J. Phys. Chem. 1995,99, 11067-11073
11067
Dissociative Resonance Raman Spectrum of Methyl Mercaptan R. E. Stevens, C. Kittrell, and J. L. Kinsey* Department of Chemistry and Rice Quantum Institute, Rice University, P.O. Box 1892, Houston, Texas 77251 Received: March 16, 1995; In Final Form: May 11, 1995@
Dissociative resonance Raman spectroscopy has been used to probe the dissociation dynamics of methyl mercaptan. A tunable, narrow-band UV laser system has been developed capable of generating 10 ns pulses of -10 mJ energy, with a bandwidth of less than 1 cm-’ (fwhm). The Raman spectrum of methyl mercaptan shows a progression which we assign to the C-S stretching mode. A strong feature we assign to the S-H stretching fundamental exhibited no overtones. The Raman spectrum of methyl mercaptan is complicated by the appearance of CS and atomic carbon fluorescence. A pathway is proposed for the multiphoton formation of these emitting species at 193 nm excitation.
I. Introduction The W photochemistry of methyl mercaptan has been actively studied, both for its practical importance in atmospheric chemistry and because it presents an interesting set of fundamental questions.’-6 Investigations within the past few years have included angular and recoil velocity analysis of photofragment^,^,^,^ absorption spectroscopy and product branching emission spectro~copy,~.~ and ab initio calculat i o n ~ . “ ~ . The * . ~ main findings and the discrepancies among them are briefly reviewed in the following section. Especially interesting from our point of view are the studies by Jensen et ale5and Keller et aL4which mounted a two-pronged attack by combining time-of-flight (TOF) measurements of the products’ angular and velocity distributions with “emission spectroscopy” for excitation at 248,222, and 193 nm wavelength radiation. Emission spectroscopy, or dissociative resonance Raman spectroscopy (DRRS), can provide a very clear indication of the earliest motion of the system toward photodissociation in the Franck-Condon region of the excited-state potential energy surface (PES) . The angular and product distributions, on the other hand, tell the story of the ultimate outcome of the dissociation, from which details of earlier dynamics can also often be inferred. The DRRS studies of the two groups mentioned in the previous paragraph were carried out with incident radiation from broad-band excimer lasers. The bandwidth in their experiments is not stated in the references, but such devices are well-known to have bandwidths (fwhm) of 100 cm-’ or more. When combined with the spectrometerresolution in their experiments, this uncertainty prohibits unequivocal identification of observed features. We have recently developed a tunable, narrow-band source for use in the 193 nm region and decided to revisit this important and interesting problem with a view to clearing up any ambiguity. Our results do lead to considerably different conclusions regarding the DRRS spectrum at 193 nm, as indicated in the section on results. 11. Brief Review of Previous Work on Methyl Mercaptan The W spectrum of methyl mercaptan is a continuum starting at about 310 nm and extending to the blue.2.6 There are two features, one centered at approximately 230 nm with a cross section of about 68 x cm2 and the other at approximately @
Abstract published in Advance ACS Absfracfs,June 15, 1995.
