J. Phys. Chem. lQ82, 86,1178-1184
1178
Systematics of Multiphoton Ionization-Fragmentation of Polyatomic Molecules Richard B. Bernsteln Department of Chemistry, Cohmbia University, New Y&, I n Final Form: September 15, 1981)
New York 10027 (Receivd: September 8, 1981;
This paper reviews the work of the author’s group (co-workersL. Zandee, D. A. Lichtin, S. Datta-Ghosh, K. R. Newton, and D. H. Parker) in the field of laser multiphoton ionization mass spectrometry. An attempt is made to extract the systematics of the multiphoton ionization (MP1)-fragmentation phenomenon from the large body of experimental data on a variety of representative polyatomic molecules. The overall process can most simply be interpreted in terms of three separate steps: multiphoton excitation of the molecule to a resonant intermediate state; ionization of the excited neutral to the continuum, forming the parent ion plus an electron; and uni- or multiphoton fragmentation of the parent ion yielding a distribution of daughter ions comprising the observable laser-induced mass spectrum. The applicability of the maximal entropy (statistical) theory of multiphoton ionization-fragmentation is briefly discussed.
Introduction The field of laser multiphoton ionization mass spectrometry (MPIMS) is the 3-year old offspring of a staid, if not tired, elderly father (namely, mass spectrometry) and a glamorous, if not provocative, young mother (multiphoton ionization spectroscopy). We shall not delve too deeply into genealogy but simply note that the first viable offspring of this winter-spring union made its appearance on the scene in 1978. The background of the “elderly father”, mass spectrometry, is too well-known to warrant further comment, but that of the “glamorous mother”, MPI spectroscopy, deserves a few words. The pioneering efforts of Johnson’ and Dalby,2who introduced the MPI technique into molecular spectroscopy, were soon followed by significant contributions by El-Sayed? Colson,4 Robin,6 Goodman: and others. The field of MPI molecular spectroscopy has recently been reviewed by J o h n ~ o n . ~ The use of a mass spectrometer to identify products of a two-photon ionization (of Csz) was introduced by Los.* Zare: Schumacher,’O and Rothell carried out two-photon ionization (TPI) studies of beams of easily ionizable molecules. A big step forward was made in 1978 by Letokhov and co-workers,12who reported two-laser, two(1)(a) P. M. Johneon, M. R. Berman, and D. Zakheim, J. Chem. Phys., 62,2500 (1975);(b) P.M.Johnson, ibid., 62,4562(1975); (c) ibid., 64, 4143 (1976);(d) ibid., 64,4683(1976); (e) D. Zakheim and P. M. Johnson, ibid., 68, 3644 (1978). (2)(a) G.Petty, C. Tai, and F. W. Dalby, Phys. Rev. Lett., 34,1207 (1975);(b) F. W.Dalby, G.Petty-Sil, M. H. Pryce, and C. Tai, Can. J. Phys., 56,1033 (1977); (c) C. Tai and F. W. Dalby, ibid., 66,183 (1978). (3)D. H. Parker, S. J. Sheng, and M. A. El-Sayed, J. Chem. Phys., 66, 5534 (1976). (4)G.C. Nieman and S. D. Colaon, J. Chem. Phys., 68,5656 (1978). (5)M. B. Robin and N. A. Kuebler, J. Chem. Phys., 69,806 (1978). ( 6 ) K. K.Lehmann, J. Smolarek, and L. Goodman, J. Chem. Phys., 69, 1569 (1978). (7)(a) P. M. Johnson, Ace. Chem. Res., 13, 20 (1980); (b) P.M. Johnson and C. E. Otis, Annu. Rev. Phys. Chem., 32, 139 (1981). (8)M. Klewer. M. J. M. Beerlaee. - . J. LOB.and M. J. Van der Wiel. J. Phys. B, 10,2~ (1977). (9)D. L. Feldman, R. K. Lengel, and R. N. Zare, Chem. Phys. Lett., 52,413 (1977). (10)(a) A. Herrmann, S. Leutwyler, E. Schumacher, and L. Woste, Chem. Phys. Lett., 62,418(1977); (b) Helu. Chim. Acta, 61,453(1978); (c) A. Herrmann, E. Schumacher, and L. Woste, J. Chem. Phys., 68,2327 (1978). (11)(a) E. W. Rothe, B. P. Mathur, and G. P. Reck, Chem. Phys. Lett., 63,74 (1978); (b) B. P. Mathur, E. W. Rothe, G. P. Reck, and A. J. Lightman, ibid., 56,336 (1978). (12)(a) V. S.Antonov, I. N. Knyazev, V. S. Letokhov, V. M. Matjiuk, B. G.Movshev, and V. K. Potapov, Opt. Lett. 3, 37 (1978); (b) V. S. Antonov and V. S. Letokhov, Appl. Phys., 24,89 (1981);(c) V. S. Letokhov, Comm. At. Mol. Phys., D7, 107 (19771,and references cited therein.
