in such a way that no further absorption can occur in the ion, unwanted ion fragmentation can be avoided (see section IV). Such two-color excitation does not increase the experimental effort considerably. With modern YAG or excimer lasers two dye lasers can be pumped with the second one being only a broad-band dye laser; in some cases even a higher harmonic frequency of the pumping YAG laser may be appropriate. In the case of furan, Table I shows that nonresonant two-photon ionization may be the better choice as ionization mechanism even if the wavelength necessary for such a two-photon absorption is much farther in the UV region and the available laser intensity much lower than for a (2 1)-photon ionization. In the case of naphthalene, the reason for the unusually small rate constant 8 is the effect of very small Franck-Condon factors: at a wavelength of 2990 A, the excess energy above the ionization threshold is so small that no vibrational levels of the ion can be excited that have a good transition probability from the neutral vibronic intermediate state. In this case one has to choose vibronic intermediate states at higher energies. In summary, for efficient multiphoton ionization a careful choice of the intermediate neutral state is necessary. In general, the ion yield for (1 + 1)-photon ionization at such low laser intensities as lo7 W/cm2 lies in the range of 10-2-10-1 of all illuminated molecules; this is a fairly large number considering the short ionization time (several nanoseconds). In fact, ion densities can even be so high that space charge effects induce large initial kinetic ion energies, leading to a drastic reduction of mass resolution in linear TOF analyzers and even strong effects in energy correcting reflectrons (see section V); furthermore, space charge effects lead to a blowing up of the ion cloud, which induces a strong divergence of the ion trajectories and therefore a low transmission of timeof-flight analyzers. At the same time, very high laser intensities, which may allow very large ion yields, may severely disturb the spectroscopy of neutral or ionized molecules by saturation or lifetime broadening effects or by inducing further unwanted photon processes, Le., production of stable or dissociating highly excited molecular ions. In conclusion, for many applications of multiphoton ionization a careful choice of the ionization mechanism, the neutral intermediate states, and the focusing conditions is necessary.

+

+

111. Multiphoton Ion Source

The dominant characteristics of multiphoton ionization as an ion source are soft ionization and species- and state-selective ionization as well as optimum features for ion optics of mass selectors. (i) Soft ionization is of interest for neutral as well as ion spectroscopy. For most (1 1) and (2 + 1) one-color absorption processes the excess energy above the ionization threshold is too

+

2952 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991

Boesl

small to reach the first dissociation threshold. To give an example, the benzene time-of-flight spectrum with a laser intensity of lo7 W/cm2 is displayed at the top of Figure 3. Another example is displayed in Figure 13 (section VIII). Here, a large biomolecule, namely, chlorophyll with a mass of 980 amu, has been ionized by two-photon absorption.20 These examples show that soft ionization conditions (low laser intensity) give little, if any, fragmentation. (ii) Species-and state-selective ionization is particularly suitable for ion spectroscopy. Species-selective ionization is possible by choosing a laser wavelength that can be absorbed preferentially by the molecule of interest, thus resulting in a fairly pure ion source. This is an especially valuable option of multiphoton ionization if the parent molecule exists in a mixture of species (e.g., natural isotopic mixture). On the other hand, by selection of special intermediate states, molecular ions can be prepared in a few vibrational levels of their ionic ground state or even in a single one. This can be tested by measuring the kinetic energy of the emitted photoelectrons and has been shown for molecular ions such as the toluene cation,2’ CH31+,10NH3+,22*23 and many othe r ~ (For . ~ ~a review of laser-induced photoelectron spectra giving information about ionic ground state excitation after multiphoton ionization see ref 24.) Even rotationally cold ions can be formed by selective excitation of rovibronic intermediate states or by cooling the neutral parent molecules in a supersonic molecular beam. Particularly for larger organic molecular ions (e.g., benzene), these advantages of laser ionization are very interesting because conventional ionization techniques produce hot ions; this results in a strong congestion of the ion spectra due to hot bands, thus precluding the observation of assignable ion spectra. Some examples of multiphoton-induced photoelectron spectra (deuterated methyl iodide, benzene, and monofluorobenzene) demonstrating state selective ion preparation are shown in Figure 2. For all of these molecules it was possible to find an intermediate state (usually the vibrationless SIor a vibrationless Rydberg state) to form molecular ions in their vibrationless ionic ground state. Due to the similar geometry of these intermediate states and the ionic ground states, these do = 0 transitions are favored by Franck-Condon factors. In the case of benzene even the population of one quantum of a special vibration in the ionic ground state was possible. (iii) These features of a resonant multiphoton ion source are especially useful in combination with mass-selective detection. In particular, time-of-flight mass analyzers can easily be combined with these kinds of ion sources due to some of their additional characteristics, i.e., formation of ions in a very short time (due to laser pulse lengths of 10 m and less), in a very small and defined volume (due to laser foci with a diameter of 100 pm and less) and with high ion densities.

IV. Multiphoton Fragmentation Multiphoton fragmentation may be a problem with laser ion sources when used for neutral or ion spectroscopy. On the other hand, controlled multiphoton fragmentation may help to distinguish isomers or to perform dissociation spectroscopy of ions. Therefore, some aspects of multiphoton ionization concerning fragmentation will be discussed in this section. In the early days of molecular multiphoton mass spectroscopy, multiphoton ionization followed by extremely strong fragmentation was observed25,26at intensities above the soft ionization limit.6q2’ (20) Boesl, U.; Grotemeyer, J.; Walter, K.; Schlag, E. W. A n d . Instrum.

1987, 16, 15 1.

(21) Meek, J. T.; Long, S. R.; Reilly, J . P.J . Phys. Chem. 1982,86, 2809. (22) Glownia, J. H.; Riley, S. J.; Colson, S.D.; Miller, J. C.; Compton, R. N. J . Chem. Phys. 1982, 77, 68. (23) Conaway, W. A.; Morrison, R. J.; Zare, R. N. Chem. Phys. Lett. 1985, 113, 429. (24) For a review see: Kimura, K. Inr. Reu. Phys. Chem. 1987, 6, 195. (25) Zandee, L.; Bernstein, R. B. J . Chem. Phys. 1979, 71, 1359. (26) Boesl, U.; Neusser, H. J.; Schlag, E. W. J . Chem. Phys. 1980. 72, 4327. (27) Boesl, U.; Neusser, H.J.; Schlag, E. W. Chem. Phys. 1981,55, 193.

6 H6+

~-f’16~

bll

101

0 I-

w

w

2

P I L

TI

0

0

100

1

0

100

C,H,F+

100

ION INTERNAL ENERGY lmeVl

Figure 2. Photoelectron spectra of the multiphoton-ionized molecules CD,I (top), C6H6 (middle), and C6H5F(bottom), measured in our laboratory. At the right side the excitation schemes are displayed. For C6H6,two different intermediate states have been involved, giving rise to two photoelectron spectra.

By use of a UV intensity of lo9 W/cm2, benzene can easily be fragmented so that over 50% of all fragment ions are ionized carbon atoms. By variation of the laser intensity or laser pulse length, different degrees of fragmentation can be reached. Several models have been postulated to explain this multiphoton ionization fragmentation behavior.28-31 In the autoionization-ladder model2*the neutral molecule is excited via autoionizing states to very high energies above the ionization threshold until the molecule breaks up into different neutral and ionized fragments. However, this process should only be active at extremely high laser intensities. One exception is the accidental excitation of a long-lived autoionizing state by multiphoton ionization. For some molecules (e.g., NH,22) vibrational autoionization has been found. In general, it has been found that further absorption above the ionization threshold takes place in the parent ion.26*32 When exciting with conventional lasers (nanosecond pulses) this absorption is interrupted by dissociation channels to fragment ions. Absorption within these fragment ions than continues until the next dissociation channels “switch over” to a smaller fragment ion. This “ladder switching” model was proposed by our group29and verified by a two-color experiment with two laser pulses delayed in time and their foci shifted in (28) Lubman, D. M.; Naaman, R.; &re, R. N. J . Chem. Phys. 1980,72, 3034. (29) Rebentrost, F.; Ben-Schaul, A. J . Chem. Phys. 1981, 74, 3255. (30) Silberstein, J.; Levine, R. D. Chem. Phys. Lerr. 1980, 74, 6. (3 I ) Dietz, W.; Newer, H.J.; -1, U.; Schlag, E. W. Chem. Phys. 1982,

66, 105.

(32) Boesl, U.; Neusser, H.J.; Schlag, E. W. Chem. Phys. Leu. 1982,87, 1.

