J . Phys. Chem. 1984, 88, 5197-5204 diphenylethylene chromophore. At low temperature in rigid glass solvents the chromophores are trapped in this arrangement, but the large angle between the planes of the two ring systems (ca. 50’) precludes any significant interchromophore interaction. We identify this species with that observed in the low-temperature photochemical experiments. At higher temperatures, the chromophores can rotate away from each other around the C(9)-C(15’) bond to give the open compound observed near room temperature. Comparison with Dianthracene Photodissociation. Because of their structural similarity, it is interesting to compare the photodissociation of lepidopterenes with that of the anthracene photodimers. Although the dissociation proceeds by the same general pathway in both cases, the efficiency of the adiabatic path shows a dramatic difference: for lepidopterene it approaches 90% a t room temperature, whereas in the anthracene photodimers it amounts to only 0.05% of the total product yield.’* This difference can be explained in terms of the qualitative theoretcial ideas developed by Michl,19 which are based on the WoodwardHoffmann rules for the conservation of orbital symmetry. In the case of photochemically allowed [4+4; 2+2] eliminations (e.g., dianthracene photodissociation), the first excited singlet (S) state of reactants correlates with that of the products, the ground (G) state of reactants correlates with a doubly excited (D) state of products, and vice versa. Avoided crossing of the D and G states leads to a “pericyclic minimum” on the excited electronic surface close to a local maximum on the ground-state surface, an ideal geometry for nonradiative transitions. l9 Trapping of electronically excited reactants at the pericyclic minimum leads predominantly to partitioning between ground-state reactants and products (18) S. Yamamoto and K.-H. Grellmann, Chem. Phys. Lett., 85, 73 ( 1982).
(19) J . Michl, Photochem. Photobiol, 25, 141 (1977), and references therein. (20) J. B. Birks, “Photophysics of Aromatic Molecules”, Wiley-Interscience, London, 1970, p 121.
5197
(diabatic path), although escape from the pericyclic minimum can lead to products via an adiabatic path in small yield. In contrast, concerted [4+2] photocycloelimination reactions (e.g., lepidopterene photodissociation) are symmetry forbidden (ground-state allowed). Here, the first excited singlet state of reactants correlates with a higher excited state of the products, leading to a barrier on the excited-state surface.19 Since there is no pericyclic minimum, diabatic leakage to the ground state is unimportant. The adiabatic process can become more efficient, depending only on the size of the excited-state barrier and the rates of competing nonradiative processes. Wavelength Dependence of Photodissociation. The spectra shown in Figure 4 demonstrate that excitation below 255 nm leads to more efficient photodissociation than for excitation at longer wavelengths. In fact, if lepidopterene is excited at ca. 230 nm, the photodissociation yield remains close to unity at temperatures down to 90 K. This is a striking observation, since it implies that, at low temperatures, photodissociation competes very effectively with thermal energy relaxation, the latter process generally having relaxation times of to s for large molecules in condensed media. Similar observations have recently been made for dianthracene’* and the photoisomer of 1-(9-anthryl)-3-(1naphthy1)propane (ANP),15 molecules with structural features in common with lepidopterenes. At this stage it is not clear whether this is due to inefficient coupling between the internal molecular modes involved in photodissociation and the solvent bath or to extremely rapid dissociation from higher vibrational levels of the first excited singlet state. However, this does not affect the main conclusions of the present work, which were derived from experiments at excitation wavelengths longer than 260 nm.
Acknowledgment. We gratefully acknowledge M. Puza for preparing the lepidopterene and dimethyllepidopterene used in this work, and J. Webster and G. Breu for able technical assistance. G.J.W. was supported by an ANU Research Scholarship. Registry No. 1-L, 55614-27-2; 2-L, 55657-77-7.