photon ionization of molecular beams with mass spectrometric detection. At the same time Schlag and co-workers13reported the TPI of a molecular beam of benzene with mass spectrometric analysis of the ions. The first studies of multiple photon (i.e., more than two-photon) ionization of molecular beams, with mass analysis to provide identification (and to measure branching fractions for the formation of different ions), were those carried out on I2 in our laboratory by Zandee et al.14 in early 1978. The first multiphoton ionization mass spectrometric study of a polyatomic molecule (benzene) was that of Zandee and the author,l6 carried out in late 1978. The field of multiphoton ionization mass spectrometry (MPIMS) (or “vibronic/mass spectroscopy via multiphoton ionization”) thus achieved viability in 1978; the infant has been growing at a healthy rate thereafter. This paper will not attempt to review all the developments in the field since that time, but concentrate upon our own work. It will, however, comment on published MPIMS contributions of other laboratories. Our own research since the publication of ref 15 has been summarized in eight additional reports (ref 16-23), which together comprise the basis for the present review.
Resonance-Enhanced and Nonresonant Multiphoton Ionization The general phenomenon of resonance-enhanced multiphoton ionization (REMPI) is thought to proceed by a coherent n-photon (resonant) excitation of the groundstate molecule in a particular rovibrational state to a so(13)u. Boesl, H. J. Newer, and E. w. Schlag, 2.Naturforsch. A, 33, 1546 (1978). (14)L. Zandee, R. B. Bernstein, and D. A. Lichtin, J. Chem. Phys., 69,3427 (1978). (15)L. Zandee and R. B. Bernstein, J. Chem. Phys., 70,2574(1979). (16) L. Zandee and R. B. Bernstein, J. Chem. Phys., 71,1359(1979). (17)D. A.Lichtin, S. Datta-Ghosh, K. R. Newton, and R. B. Bematein, Chem. Phys. Lett., 76,214 (1980). (18)K. R. Newton, D. A. Lichtin, and R. B. Bernstein, J. Phys. Chem., 86, 15 (1981). (19)D. A. Lichtin, L. Zandee, and R. B. Bernstein in “Lasers in Chemical Analysis”, G. N. Hieftje et al., Ed., Humana Press, Clifton, NJ, 1981,p 125. (20)D.H. Parker, R. B. Bernstein, and D. A. Lichtin, J. Chem. Phys., 75,2577 (1981). (21)D. A. Lichtin, R. B. Bematein, and K. R. Newton, J. Chem. Phys., 75,5728 (1981). (22)D. H. Parker and R. B. Bernstein, J. Phys. Chem., 86,60(1982). (23)(a) K.R. Newton. Ph.D. Thesis.Columbia Universitv. New York. Aug, 1981; (b) K. R. Nekton and R. B. Bernstein, manus&pt in prep: aration.
0022-3654/82/2086-1178$0 1.25/0 0 1982 American Chemical Society
Systematics of Multiphoton Ionization-Fragmentation
called intermediate, excited vibronic state of the neutral molecule, often (but not always) a readily ionizable Rydberg state. As in laser-induced fluorescence (LIF) from an n-photon-excited state19p24t25where the excitation spectrum reveals information on the rovibrational levels of this upper as well as the lower electronic state, so with laser-induced ionization, the spectrum of the resonance peaks in the ion current (vs. laser wavelengths) yields similar information on the n-photon-excited intermediate state? The next stage in the MPI process is the absorption of one or more additional photons by this resonant intermediate state to excite it to the continuum, Le., past the molecular ionization potential, thus forming the parent molecule ion plus an electron. Often this step is very efficient, so the overall rate of ion production is limited by the “slow step” of resonant excitation. For the nphoton, coherent resonant excitation the yield of the intermediate (and thus the ion) is proportional to the nth power of the radiation density or the laser power, so that it is advantageous to use a high power, pulsed (as well as tunable, of course) laser source for REMPI spectroscopy. In general the sensitivity of REMPI is greater than that of LIF since the efficiency of the fluorescence light gathering for LIF is usually 510%, compared to the near 100% ion collection efficiency for MPI. Also with efficient pumping to the continuum one easily exceeds the 50% “saturation limit” of LIF, ionizing essentially all the resonantly excited intermediate state molecules. Just as for the case of atoms, however, multiphoton ionization can take place directly, even without passing through a resonant intermediate state. Of course, this requires a larger number of photons, so the rate of ion production is a higher order in laser power. This has the experimental consequence that at relatively low laser pulse levels the resonance enhancement of ion current is extremely important whereas a t very high pulse power the nonresonant contribution to the ion yield becomes dominant, and all spectroscopic information on the molecule is lost. Also lost is the specificity and structure sensitivity which makes REMPI an attractive analytical tool. Nevertheless the NRMPI process can be so exploited when combined with mass spectrometry, as will be pointed out later. NRMPI fragmentation patterns are indeed revelatory of the molecular structure (though not the spectrum, of course). This leads to a discussion of the third stage of the overall laser-induced ionization process, namely, the fragmentation, i.e., dissociation of the parent ions into a “mass spectrum”. This fragmentation may require zero, one, two, or more additional photons as the case may be, with various consequences (to be noted).
