J . Phys. Chem. 1989, 93, 2383-2387
2383
Two-Color Multiphoton Excitation of Methanol: Observation of Structure in the Quasi-Continuumt Michael Ivanco,* D. K. Evans, and Robert D. McAlpine Physical Chemistry Branch, Atomic Energy of Canada Ltd., Research Company, Chalk River Nuclear Laboratories, Chalk River, Ontario KOJ 1 JO, Canada (Received: April 1 , 1988; In Final Form: September 14, 1988)
Infrared (IR) multiphoton absorption (MPA) in the quasi-continuum of methanol, in the vapor phase at a pressure of 400 Pa (3 Torr), was investigated by using a two-color, two-laser, photoacoustic technique. An HF laser operating on the Pl(6) line (3693.5 cm-I) was used to excite the OH stretch, and following a 500-ns delay, MPA was probed with a C02 laser. A comparison of one- and two-color MPA across the C 0 2 laser emission spectrum shows relatively sharp structure in the two-color spectrum where there was either no structure or very weak structure in the one-color MPA spectrum at the same wavelengths. This structure is attributed to red-shifted absorption in the CO stretch and OH bend.
1. Introduction Infrared (IR) multiphoton absorption (MPA) leading to dissociation has been shown to be a widespread phenomenon since it was first observed in 1971 by Isenor and Richardson.' Despite the fact that the study of MPA is now a relatively mature field, the mechanism for the excitation, particularly processes that take place in the vibrational manifold at higher levels of excitation, is not fully understood. Theoretical descriptions of MPA generally separate the pumping process into three energy regions.2-E The lowest energy region, El, involves excitation of the discrete low-energy levels of a molecule and is characterized by a low density of states. The excitation is expected to be strongly dependent on laser wavelength, fluence, and intensity. El is also called the coherent pumping regionEbecause the Rabi flopping frequency, wR,is much greater than the rate of intramolecular vibrational energy redistribution, W, a TI type process. In addition, T2 type dephasing processes should be slow compared to the response time of the molecule to the radiation field. In the highest energy region, EIII,above the dissociation level of the ground state, the energy levels are continuum levels and W is much greater than wR. The EIIIthe pumping is incoherent and competes with molecular dissociation. The excitation in this region can be well-described by a rate equationsEb The excitation processes in EI and EIIIare quite well-understood. It is in the intermediate region, E , also called the quasi-continuum (QC), where the dynamics of the vibrational levels and their interaction with radiation are uncertain. 4n understanding of processes that take place in the Q C is desirable because the chemical reactivity of most molecules is determined by these proces~es.~Early theoretical studies used rate equations to describe IR pumping in EII and excluded molecular dissociation processes. Inherent in this model is the assumption that Wis much greater than wR and that T2 type dephasing processes are fast compared to 1/wR. Consequently, the picture of localized vibrations, whether normal or local mode, which is valid in EI, breaks down and vibrational quantum numbers are considered to be "bad" quantum numbers. As a result, a rate equation picture predicts a uniform distribution of transition probabilities and only the total vibrational energy is considered to be important. The observation of overtone spectra, to high levels of excitation in some molecules,1b17 stimulated emission pumpinglE-*' and two-color MPA experiment^^^-^^ have suggested that the integrity of some localized vibrations does not necessarily break down in region EII. These experiments have led to the development of new theories of vibrational dynamics in the QC which have application to MPA. These theories have described intramolecular vibrational coupling from both and quantum mechanical33 perspectives and show that it is possible for strongly localized 'Issued as AECL Contribution No. 9719.
0022-365418912093-2383$0 1 SO10
vibrations to exist embedded within the QC. This means that the magnitude of W is a function of the vibration type and, for certain
(1) Isenor, N. R.; Richardson, M. C. Appl. Phys. Lett. 1971, 18, 224. (2) Mukamel, S.; Jortner, J. Chem. Phys. 1976, 65, 5204. (3) Letokhov, V. S.; Moore, C. B. Sou. J . Quantum Electron (Engl. Transl.) 1976, 6, 129. (4) Ambartzumian, R. V.;Letokhov, V.S. Acc. Chem. Res. 1977, 10,61. (5) Lyman, J. L. J . Chem. Phys. 1977, 67, 1868.