Multiphoton Ionization-Fragmentation Spectroscopy of Methylnaphthalenes David Mordaunt, Gary Loper, and John Wessel* Chemistry and Physics Laboratory, The Aerospace Corporation, El Segundo, California 90009 (Received: May 21, 1984)
One-color and two-color laser spectroscopic studies of multiphoton ionization (MPI) and fragmentation spectra of 1methylnaphthalene and 2-methylnaphthalenewere performed in order to assess the potential capability for selective excitation and for identification of large aromatic hydrocarbons. The MPI spectra basically resemble the ultraviolet absorption spectra. Although the MPI/mass spectra (MPI/MS) have complex intensity dependence, they are similar for both isomers. Thus effective differentiation from other compounds is readily achievable; however, differentiation between methyl-substituted isomers is not readily achievable. Fragmentation spectra of parent and smaller ions were studied in detail. Whereas optical absorption spectra of the parent ions obtained by ion cyclotron resonance were reported to differ for the isomers, the MPI/MS parent ion spectra were similar. A kinetic model for the excitation processes is suggested that explains the MPI/MS results on the basis of a sequential fragmentation mechanism, involving the methylnaphthalene analogues of tropylium and benzyl ions. More sophisticated detection methods are suggested that may provide the desired selectivity.
Introduction Multiphoton ionization spectroscopy can provide highly sensitive detection for atomic and molecular species. Single-atom detection has been achieved’ and aromatic hydrocarbon molecules have been detected down to densities of about lo4 molecules/cm3 in the presence of 1019molecules/cm3 buffer gas.2 The major problem (1) G. S. Hurst, M. H. Nayfeh, and J. P . Young, Appl. Phys. Lett., 30, 229 (1977).
0022-3654/84/2088-5 197$01.50/0
encountered in extending this capability to lower densities and in successfully applying it to practical large molecule detection is the limited optical selectivity associated with the excitation pro~ess.~Important problems such as detection of trace quantities (2) C. Klimcak and J . Wessel, Appl. Phys. Lett., 37, 138 (1980). (3) D. E. Cooper, C. M. Klimcak, and J. E. Wessel, Phys. Reu. Lett., 46, 324 (1981); J. E. Wessel, D. E. Cooper, and C. M. Klimcak, Proc. SPZE, 286, 48 (1981); D. E. Cooper, R. P. Frueholz, C. M. Klimcak, and J. E. Wessel, J . Phys. Chem., 86, 4892 (1982).
0 1984 American Chemical Society
5198 The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
k+ I
%
Ik>
IO>
Figure 1. Ionization and fragmentation processes.
of carcinogenic hydrocarbons in the presence of inactive isomers are difficult or impossible to solve using existing real time detection methods. We have investigated new approaches based on resonance-enhanced multiphoton ionization spectroscopy combined with laser-induced fragmentation and mass spectrometry in order to develop improved selectivity based on the combination of optical spectra of both the neutral and ion species and the associated mass spectra. In recent years multiphoton ionization/mass spectrometry (MPI/MS) has been studied for a variety of molecules, primarily in order to probe the dynamics of the processes. The technique has been suggested as a general analytical method for aromatic hydrocarbon^,^,^ either by itself, or in combination with other separation techniques, such as gas chromatography.6 The multiphoton ionization process has major advantages with respect to other detection methods in that it provides optical selectivity and near unit ionization efficiency for aromatic hydrocarbons. The photoelectrons and photoions can be detected with near unit quantum efficiency in the presence of extremely low background noise. The photoions can also be mass analyzed without sacrificing detection efficiency, thereby improving the selectivity of multiphoton detection. Several prior investigations addressed state selectivity of .MPI/MS.’