Laser-Induced Fragmentation of MPI-Formed Molecular Ions It was first demonstrated by Zandee et al.4J5that a larger number of photons than the minimum required to form the parent molecular ion could be absorbed by the system. In the case of the benzene molecule, the observation of many “energetically costly” fragment ions including C+ from the REMPI process at 391 nm implied that as many as nine of the 3.17-eV photons must have been absorbed within the 6-ns laser pulse duration.l6J6 Nevertheless the wavelength selectivity (the resonance structure in the spectrum of the ions collected) is retained; the wavelength spectrum of each fragment ion serves as a “fingerprint” (24)R. G.Bray, R. N. Hochstrasser, and J. E. Weasel, Chem. Phys. Lett., 27, 167 (1974). (25)J. H. Brophy and C. T. Rettner, Chem. Phys.Lett., 67,351(1979).
The Journal of Physical Chemlstry, Vol. 86, No. 7, 1982 1179 LASER MONITOR MOLECULAR LEAK INLET I O N DRIFT TUBE I l m l
MULTIPLIER DETECTO;
QUANTA- RAY
TRIGGER
MIXING
CRYSTAL DICHROIC MIRROR
--
Figure 1. Schematic drawing of the Columbia laser MPI-TOF mass spectrometer, based on ref 17, 18, and 20-23, combining various possible configurations.
-
of the intermediate state of the benzene molecule (i.e., that of the lEl, ‘Al transition). (Similarly16for the butadiene molecule, wkere the extensive fragmentation at 372 nm implied absorption of at least six photons, and for Iz at 585 nm, six photons.14J6 Analogous fragmentation results were reported by other workers.z631 This contrasts with the NRMPI of benzene (and other organic compounds) which can be readily observed with intense focused W pulses from excimeP.33and Nd:YAG21*23 lasers. However, using low-power irradiation, Boesl et al.13 observed a true “1+ 1”REMPI process on benzene, i.e., a one-photon ionization following a one-photon excitation to an intermediate state, yielding only the C6HB parent ions (plus electrons), Le., a two-photon ionization (2PI) process. Further experiments by Schlag and co-workers%have shown that the fragment ions from the TPI of benzene arise from photodissociation of the parent ions rather than from autoionization of excited neutral benzene molecules. Kinetic energy analysis36” of the photoelectrons, cations, and anionsn has established for a number of m o l d e a that fragmentation is accompanied by low-energy electrons and is thus not occurring via an autoionization ladder (as had been tentatively proposed earlier16*31).Separate two-color (26)C.D. Cooper, A. D. Williamson, J. C. Miller, and R. N. Compton, J. Chem. Phys.,73, 1527 (1980). (27)(a) G.J. Fisanick, T. S. Eichelberger, B. A. Heath, and M. B. Robin, J. Chem. Phys.,72, 5571 (1980); (b) G.J. Fisanick and T. S. Eichelberger, ibid., 74,6692 (1981). (28)C. T. Rettner and J. H. BroDhv. Chem. Phvs..56. 53 11981). (29)J. W. Hudgens, R. Pandolfi, P:R: Stannard, &d M. A. El-Bayed, Chem. Phys.,45,27 (1980). (30)(a) T. G. Dietz, M. A. Duncan, M. G. Liverman, and R. E. Smalley, J. Chem. Phys.,73,4816(1980);(b) Chem. Phys.Lett., 70,246 (1980). (31)D.M.Lubman, R. Naaman, and R. N. Zare, J. Chem. Phys.,72, 3034 (1980). (32)(a) S.D. Rockwood, J. P. billy, K. Hohla, and K. L. Kompa, Opt. Commun., 28,175(1979);(b) J. P.billy and K. Kompa, J. Chem. Phys., 73,5468(1980);also in ‘Laser Spectroscopy”,Vol. IV, Springer-Verlag, Berlin, 1979,p 631. (33)(a) J. W. Hudgens, M. Seaver, and J. J. DeCorpo, J. Phys.Chem., 85,761 (1981);(b) M. Seaver, J. W. Hudgens, and J. J. DeCorpo, Int. J . Mass Spectrom. Ion Phys.,34, 159 (1980). (34)(a) U.Boesl, H. J. Neusser, and E. W. Schlag in ‘Laser-Induced Processes in Molecules”, Vol. 6,J. D. Smith and K. L. Kompa, Ed., Springer, Berlin, 1979,p 219; (b) U. Boesl, H. J. Neusser, and E. W. Schlag, J. Chem. Phys.,72,4327(1980); (c) U.Boesl, H. J. Neusser, and E. W. Schlag, J. Am. Chem. Soc., in press. (36)J. T.Meek, R. K. Jones, and J. P. Redly, J. Chem. Phys.,73,3503 (1980). (36)J. C. Miller and R. N. Compton, J. Chem. Phys.,75,22 (1981). (37)M. S. DeVries, N. J. Van Veen, T. Ballen, and A. E. DeVries, Chem. Phys.,56, 157 (1981).