Feature Article space.32 In recent experiments we were able to follow this fragmentation tree step by step using a new tandem time-of-flight techniq~e-’~--’~ (for an explanation of this technique see section V). For picosecond laser excitation it has been shown35that absorption takes place mainly in the ladder of parent ion states. The reasons are (i) at the much higher laser intensities laser excitation overcomes the dissociation processes within the molecular ion and (ii) fragment ions are formed with a certain time delay after parent ion excitation and therefore cannot absorb because of the short laser pulse length. The same authors demonstrated that for nanosecond laser pulses absorption and ionization of neutral fragments do not play an important role as a channel for fragment ions.36 However, if fast dissociation of neutral intermediate states is dominating or even suppressing the multiphoton ionization of the parent molecule, multiphoton ionization of neutral fragments may be the main source of ions. This is a sort of neutral ladder switching and has been shown for several molecules, e.g., NH3.a37 Multiphoton ionization has two features caused by the ladder switching process: (a) large production of metastable parent and fragment ions and (b) small kinetic energy release during fragmentation. This has several consequences for neutral and ion spectroscopy. (i) By absorption of one photon above the ionization threshold, slowly decaying (metastable) or stable, excited ions, which induce a strong background signal, may be formed. (ii) Neutral and ionized fragments produced by uncontrolled multiphoton processes may cause an additional structure in the spectrum of neutral or ionized molecules. Therefore, mass-selective detection or at least a preliminary check is often important for laser spectroscopy of molecules and molecular ions, of large molecules and clusters in particular. (iii) On the other hand, multiphoton excitation can serve as a source of neutral and ionized transients for spectroscopy or beam experiments. In comparison with other sources like electron-impact or plasma ionization, fragments formed by multiphoton excitation may have small internal energies, especially when they are products of metastable precursors. By adjustment of the laser intensity and wavelength, the yield of the species of interest can often be optimized. (iv) Small kinetic energy release is essential for a high mass resolution in time-of-flight mass spectrometers, even in energy-correcting reflectrons. A high mass resolution allows a narrow observation time gate ( ~be * discussed. Multiphoton dissociation of the benzene cation can be described by a fragmentation tree whose levels correspond to a certain number of absorbed photons. Within these levels, sequential and parallel fragmentation channels can be active. After absorption of two UV photons, the molecule is ionized; after three photons two metastable decay channels lead to C6H5+and CaH4+; after absorption of four photons several channels open up, whose products C4H4+,C4H3+,C4H2+,and C3H3+are the starting points for further absorption-dissociation ladders. The degree of fragmentation can be controlled by the laser intensity inducing the absorption of more or less photons. This behavior is illustrated in Figure 3 where benzene has been ionized with four different laser intensities; the degree of fragmentation varies from very soft fragmentation (at the top of Figure 3) to mainly production of carbon ions (at the bottom of Figure 3). The ladder switching is illustrated in a schematic way on the right side of each mass spectrum. In the future, this variable fragmentation may be a new tool for investigating the structure of large molecules as in the case of angiotensin (a decapeptide with a mass of 1995 amu).m At low intensity, only the molecular ion peak appeared in the mass spectrum. At high intensity, groups of fragment ions corresponding to the stepwise separation of single amino acids dominate the mass spectrum.

V. Mass-Selective Detection by Time-of-Flight Analyzers The wealth of experimental possibilities offered by multiphoton ionization can only be exploited when used in combination with mass-selective ion optical techniques. One of the most promising of such techniques is the time-of-flight analysis. It is highly suitable for a combination with pulsed lasers. In particular, reflectrons” offer several features that are very useful for neutral and ion spectroscopy. In conventional linear time-of-flight instruments the ion energy distribution AU resulting from (1) the initial kinetic energy of the neutral parent molecule, (2) the kinetic energy released during fragmentation, (3) space charge effects, and (4) ion production within a finite volume causes a large distribution of flight times and therefore low mass resolution. This (41) Baer, T. Comments At. Mol. Phys. 1983, 13, 141.

2954 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 DETECTOR

I LASER 1

L1

LZ

A

;h

b high energy !on 101 energy >on

0

i'

ION R E F L E C T O R

40ns

+ U t i LASER1

HV

L2

A -rL

-?-

L1

'

I

ION R E F L E C T O R

i5 ns' 25 ns

@ LASER 2 LASER 3

DETECTOR

I

i"

Figure 4. Different arrangements involving a reflection time-of-flight analyzer and primary and secondary laser excitation. (A) Illustration of a conventional reflectron, (B)reflectron with a high-voltage pulsed ion source, (C) reflectron with secondary excitation in the space focus of the ion source.

energy distribution is corrected in a reflectron analyzer using special reflecting fields (see Figure 4A): high-energy ions penetrate further into this reflecting field before being reflected than lowenergy ions of the same mass; this causes a longer dwell time in the reflecting field for the high-energy ions which, on the other hand, spend less time in the field-free drift region due to their higher velocity. By choice of the correct electric fields within the ion mirror, both time shifts can be compensated and ions of different kinetic energies but with the same mass reach the detector at the same time. This correction condition for the ion energy is independent of the ion mass. In the following, three advantages of reflectrons are discussed. ( 1 ) The high mass resolution M/AM of the reflectron is due to a favorable ratio of time-of-flight t to ion peak width At ( M I A M = (1 /2)t/At); it enables a very efficient suppression of noise, which may be caused by specific unwanted ions or random background. A mass resolution of 4000 can easily be reached, with some effort even M/AMof 10000 is possible.42 Recently, in a newly constructed reflectron M/AM of 35 000 has been achieved.43 (ii) There are operation modes of reflectrons which allow the measurement of decay times in the range of 20 ns to l ps" and l to 20 ps.45*46 As for spectroscopy, the decay behavior of molecules and molecular ions is particularly important for photodissociation spectroscopy. For an introduction into these techniques and further references see refs 44, 45, and 46. (iii) The reflector end plate can be used as a low-energy filter for eliminating all ions with (42) Walter, K.; Boesl, U.; Schlag, E. W. Inr. J. Mass.Spectrom. Ion Processes 1987, 78, 69. (43) Bergmann, T.; Martin, T. P.; Schaber, H. Rev. Sci. Instrum. 1989, rsn. - -,-1411. .-, 792 . - -. (44) Boesl, U.;Weinkauf, R.; Walter, K. In Aduances in Laser Science-IY; Gole, J. L., Heller, D. F.,Lapp, M.,Stwalley, W. C., Eds.; American Institute of Physics: New York, 1989; p 488. (45) h l , U.;Weinkauf, R.; Neuuer, H. J.; Schlag, E.W. J . Phys. Chem. 1982, 86, 4857. (46) Neusser, H. J. J . Phys. Chem. 1989, 93, 3897.

Boesl energies higher than its potential. This is important when ions of the same mass are produced by different photon processes but only those of one process are to be detected. For instance, for multiphoton ionization of smaller molecules (2 1)-photon absorption is often necessary. In this case, high laser intensities have to be used to obtain reasonable ion yields; on the other hand, these high intensities favor further absorption within the molecular ions and finally their fragmentation (see section IV and Figure 3 ) . If fragmentation by secondary excitation (e.g., dissociation spectroscopy of ions, see section IIV) is to be achieved, primary fragmentation during ion production may disturb or even prevent the detection of these secondary fragment ions. In Figure 4, B and C, two arrangements for solving this problem are displayed. In both cases, ion kinetic energy differences are introduced between primary and secondary fragment ions. The arrangement in Figure 4A can be used for secondary excitation if no primary fragment ions interfere with secondary ones. Arrangement E (Figure 4B). By delaying primary (multiphoton ionization) and secondary (multiphoton dissociation) laser excitation in time and applying a short high-voltage pulse to the repeller electrode of the ion source (see Figure 4B) an energy difference between primary and secondary fragment ions can be introd~ced.~' The advantage of this arrangement is that only a delay of about 50 ns between the two lasers is required. This can be achieved by optical delay lines so that only one pump laser is needed for two dye lasers. An example of such a discrimination is given in Figure 4B (insert). Laser 1 produces C H 3 P and CH3+fragment ions by ( 2 1)-photon ionization of CH31+. Laser 2 excited CHJ+ into the lowest excited electronic state; the absorption of a second photon also produces CH3+fragment ions. If a 50-11stime delay between laser 1 and laser 2 is applied, the secondary CH3+ fragments will be formed a t a shifted position in the ion source (due to the moving molecular ion cloud) inducing a difference in energy. This effect is, however, so small that these secondary methyl ions appear as an unresolved shoulder in the time-of-flight spectrum of the primary methyl ions (Figure 4B, insert a). The application of a high-voltage pulse after laser 1, but before laser 2, leads to a difference in momentum inducing different times of flight (Figure 4B, insert b), but also different ion kinetic energies. The latter effect is used in Figure 4B, insert c: The reflector end plate can be set at a potential lying between the ion kinetic energy or primary and secondary fragment ions; then the primary ions (higher energy) will hit the reflector end plate and disappear from the time-of-flight mass spectrum (Figure 4B, insert c). Arrangement C (Figure 4 0 . Another operation mode of the reflectron, a tandem time-of-flight technique, is very useful for spectroscopy if, in spite of soft and selective resonance-enhanced multiphoton ionization, unwanted ions are created in the ion source and the arrangement of Figure 4B is not selective enough. As displayed in Figure 4C, laser 1 creates ions by multiphoton ionization; laser 2 delayed by 20-50 ns excites the molecular ions (for ion spectroscopy see section VII). When the ions reach the field-free drift region they are already separated in space due to different times of flight. In the space focus, a special point in the field-free region (seebelow), the molecular ions are further excited by laser 3, inducing dissociation of the molecular ions excited by laser 2 . Secondary fragment ions have a considerably lower ion kinetic energy than primary fragment ions and can be selected as described above. As for the space focus of an ion source$8 at this point all ions of the same mass are focused in time despite their different kinetic energies; ions of different masses are, however, separated by the different velocities they have. The field-free drift region up to the space focus, therefore, represents a very short, simple timeof-flight mass separator. By means of a special construction of the ion source (i) this space focus can be shifted away from the

+

+

(47) Weinkauf, R.; Walter, K.; Boesl, U.; Schlag, E.W. Chem. Phys. Len. 1987, 141, 267. (48) Wiley, W. C.; McLaren, I. H. Rev. Sci. Insrrum. 1955, 26, 1150. Poschenrieder, W. P. Int. J . Mass. Spectrom. Ion Phys. 1975, 16, 353.