-I4 It was immediately apparent that for aromatic hydrocarbons the extent of fragmentation can be varied over wide ranges, from production of parent ions exclusively, to nearly complete fragmentation into C1+ and C2+ species, provided the excitation process involves resonance enhancement via stable intermediate states. This type of excitation process is generally applicable to the aromatic hydrocarbons using near ultraviolet laser sources. Investigations, based on double pulse excitation with time delaylOJ1and by two-color laser excitation,I2 have established that for a variety of aromatic hydrocarbon molecules the overall excitation and fragmentation process consists of pro(4) V. S. Antonov, V. S. Letokhov, and A. N. Shibanov, Appl. Phys., 22, 293 (1980). (5) R,p. Frueholz, J, E,We&, and E. Wheatley, Anal. Chem., 52, 281 (1980). (6) C. Klimcak and J. E. Wessel. Anal. Chem.. 52, 1233 (1980). (7j V. S. Antonov and V. S. Letokhov, Appl. Phys., 24, 89 (1981). (8) L. Zandee and R. B. Bernstein, J . Chem. Phys., 70,2574 (1979); 71, 1359 (1979); D. A. Lichtin, S. Datta-Ghosh, K. R. Newton, and R. B. Bernstein, Chem. Phys. Lett., 75,214 (1980); D. H. Parker, R. B. Bernstein, and D. A. Lichtin, J . Chem. Phys., 75, 2577 (1981); D. A. Lichtin, R. B. Bernstein, and K. R. Newton, J . Chem. Phys., 75, 5728 (1981); D. H. Parker and R. B. Bernstein, J . Phys. Chem., 86, 60 (1981). (9) D. M. Lubman, R. Naaman, and R. N. Zare, J . Chem. Phys., 72,3034 (1980). (10) R. Pandolfi, D. A. Gobelli, and M. A. El-Sayed, J . Phys. Chem., 85, 1779 (1981). (11) U. Boesl, H. J. Neusser, and E. W. Schlag, Chem. Phys., 55, 193 (1981). (12) K. R. Newton, D. A. Lichtin, and R. B. Bernstein, J . Phys. Chem., 85, 15 (1981). (13) D. H. Parker and R. B. Bernstein, J . Phys. Chem., 86, 60 (1982). (14) G. J. Fisanick, T. S. Eichelberger, B. A. Heath, and M. B. Robin, J . Chem. Phys., 72, 5571 (1980).
Mordaunt et al. duction of a parent ion which is subsequently excited by absorption of one quantum or more as shown in Figure 1. When sufficient energy is accumulated to reach the ion dissociation threshold, fragmentation occurs with a product distribution controlled by the deposited energy and by the product channel energies. This is the statistical model that predicts thermal type product distributions. Laser intensity and pulse duration provide control of the degree of fragmentation in this model. Variation of parameters such as laser wavelength is predicted to be relatively unimportant except that the ion absorption cross section and the number of photons required to reach product thresholds will change with wavelength, thereby shifting the threshold intensities for fragmentation. This model has been investigated by several research groups.‘-I4 Initial studies suggested large deviations from thermal behavior with fragmentation patterns being uniquely dependent on laser color.’ Subsequent studies revealed that, with careful control of excitation intensity, any particular fragmentation pattern produced for an aromatic by one wavelength (or a combination of two wavelengths8) can be reproduced moderately well by another wavelength and intensity. This implies that only a limited amount of additional selectivity will be available from ion fragmentation patterns. In the related problem of isomer fragmentation, the statistical modells predicts identical fragment distributions for isomers, as a consequence of identical product thermodynamics. (However, Parker and BernsteinI3 discovered fundamentally different MPI fragmentation patterns for alkyl iodide isomers.) The main objective of work described in this report was to investigate the possible differences in ion fragmentation spectra in order to achieve highly selective detection of the parent molecule. Considerable information about cation spectra is available from prior studies involving ions formed in low-temperature matrices,I6 and ions trapped in ion cyclotron resonance ~pectrometers.’~,’~ In the matrix studies ions were formed by subjecting low-temperature matrices to ionizing radiation or by adding strongly oxidizing chemicals to the matrices. The absorption spectra were measured directly. Ion cyclotron resonance (ICR) experiments measured absorption indirectly for ions formed by electron beam impact. The parent ions thus formed absorbed optical radiation from a low-intensity source, thereby inducing dissociation. Absorption was measured by monitoring loss of the parent ion signal or by appearance of a signal at a mass corresponding to one less hydrogen atom. Dunbar and Klein obtained ICR spectra of the methylnaphthalene^.'