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The Journal of Physical Chemistry, Vol. 86, No. 7, 1982
(or two-laser) experiment^'^*^^,^^ have confirmed this for several molecules and have shown the relevance of the absorption of the parent ion to the MPI fragmentation (its extent and dependence upon laser pulse power). Direct experiments on laser-induced photodissociation of ions are available from the ICR studies of Beauchamp and co-workers38and of Dunbar et al.;39our own studiesz3 (to be described) have made use of a combined electron impact ionization-laser fragmentation technique. Several other recent experiments including those of E1-Sayed,a Baer,4I Zaret2Welge,43and others have added to the store of knowledge about the MPI-fragmentation process. However, in the next sections we will confine our attention mainly to experiments (and their theoretical interpretation) carried out in the author’s laboratory. Most of the results to be presented have been obtained by use of a time-of-flight mass spectrometer (TOFMS) adapted especially for laser multiphoton ionization.” Provision has been made for electron impact (EI) ionization of molecules admitted to the ionizing region (through a molecular leak) at a pressure in the range 1 X 10+-2 X torr. Figure 1is a schematic drawing of a recent embodiment of the laser ionization mass spectrometer, in which a Quanta Ray NdYAG pumped dye laser system provides 5-ns pulses of visible or W radiation (at a 10-Hz repetition rate) which are focused down on the molecules in the appropriate region of the ionizer. The TOFMS is synchronized with the laser (and the entire operation is controlled) by a minicomputer. For details, including the evolution of the instrumentation, see ref 17, 18, 20-23.
Extremes of MPI-Fragmentation Behavior Before getting into the details of the large body of MPIMS data, let us begin by considering a few examples of extremes of behavior which point up the problem of interpretation of fragmentation patterns in MPI mass spectrometry. Then we shall be in a better position to “dig in” and search out the systematics. Figure 2 shows NRMPI mass spectra21 of tert-butylbenzene, i.e., ionic fragmentation patterns recorded under different conditions of laser power (laser pulse energy) and wavelength, showing the extreme range of fragmentation possible, ranging from “soft ionization” (to parent ion exclusively) to atomization yielding extensive formation of C+ (as first seen in the REMPI of benzene15). Another interesting case is that of the triethylenediamine molecule (Dabco), a caged tertiary amine. Figure 3 presents a compari~on’~ of REMPI mass spectra of Dabco at two different wavelengths with an E1 mass spectrum as a reference. The ionization process at h = 559 nm requires a minimum of four photons absorbed by a molecule to reach the continuum (the ionization potential of Dabco being 7.23 eV); at 425 nm, 1 3 photons are required. But it is clear from the MPIMS fragmentation patterns that at least two or three more photons must be needed to meet the energetic requirements to break the bonds of the parent ion and produce the extensive (and “expensive”) fragmentation observed. Thus, although the REMPI (38)See, e.g., B. S. Freiser and J. L. Beauchamp, Chem. Phys. Lett., 35,35 (1975). (39)See, e.g., (a)P. P. Dymerski, E. Fu, and R. C. Dunbar, J. Am. Chem. SOC.,96,4109(1974);(b) R. C. Dunbar, H. H. Teng, and E. W. Fu, ibid., 101,6506 (1979). (40)R. Pandolfi, D. A. Gobell, and M. A. El-Sayed, J. Phys. Chem., 85,1779 (1981). (41)T.Carney and T. Baer, J. Chem. Phys., 75,477 (1981). (42)D.Proch, D.M. Rider, and R. N. b e , Chem. Phys. Lett., 81,430 (1981). (43)H.Zacharias, H.Rottke, and K. H. Welge, Appl. Phys., 24, 23 (1981).