0022-365419512099-11067$09.0010
204 nm with a cross section of about 850 x cm2. These features are attributed to the 1’A” and 2’A“ (C, point group) excited electronic states. Primary Photoproducts. The two principal photoproduct channels in the energy region of interest are believed to be those of processes I and 11:
+ hv - CH,S + H CH3SH + hv - CH, + SH
CH3SH
(1) (11)
Although there are several additional product channels that are energetically accessible, both Vaghjiani6 and Jensen et aL5 find only the products of process I on the red side of the center of the 1’ A” system (248 nm). Actually, neither of the measurements would be able to distinguish between CH3S and its isomer CHzSH, which lies some 2200 cm-’ higher in energy.’(’ However, Segall et al. investigated the photolysis of CH3SD at 193 nm and found no evidence for CH bond-breaking at that energyS7 At 222 and 193 nm there is also some p r o d ~ c t i o n ~of- ~CH3 SH. Vaghjiani’s results appear to give the more reliable branching ratios. His values for the quantum yield a1 of process I are 0.95 f 0.10 at 222 nm and 0.49 f 0.08 at 193 nm. Jensen et al. could not directly determine the branching ratio without a calibration of SH+ from SH vs from CH3S as the parent. Their data do show that the C-S/S-H branching ratio goes up by a factor of 8 at 193 nm relative to the branching ratio at 222 nm. Recent work by Barone et al. in measuring the quantum yields of H and CH3S in organosulfur compounds agrees quite well with Vaghjiani. Most of these groups have assumed that the quantum yield a11for process I1 is a11 = 1 - ar. However, Nourbakhsh et aL3 claim to have some evidence for a third channel at 193 nm:
+
CH,SH
+ hv
-
CH,S
+ H2
(111)
No estimates are given for the branching ratio to this channel, possibly because detection of CH2S is complicated by a large CH2S+ signal from CH3SH clusters in their supersonic expansion. Velocity and Angular Distributions. Time-of-flight (TOF) velocity and angular distributions have been investigated by three groups, all employing supersonic molecular beams of CH3SH seeded in He and crossed by wide-band light from excimer lasers. Keller et aL4 and Jensen et aL5 studied the wavelengths 248, 222, and 193 nm, using mass spectrometry
0 1995 American Chemical Society
Stevens et al.
11068 J. Phys. Chem., Vol. 99, No. 28, 1995
-Segall
------Nourkbakhsh Keller
I 10.0
20.0 30.0 Translational energy (kcal/mol)
.O
40.0
Translationd Energy (kcal/mol)
Figure 1. TOF measurements for S-H bond fragmentation in methyl mercaptan measured by two different experiment^^,^ at 248 nm. and electron impact ionization. They detected the species CH# and SH+. Segall, et aL7 worked at 248 and 193 nm, using high-n Rydberg time-of-flight (HRTOF) detection (limited to detection of H or D only). The third group, Nourbakhsh et al.,3 worked at 193 nm, again using mass spectrometry with electron impact ionization. They detected the species SH+, CH3+ , CH2S+ , H+, and S+. The S+ was attributed to fragmentation of SH, and much of the CH2Sf was attributed to cluster fragmentation. The polarization results in both of the studies of dissociation via process I at 248 nm yield an asymmetry parameter p - 1, indicating both a rapid dissociation on the time scale of molecular rotation and a transition moment perpendicular to the C, symmetry plane (in accordance with the electronic assignment of ’A” symmetry). The TOF results of the two studies are qualitatively similar but in quantitative disagreement. Segall et a1.l resolve what they identify as a vibrational progression with peak-to-peak spacings of roughly 800 cm-I, which they attribute to v = 0, 1 , 2 excitation of the CS stretching vibration in the CH3S radical. It would not be surprising for them to have better effective energy resolution, since their H atom detection is on the favorable end of the kinematic lever for process I. When the two results are viewed in the same graph, as in our Figure 1, the difference between the two measurements appears to be explainable by differences in resolution. The situation at 193 nm is more puzzling. Here there are three different experiments: Keller et al.,4 Segall et al.,7 and Nourbakhsh et ale3 Keller, et al. again find a p value near - 1 at this wavelength. They report a distribution in translational energy that rises from a threshold near 60 kcal/mol to a maximum at slightly lower translational energies ( E T ’ S ) and , then falls smoothly to lower ET’Sto nearly zero below about 20 kcal/mol. In contrast, Nourbakhsh et al. find a distribution with about the same threshold value on the high side, an essentially linear rise to a plateau at roughly 30 kcal/mol which persists down to 20 kcal/mol, the lowest value for which they show data. Segall et al. obtained yet another result. They find a bimodal ET distribution. Polarization studies of their “fast” component agree with the p value of about - 1, but the “slow” component is best fit by a value for 8 , of -0.65 & 0.07, which would indicate a much longer-lived existence of the photoexcited