^ They demonstrated that hydrogen loss is the lowest energy fragmentation process, requiring about 2.6 eV. Therefore one-photon absorption induces fragmentation for wavelengths shorter than 470 nm, whereas two or more photons are required at longer wavelengths. Dunbar and Klein found that the methylnaphthalene parent ion ICR spectra are similar to that of the naphthalene ion. There are strong UV transitions arising from electrons promoted from normally filled orbitals to normally empty orbitals. Strong red absorption bands arise from transitions between normally filled orbitals. In general the-aromatic hydrocarbon cations have strong red or near IR absorption bands that arise from promotions between in a region completely Occupied Orbitals* Absorption free of interference from the higher energy neutral molecule transitions. The ion absorDtion sDectra are structured and differ. even for closely related isdmers. ‘The red absorption band of the 1-methylnaphthalene ion reported by Dunbar and Klein is stronger than for 2-methylnaphthalene ions.” The UV band structure is substantially different for the two isomers. Low-temperature matrix studies reveal discrete vibronic structure1’ in both red and (15) J. Silberstein and R. D. Levine, Chem. Phys. Lett., 74, 6 (1980); J. Silberstein and R. D. Levine, J . Chem. Phys., 75, 5735 (1981); F. Rebentrost and A. Ben-Shaul, J . Chem. Phys., 74, 3255 (1981). (16) T. Shida and S. Iwata, J . Am. Chem. SOC.,95, 3477 (1973); G. J. Hoijtink and P. J. Zandstra, Mol. Phys., 3, 371 (1960). (17) R. C. Dunbar and R. Klein, J . Am. Chem. SOC., 98, 7994 (1976); M. S. Kim and R. C. Dunbar, J . Chem. Phys., 72, 4405 (1980). (18) B. S. Freiser and J. L. Beauchamp, Chem. Phys. Lett., 35, 35 (1975); R. C. Dunbar, “Ion Photodissociation” in “Gas Ion Chemistry”, Vol. 11, M. T. Bowers, Ed., Academic Press, New York, 1979.
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5199
Multiphoton Spectroscopy of Methylnaphthalene INTiNSITY CONTROL
BEAMSPLITTER k YAG
O K LASER IPUL 11
VISI0.E LIGHT AT
132 nm
Y?
1 M I I LENS
BOXCAR INTEGRATOR
SCAN CUNTSOL
532 nm
INClOENCi
I TO VACUUM PUMP
Figure 2. Experimental apparatus.
UV bands. Therefore if fragmentation in MPI involves interaction with these ion absorption bands, then the fragmentation spectra should differ for the two isomers. Moderately small shifts in ion absorption band intensities and positions should greatly shift threshold intensities a t which different fragments are observed. This should be generally true for the aromatic hydrocarbons. In prior work Lubman and Kronick reported the MPI/MS spectra of an extensive series of aromatic hydrocarbons including the methylnaphthalenes.2o The mass spectra reported for the methylnaphthalenes consisted principally of the parent ion, which was produced by UV ionization. Thus, in these mass spectra, as in the conventional electron beam induced mass spectra, there was no discernible differenence between methylnaphthalene isomers.
Experimental Section The apparatus used for the MPI/MS experiments is shown in Figure 2. The pump laser was a Quanta-Ray Model DCR-1A pulsed ND:YAG oscillator and amplifier. This laser was frequency doubled and used to pump two separate dye laser systems. The first dye laser was a home-built grazing incidence pulsed dye laser.21,22Output from this laser was amplified and then frequency doubled external to the laser to provide tunable UV output at frequency wl. This UV beam was filtered to remove any residual visible light and the amplifier pump intensity was adjusted to obtain constant UV output power. The UV was focussed into the ionization cell by a IO-cm focal length lens (beam dimensions 0.1 X 0.2 cm at lens). The dye laser and doubling crystal were tuned by a pair of stepping motors controlled by a Cromemco (Z-80) laboratory computer. The laser line width was -0.2 crn-', the pulse duration was - 5 ns, and typical pulse energies were 1 m J in the visible and 0.1 mJ at the second harmonic. The second dye laser was a Quanta-Ray PDL-1 dye oscillator-amplifier system. This laser produced the visible beam at frequency w2 and had a spectral width of -0.2 crn-', a pulse duration of 5 ns, and output energies up to 50 mJ per pulse. The beam, with dimensions 0.7 X 0.7 cm, was focused into the ionization cell by a 1-m focal length lens. The dye laser intensities were controlled by adjusting the individual pump intensities. The pulse energy was maintained constant during wavelength scans. The dyes used in these experiments were rhodamine 6G, rhodamine B, rhodamine 640, and DCM. All of the laser dyes were obtained from Exciton and used without further purification. The 1-methylnaphthalene was obtained from ICN Pharmaceuticals and used without further purification. The 2-methyl(19') L. Andrews, B. J. Kelsall, and T. A. Blankenship, J. Phys. Chem.,
16 (1982). 86,29% ,..._,.