Bernstein r
,
,
I
I
I
I
ip
266 nrn 1.5 rnj
I\
i I
0
20
40
60
80
100
120
140
MASS
Figure 2. MPI fragmentation patterns of tert-butylbenzene obtained under different conditions of laser wavelength and pulse energy. Adapted from ref 21.
processes at 559 and 425 nm are termed 4PI and 3PI (fourand three-photon ionization), respectively, there is an unspecified additional number of photons to be absorbed, needed to account for the MPIMS. Figure 4 shows the dependence upon laser power of the MPIMS fragmentation patternsz0for Dabco, comparing the 3PI and 4PI results. Analogous data are available for the closely related caged tertiarly amine, quinuclidine (Abco). Clearly the fragmentation increases strongly with increasing laser pulse energy (although the total ion yield is found to increase only quadratically with laser power, as expected for a two-photon resonance intermediate “bottleneck”). Figure 5 presents results of some analogous experiments on an nopen”tertiary amine, triethylamine. At the 480-nm resonance, note the simplicity of the fragmentation processes, Le., loss of CH3radical, with a power-independent daughter/parent branching ratio. (Similarly for trimethylamine, but loss of H atom instead of CH3). For the NRMPI process at 355 nm, power-dependent fragmentation of the ions is observed.20 Figure 6 presents MPIMS fragmentation dataz2for npropyl iodide compared to isopropyl iodide. Several points can be noted: first, the clear difference in the mass spectral fragmentation patterns of the isomers, and second, the power independence of certain ratios of peak intensities reminiscent of the results shown for triethylamine (Figure 51, but distinctly different than found in the case of the caged amine (Figure 4). These are just a few examples of the great diversity of MPIMS fragmentation behavior, a few of the “problems” for whose solution a “systematics” is called for.
Qualitative Systematics Based on Energetics Many of the observed MPIMS fragmentation results (both REMPI and NRMPI) can be understood qualitatively by consideration of the energetics of the three
Systematics of Multiphoton Ionization-Fragmentation
The Journal of Physical Chemistry, Vol. 86, NO. 7, 1982 1181
0
111. photodissociation M+ I
1
E1 MRSS SPECTRUM
rho,
F+ + R
In previous discussions of REMPI processes, they were m ionizations; here we have usually described as n broken up m into q + r. For case a, n = 3, q = 2, and r is not specified, i.e., the photofragmentation of the parent ion M+ is not indicated. Step I is the coherent, n-photon resonant excitation of the parent molecule M to some intermediate rovibronic state M*. In case a it is a three-photon resonance at 3wl. Step 11is the (probably) coherent uppumping of M* to the continuum, forming the parent ion M+ plus an electron. If the slow step in the combined ionization process is step I, the rate of M+ production will be proportional to the nth power of the laser pulse intensity. The 3R 2 process shown yields only M+ + e- with a combined energy (the sum of the internal energy of M+ and the relative energy of the recoiling electron-ion pair) of less than h q . Depending upon the initial degree of internal excitation of M+ (and intramolecular relaxation rates) it may be possible to photodissociate M+ to F+ R by r = 1, 2, etc., photons, as will be discussed later. For the NRMPI situation, case b, in which there is no resonant intermediate state, the 5WR process shown is equivalent to the 3R + 2 with the exception that the yield of M+ is proportional to a higher power of the laser pulse energy (or peak power). For NRMPI-fragmentation, processes I and I1 are combined
+
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+
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+
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k ho,
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where k = n + q, followed by the photofragmentation step 111. In the simplest case the relative rate of M+ formation via NRMPI vs. REMPI at the same wavelength will be proportional to am/aREtimes the laser intensity to the power k/n (>l,here 5/3) so that NRMPI will always dominate REMPI at high laser power levels. At low laser powers, of course, the lower order processes are preferred. Next consider case c, a 2R resonant excitation at o2to another excited state M*’. As in case a, one additional photon is insufficient for ionization, but in contrast to case a, the second (additional) w2 photon brings the molecule up to an energy above the appearance potential (AP) of the low-lying fragment ion F+. The excited neutral, say M**, can either ionize to the parent ion M+ (p) plus an electron, releasing considerable energy (into kinetic energy and/or M+ internal excitation), or directly fragment to the daughter ion F+ (d) plus e- + R. The branching ratio (daughter/ parent) will depend upon the excess energy in M*’ after absorption of the two w2 photons. Thus following step I we have 11. ionization-fragmentation
M*‘ .!!3+, M**
F+ + e- + R
\M+ + eFigure 3. Comparison of REMPI mass spectra of triithylenediamine (Dabco) obtained at A 559 and 425 nm, with a 70-eV E1 mass spectrum reference. Adapted from ref 17.
%eparate” steps comprising the overall process, as illustrated in Figure 7. Consider the REMPI situations, case a.
I. excitation 11. ionization
nho,
M-M* M*
qhoi
M++ e-
where the d/p branching ratio is expected to increase with w2 (see below). Case d is the NRMPI analogue of c. Just as for the 2R + 2 REMPI of case c, the 4NR process d yields both F+ and M+, with the branching ratio (d/p) governed by the ratio of the excess energies, [4h02- AP(F*)]compared to [4ho2 - IP(M+)]. A t a fixed w2, the d / p branching ratio should be essentially independent of laser power (although the total ion current will, of course, increase strongly with increasing pulse power).