Feature Article

The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 2955

ion source (longer drift region) and (ii) an even tighter time focusing can be reached by second-order energy c~rrection.’~*’~ This allows a mass resolution of up to 700 in a drift region as short as 12 cm. The excitation of one molecular ion out of a mixture of ions is then possible by choosing the right time delay between laser 1 and laser 3; this delay time is equal to the time-of-flight of the selected molecular ions from the point of ionization to the space focus. The focus of laser 3 overlaps with the space focus. By choosing a wavelength for laser 3 (maybe even a fixed frequency) that cannot be absorbed by the unexcited molecular ions, we can record the excitation of these ions by laser 2 (tunable for spectroscopy) with high selectivity. A special advantage of arrangement C for ion spectroscopy is that a low intensity of laser 2 (only one-photon process) can be applied, avoiding power broadening of the spectra while a high intensity of laser 3 can be used for detection. As for neutral spectroscopy, for some cases of multiphoton ionization spectroscopy, e.g., isomers, some further characterization in addition to the molecular mass may be necessary. For example, a particular neutral molecule may be identified by characteristic fragments. In these cases, the tandem time-of-flight technique described above can also be used very effectively. As laser 1 is now responsible for the spectroscopy, laser 2 is not needed; but laser 3 creates a specific secondary mass spectrum, which can easily be separated from all other masse^.^',^^

VI. Mass-Selective Laser Spectroscopy of Neutral Molecules With resonance-enhanced multiphoton ionization spectroscopy? the neutral intermediate state is investigated spectroscopically; the ionization itself acts as a detection mechanism. Since its introduction in 1975,49 this spectroscopic technique has become a very popular method. Two-photon ionization with tunable lasers had already been achieved in 1971 for Rb atoms.50 The advantages of multiphoton ionization spectroscopy compared with absorption or fluorescence spectroscopy are very high sensitivity (in principle all ions can be detected), no problems with stray laser light, and nonreliance on fluorescing conditions. The latter aspect is particularly important for the spectroscopy of highly excited states, Rydberg states, or metastable atomic states (e.& the 2 s state of atomic hydrogens1). For most problems of low particle densities multiphoton ionization spectroscopy is the appropriate spectroscopic method. A simple but sensitive arrangement for this method is a gas cell very similar to a proportional counter6 which, however, does not allow mass-selective spectroscopy. One outstanding advantage of multiphoton ionization spectroscopy with mass-selective detectors is the suppression of background. Sources of background are mostly ions of other species or fragment ions of the investigated molecule itself, as already mentioned in section 111. Even species-selective spectroscopy of a mixture of molecules is possible (e.g., spectroscopy of rare isotopic molecules in their natural isotopic mixture), as well as the spectroscopy of larger, fragile molecules (Le., molecular clusters, biomolecules) without interference from absorbing, fluorescing or ionized fragments. On the other hand, the introduction of supersonic beams (particularly pulsed beams) into molecular spectroscopy was a major breakthrough because of the considerable simplification of the molecular spectra. Since a two-stage vacuum system is generally needed for this technology, the final step of adding simple ion optics, a field-free drift region, and an ion detector is now straightforward. Thus, with fairly little effort, mass-selective laser spectroscopy can be combined with supersonic beam techniques. Mass-selective detectors in combination with laser spectroscopy have been applied to di- and triatomic molecules since 1977*s and to the first polyatomic (more (49) Johnson, P. M.;Berman, M.R.; Zakheim, D.J . Chem. Phys. 1975, 62, 2500. Petty, G.;Tai, C.; Dalby, F. W. Phys. Rev. Lett. 1975, 34, 1201. Hurst, G.S.;Payne, M.G.;Nayfeh, M.H.; Judish, J. P. Phys. Rev. Letr. 1975. 35, 82. (50) Ambartzumian, R. V.; Kalinin, V. P.; Letokhov, V. S . JETP Len. 1971. 13. -. -217. (51) Hildum, E. A.; ~

~1

Boesl, U.;McIntyre, D. H.; Beausoleil, R. G.;Hlnsch, T. W.Phys. Rev. Lett. 1986, 56, 516.

2590

-

2589

2722

Laser Wavelength

2721

2669

26625

iAI-

Figure 5. Multiphoton ionization spectra (spectroscopy of the neutral intermediate state) of vibronic bands of the &-So transition of (a, left) C6H6,(b, center) CBHIO, and (c, right) C7H8and their ”C, isotopomers. Spectra of isotopomers have been measured simultaneously.

than triatomic) molecule by our group in 1978.6 From the many applications (already carried through or still possible) of laser mass spectrometry to the spectroscopy of neutral molecules, one special problem is referred to in the following: isotope selective spectroscopy. Isotope-Selective Spectroscopy. The measurement of isotope shifts may be a valuable aid for assigning vibronic spectra of molecules. Vibrational isotopic shifts contain information about the motion of particular atoms and thus help to identify vibrations or calculate force fields. Zero-point isotopic shifts help to identify electronic origins and/or provide information about the nature (i.e., bonding, nonbonding, etc.) of the promoted electron. Knowledge of isotopic spectra may also allow a photochemical enrichment of isotopic molecule^'^ and, in some cases, the tracing of chemical reactions. In this field, laser spectroscopy with mass-selective detection opens up new possibilities; particularly, laser Spectroscopyof natural isotopic mixtures or enriched mixtures with low purity makes expensive synthetic isotope substitution unnecessary. As an example, some spectra of organic molecules observed in their natural isotopic abundance are presented in Figure 5 . They have been measured by setting the time window of a gated integrator just at the top of a preselected ion peak within a time-of-flight mass spectrum. Thus mass-selected laser spectra of one or even several molecules (e.g., isotopomers) can be registered simultaneously. In Figure 5a a vibronic band of benzene and its 13CIisotopomer is displayed. This band represents the transition from the vibrationless molecular ground state So to the first excited singlet state S1with one quantum of the u6 vibration excitation. It has been measured using the natural isotopic mixture of benzene; the rotational temperature corresponds to a Trot= 5 K due to cooling in a supersonic molecular beam (3 bar, Ar as carrier gas). As the S,(0, = 1) is a degenerate state the reduction of symmetry by substitution of one I3C causes a splitting of this band. The two isotope bands have been shifted by +2.5 and -2.5 cm-l, respectively, and have changed their rotational envelope considerably. Thus, isotope substitution delivers information about the symmetry of vibronic levels. In Figure 5b similar spectra can be seen for p-xylene and its I3C1isotopomer; they are also measured simultaneously by using the natural isotopic mixture and a supersonic molecular beam. The band represents the vibrationless transition from So to S1 (origin), which is allowed in p-xylene in contrast to benzene. As no degenerate state is involved in this transition, no splitting of the band is induced by 13C substitution, although a symmetry reduction is also induced in this case, too. The somewhat broader envelope of the isotopic band is now caused by two different positions that are possible for the substituted 13Catom: in one of the methyl groups or in the carbon ring. In the latter case, two sites are possible (next to a methyl group or in meta position), which may, however, not differ noticeably in their isotopic shifts. The intensities of the bands in the spectrum as a result of the two isotope positions (ring or methyl group) should be 6:2 according to the number of replaceable carbon atoms. The m*transition SI-Sohas mainly an effect on the *-system of the ring; the I3C

2956 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991

h 7.9 cm-1

i

c

1

71 761

71782

v

1I"[

Figure 6. Multiphoton ionization spectra of an intermediate Rydberg state of OCS (one vibronic band). The isotope shifts of O"CS and

OC"S (measured at the natural isotopic mixture) deliver information about the type of vibration. Spectra are recorded simultaneously.