(20) D. M. Lubman and M. N. Kronick, Anal. Chem., 54, 660, 2289 (1982); 55, 867, 1486 (1983). (21) M. G. Littman a_"d H. J. Metcalf, Appl. Opt., 17, 2224 (1978); M. G. Littman, Opt. Lett., 3,, 138 (1978). (22) I. Shoshan, N. N. Danon, and P. Oppenheim, J. Appl. Phys., 48,4495 (1977).
naphthalene was zone refined before use. The vacuum system torr. Methylmaintained a background pressure of 2 X naphthalene was stored in an evacuated reservoir and admitted into the detection system through a leak valve. Typical MPI/MS experiments were performed with 2 X torr of methylnaphthalene in the system. The UV beam at w1 was focused into the ionization region with a IO-cm focal length lens and the visible beam at w2 was focused to overlap in the counterpropagating configuration with a 1-m focal length lens. Typical voltages for the repeller plate and acceleration grid were + 1400, and + 1200 V, respectively. The ions were detected at the end of the 70em-long drift tube with a Johnston Labs ion multiplier with a typical gain of 10'. The output from the ion multiplier was averaged by an EG& G-PAR Model 162 boxcar integrator equipped with a Model 164 integrator module. The boxcar integrator was interfaced with the Cromemco computer which could control the time delay and gate position of the boxcar. This computer also performed data collection and signal processing. The boxcar output was also applied to a chart recorder. The output of the ion multiplier was displayed on an oscilloscope for direct observation. Typical mass spectral resolution was such that the broadened parent ion signal ( m / e 142) fell to 10% of its peak intensity at positions separated by one mass unit.
Results The electronic absorption spectra of the methylnaphthalenes closely resemble those of naphthalene. There is a slight red shift of the weak S1 So absorption relative to naphthalene. Spectral congestion in the methylnaphthalenes due to thermal excitation of low-energy vibrational modes associated with the methyl group obscures all discrete vibrational structure in this transition. Nevertheless there are clear differences in the band structures for the two isomers. However, it would be difficult to use the differences to detect one isomer in the presence of larger amounts of the other isomer. The stronger S2 So absorption peaks near 270 nm for both isomers. This transition is more lacking of resolved vibrational structure due to both spectral congestion and vibronic interference between S2and S1. The electron impact mass spectra of the isomers are essentially indistinguishable. Excellent spectral discrimination can be achieved at cryogenic temperatures in a matrix environment or in a supersonic jet expansion, wherein the initial thermal distribution of molecules is confined to a few rotational-vibrational levels. Resonance-enhanced two-photon ionization spectra were recorded for the methylnaphthalenes by monitoring total ion current as frequency-doubled output from a dye laser was scanned through the UV absorption bands of the compounds. Laser intensity was maintained relatively constant and pressure was about 2 X torr. Two-photon ionization starts at 312 nm and increases to shorter wavelength. This is consistent with two-photon excitation with a long wavelength limit corresponding to the ionization potential of 7.96 eV (for both isomers). For wavelengths less than 3 12 nm the spectra are similar to the one-photon absorption spectra. The lack of discrete vibronic structure severely limits the potential for selective detection based on these transitions observed in an ambient temperature sample. One-color intensity dependences were measured for the parent and fragment ions. Parent ion (CIIHlo+)current (not shown) is quadratic in laser intensity at low intensities. This is consistent with two-photon ionization. The parent ion slope exponent becomes substantially less than 2 before fragment ions are observed. This indicates that the ionization cross section is larger than cross sections for fragmentation processes. (uF = [ZT~p]-l where Z, is the threshold intensity, uF is the fragmentation cross section, and T~ is the laser pulse duration.) Intensity dependences are similar within our limits of measurement for both isomers at wavelengths of 274, 279, and 295 nm. The first fragment is ClIH9+,corresponding to loss of one hydrogen atom from the parent ion. This process occurs when the parent ion absorbs one ultraviolet photon. As a consequence the ratio C11H9+/CIIHlo+ is observed to have unit slope at low
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The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
Mordaunt et al.
1 ,
0 2-METHYLNAPHTHALENE
1-1
j
\lCR
p_--c + 0+---7