1182
The Journal of Physical Chemistry, Vol. 86, No. 7, 1982
Bernstein r2
DA8CO N(C2HqI3N
3PI
4PI 559nm
420nm
559nm ‘25rnjI
4 2 0 nm
1
MULTIPHOTON E X C I T A T I O N
,
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559nm
;
:420nm lOml 155 ,:,!
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100
2 0 40
l . , I , ! l
~
PI
Figure 7. Scheme to systematize variety of observed overall MPIfragmentation behavior.
,
iodide and isopropyl kdlde. Lower panels show electron impact mass spectra. Adapted from ref 22.
excited M+ absorbs the next photon (i.e., a + 1 process) the daughter ion will be formed; but if M+ internally relaxes quickly (51ns) it will require two photons (i.e., the +2 process) to yield daughter ion. Since the one-photon (+1)process would be favored over the +2 (multiphoton dissociation, MPD) route, the d/p ratio should be strongly power dependent. This MPD situation contrasts with the one-photon dissociation case f; here one additional o4 photon (i.e., the +1process) suffices, regardless of relaxation rate, to accomplish fragmentation to F+. Let us now extend these considerations to deal with the formation of several daughter ions (e.g., dl, dz, etc.) by one-photon and multiphoton dissociation of the parent ion. Case g refers to the situation in which the photon energy hog is insufficient to reach the AP of the lowest-lying daughter ion (dJ, so a +2 (multiphoton dissociation) process is required. For the production of dz,a +3 MPD process is called for. Thus the d,/p branching ratio will be strongly power dependent, with the d2/dl ratio increasing linearly with power. For case h, dl arises via a +1process (with 0 6 photons), d2from a +2 MPD as shown. In contrast with case g, here the dl/p branching ratio is only linear in power, but the dz/dl ratio increases with power as in case g. Case i is interesting because the +1 o7 photon excitation of M+ produces an excited state of sufficient energy to fragment to both dl and d2 Recalling the related situation in cases c and d, here the ratio dz/dl should be power independent, governed only by the ratio of the energy excesses [ hw7 AP(dz)] compared to [ho,- AP(dl)]. The discussion above has been qualitative and oversimplified. Nevertheless, most of the observations to date can be readily understood in terms of these simple considerations, as will be seen in examples to follow. It is interesting to note that the full “rate equations” approach, widely used in the IR-MPE and IR-MPD field (e.g., ref 44), has been frequently employed in the MPI area as well (e.g., ref 1, 3, 27, 32, 45, etc.), but because of the large numbers of rate coefficients involved it has been difficult to determine an unequivocal set which can account for all the observations. At the present stage, the qualitative behavior of most molecules undergoing MPI can best be interpreted, i.e., codified, as above via cases a-j. Under certain conditions a statistical theory becomes applicab1e.21&47
Next consider the photofragmentation steps per se. In case e, the last of the ionizing photons o3produces the M+ plus electron with excess energy as shown; if the internally
(44)M.Quack and J. Troe, Znt. Rev. Phys. Chem., 1, 97 (1981). (45) F.Rebentrcat and A. Ben-Shad, J. Chem. Phys., 74,3255 (1981).
20 40 60 SO MASS
60 80 100 MASS
FlQllre 4. Dependence upon laser power of the MPIMS fragmentation patterns for triethylenedlamine (Dabco), comparing the so-called “three-photon ionization” (3PI) with the “four-photon ionization” (4PI) patterns. Adapted from ref 20. /P
80 85 90 95 100
1 4 8 0 nm
/P
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80 85 9 0 95 100
1 480nm loom)
lp
80 85 90 95 100
,I,
,j g i$1, i‘,“,” y
,
I
,i I P , 20 40 60 80 100 20 4 0 60 80 100 2 0 ’ 4 0 ’ 6 0 ’8 O T O O MASS
MASS
~
MASS
Flgwe 5. Results analogous to Figure 4 for triethylamine: upper, h
= 480 nm; lower, h = 355 nm. Adapted from ref 20.
7’’ ’
( c y I CH’
I
I
’
MP: 368 n m ’ 5 m,
!Jji I
20
40
120 M1SS
140
160
t
20
43
I20
140
160
MASS
Figure 8. Comparison of MPIMS fragmentation patterns for n-propyi
Systematics of Multiphoton Ionization-Fragmentation
The Journal of Physical Chemistry, Vol. 86, No. 7, 1982 1183
N (C2H413CH
t - Butylbenzene
P-CH, 511nm 3 7ml
4
-A
I
L
-
i
L
02
a) 266nrn, 005rnJ; 560nrn, 9 5mJ 96
-
d
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1
P
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rnj
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1
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28
30
42
55
50
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82
90
110
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P-CH3 h
Figure 8. Examples of “alternative pathway test” of the statistical model of MPI-fragmentation, adapted from ref 21. Data for quinuclidine (Abco) at the indicated resonance wavelengths (and laser pulse energies).