-
Figure 2. TOF measurements for S-H bond fragmentation in methyl mercaptan measured by three different experiment^^,^,^ at 193 nm. The high-energy tail (60-100 kcal/mol) in the spectrum of Nourbakhsh et al. is attributed by the authors to a further photodissociation of S-H formed after an initial C-S bond cleavage in methyl mercaptan. CH3SH before dissociation (lifetime of several vibrational periods). Both their “fast” and “slow” components exhibit partially resolved structure at a spacing of roughly 700-800 cm-I. They investigated the fluence dependence of both components and found no indication that either of them comes from multiphoton processes. Our Figure 2 shows all three of the 193 nm ET distributions plotted together. All three studies appear to be quite carefully done. Those of Segall et al. are the “prettiest” and therefore invite a certain confidence. Their detection method (for the products of process I) is probably the best of the three: as previously noted, it has a considerable kinematic advantage over detection of CH3S. Because it is specific for atomic H, it is also superior to Nourbakhsh et al.’s electron impact ionization, since contaminants that give H+ on ionization are notorious in high-vacuum systems. Still, it is quite surprising that Keller et al. would have missed such a large “slow” component. What are we to make of such disparate results? Perhaps the most appropriate comment is that of Segall et al.: “This leaves open the discrepancy.” (Said discrepancy is now among three results rather than the two they discussed). Ab Initio Results. The 1’A” and 2IA“ electronic states of CHjSH have been investigated theoretically by ab initio calculation^.^^^^^^^^ These all show that the 2IA” adiabatic PES is bound in both the SH and the CS bond coordinates. The lower (1IA”) PES is directly dissociative in the SH coordinate. The work of Jensen et aL5 indicated dissociative behavior in the CS coordinate as well, but with a barrier of approximately 0.6 eV. This shallow well appeared as a shoulder in Yarkony’s result^.^ Jensen et al. observe that vertical excitation to the 1’A” state places the system near the transition state for the reaction CH3 SH CH3S H on that PES. The adiabatic BomOppenheimer approximation, however, would seem to predict that excitation to the higher electronic state would lead to population of stable levels and hence to structured absorption. The continuum nature of the actual absorption as well as the appearance of photoproducts at high quantum yield is interpreted by Jensen et al. as evidence of quite significant nonadiabatic
+
-
+
Resonance Raman Spectrum of Methyl Mercaptan coupling between the two ‘A” states in or near the FranckCondon region. Yarkonyghas taken a different and quite interesting approach, noting that what appears as “avoided crossings” in onedimensional cuts of the two PES’S actually implies nearby conical intersections of the two surfaces. He has canied out extensive ab initio calculations aimed at location of these conical intersections, using an algorithm he has recently developed for this purpose. He finds a locus of intersections that is only weakly dependent on structural parameters other than the SH and CS bond coordinates but is highly correlated in those two variables. According to Yarkony, this indicates a viable twoparameter model for CH3SH photodissociation. This is an extremely interesting possibility that certainly falls into the category of “nonadiabatic” in the sense of a model with a single Bom -Oppenheimer surface. Dissociative Resonance Raman Spectroscopy. Keller et aL4 report emission spectra for 193 nm excitation (somewhat to the blue of the center of the 2IA“ transition), and Jensen et aL5 do the same for excitation at 222 nm, where the absorption is presumably mostly in the first band. As previously mentioned, the incident light at both these wavelengths came from a broad-band excimer laser (Our estimate is a bandwidth of roughly 100 cm-’ in both cases.). The authors state their array has a resolving power of 66 cm-l. Convolution of the laser band shape with the apparatus function of their spectrometer and