(6) Grant, E. R.; Schulz, P. A,; Sudbo, A. S.; Shen, Y. R.; Lee, Y . T.Phys. Rev. Lett. 1978, 40, 115. (7) Quack, M. Adu. Chem. Phys. 1982, 50, 395, and references therein. (8) (a) Goodman, M. F.; Stone, J.; Dows, D. A. J . Chem. Phys. 1976,65, 5052, 5062. (b) Weston Jr., R. E. J . Phys. Chem. 1982,86, 4864. (9) Forst, W. Theory of Unimolecular Reactions; Academic Press: New York, 1973. (IO) Henry, B. R. Acc. Chem. Res. 1977, 10, 207. (11) Giver, L. P. J . Quant. Spectrosc. Radiat. Transfer 1978, 19, 31 1. (12) Bray, R. G.; Berry, M. J. J . Chem. Phys. 1979, 71, 4909. (13) Eastes, W.; Ross, U.; Toennies, J. P. J. Chem. Phys. 1978, 70, 1652. (14) Reddy, K. V.; Heller, D. F.; Berry, M. J. J . Chem. Phys. 1982, 76, 2814. (15) Perry, J. W.; Moll, D. J.; Kuppermann, A,; Zewail, A. H. J . Chem. Phys. 1985,82, 1195. (16) Baggot, J. E.; Chuang, M.-C.; Zare, R. N.; Dubal, H. R.; Quack, M. J . Chem. Phys. 1985, 82, 1186. (17) Scherer, N. F.; Doany, F. E.; Zewail, A. H.; Perry, J. W. J . Chem. Phys. 1986,84, 1932. (18) Kittrel, C.; Abramson, E.; Kinsey, J. L.; MacDonald, S. A.; Reisner, D. E.; Field, R. W.; Katayama, D. H. J . Chem. Phys. 1981, 75, 2056. (19) Lawrence, W. D.; Knight, A. E. W. J. Chem. Phys. 1982,76,5637. (20) Moll, D. J.; Parker, G. R.; Kuppermann, A. J . Chem. Phys. 1984,80, 4800. (21) Nebel, A.; Comes, F. J.; Stephan-Rossbach, K.-H. Chem. Phys. 1985, 94, 25. (22) Nowak, A. V.; Lyman, J. L. J. Quant. Spectrosc. Radiat. Transfer 1975, 15, 1945. (23) Ambartzumian, R. V.; Gorokhov, Yu.; Letokhov, V. S.; Markarov, G. N.; Puretzky, A. A,; Furzikov, N. P. JETP Lett. (Engl.Transl.) 1976, 23, 194. (24) Akulin, V. M.; Alimpiev, S. S.; Karlov, N. V.; Prokhorov, A. M.; Sartakov, B. G.; Khokhlov, E. M. JETPLett. (Engl.Transl.) 1977,25,400. (25) Fuss, W.; Hartman, J.; Schmidt, W. E. Appl. Phys. 1978, 15, 297. (26) (a) Fuss, W.; Hartman, J. J. Chem. Phys. 1979, 70, 5468. (b) Fuss, W. Chem. Phys. Lett. 1980, 71, 77. (27) Ambartzumian, R. V.; Letokhov, V. S.; Makarov, G. N.; Puretzky, A. A. Opt. Commun. 1978, 25, 69. (28) Borsella, E.; Fantoni, R.; Giardini-Guidoni, A. Chem. Phys. Lett. 1982, 87, 284. (29) Mukherjee, P.; Kwok, H. S. J . Chem. Phys. fi86,84,1285. (30) Christoffell, K. M.; Bowman, J. M. J . Phys. Chem. 1981, 85, 2159. (31) Jaffe, C.; Brumer, P. J . Chem. Phys. 1980, 73, 5646. (32) Hose, G.; Taylor, H. S. Chem. Phys. 1984, 84, 375.
0 1989 American Chemical Society
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Ivanco et al.