substitution in the ring will therefore induce a larger isotopic shift of the zero-point energy than the I3C substitution in the methyl group. This is reflected by the observed isotope shifts of the shoulder in the red in Figure 5b of +2.6 cm-' (methyl group) and of +3.7 cm-' of the main peak (ring). In Figure 5c the origin of the SI-Sotransition of toluene and "Cl-substituted toluene is shown; both spectra are measured simultaneously as in the other spectra of Figure 5. The better resolved structure is due to the narrower line width of the laser. Similar arguments as in the case of xylene are valid. The intensities for the ring and methyl substitution should be 6:l. Obviously, the strong R branch in Figure 5c is due to the ring substitution which results in the larger isotope shift; the isotope band due to the methyl substitution leads to a washing out of the sharp decrease between R and P branches and to the small shoulder in the red. Observed isotope shifts are 3.8 cm-l (ring) and 2.7 cm-l (methyl group). A much clearer discrimination between ring and methyl group substitution would be possible, if one could distinguish between both structural isomers. Normally, this is possible with massselective detectors when observing the fragmentation pattern of the ions of these isomers and monitoring a fragment ion which is typical of one structure. Fragmentation is easily possible with high enough laser powers (see section IV). In the case of xylene and toluene ions, however, fast isomerization washes out all structural information. But at the right wavelength and with high enough laser intensities, the ion can in some cases be excited to energies where dissociation is fast and overcomes isomerization. This has been shown for the example of alkyl iodides.52 Another example of a useful application of isotope-selective spectroscopy to the assignment of molecular spectra is given in Figure 6. Here, one vibrational band of the triatomic linear OCS molecule is displayed together with the corresponding bands of the OC3*S and OI3CS isotopes. To distinguish between O13CS and OC33S,the 13C0fragment ion rather than the molecular ion has been monitored in the upper spectrum of Figure 6; this is an illustrative example of how to discriminate spectra of isobaric molecules (here isomerization is not a problem as was the case with xylene and toluene) by laser ionization involving fragmen(52) Kuhlewind, H.; Neusser, H. J.; Schlag, E. W. J . Phys. Chem. 1985, 89, 5593, 5600.

Boesl tation. The spectra in Figure 6 have been taken with the natural isotopic mixture (1 .l% OI3CS, 4.2% 0C"S); they have also been registered simultaneously during one laser scan by setting three gated integrators at the right times according to the time-of-flight of the O C S , OC34S+,and the 13CO+ions. The band in Figure 6 lies in a spectral region where several transitions to Rydberg states overlap and therefore vibrational assignment is very troublesome. By means of isotopic substitution, vibrational assignments can be supported substantially. The spectra in Figure 6 show that substitution of 34Sdoes not change the band considerably, so it cannot be due to a C-S stretching mode (about 850 cm-I). Substitution of 13C induces a shift of 8 cm-l; for a C-0 stretching mode of about 2050 cm-I, a much larger shift is to be expected. Therefore, the bending mode (about 1050 cm-I) is the only possible mode to induce this transition. The bending mode in linear triatomic molecules and molecular ions is of special interest: it is a degenerate mode with vibrational angular momentum which can couple with electronic orbital angular momenta. The spectra in Figures 5 and 6 have been taken after cooling in a supersonic beam, but with conventional lasers of reasonable bandwidth (1-0.2 cm-I). They demonstrate that laser spectroscopy with mass-selective detectors delivers isotopic information with negligible additional effort (no additional laser scans or synthetically enriched isotopic molecules). Due to the high ion transmission of time-of-flight analyzers, the necessary sensitivity for such measurements is readily available. VII. Mass-Selective Laser Spectroscopy of Molecular Ions For the spectroscopy of molecular ions, many different spectroscopic techniques exist, i.e. absorption, fluorescence excitation, fluorescence emission or photoelectron spectroscopy. None of them, however, allows mass-selective detection, and often not mass-selective excitation either. Many successful applications of these techniques have been reported but are, nevertheless, subject to diverse further restrictions. In general, absorption spectroscopy of ions is limited to two methods involving ion sources with high ion yields. (i) Gas discharge is one such ion source. The absorption of ions and neutrals can be distinguished by the velocity modulation technique.53 This method has been applied mostly to smaller, closed-shell ions (less reactive, more stable in discharges), less to radical ions, and not at all to larger organic molecular ions. (ii) The other very successful technique of ion absorption spectroscopy is absorption in cold noble gas matrices.s4 However, matrix effects influence the energies of vibronic levels. Of course, with both methods massselective ion preparation or detection is impossible, although unwanted reactions within the gas discharge and photoproducts within the matrix would make this necessary in many cases (e.g., benzene cations). Fluorescence spectroscopy has also been very successfully applied to molecular ions. While fluorescence emission allows spectroscopy of the ionic ground state, fluorescence excitation provides information about excited vibronic states.5s In the former case, excitation has often been performed by electron impact. In the latter case, by use of modern commercial pulsed laser sources, high spectral resolution of less than 0.03 cm-I can be reached. Mass-selective preparation may be possible if ionization and excitation are performed by separate mechanisms. Of course, only fluorescing molecules can be observed by using this kind of spectroscopy. However, for low-lying levels of open-shell molecular ions fast internal conversion will often suppress any radiative emission. Photoelectron spectroscopy is one of the most generally used spectroscopic methods; most of the information about molecular ions has been provided by this technique, using vacuum-UV light (53) Gudeman, C. S.;Saycally, R. J. Annu. Rev. Phys. Chem. 1984,35,

381.

(54) Bondybey, V. E.; Miller, T. A.; English, J. H. J . Chem. Phys. 1980, 72, 2193. For review see: Shida, T.; Haselbach, E.; Bally, T. Acc. Chem. Res. 1984, 17, 180. (55) For review see: Maier, J. P. J . E / . Spectrosc. Relut. fhenom. 1986, 40. 203.

Feature Article sources to investigate excited electronic ion states.56 Since 1980 ionic ground states have been successfully observed by photoelectron spectroscopy after multiphoton ionization of the parent molecule.24 Particularly, vibrational levels of several organic radical cations have been observed since then, e.g., 1,3-butadiene,s7 or monofluorobenzene6' naphthalene,s8 aniline,22phenol?9 xyeln@ e, ' and many others.24 The major problem with photoelectron spectroscopy is its low resolution (typically worse than 100 cm-' for conventional photoelectron spectroscopy). Only the ZEKE technique (zero kinetic energy photoelectron spectroscopy)62allows a resolution of up to 1 cm-I. In principle, mass-selective detection for photoelectron spectroscopy is possible by photoion-photoelectron coincidence techniques. Both ZEKE and coincidence techniques are, however, very time consuming. There are two ion spectroscopic techniques which have massselective detection as an intrinsic feature; photoionization and photodissociiation spectroscopy. By conventional photoionization spectroscopy (Le., one-photon absorption), ionization thresholds and vibrational structures of the ionic ground states of many molecules have been observed. Multiphoton ionization spect r o ~ c o p yhas ~ ~ been applied to polyatomic organic molecules, including naphthalene,M aniline,65diazabicyclooctane,M benzene and benzene-Ar complexes,67and many others. Mass-selective detection of molecular ions, whose intensity is registered as a function of the wavelength of the exciting light source, is straightforward for photoionization spectroscopy. The very poor spectral resolution is, however, the main reason why this method is mainly limited to the measurement of ionization thresholds. There are no such restrictions for photodissociation spectroscopy; it can be applied to radiative as well as nonradiative states, mass-selective ion preparation and detection are intrinsic features and there is no principal limitation to spectral resolution other than the bandwidth of the light source, Doppler broadening, or natural line widths. There are two kinds of photodissociation spectroscopy: (i) One-photon dissociation spectroscopy (1 -PDS) and (ii) resonance-enhanced multiphoton dissociation spectroscopy (REMPDS). In the latter case, one-photon absorption into the ion state of interest is followed by absorption of one or more photons to a dissociating state. In both cases, the product ion current is measured as a function of the wavelength of the first photon. One-photon dissociation spectroscopy is, of course, restricted to predissociating states; this is not the case for multiphoton dissociation spectroscopy. In most earlier experiments of photodissociation spectroscopy, predissociating states were investigated. In these experiments, molecular ions have been formed in electron impact, plasma, or similar ion sources and preselected by mass filters. Spectroscopy was performed by using continuous wave lasers and by tuning the laser frequency or the Dopper shift of a fast ion beam. Some (56) For review see: Eland, J. H. D. Photoelectron Spectroscopy, 2nd ed.; Butterworths: London, 1984. Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S . Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies Press: Tokyo, 1981. (57) Woodward, A. M.; Chupka, W. A,; Colson, S. D. J . Phys. Chem. 1984, 88, 4567. (58) Hiraya, A.; Achiba, Y.; Mikami, N.; Kimura, K. J . Chem. Phys. 1985.82, 1810. (59) Anderson, S. L.; Goodman, L.; Krogh-Jespersen,K.; Ozkabak, A. G.; Zare, R. N.; Zheng, Ch. J. Chem. Phys. 1985, 82, 5329. (60) Walter, K.; Scherm, K.; Boesl, U.Chem. Phys. Left. 1989,161,473. (61) Walter, K.; Boesl, U.;Schlag, E. W. Chem. Phys. Lett. 1989, 162, 261. (62) Muller-Detlefs, K.; Sander, M.; Schlag, E. W. Chem. Phys. Lett 1984, 112, 291. (63) In o p p i t e to neutral multiphoton ionization spectroscopy (see section VI), at multiphoton ionization spectroscopy of the ionic ground state the first photon wavelength is fixed and the second photon wavelength is tuned around and above the ionization threshold. (64) Cooper. D. E.; Frueholz, R. P.; Klimcak, C. M.; Wessel, J. E. J. Phys. Chem. 1982, 86, 4892. (65) Smith, M. A.; Hager, J. W.; Wallace, S. C. J . Chem. Phys. 1984.80, 3097. .. (66) Fujii, M.;Mikami, N.; Ito, M. Chem. Phys. 1985, 99, 193. (67) Fung, K. H.; Selzle, H. L.; Schlag, E. W. 2.Naturforsch. 1981,360, 1257. Fung, K. H.; Henke, W. E.; Hays, T. R.; Selzle, H. L.; Schlag, E. W. J . Phys. Chew. 1981, 85, 3560.