multichannel detector is difficult for us to estimate, since there is known to be some signal crossover between channels in a detector array with an image intensifier. The emission spectrum reported with 222 nm excitation showed features identified at the fundamentals of V I (methyl asymmetric stretch) and v7 (SH stretch). No features are identified as overtones or combinations of any modes, and no v5 (CS stretch) fundamental is observed. It is not clear, however, that the v5 fundamental could have been seen since the appropriate region appears to be buried under a large background in their figure. Several smaller, unassigned peaks were attributed to fluorescence from products excited by the laser. At 193 nm excitation, a sequence of five features with smoothly declining intensities was seen. These features were spaced roughly 650 cm-’ apart and had widths in the neighborhood of 250 cm-’. These were initially assigned4 as the fundamentals of v5 and v3 (methyl umbrella mode) and overtones and combinations of these modes. In the later paper, however, they were identified as the fundamental and first four overtones of vg only.5 The results at 222 nm excitation, since they do not present clearly identified long progressions in any mode, cannot be taken as indicative of anything about short-time dynamics. It is wellknown that the intensities of fundamental bands are extremely easily “contaminated” by nonresonant contributions to their Raman amplitudes. However, the assignment of the progression seen at 193 nm excitation as an overtone sequence in vg is exactly the sort of thing that one looks for as the signature of rapid, direct dissociation along a reaction coordinate closely paralleling one of the ground electronic state normal modes. It was this striking aspect of the 193 nm results, especially in view of the inability to distinguish clearly between two alternative assignments, that fust attracted our attention for higher resolution study. III. Experimental Apparatus The Raman signals in this work were generated by a pulsed tunable 193 nm laser system capable of delivering up to -10
J. Phys. Chem., Vol. 99, No. 28, 1995 11069 SHG
NQYAG
SHG
Dye Laser LDS 698
Laser
I
I Angle Tuned BBO Crystal
Circut
PlCkoff
$--$-1 *
Dual Photodide
300 mm lens
To Cell
Spatial Filter
Figure 3. The 193 nm tunable, narrow-band laser system.
mJ of >95% polarized light in the wavelength range of 192.5194 nm, with a bandwidth less than 1 cm-l fwhm. This is accomplished by using the discharge chamber of an ArF excimer laser as a gain medium for a narrow-band seed pulse.I3-l5 The seed pulse is obtained by use of an angle-tuned BBO crystal to mix the fourth harmonic (266 nm) of a pulsed Nd:YAG laser with the output from a dye laser (LDS 698 dye) pumped by the second harmonic (532 nm) of the same Nd:YAG laser. Figure 3 shows a schematic diagram of the laser system. The phasematching angle of the BBO crystal is controlled by a feedback circuit which monitors a dual photodiode to sample the sumfrequency light generated in the crystal. The seed pulse is focused by a 30 cm focal length lens and, for its final amplification, makes two passes through the cavity of an ArF excimer laser (Questek Model 2860). A timing circuit controls the delay between the firing of the YAG laser and that of the excimer laser to guarantee that the seed pulse is inside the excimer cavity while the gain medium is excited by the discharge. The desired narrow-band pulse is accompanied by some broad-band background from the ArF laser used as the final amplifier, but this constitutes only a total of -4% of the photons reaching the cell. This background has an approximately Gaussian profile with a maximum at 193.3 nm and a fwhm of -100 cm-I. Shot-to-shot jitter of the pulse energy was partially compensated by use of a total emission monitor to be described below. We have demonstrated that the use of a stimulated Raman cell for Stokes shifting of the light provides tunable light at -0.5 mJ per pulse in the range 194-205 nm. This capability was not used in the work reported here, however. The interaction region in which the mercaptan sample is irradiated by the laser is cylindrical with a 1 mm diameter and an 8 mm length. The mercaptan was diluted with helium to a total pressure of 550 Torr before entering the cell, giving a 15% molar concentration which optimized the detected Rayleigh emission. The laser beam was focused 15 cm past the target
Stevens et al.
11070 J. Phys. Chem., Vol. 99, No. 28, 1995 6.0
V 7 A
0
Spectrometer
1
p=l=k
4.0
.$ C
Boxcan
Sample enters from above
V
$-
V
x ._
6 5
h
u)
al
d .