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
I 1
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Figure 1. Experimental setup for two-color experiments. E, and EB are C 0 2and HF laser radiant energy monitors, respectively; D, and D2 are fast photodetectors for the C 0 2and HF lasers, respectively; EB triggers the DAU for two-color experiments. L1 is a 50-cm focal length germanium lens, and L2 is a 2-m focal length BaF2 lens. Neutral density filters, calibrated for H F laser wavelengths, are used to adjust the HF laser fluence and CaF2 filters for the C 0 2 laser.
vibrations in the QC, is not necessarily much greater than wR. Earlier two-laser experiments on SF622-26and SiF424showed relatively broad singly peaked structure, as a function of wavelength, that was red-shifted from the one-photon IR absorption and the one-color MPD maxima. One-color MPA or MPD spectra represent a convolution of all of the excitation steps involved, and, prior to the two-color SF6 and SiF4 experiments, it was thought that the structure in one-color (MPD) spectra was completely due to the wavelength-dependent excitation steps in region EI. These experiments, however, implied that absorption in the Q C is also wavelength dependent. Other two-color MPD ~ ~ ’ a red shift in the dissociation experiments on 0 ~ 0 showed spectrum due to the second photon as well as some weak structure. It is, however, difficult to draw general conclusions from the experiments on SF6, SiF4, and oso4for two reasons. Firstly, all of these molecules are highly symmetrical and, thus, are not typical of most molecules that undergo MPD. Secondly, molecular dissociation, which was measured in all three cases, is not necessarily a good measure of absorption. Recent two-color MPD experiments on C2F5ClZghave shown a great deal of sharp structure in the Q C of this molecule. This molecule, being unsymmetrical, is more typical of molecules that undergo MPD than SF6, SiF,, and 0sO4 A subsequent two-color experiment,29in which the small signal transmission of the second color was measured instead of dissociation, corroborated the MPD results in part of the same spectral region. These C2F5Clresults also run contrary to expectations based on a rate equation picture of excitation in the QC. The relatively sharp structure observed in the Q C strongly suggests that vibrational selection rules are important in excitation through region E,[, so that a purely statistical approach is not appropriate. The purpose of this paper is to study absorption in the quasi-continuum of CH30H. Methanol has been chosen for this study because of its unique spectral properties and because it is relevant to studies of laser isotope ~ e p a r a t i o n . ~ While ~ - ~ ~ very high se(33) Abram, I.; deMartino, A,; Frey, R. J . Chem. Phys. 1982, 76, 5727. (34) (a) Borden, A.; Barker, E. F. J . Chem. Phys. 1938, 6, 553. (b) Burkhard, D. C.; Dennison, D. M. J . Mol. Specrrosc. 1959, 3, 299. (c) Bames, A. J.; Hallam, H. E. Trans. Faraday SOC.1970, 66, 1920. (35) Bialkowski, S. E.; Guillory, W. A. J . Chem. Phys. 1977, 67, 2061. (36) Bialkowski, S. E.; Guillory, W. A. J . Chem. Phys. 1978, 68, 3339. (37) Bhatnagar, R.; Dyer, P. E.; Oldershaw, G. A. Chem. Phys. Lett. 1979, 61, 339.