The Journal of Physical Chemistry, Vol. 95, No. 8,1991 2957

i

2n 2" REMPDS lPDS

10.5-3mJI {QlmJI Laser pulse energy

CD;

TIME-OF-FLIGHT PROFILE AT

h*

Figure 7. Illustration of 1 and (1 + 1) photodissociation spectroscopy (1-PDS and REMPDS) of molecular ions (Le., CD31+). At the left side the excitation scheme is displayed, and at the right side the ion spectra observed by 1-PDS and REMPDS. The predissociation threshold is marked by the dashed line. At the bottom (right) time-of-flight profiles are shown. The same vibronic level of the A state (at X*) has been excited during I-PDS and REMPDS. From the different decay times it is clearly seen that different final states are reached by I-PDS and REMPDS.

excellent experiments have been performed applying this spectroscopic method, i.e., on H30+P8N20+P9and SO+70(for a review see ref 9). Later on,pulsed dye lasers have been used, which allow a considerable extension into the UV spectral range and have been used for predissociation spectroscopy of N20+,71 02+,72 0CS+,l6 or CH31+.10*uIn the latter case, two new techniques have been used: multiphoton ionization as an ion source and time-of-flight spectrometry as mass-selective ion detection. Finally, our group succeeded in measuring the first well-resolved multiphoton dissociation spectra of polyatomic molecular ions,e0 fully utilizing the advantages of multiphoton ionization and reflectron time-of-flight analyzers. There is already a short history of multiphoton dissociation spectroscopy on polyatomic molecular ions, Le., halogenated benzenes,') methylnaphthalene, and naphthalene74or butadiene.s7 However, no structure and in one case only a crude vibrational structure74has been observed. Very shortly after our multiphoton dissociation spectra of CH31+, high-resolution spectra of N20+ and CS2+,75of the monochlorobenzene cation," and of the pfluorobenzene cation (detected by ion dip spectro~copy)'~ were reported. In the following, the multiphoton dissociation spectroscopy on CH31+,u,78the benzene ~

~~~

~~

(68) Carrington, A.; Kennedy, R. A. J . Chem. Phys. 1984, 81, 91. (691 Abed. S.: Brover. M.: Carre.. M.:. Gaillard. M. L.: Larzillitrc. M. Chem.'Phys. i983,74; si. (70) Cosby, P. C. J . Chem. Phys. 19%4,81, 1102. (71) Frey, R.; Kakoschke. R.; Schlag, E. W. Chem. Phys. Letf. 1982, 93, .)*.I

LLI.

(72) Frey, R.; Kakoschke, R.; Milller-Detlefs, K.; Schlag, E. W. Z . Phys. 1982, A307, 25. (73) Dunbar, R. C.; Teng, H. H.-I.; Fung, E. W. J . Am. Chem. Soc. 1979, 101,6506. (74) Syage, J. A.; Wessel, J. E. J . Chem. Phys. 1987,87, 3313. Mordant, D.; Looper, G.; Wessel, J. E. J . Phys. Chem. 1984, 88, 5197. (75) Danis, P. 0.;Wyttenbach, T.; Maier, J. P. J . Chem. Phys. 1988,88, 3451. (76) Ripoche, X.;Dimicoli, I.; Lecalve, J.; Piuzzi, F.; Botter, R. Chem. Phys. 1988, 124, 305. (77) Tsuchiya, Y . ; Fujii, M.; Ito, M. J. Chem. Phys. 1989, 90, 6965. (78) Walter, K.; Weinkauf, R.; Boesl, U.;Schlag, E. W. J . Chem. Phys. 1988, 89, 1914.

2958

The Journal of Physical Chemistry, Vol. 95, No. 8. 1991 electronic

l l x v z I origin

12xv21 origin

origin

of v3 p,rogression

of v3 progression

0

+loo0 FREQUENCY [cm-ll

+ZOO0

Av [crf'l

Figure 8. A-X ion spectrum of CDJ'. The molecular ions are prepared by multiphoton ionization; electronic and vibronic origins are marked. The two spectra are due to the two spin-orbit components of the ionic

ground state. cation,79 and the monofluorobenzene cation6' will be presented. Multiphoton Dissociation Spectroscopy of Methyl iodide Cations. The CH31+ cation is an illustrative example of the difficulties arising in the spectroscopy of molecular ions. Even though the methyl iodide cation has been studied extensively (see references cited in refs 10, 44, and 78), no unapbiguous assignment existed for the first electronic transition A-X. The reason is that fluorescence spectroscppy is not applicable due to fast nonradiative processes in the A state and that conventional onephoton dissociation spectroscopy is limited to energy levels above the pcedissociation threshold which lies far above the vibrationless A state. Only with multiphoton dissociation spectroscopy was it possible to obtain a well;res$ved spectrum reaching down to the electronic origin of the A-X transition. As has been shown for the first time by Colson et a1.I0 and illustrated in section 111, cold CH$+ ions can be formed by multiphoton ionization, an ion source which is best suited for ion spectroscopy with lasers. We used the same ionization scheme for our multiphoton dissociation spectra. In Figure 7 an excitation scheme within the ion for one-photon and for multiphoton dissociation is displayed together with a section of the resulting ion spectra. It can clearly be seen that the one-photon spectrum breaks off at the predissociation threshold in contrast to the multiphoton dissociation spectrum which continues to lower excitation wavelengths. In the case of CD31+, both excitation schemes can also be distinguished by typical decay times of the ion, which show up in the time profile of the mass peaks in the time-of-flight mass spectrometer (for optimum observation, a special operation mode of the reflectron has to be used4'). The inserts in Figure 7 (bottom) display such time-of-flight profiles for the bands at the wavelength A*. Furthermore, these decay time measurements are an additional aid for the assignment of spectra. For example, the small peaks at the red end of the I-PDS spectrum could clearly be identified as hot bands (and not the onset of multiphoton absorption) due to their decay times being much longer than for transitions in the REMPDS spectrum.45 In Figure 8 another section of the CD31+spectrum is shown. As the ionic ground state is split into two spin-orbit components, spectra of ions from conventional ion sources always show a mixture of both components. Using multiphoton ionization, the seear_ation of both compon_ents is no problem so that in Figure 8 A-X transitions for both X components can be shown separately. The sharp vibrational bands are to be attributed to narrow rotational envelopes of about 5 cm-'; this is caused by rotational cooling of the neutral methyl iodide molecules before multiphoton ionization. Typical rotational envelopes of CHJ+ at room temperature show a width of 50-100 cm-I. Due to this rotational cooling and to vibrationless preparation of the molecular ions, congestion of bands is avoided in the spectra of Figure 8. Con(79) Walter, K.; Weinkauf, R.: Boesl, U.;Schlag, E. W. Chem. Phys. Len. 1989, 155. 8.

Boesl gestion of vibrational bands is a severe problem in the spectroscopy of polyatomic molecular ions which are formed in conventional ion sources. The spectrum in Figure 8 is dominated by u3 (C-I stretch) progressions starting at several origins. Some of these origins, Le., the electronic origin, the u2 (C-H umbrella) origin and the 2u2 origin are marked_in Figure 8. In total, 300 bands have been observed for the A-X transitions of CH31+and CDJ+, 90% of which could be assigned. Although the hot bands are very weak, those to the red of the electronic origin could also be assigned unambiguously, delivering information about the ionic ground state and its spin orbit components. Multiphoton Dissociation Spectroscopy of Benzene Cations. A very famous example of ion spectroscopy, where neither fluorobenzene nor prediss_oci_ationmethods can be applied, is the first electronic transition B-X of the benzene cation. This transition is dipole-forbidden and the B(E2J s t a F is neither fluorescing (owing to fast internal conversion to the X state) nor dissociating. Even in matrices with high ion densities@no absorption, and only weak, nearly uns_tru_cturedfluorescence was found, which has been assigned as the C-X transition. Most information on the benzene cation comes from photoelectron spectroscopy of electronic statesa1 (with little vibrational informatioin), from laser photoelectron spectra of the ionic gtound state,82and from theoretical Nevertheless, the 8-X transition is of great interest, because it is expected to