Illustrations of Qualitative Differences in MPI-Fragmentation Behavior A very common situation is that exemplified by Figure 2 for tert-butylbenzene,21 namely, a wide range of fragmentation patterns encompassing “soft” ionization (to parent ion exclusively) as well as extreme fragmentation (all the way to C’, as also found for benzene in ref 26). Clearly, the photodissociation spectrum of the parent ion plays a dominant role: at 266 nm it is only weakly absorbing, while at 355 nm the parent ion (p) and the two daughter ions (p - CH3and C7H7+)all absorb strongly and have been completely photodissociated during the 5-11s laser pulse. Considering the two MPI mass spectra in Figure 3, initial inspection would suggest that the particular doorway state (425 vs. 559 nm) chosen is a dominant influence on the fragmentation pattern. However, it has been found2I that by varying the laser pulse energy it is possible to obtain very similar fragmentation patterns via the two different doorway states, not only for triethylenediamine but for a variety of other polyatomic molecules. This has been taken as evidence in support of the SilbersteinLevine statistical t h e o r p of MPIMS. The results20 shown in Figure 4, however, do show a significant difference between the fragmentation patterns at the two wavelengths, namely, that the parent ion is much more strongly photodissociated at 559 nm than at 420 nm. The alternative pathway test of the statistical theory is well illustrated by the results21displayed in Figure 8 for quinuclidine (Abco). Equivalent fragmentation patterns were recorded for two doorway states (at 441 and 511 nm) a t each of two sets of laser pulse energies. The data of Figure 5 (also from ref 20) show that at 480 nm a mechanism analogous to that of Figure 7c applies, (46) (a) J. Silberstein and R. D. Levine, Chem. Phys. Lett., 74, 6 (1980); (b) J. Silberstein and R. D. Levine, J. Chem. Phys., 75, 5735 (1981). ’ (47)F. Rebentrost, K. L. Kompa, and A. Ben-Shaul, Chem. Phys. Lett., 77, 394 (1981).
40
50
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80 90 100 1 1 3 120 130 141 MRSS
m
e 0. T w m l o r MPI-fragmentation patterns for tert-butylbenzene: lower panel (d), weak UV pulse, soft ionization; upper panels (c, b, a), same as d but with subsequent visible pulses of indicated (increasing) pulse energies. Adapted from ref 23.
Le., that the “first photon” of a 2R + 2 process yields F+ and M+ (see Figure 7) with a branching ratio independent of laser power. At 355 nm a more common 2R 1 sequential process occurs, i.e., the daughter ion H2CN(C2H5)2+photodissociates and the results are much like the situation for triethylenediamine at 559 nm. The resultsn shown in Figure 6 for the isomers n-propyl and isopropyl iodide are of considerable interest for several different reasons. First, from the viewpoint of mechanism, i.e., dynamics and spectroscopy: clearly the n-propyl cations do not isomerize appreciably within the 5-ns laser pulse duration, but absorb the 368-nm radiation and photodissociate, whereas the isopropyl cations are essentially transparent at this wavelength and thus survive without fragmentation. The n-propyl ion/C2H3+daughter ion ratio (at 268 nm) are nearly independent of laser power; this represents another example of a Figure 7c process. (Somewhat analogous results were found22for C4H91isomers.) Second, from a practical viewpoint, the data of Figure 6 show that easy distinguishabilityof the MPI mass spectra for the two isomeric molecules whose electron impact mass spectra are virtually identical.
+ +
Fragmentation of Ions In an attempt to clarify the role of photodissociation of ions in the overall MPI fragmentation process, an extensive study has been carried outB with the Columbia laser-TOF mass spectrometer. Several modes of operation were employed (a) a 5-11s laser pulse (at A,) from the NdYAG laser system preparing ions, followed (in 10-50 ns) by a second 5-11s laser pulse (at A,) photodissociating (some of) the ions; (b) a 1-ps electron beam pulse (at a selected electron energy, between 13 and 70 eV), followed (in 50.5 ps) by a 5-ns laser pulse; (c) a 5-ns laser pulse (at AI), followed (in 50.5 ps) by a 200-ns laser pulse (at A,) from a flashlamp-pumped dye laser (high fluence, relatively
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J. Phys. Chem. 1982, 86, 1184-1192
broadband, low power density). One example, of mode a, should suffice to indicate the methodology. Figure 9 shows resultsz3of a two-color experiment on tert-butylbenzene. The first pulse is a LIweak” UV pulse at 266 nm, which produces essentially only the parent ion (panel d). The second pulse, in the visible (at 560 nm), is absorbed by the tert-butylbenzene cation and causes photodissociation. For a 9.5-mJ pulse, more than 90% of the original parent ion is destroyed via the loss of a CH3 radical (panel a). A series of e ~ p e r i r n e n t scarried ~ ~ out in mode b served to elucidate the fragmentation pathways relevant to the MPI (in the visible) of the benzene molecule. A quantitative “pathway diagram” has been developed showing the relative rates of photodissociation of the following eight ion frgments relative to that of the C6H6+parent ion: C6H5+,C&+, C6H3+,C6H2+, C4H4+,C4H3+,C4H2+, and C3H3+.Some two dozen branching ratios were established for the fragmentation of these benzene-derived ions. From a large variety of experiments using the different modes of operation as described above, it has been possible to observe all of the processes designated e, f, g, h, and i in Figure 7. The results show the prevalence of multiphoton dissociation (MPD) of ions (vis-a-vissingle-photon fragmentation) at the typical laser pulse power levels used in MPIMS experiments.