lectivity for excitation of the OH stretch relative to OD has been demonstrated with an H F laser, there is evidence that methanol is very hard to dissociate with such a laser because of the large anharmonicity in the O H / D ~tretch.~~*,O In contrast, methanol is relatively easy to dissociate with a C 0 2 laser, exciting the CO stretch, but the isotopic selectivity is much lower. In this work we have investigated multiphoton absorption in methanol using both H F and C 0 2 lasers. Methanol has strong IR absorptions at 2.7 Km (OH stretch), 3.4 pm (CH stretch), and 9.6 pm (CO stretch). These three vibrations can be excited by HF, DF, and COz lasers, respectively. Methanol has C, symmetry, which makes it typical of most molecules that undergo MPD. In addition, it has been studied extensively in one-color experiments in the infrared region of the spectrum so that a large data base on the molecule’s IR spectra,34 MPA and MPD, exists.3542 The studies described in this paper have been carried out using a two-color, two-laser, photoacoustic pump/probe technique to measure directly MPA in the QC. 2. Experimental Section In these experiments, two lasers have been used to excite gas-phase methanol at a pressure of 400 Pa (3.00 Torr), and the absorption has been measured photoacoustically. Figure 1 shows the experimental setup for these measurements. The first laser, an HF laser (Lumonics 21 2A), excites the O H stretch in methanol and is used to preheat the molecules vibrationally into the QC. The H F laser is an unstable resonator running on several longitudinal modes with a pulse length of approximately 300 ns. The P,(6) line (3693.5 cm-I) was used in all experiments. The second laser, a CO, laser (Lumonics short pulse43), was used to probe the excitation of the preheated molecules following a 500-11s time (38) Schmeidle, R.; Meier, U.; Welge, K. H. Chem. Phys. Len. 1981, 80, 495. (39) McAlpine, R. D.; Evans, D. K.; McClusky, F. K. Chem. Phys. 1979, 39, 263. (40) Chin, S. L.; Evans, D. K.; McAlpine, R. D.; McClusky, F. K.; Selkirk, E. B. Opt. Commun. 1979, 31, 235. (41) McAlpine, R. D.; Evans, D. K.; McClusky, F. K . J . Chem. Phys. 1980, 73, 1153. (42) Evans, D. K.; McAlpine, R. D.; Adams, H. M. Isr. J . Chem. 1984, 24, 187. (43) Pasternak, A. W.; James, D. J.; Nilson, J. A,; Evans, D. K.; McAIpine, R. D.; Adams, H. M.; Selkirk, E. B. Appl. Opt. 1981, 20, 3849.
Two-Color Multiphoton Excitation of Methanol
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2385 9P
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Figure 2. One-color MPA of methanol as a function of laser wavelength, measured photoacoustically,is shown with the HF laser blocked. The abscissa is uncalibrated but is directly proportional to the microphone voltage and hence the absorption. The C 0 2 laser emission regions are shown (inset), and the one-photon IR absorption spectrum of methanol in the CO stretch region is shown for comparison. The IR absorption spectrum was obtained on a Bomem DA3.02 Fourier transform IR spectrometer with the resolution degraded to 1.5 cm-' in order to show the rotational envelope of the vibrational transition without fine structure.
delay. The wavelength of the C 0 2laser, which is only line tunable, was varied over the entire laser spectrum with some gaps between bands. COz laser pulses of 60-11s duration were used, and they were both single transverse and single longitudinal mode pulses. The synchronization of the two lasers was accomplished by using a pulse delay generator (Gould PG58A), triggered by the C 0 2 laser amplifier trigger pulse. The delay between the amplifier trigger pulse and the COz laser output pulse was sufficiently long to allow the H F laser to fire well before the COz laser. A second portion of each laser beam was split off and the energy per pulse measured by using EA for the C02 laser and EB for the HF laser (Laser Precision Corp. Rj-7200, both monitors). Part of each laser pulse was also split off and observed with fast photodetectors, D, (photon drag) for the C 0 2laser and D2 (InAs) for the H F laser. In this way the time delay between the two lasers could be continuously monitored. Both lasers are gained switched, but it was possible to reduce the jitter between the two laser pulses to f 7 0 ns after reconditioning the spark gaps in both lasers and running them closer to their self-breakdown pressures. The laser beams were focused and spatially overlapped at the center of a photoacoustic cell with counterpropagating geometry. The H F laser beam diameter was approximately 1.5 times that of the C 0 2 laser at the focus. A sapphire flat was used to block the C 0 2 laser light while allowing transmission of the H F laser. The H F laser pulse, which has a doughnut-shaped radial intensity profile, was blocked by an iris. The COz laser was aimed through the center of the iris, which was substantially larger in diameter than the C 0 2 beam; hence there were no diffraction effects. The lasers were aligned by overlapping burn patterns on opposite sides of photosensitive paper. Alignment apertures were then placed in the beam path so that the beam overlap could be adjusted with the cell closed. The alignment was frequently checked by opening the cell through the course of the experiments and was found not to wander. Methanol HPLC grade was dried over 4A molecular sieve and was vacuum degassed at regular intervals throughout the experiments. 3. Results
Three types of experiments were performed: 1. The MPA (microphone) signal was measured with the HF laser operating on the Pl(6) and the C 0 2 laser blocked. The energies of the H F laser pulses together with the MPA signals were measured with the data acquisition unit (DAU)@and stored
Wr
CO2 LASER FREWLNCY Ikm-!