show sharp structure and therefore to rovide information about the strong coupling between the B and states, which have an energy gae of only a few tenths of an electronvolt. According to theory this C state should d p y on the femtosecond time scale.83 In the spectroscopy of the B state this very strong coupling should mainly influence the-co_upling eZumodes, VI6 and ~ 1 7 . These modes also induce the B-5 tzansition by intensity borrowing from the strongly allowed C-X transition. As pointed out in section 111, benzene cations can be prepared with an efficiency of about 90% in the ionic ground state with excitation of either no vibration or one quantum of the q 6 . For such an ion preparation it is necessary to choose the right neutral intermediate state: the vibrationless SI and the S, with one quantum of the 4 6 - However, these intermediate states can only be reached via hot bands of the S& transition which have just a few percent of the intensity of the main band a t room temperature. Therefore, in the supersonic beam a compromise had to be found between optimum rotational cooling and efficient population of the and V g + VI6 vibrational levels in the neutral ground state. One is tempted to compensate the low hot band intensity by higher laser pulse energies. On the other hand, this may lead to an unwanted absorption of benzene cations into stable excited ion states and disturb or even prevent the measurement of interpretable ion spectra. In Figure 9 the two multiphoton ionization schemes, the laser excitation for the ion spectroscopy (black arrows) and the multiphoton dissociation (white arrows) are shown. For resonant excitation and dissociation of the ions the same laser has been used with a wavelength between 515 and 495 nm. For effective dissociation at least two photons (white arrows) have to be absorbed; after one-photon absorption the dissociation threshold is exceeded, but the dissociation rate is still too slow to be detected in a time-of-flight analyzer. High intensities of the second laser, which may lead to saturation effects or power broadening of the ion spectra, are therefore necessary. The ionic cold band and hot band spectra are displayed in Figure 10. In the cold band spectrum the inducing modes V I 6 and ~ 1 can 7 be made out clearly together with combination bands with the Jahn-Teller active ezBmodes 4 us, and ug and the totally

P

(80) Bondybey, V. E.; Miller, T. A.; English, J. H. J . Chem. Phys. 1980, 72, 2193. Miller, J. H.; Andrews, L.; Lund, P. A.; Schatz, P. N. J . Chem. Phys. 1980, 73, 4932. (81) Karlson. L.; Mattson, L.; Jadrny. R.; Bergmark, T.; Siegbahn, K. Phys. Scr. 1976, 14, 230. (82) Long, S.R.; Meek, J. T.; Reilly, J. P.J . Chem. Phys. 1983, 79, 3206. (83) KBppel, H.; Cederbaum, L.S.;Domcke, W. J . Chem. Phys. 1988,89, 2023.

The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 2959

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P

-

19bOO

laboo

HOT BAND V16 = 1 IONIZATION

TT 38090

10061 608 0

t 1

T

x1

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T

L

v6=l,v16=l

Figure 9. Excitation scheme for (i, ionization) state-selective preparation of C6H6+with two different intermediate states (left and right scheme), for (ii, spectroscopy) excitation of the B state, and for (iii, dissociation) photodissociation as detection mechanism of the excitation of the B state.

symmetric uI mode. From the hot band spectrum the energy of the electronic origin and of the 2uI6level can be deduced when the V I 6 ground-state frequency is used from laser photoelectron spectra.** The 2uI6 frequency of 519 cm-’ turns out to be considerably larger than twice the frequency of 224 cm-’ of the singly excited ulb. This large-inverse anharmonicity is caused by strong coupling between the B and states. This effect and the major features of our cold band spectrum are in good agreement with the theoretical work of Domcke et al. concerning Jahn-Teller and pseudo-Jahn-Teller interactions in the benzene catiomS3 The bandwidths in both spectra are mainly due to rotational structure; power broadening is also a problem. In summary, despite severe difficulties (e.g., multiphoton ionization via hot bands, low intensities of the ionizing laser to avoid unwanted excitation of benzene cations, no optimum rotational cooling, a dipole-forbidden transition in the ion, and the need to absorb another two photons for effective dissociation), benzene cation spectroscopy by REMPDS is obviously possible. Surely, this is also due to the highly selective detection technique involving a reflectron analyzer. A further development of this technique, already displayed in Figure 4C, helps to minimize power-broadening effects and has been applied to the cation of another aromatic molecule, monofluorobenzene. Multiphoton Dissociation Spectroscopy of the Fluorobenzene Cation. Many halogenated benzene cations have been studied by fluorescence spectroscopy in the gas phases4 and in matrices.*’ Nevertheless, there is a large class of halogenated benzene cations that do not fluoresce. All monohalogenated and some dihalogenated derivatives belong to this class. The reason for the suppressed fluorescence is the same as in th_e benzene cation itself, namely the very fast relaxation of the C state to the nearby nonradiative B state. For several other species, such as di(84) Maier, J . p.; Marthaler, 0.;Misev, L.; Thornmen, F. In Molecular Ions, Geometry and EIectronic Srrucrure; Berkowitz, J., Groenewald, K.O., Eds.; NATO AS1 Series 9 Plenum Press: New York, 1983; Vol. 90, p 125. (85) Bondybey, V. E.; Miller, T. A. In Moleculur Ions, Spectroscopy, Structure and Chemistry; Miller, T. A,, Bondybey, V. E., Eds.; North-Holland: Amsterdam, 1983; p 125.

18000

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Figure 10. Hot band (top) and cold band (bottom) spectrum of the B-X transition of C6H6+,formed by resonance-enhanced multiphoton ionization and detected by resonance-enhanced multiphoton dissociation.

bromob-enzene cations and their mono- and difluoro derivative^,^^ the B-X transition is dipole-allowed, but inter@ conversion is very fast due to a small energy gap betwee! the B and X states. In addition, in all these species the B and C states lie below the first dissociation threshold, so that neither fluorescence nor one-photon dissociation spectroscopy can be applied. In some cases absorption spectroscopy in matrices was possible, as for m- and p-difluorobenzene catiomS6 But no such spectra could be observed for benzene or monofluorobenzene ions. We decided to apply REMPDS to monofluorobenzene cations; for monochlorobenzene cations such spectra have already been ob~erved.’~ A comparison of these molecular ions with the benzene cgion thould deliver information abo_ut ;he coupling between the B and C states as a function of the B-C energy gap and may help to assign the ion spectra. The three-laser excitation scheme used in our experiment is shown in Figure 11 and is described in the following. As an ion source, multiphoton ionization via the vibrationless neutral SI intermediate state in a supersonic beam was used; thus, the preparation of rotationally and vibrationally cold monofluorobenzene ions was possible (see Figure 2). As for the spectroscopy of the ion, in this experiment the spectroscopic excitation step and the photodissociation step have been separated by using two different lasers delayed in time and shifted in space (for experimental scheme see Figure 4). This enables one to use a low-intensity laser pulse for the spectroscopy, avoiding spectral broadening. On the other hand, a very intense laser pulse can be applied for multiphoton dissociation to effectively fragmentate and therefore detect the excited ions. To avoid interference by the third laser, its wavelength has to be chosen in such a way that no absorption of photons, due to laser 3, from the ionic ground ~

~~~

(86) Bondybey, V. E.; Miller, T. A,; English, J. H. J . Chem. Phys. 1980, 72, 2193.

2960 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991

Boesl

t

-I.C.