Concluding Remarks The systematics of the multiphoton ionization-fragmentation phenomenon have been emerging over the past 2 or 3 years. The overall process can be interpreted in terms of three separate steps: (1)MPE (to a resonant intermediate state of the neutral), for the REMPI situation, (2) ionization of the excited neutral (or the groundstate molecule, for the NRMPI case) to the continuum forming the parent ion and an electron, and (3) uni- or multiphoton dissociation (usually MPD) of the parent ion yielding a distribution of daughter ions comprising the observable MPIMS fragmentation pattern. The roles played by energetics, spectroscopy, and dynamics in the overall MPI-fragmentation process are becoming clearer, as well as the conditions for applicability of the statistical theory of MPI-fragmentation. Acknowledgment. The author acknowledges the many significant contributions of his co-workers (named in the abstract), and to the National Science Foundation for financial support of this research. This research was supported by the National Science Foundation Grants CHE 77-11384 and CHE 78-25187. In addition, fond memories of a decade of close encounters with and inspiration from Professor J. 0. Hirschfelder at the University of Wisconsin are deeply appreciated.
Laser Probing of Vibrational Energy Redistribution and Dephasingt A. Zewail;
Wm. Lambert, P. Felker, J. Perry, and W. Warren’
Artbur A m Noyes Laboratory of Chemlcal Physlcs, CalHornie Institute of Technology, Pasadena, Callfornie 9 1125 (Received: September 8, 198 1)
This paper addresses questions important to the origin of optical dephasing and vibrational energy redistributions in molecules. Several laser techniques are discussed and three major findings are presented. These findings are related to (a) optical dephasing of molecules in the gas phase and in beams, (b) dephasing of high-energy vibrational overtone states of large molecules, and (c) energy randomizationand quantum beats in large molecules (anthracene) excited by picosecond pulses and cooled by supersonic jet expansion.
When molecules interact with coherent laser fields, they either rupture and produce new chemical species or they redistribute the deposited energy among the many internal degrees of freedom. The primary processes that govern the reactive chemical changes-laser selective chemistry1-or the nonreactive radiationless relaxation are of great current interest in at least three major areas of research: (a) multiphoton absorption, dissociation, and ionization by intense or weak laser fields; (b) chemical activation by selective laser pumping of certain states in molecules; and ( c ) intra- and intermolecular dephasing following selective and coherent excitations in molecules. Lasers can probe many of these important events that are not amenable to conventional spectroscopic methods. It is the purpose of this article to show how lasers with short ‘Based on a lecture given by A. Zewail in honor of Professor J. 0. Hirschfelder at the International Symposium: New Directions for the Molecular Theory of Gases and Liquids, Madison, WI, 1981. *Alfred P. Sloan Fellow and Camille & Henry Dreyfus Foundation Teacher-Scholar. National Science Foundation Postdoctoral Research Fellow. *Contribution No. 6517.
*
0022-3654/82/2086-1184$01.25/0
duration (picoseconds) or narrow bandwidth ( -lo4 cm-’) can be used to probe some of the dynamics that describe “what goes on between and inside molecules”. Focus will be on (a) the origin of dephasing, (b) the relaxation of selective bond states following the preparation by light, and (c) vibrational energy redistribution in isolated large molecules. There will be no discussion of the multiphoton effects or the influence of lasers on chemical reaction yields (see companion articles in this issue by Professors Bernstein and Zare on these topics).
Optical Phase Coherence and Energy Relaxation The coherent interaction between a laser and molecules can be handled in a variety of ways including the rigorous method developed recently by Professor Hirschfelder’s group. On the molecular level one would like to know, in a general sense, the answer to the following questions: (a) What is the nature of the states that we excite with light? (1) See the articles by A. H. Zewail; V. S. Letokhov; R. Zare and R. Bernstein; and Y. Lee and Y. R. Shen in the special issue on Laser Chemistry, Phys. Today, 33, No. 11 (1980).
0 1982 American Chemical Society