Figure 3. Two-color MPA of methanol, with the dc component due to the HF laser subtracted (0),is compared to the one-color MPA of Figure 2 ( X ) . Lines between points are linear interpolations.
on the microcomputer. An H F laser fluence (4) of 25 J/cm2 was used in all cases. These results are not shown, but previous work in this laboratory,40using the same laser, has shown that methanol molecules at a pressure of 400 Pa and a fluence of 25 J/cm2 absorb five HF photons on average. This level of excitation corresponds to approximately 18000 cm-'. The one-color H F laser experiments were necessary because they establish a base line for the two-color experiments, to which the H F laser adds a dc offset. 2. The energy of the CO, laser was measured, together with the MPA signal at each COz wavelength with the HF laser blocked. Figure 2 shows the one-color C 0 2 laser signal as a function of wavelength. The C 0 2 laser is only line tunable so that the lines drawn between points are linear interpolations as well as the dotted lines between bands. It is interesting to note that the MPA spectrum at a fluence of 50 J/cm2 follows the one-photon IR spectrum fairly closely. Previous studies in this have shown that at a fluence of 50 J/cm2 and a pressure of 400 Pa, using the 9P(34) line of the COz laser, approximately five C 0 2 laser photons are a b s ~ r b e d . The ~ ~ .9P(34) ~~ line of the COz laser corresponds to absorption in the center of the Q branch of the C O stretch. Hence, the estimate of five photons absorbed (( n ) = 5 ) should be an upper limit to absorption at 50 J/cm2. Since the MPA spectrum closely follows the one-photon spectrum, it appears that the anharmonicity in the C O stretch is quite small. This is also born out by the fact that collisionless MPA is very facile with C 0 2 laser pumping.42 As expected, there is very strong MPA of photons from the 9P branch of the C 0 2 laser and somewhat weaker absorption in the 10R branch. In addition, there is weak absorption in the 9R branch and negligible absorption in the 1OP branch. 3. The MPA signal was measured with both lasers unblocked. The time delay between the lasers was 500 ns with the H F pulse leading. The data were stored on a computer together with the energy/pulse of the C 0 2 laser. The output of the HF laser was also measured in the two-color experiments using energy monitor Eg, and EBwas also used to trigger the DAU. EB was triggered internally with a preset threshold trigger level which ensured that only the contribution from HF laser pulses within a narrow energy (f5%) were recorded. A minimum CO, laser pulse energy rejection of 1.0 mJ was built into the data acquisition software. The two color results are shown in Figure 3 with the one-color results shown for comparison. A comparison of the one- and two-color MPA reveals that there is enhanced MPA in the lop, 10R, and 9R branches of the C 0 2 laser emission spectrum and very little apparent enhancement in the 9P branch. The results are plotted, in Figure 4, in terms of two-color enhancement (TCE) of the MPA versus C 0 2 laser wavelength. TCE is defined such that TCE = A ( ~ ) H F + c o-, [AHF+ A(w)co,l
(44) Adams, H. M.; Selkirk, E. B.; Gocdale, J. W.; Evans, D. K.; McAlpine, R. D. Rev. Sci. Instrum. 1984, 55, 1298.
(1)
where AHFis the dc offset of the MPA signal due to HF laser
2386 The Journal oJPhysical Chemistry. Val. 93. No. 6. 19x9
lvanco et al.