II 2odoo SPECTROSCOPIC STEP

5

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MULTIPHOTON IONIZATION

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so 0 Figure 11. Excitation scheme for (i, ionization) state-selectivepreparation of C6HsF+, for (ii, spectroscopy) excitation of the B state and for (iii, dissociation) photodissociation as detection mechanism of the excitation of the B state. state is possible. For monofluorobenzene cations this is easily achieved by the second harmonic of an YAG laser. Due to the spatial distance between the foci of laser 2 and laser 3 it takes 4 ps for the ions to arrive at laser focus 3. This flight time is long enough for different masses to arrive at laser focus 3 at significantly different times. Hence, this arrangement allows massselective multiphoton dissociation and thus an elimination of background ions produced by laser 1-or 2 (see section V). The observed spectrum of the 8-X transition of the monofluorobenzene cation is shown in Figure 12. Of the three inducing modes ut&, and ~ 1 7 only ~ , the latter two appear in the spectrum with strong intensity; the frequencies of all three of these modes are fairly independent of substitution by one halogen atom and are therefore comprable to the corresponding benzene frequencies. In the benzene B-X ion spectrum, the ut6 and ~ 1 modes 7 have been observed as inducing modes (see Figure lo), which is very similar to the monofluorobenzene ion spectrum (Figure 12). In the latter, however, the intensity of the ~ 1 band 7 ~ is lower in comparison with the V I 6 band. In comparison with the benzene cation, the and ~ 1 frequencies 7 ~ of the monofluorobenzene cation also show a drastic but not such a strong decrease in the ionic B state in comparison to the X state. This may be a hint that there is less strong coupling between the B and C states. A similar effect can be deduced from the monochlorobenzene cation spectrum for uIk. For the monochlorobenzene cation,’6 however, the second strongly inducing mode has been assigned as the vlOamode instead of the ~ 1 mode. 7 ~ On the basis of our results of the benzene and monofluorobenzene cation, the Y~~~ may also be the second inducing mode for the monochloro_benzene cation; an observed decrease in its frequency from the X state to the B state, which is similar to the benzene and monofluorobenzene cation, would support this assignment. VIII. Outlook for Future Work and Summary Rydberg States. Due to the high density of electronic levels the assignment of molecular Rydberg states is very complicated and in many cases not possible without an additional application of new spectroscopic methods. One of these is multiphoton ab-

21boo 2 FREQUENCY I cm”

- LASER

22boo

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Figure 12. Spectrum of the B-X transition of C6HSF+,formed by resonance-enhanced multiphoton ionization and detected by resonance-enhanced multiphoton dissociation.

sorption spectroscopy by coherent two, three, or more photon excitation. Due to different selection rules for two- or three-photon absorption, electronic transitions with different symmetries may be enhanced and therefore distinguished. In most cases, absorption of one further photon induces multiphoton ionization, which is one of the most sensitivie detection schemes for Rydberg states. Even the kinetic energy of the emitted photoelectrons carries information (Franck-Condon factors) about the intermediate Rydberg states. Thus, photoelectron spectroscopy has been used for the assignment of molecule such as OCSg7or C2H2.88 However, any application of photoelectron spectroscopy is restricted by its low resolution, which does not allow unambiguous assignment; this is the case for OCS,where the spin-orbit splitting of the lowest ionic state has not been resolved.87 Hot band ion spectra may allow a much more accurate determination of vibrational frequencies (and their population) in the ionic ground state than photoelectron spectroscopy. Even Fermi resonances within the vibrational manifold of one Rydberg state or coupling between different Rydberg states may be detected by this additional spectroscopy as these perturbations will result in the pop ulation of different vibronic levels of the ion. In addition to ion dissociation spectroscopy, mass-selective detectors offer methods for supporting assignments of spectra. Many molecular Rydberg states show strong dissociation. The fragments can be analyzed in a laser time-of-flight apparatus for their mass, identity (fragment mass spectrum), and kinetic energy release and thus help to discriminate between vibrational levels of different Rydberg states. On the other hand, if such fragmentations play a major role, the multiphoton spectra of the parent molecule and fragments may overlap and prevent any meaningful assignment. It is only by means of mass-selective detectors that these spectra can be disentangled. Large Molecules. High-resolution spectroscopy of large molecules, particularly those of biological interest, is problematic because of their vanishing vapor pressure. Special methods have been developed for the vaporization of nonvolatile species, i.e., plasma desorption, secondary ion mass spectroscopy (SIMS), electro- and thermospray, or laser d e ~ o r p t i o n . ~Especially ~ laser desorption has been used as a neutral and not only as an ion source. But all desorption techniques produce fairly hot ionic and neutral molecules (in the case of neutral desorption with lasers: up to 0.7 evgO). Therefore, at our institute, a combined laser desorp(87) Yang, B.; Eslami, M. H.; Anderson, S.L. J . Chem. Phys. 1988,89, 5527. (88) Orlando, T. M.; Anderson, S. L.; Appling, J. R.; White, M. G. J . Chem. Phys. 1987, 87, 852. Ashfold, M. N. R.; Tutcher, B.;Yang, B.;Jin, Z. K.;Anderson, S. L. J . Chem. Phys. 1987,87, SIOS. (89) For review: Morris, H.R. Sofr Ionizarion Biological Mass Specrromerry; Heyden: London, 1980. Benninghoven, A. Ion Formation from Organic Solids; Springer Series in Chem. Phys. Vol. 25; Springer: Berlin, 1983. Lyon, Ph. A. Desorprion Mass Specrrometry; American Chemical Society: Washington, DC, 1985.

The Journal of Physical Chemistry, Vol. 95, No. 8. 1991 2961

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c

91

I

1

0

,248

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, , , , , ,

100

,

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200

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300

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Figure 13. Time-of-flight mass spectrum of chlorophyll, measured after neutral laser desorption and resonant multiphoton ionization. The sample (methanolic extraction of a cyanobacterium) contained less than 10% chlorophyll, the components at mass 870, 892, and 908 are natural constituents of the probe. The spectrum demonstrates the capabilities of soft and selective detection by the combination of laser desorption, supersonic beam, and resonant multiphoton ionization.

tion/supersonic beam apparatus has been d e v e l ~ p e d ~with ',~~ secondary multiphoton ionization of the desorbed neutral molecules. In Figure 13, the multiphoton ionization mass spectrum of chlorophyll is displayed;20 the probe sample was obtained by extraction of a bacterium. It contained less than 10%chlorophyll and more than 50 other species. The spectrum in Figure 13 illustrates (i) the possibility of soft desorption and ionization of fragile molecules and (ii) high selectivity (extremely low chemical background). The three ion peaks are due to natural constituents of the sample (and not to fragmentation); each of them shows the typical isotopic mass pattern. Using a supersonic beam allows the cooling of internal degrees of freedom: for large molecules, medium- to high-resolution laser spectroscopy can only be carried out at low temperatures. With a similar arrangement (combination of thermospray, thermal desorption, and supersonic molecular beam) the first mass-selected laser spectra of nonvolatile biomolecules ( t r y p t ~ p h a n ,di~~ p e p t i d e ~ ~have ~ ) been observed. Laser spectra have also been obtained when a combination of laser desorption of neutrals with a supersonic beam is used (e.g., smaller peptides9s). These latter spectra show only broad features, but laser desorption is one of the most promising methods for the vaporization of thermally very fragile biomolecules, e.g., chlorophyll or larger peptides.92 For the future it is necessary to develop a laser desorption source that is stable for an hour or longer as well as a better coupling of laser-desorbed molecules into the supersonic beam. Of course, mass-selective detectors are essential for this kind of spectroscopy. Neutral and Ionized Clusters. Another application of laser mass spectrometry is in the large field of neutral and ionized clusters. Unfortunately, most cluster sources produce a broad distribution of cluster sizes and many molecular clusters dissociate easily. Therefore, many experiments (spectroscopy in particular) are not possible without mass-selective detectors.96 Neither (90) Antonov, V. S.;Letokhov, V. S.;Matveyets, Yu. A,; Shibanov, A. N. Luser Chem. 1982, I , 37. (91) von Weyssenhoff, H.; Selzle, H. L.; Schlag, E. W. Z . Naturforsch. 1985, 40A. 614. (92) Boesl, U.;Grotemeyer, J.; Walter, K.; Schlag, E. W. Anal. Instrum. 1987, 16, 151. Grotemeyer, J.; Boesl, U.; Walter, K.; Schlag, E. W. Org. Mass Spectrom. 1986, 21, 645. (93) Rizzo, T. R.; Park, Y. D.; Peteanu, L.; Levy, D. H. J . Chem. Phys. 1985, 83, 4819; 1986, 84. 2534. (94) Cable, J. R.; Tubergen, M . J.; Levy, D. H. J . Am. Chem. SOC.1987, 109, 6198. (95) Trembeull, R.; Lubman, D. M. Anal. Chem. 1987,59, 1082. (96) For review see: Faraday Symposium No. 25; J . Chem. Soc.,Faraday Trons. 1990, 86 (13). Maier, J. P. Ion and Cluster Ion Spectroscopy and Structure; Elsevier: Amsterdam, 1989.