40.000
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Figure 4. Two-color enhancement of the MPA signal, as defined by y 1. The lines between paints are, as before, linear interpolations.
excitation, A ( w ) is~the ~ one-color ~ absorption (from Figure Z), and IS the absorption measured with both lasers operating (from Figure 3). Figure 4 shows three primary features, centered at approximately 945,975, and 1080 cm-’, with, possibly, another feature a t 985 cm-’. The wavelength could not be extended any further to the blue for the latter. 4. Discussion
Since the two-color MPA technique that has been used to obtain our results is unique, it is important to examine exactly what Observablesare being measured. Figure 5 illustrates the excitation scheme that was employed in these experiments. The H F laser, which excites the O H stretch, preheats the methanol molecules to an average level of excitation of approximately five photons absorbed, or 18000 Earlier one-color studies of this m o l e c ~ l e with ’ ~ ~the ~ ~same ~ laser have estimated that the onset of the Q C is a t approximately three HF photons absorbed, or 11 000 cm-I, so that, in the experiments described in this paper, methanol is excited well into the QC. With a 300-11sH F laser pulse, a t 400-Pa methanol pressure, the excitation is collisionally enhanced, with about six hard-sphere collisions occurring during the pulse so that the excitation of methanol into the QC can be said to be a statistical process. The actual mechanism of the apparent collisional enhancement is, however, unknown. Previous studiesa have shown that collisionless absorption of methanol with an H F laser is very difficult, owing to the large anbarmonicity in the O H stretch, and only collisional processes that occur during the pulse make it possible to excite into the QC. A 500-ns delay ensues before the molecules are probed with the CO2 laser. This delay is long enough to ensure that vibrational-rotational (V-R) and vibrational-vibrational (V-V) relaxation processes are almost certainly complete45but short enough that vibrational-translational (V-T) relaxation is negligible. The molecules in the sample should, therefore, have a well-defined vibrational and rotational temperature. The CO, laser pulse then induces further excitation of those preheated molecules in the QC to an additional level of approximately five photons (maximum) absorbed, or about 5000 cm-I, based on comparison with earlier results for the 9P(34) line!1.42 The two-color MPA spectrum (Figure 4), then, represents a convolution of a maximum of five absorption steps in the QC. The fluences employed in these studies are sufficiently low that multiphoton dissociation is negligible and does not complicate the interpretation of the results. There will only be structure in Figure 4, the T C E spectrum, if there is absorption in the QC that is greater than the sum of the two one-color signals. Consequently, the most important result of this study is the observation of structure in Figure 4, where there is no one-photon IR absorption or one-color MPA near 945 Em-’, in the IOP branch of the CO, laser emission spectrum. Because of the way in which TCE is defined, this structure must be due to absorption processes that take place in the Q C of methanol. (45) Lambert. 1.
D.J . Chem. Soc., Faraday Trans. 2 1972, 68, 364.
Figure 5. Two-color excitation scheme. The methanol vibrational energy ladder in the OH stretch is shown schematically with the average number of HF and C02 laser photons absorbed indicated with arrows. The total vibrational density of states is given at each level of excitation, obtained by State counting and assuming that all modes are harmonic oscillators except for the OH stretch.
The largest two-color enhancement that we have observed is near 975 cm-’. This is a region of the spectrum where there is one-color absorption although Figure 3 shows that this absorption is much stronger when the molecules are preheated into the QC. TCE is also observed near 1080 cm-I which is a region where there was weak one-color MPA. In the 1040-cmP region, where the one-color MPA was the strongest, there is little or no TCE. In the latter three spectral regions there is the possibility that molecules unexcited by the H F laser contribute to the observed TCE. That is, in addition to the absorption sequence shown in Figure 5, there may be one-color absorption of CO, laser radiation by unexcited methanol molecules. Because of this possibility, no strong conclusions can be drawn about the relative magnitudes of MPA in the Q C where there is some one-color absorption. However, it can be said that in regions of the spectrum where there is one-color absorption, near 915 and 1080 cm-’, the absorption in the QC is stronger than it is for unexcited molecules, as a comparison of Figures 2 and 3 illustrates. It should also be pointed out that TCE measured in Figure 4 is an underestimation of the true TCE because the area of the CO, laser probe beams is only 0.45 times that of the H F laser beam. The CO, laser only probes a fraction of the preheated molecules, although all of the molecules in the volume element swept out by the H F laser beam contribute to the measured MPA signal. The two-color MPA results indicate that there is relatively sharp structure in the QC. The peak near 945 cm-l, for example, where there is no one-color absorption, is only 10-15 cmP wide. That the spectrum is not more uniform strongly implies that vibrational selection rules apply to transitions in the Q C and that there are localized vibrational modes a t very high levels of excitation. The origin of the structure in the T C E spectrum is not completely clear. In the OsO, experiments,” the weak structure in the two-color MPD was attributed to anharmonic splitting of vibrational levels in the QC. However, this can play no role in C H 3 0 H since it has C, symmetry. In C,F,CI?* the structure in the two-color MPD was related to red shifts of several fundamental bands and combination bands that, in Born-Oppenheimer language, “borrow intensity” from strong IR transitions. In CH,OH, on the other hand, there are not as many low-frequency modes or combination bands that overlap the CO, laser emission spec-
Two-Color Multiphoton Excitation of Methanol trum, and it is difficult to account for the structure in Figure 4 unambiguously. The one-photon IR spectrum of methanol has been extensively investigated in the vapor phase.34 The most intense peak in the one-photon IR absorption spectrum is centered at 1034 cm-l (Q branch), which is associated with the v4 mode, CO stretch vibration. The only other fundamental vibration that appears with at least medium intensity in the one-photon spectrum that is close to the C 0 2 laser emission spectrum is the v6 vibration (the O H bend at 1340 cm-’). It would require a red shift of over 250 cm-’ to have the C02 laser emission spectrum overlap the center of this transition, but some rotational components may only require a 150-200-~m-~ red shift at the higher temperature of the excited molecules. The anharmonicity of the v6 mode is not known, but, because the v I mode (OH stretch) has a very large anharmonicity of -86.2 cm-1,34the cross-anharmonicity X I 6 may be substantial. It has been shown in other moleculesa4* that coupling between a stretching and bending vibration can be substantial when the stretching vibration is anharmonic. If the OH stretch is very anharmonic, then the vibrational kinetic energy and vibrational potential for all other coordinates can be a strong function of the ~~ a red shift of level of excitation in the OH o ~ c i l l a t o r .Hence, about 250 cm-I in the v6 (OH bend) mode may not be unreav6 combination bands, particularily at sonable, at least in v 1 ihigh levels of vibrational excitation in the QC. C H 3 0 H also has very strong absorption features involving the O H torsional, v I 2 mode, which has a fundamental vibrational frequency of 232 cm-I. The vibrational potential of this mode, however, has three identical maxima with a low barrier height of approximately 470 cm-1.34cThe absorption of a second photon in this mode transforms the torsional oscillation into a near free internal rotation. The second overtone in this mode has been observed to extend from 380 t3 800 cm-’ with medium intensity. Although a higher rotational temperature, such as would exist in a gas that has been preheated to the QC, might shift higher energy rotational components of the second overtone of this vibration into the C 0 2 laser emission region, it is unlikely that excitation in this mode plays much of a role in the MPA. Because absorption of two photons converts the v12 vibration into free internal rotation, for most rotational components the absorption of any further photons, with wavelengths as short as 9 or 10 pm, into pure rotation is a highly unlikely process. Hence, it is doubtful that there could be localized structure in the QC, separated by approximately 1000 cm-’, that could be due to the torsional mode of CH30H. There are also two very weak transitions at approximately 1100 and 11 50 cm-’, v, and v l l modes, that have been identified with the CH3 rocking vibrations. For excitation in these modes to be important in the