absorption nor fluorescence but ionization and dissociation spectroscopy are adequate techniques for cluster spectroscopy. Therefore, photon excitation of neutral or ion clusters on the one hand, and ion detection on the other hand, have to be mass selective. For neutral cluster spectroscopy the first mass selection can be reached by exploiting the mass-dependent slip of large molecules within a supersonic beam of light atoms; another method to obtain mass-selected neutrals is to mass select and reneutralize ions. For ionized or reneutralized clusters tandem time-of-flight mass techniques can be used. For small metal cluster ions, this has already been carried out by integrating a reflectron,97but the mass resolution (Rprim< 200, R, < 50) was not very large. In a new a p p r o a ~ hwe ~ ~constructed .~~ an ion source with a second-order space focus (a mass resolution of Rprim> 700 was reached); secondary excitation can now take place within this space focus (see section V). A reflectron has also been used as mass-selective detector. Its mass resolution R,, should easily exceed 1000 (in a preliminary experiment R , > 350 was reached). A further advantage of excitation within the second-order space focus is the high ion concentration which allows strong focusing of the secondary laser and therefore efficient excitation and dissociation of molecular ions and molecular cluster ions with medium and low transition moments. This principle has been mentioned in section V and applied to the spectroscopy of monofluorobenzene cations in section VII. It may also be applied efficiently to cluster spectroscopy. Isomers. Another field of application for tandem laser mass spectrometry is the study of isomers. Conventionally, one tries to distinguish isomers by analyzing their fragmentation patterns. This is usually done by secondary collision induced fragmentation after a first mass selection.98 Similar measurements are possible with secondary laser excitation and dissociation. When recording the ion current of one specific fragment ion as a function of the secondary laser wavelength, it may even be possible to obtain the spectrum of one isomeric ion out of a mixture of isomers. When recording the ion current of one specific fragment ion as a function of the wavelength of the primary, ionizing laser, spectroscopy of a neutral isomer is possible (see section VI, Figure 6 , top). Unfortunately, fast isomerization processes within the ion (or even in the resonant intermediate state) very often wipe out each characteristic feature within a fragmentation pattern (see p-xylene or toluene in section VI). One solution to the problem is multiphoton absorption at high laser intensities (to overcome isomerization after one-photon absorption) up to high ion internal energies: dissociation processes whose thresholds lie energetically higher than that of isomerization but which show a steeper rise in the rate than those (crossing of isomerization and dissociation rates) can now overcome isomerization and reveal structural characteristic^.^^ On the other hand, spectroscopic information can be used for analyzing isomers by isomer-selective ionization. An illustrative example is that of dichlorotoluene isomers, whose neutral intermediate spectra differ considerably.99 Other examples are CIOH8,'"0C7H80, and C8HloOIo' isomers. In the case of isotopic isomers (e.g., in section VI) strong rotational cooling and a narrower laser bandwidth than in Figure 5 have to be used. Analysis of Traces of Atoms and Molecules. The high selectivity and sensitivity of resonance-enhanced multiphoton ionization and the high resolution of time-of-flight mass spectrometers (reflectrons) make laser mass spectrometry one of the most promising techniques for detection and analysis of traces of special atomic and molecular species. Excitation schemes for resonant (97) Alexander, M. L.; Levinger, N. E.; Johnson, M. A.; Ray, D.; Lineberger, W. C. J . Chem. Phys. 1988,88,6200. LaiHing, K.; Cheng, P.Y.; Taylor, T. G . ;Willey, K. F.; Peschke, M.; Duncan, M. A. Anal. Chem. 1989, 61, 1460. (98) For a review see: Bush, K. L.; Glish, G . L.; Luckey, S. A. Mass SpectrometrylMass Spectrometry; VCH Publishers: Weinheim, FRG, 1988. (99) Lubman, D. M.; Tembreull, R.; Sin, C. H. Anal. Chem. 1985, 57, 1084, 1186. (100) Lubman, D. M.; Naaman, R.; &re, R. N . J . Chem. Phys. 1980,72, 3034. (101) Chang, T. Ch.; Johnston, M. V. J . Phys. Chem. 1987, 91, 884.

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multiphoton ionization of all elements of the periodic system have been publishedto2and "single-atom sensitivity" is claimed by the authors. Others claim a similar sensitivity also for molecules.'03 (For a review see, ref 104.) The highest sensitivity may be reached by a combination of desorption, laser ionization, and time-of-flight analysis. A gaseous probe may be gathered by freezing on a small surface and desorbed by a short laser pulse. The probe is then concentrated in a short gas pulse which can be ionized by pulsed lasers in a much more efficient way due to a favorable duty cycle. It has been possible to detect 1 part in IO8 of a monolayer of organic molecules after freezing, laser desorption, and laser ionization.Io5

IX. Summary In this report, the advantages and problems of multiphoton ionization and fragmentation for neutral and ion spectroscopy are (102) Hunt, G. S.;Payne. M. G.;Kramer, S.D.; Chen, C. H. Phys. Today 1980, Sepr., 24. (103) Letokhov, V. S. Opr. Acra 1985, 32, 1191. (104) Hurst, G. S.;Morgan, C. G.Resonance Ionization Spectroscopy 1986 Inst. Phys. Conf. Ser. Vol. 84; Institute of Physics: Bristol, U.K., 1987. (105) Antonov, V. S.;Letokhov, V. S. In Multiphoton Processes; Lam-

bropoulos, p., Smith, S.J., Eds.; Springer: Berlin, 1984; p 182.

explained and some information about the employment of modern time-of-flight analyzers as mass-selective ion detectors is given. As examples of mass-selective neutral spectroscopy, isotope-selective but simultaneously measured spectra of vibronic bands of benzene, toluene, xylene, and OCS are shown; positive features and problems have been discussed. As for ionized molecules, multiphoton ionization as an ion source and multiphoton dissociation spectroscopy of the cations of methyl iodide, benzene, and monofluorobenzene are presented. These examples as well as section VI11 illustrate the usefulness of laser mass spectrometry (Le., combination of resonant laser excitation and time-of-flight analysis) for the spectroscopy of neutral and ionized molecules. On the other hand, due to the simple arrangement of time-of-flight analyzers (even those with high resolution), many experimentalists should be able to build their own instruments, thus straining their budgets much less than with the laser system which is mostly used for spectroscopy anyway. If a supersonic beam is already used, the additional effort for a time-of-flight analyzer is even smaller.

Acknowledgment. The author is indebted to Prof. E. W. Schlag for his continuous interest in this work and to his co-workers Drs. R. Weinkauf and K. Walter for their numerous important contributions. He is very grateful to Prof. s. D. Colson for carefully reading and discussing the manuscript.

ARTICLES Resonance Raman Spectroscopy of IV-Methyiacetamide: Overtones and Combinations of the C-N Stretch (Amide 11') and Effect of Solvation on the C=O Stretch (Amide I) Intensity Leland C. Maynet and Bruce Hudson* Department of Chemistry, University of Oregon, Eugene, Oregon 97403 (Received: June 11, 1990; In Final Form: October 24, 1990)

Resonance Raman spectra of N-methylacetamide(NMA) and its isotopic derivatives are reported for the vapor phase, acetonitrile, and aqueous solution with excitation wavelengths of 218,200, and 192 nm. The 192-nm vapor-phase spectra of NMA and that of N-deuterated NMA (D-NMA), as well as that of an aqueous solution of D-NMA obtained with 200-nm excitation, exhibit identifiable overtones and combinations with considerable intensity. This intensity distribution is discussed in terms of the change of the molecular geometry along the C-N stretching coordinate upon electronic excitation. The resonance Raman spectra of NMA in the gas phase and in acetonitrile solution have considerable intensity in the amide I (C=O) stretching vibration. This mode is absent or very weak in the spectra obtained with aqueous solutions. Spectra obtained in mixtures of acetonitrile and water show that the amide I vibration decreases roughly linearly in intensity with water content. Various hypotheses concerning this dramatic effect of solvation on the resonance Raman spectrum of NMA are presented and discussed.

Introduction The simple peptide N-methylacetamide (NMA) has long served as a model compound for understanding the spectroscopy of the peptide bond. This has been the case for the recently developed technique of ultraviolet resonance Raman spectroscopy.'-5 These studies have illustrated several interesting points concerning the resonant excited-state and the ground-state modes of motion of this simple peptide. Isotopic effects such as N-deuteration result 'Present address: Department of Biochemistry and Biophysics, University of Pennsylvania. Philadelphia, PA 19104.

in very substantial changes in the resonance Raman spectra.' Despite rather extensive study, new aspects of the resonance ( I ) Mayne, L. C.; Ziegler, L. D.; Hudson, B. Ultraviolet Resonance Raman Spectroscopy of N-Methylacetamide. J. Phys. Chem. 1985, 89, 3395. (2) Dudik, J. M.; Johnson, C. R.; Asher, S. A. UV Resonance Raman Studies of Acetone, Acetamide and N-Methylacetamide: Models for the Peptide Bond. J . Phys. Chem. 1985,89, 3805. (3) Mayne, L. C.; Hudson, B. Far Ultraviolet Resonance Raman Spectroscopy: Studies of the Peptide Bond. In Proceedings of the XIth fnternarional Conference on Raman Sperroscopy; Clark, R. J. H., Long, D. A., Ed.; John Wiley & Sons: Chichester, 1988; p 745.

0022-365419 112095-2962$02.50/0 0 1991 American Chemical Society