QC, they would have to “borrow intensity” from strong IR-active transitions or combination bands. In methanol, the strongest IR transitions are associated with excitation of the C O stretch, C H stretch, OH torsion, OH stretch, and O H bend. The energy mismatch between the rocking modes and the C H and O H stretches, however, is very large. In addition, it seems unlikely, on the basis of classical considerations, that any of these five vibrations could couple strongly enough to the CH3 rocking motion to facilitate intensity borrowing. It would seem, then, that the structure observed in the T C E spectrum is due to either the CO stretch (v4), the OH bend (v6), or both. The CO stretch band spans the region from 970 to 1080 cm-l in the one-photon spectrum. It is reasonable to propose that the peaks in the TCE spectrum that overlap the IO-pm bands of the C 0 2 laser emission spectrum are due to red-shifted MPA in the CO stretch. The peak at 1080 cm-I, blue-shifted from the C O stretch absorption maximum, is more difficult to account for. If absorption in the C O stretch, in the QC, is red-shifted, as the appearance of the peaks at 945 and 975 cm-I would suggest, then
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2387 attributing the absorption at 1080 cm-I to the C O stretch would seem inconsistent. Because the preheated molecules have undergone complete V-R relaxation before the C 0 2 laser probes the sample, however, the rotational envelope of the C O stretch may be considerably broadened. Hence, it is possible that some high-energy rotational components in the P branch are blue-shifted with respect to the one-photon spectrum. Alternatively, the O H bend may have an anharmonicity large enough to shift its absorption into the 9-pm region at high vibrational and rotational levels of excitation, which could account for some of the observed structure. Without further knowledge about the anharmonicity of the 16 bending vibration, it is difficult to make a strong statement about its potential importance to excitation in the QC. A mechanism for the observed decoupling of certain vibrational modes in the Q C has been suggested in some recent theoretical The decoupling mechanism has been called “extreme motion”. That is to say, in systems where vibrations are excited to large amplitude, there may be dynamical barriers in the molecular potential that keep certain vibrational modes from coupling to others even if the state density is large. Most of these calculations have been done using systems with two vibrational degrees of freedom, but it has been shown that the same principles can be extended to many-degrees-of-freedom systems with similar conclusion^.^^ The observation of relatively sharp structure in the TCE spectrum of methanol, and observations in ofher systems that have been previously mentioned, are consktent with these theories. In particular, where there is no ohe-color MPA or one-photon absorption, unexcited molecules cannot contribute to the observed structure in the two-color spectra. From our experiments, then, it seems that the C O stretch is one vibration that is not strongly coupled to others in the Q C and possibly the O H bend as well. The observation of TCE over narrow regions of COz laser wavelengths also has importance in a potential laser isotope separation scheme. The possible use of two-color techniques to enhance isotopic selectivity has been recognized for a number of years.49*s0 Because of the high laser pulse intensities required to cause MPD of most molecules, power broadening in region EI can significantly reduce isotopic selectivity. In a two-color MPD experiment a lower intensity of the pump laser (wl)is used to excite the molecules into the QC but not to dissociation. A second laser pulse (w2), not resonant with any one-photon absorption, can then excite the molecules to dissociation. The isotopic selectivity of the entire process should be improved because of smaller power broadening of the spectral lines in EI, with the lower intensity pump pulse. Our two-color data show that additional restrictions must be placed on the wavelength, w2, of the second pulse. Hence, it is not sufficient that there be no one-photon absorption at u2,but it may also be necessary that w2 overlap resonances within the QC. 5. Conclusion
We have studied the MPA of methanol using a two-laser, two-color, photoacoustic, pump/probe technique. Relatively sharp structural features have been observed in the two-color spectrum as a function of the wavelength of the probe laser. The spectral features have appeared both red- and blue-shifted from the one-photon C O stretch absorption maximum. The structure has been attributed to both red-shifted absorption in the QC, due to the CO stretch, and possibly more strongly red-shifted absorption in the O H bend. There also exists the possibility that the blueshifted absorption, near 1080 cm-’, is due to high-energy rotational components in the P branch of the C O stretch. The data indicate that absorption in the Q C of methanol is wavelength dependent and contribute to an emerging picture of the Q C as a region in which coherent excitation competes with statistical processes in multiphoton absorption. Registry No. Methanol, 67-56-1.
(46) Mortensen, 0.S.; Henry, B. R.; Mohammadi, M. A. J . Chem. Phys. 1981, 75, 4800. (47) Child, M. S.; Lawton, R. T. Faraday Discuss. Chem. SOC.1981, 71, 212. (48) Carrington Jr., T. J . Chem. Phys. 1987, 86, 2207.
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