-
4v!5
2.0
6
I
Figure 4. Light detection system used in this laboratory. 0.0
region by a 50 cm focal length lens to reduce the likelihood of multiphoton processes. Light emitted from the interaction volume is collected at a 90" angle relative to both the direction of laser propagation and sample flow (see Figure 4). The 8 mm long image is magnified 6 times and focused on a 50 mm, long slit of a SPEX 1400 Series monochromator. A small fused silica pick-off inside the monochromator diverts a portion of the light to a solar-blind phototube (Hamamatsu R2078), which monitors the total amount of UV light entering the monochromator. This phototube total emission monitor (TEM)16 serves to monitor fluctuations in laser intensity and sample concentration. The rest of the light entering the monochromator impinges on a scanning dispersive grating (1200 lines/mm blazed at 1000 nm) used in fourth order and is reimaged on an exit slit, where it strikes a different solar-blind phototube (Hamamatsu R166). The Raman spectrum was collected with 0.5 mm wide slits, giving an -35 cm-' resolution. The signals from the phototubes are amplified and collected via a boxcar integrator with a 10 ns gate, which corresponds to the duration of the laser pulse. A data acquisition program (Labview 3) is used to drive the grating on the monochromator, to collect the signal from the Raman and TEM boxcars, and to monitor absorption measurements from beam pick-offs to photodiodes before and after the cell. The program is capable of averaging a large number of shots corresponding to each grating position of the monochromator. After every four steps of the grating, the light is blocked via a shutter, and 10 dark shots are taken from the phototubes to compensate for electronic noise in the system, thereby providing a base line.
IV. DRRS of Methyl Mercaptan Our first attempt at observing Raman scattering by methyl mercaptan was overwhelmed by intense fluorescent light from multiphoton processes involving photoproducts of the primary dissociation of interest. These effects are further discussed in the following section, where plausible mechanisms for the prominent features are presented. In order to suppress this unwanted intensity insofar as possible, we decided to use the grating in fourth order and to work at a wavelength of 193.3 nm, just to the red of an 0 2 absorption feature.I7 Spectra were also taken just to the red of the 0 2 features at 193.5 and 193.7 nm. These additional spectra serve to identify Raman features from fluorescence features, and the Raman spectra are essentially similar to the spectrum obtained at 193.3 nm. The inherent advantage of working at 193.3 nm is that it is at the center of
500.0
1000.0
1500.0
2000.0
2500.0
3C 0.0
Raman Shift (cm-1)
Figure 5. DRRS of methyl mercaptan measured in this laboratory for an excitation wavelength of 193.3 nm. v5 corresponds to the C-S stretch and is the only feature seen to give a progression. v7 is the S-H stretch and has the strongest fundamental of the Raman features in this spectrum.
TABLE 1: Methyl Mercaptan Fundamental Frequencies Taken from Thompson and SkerretP vibration
description methyl sym stretch methyl asym stretch methyl umbrella methyl deformation
C-S stretch H-C -S bend S-H stretch S-H wag C-S-H bend
freq, cm-' ~2870 3000 1335 1475 1430 704 1060 957 2597 600 803
the ArF gain profile and thus produces the lowest broad band background and minimizes jitter. Figure 5 shows a cumulative set of fourth-order scans of methyl mercaptan taken in our apparatus at 193.3 nm excitation wavelength at 0.2 rdlpulse laser energy. Table 1 shows the normal mode frequencies of methyl mercaptan taken from Thompson and Skerrett.I8 The Raman cross sections for methyl mercaptan at 193 nm are very weak. The data in Figure 5 required the compilation of nine separate scans, each of which averaged 160 shots per point, for a net of 1440 shots per point on a 0.02 nm grid. The prominent features of the spectrum are the C-S stretching fundamental (215) at 704 cm-I, the H-C-S bending fundamental (216) at 1060 cm-I, and the S-H stretching fundamental (217) at 2597 cm-l. Weaker features include the C-S-H bending fundamental (219) at 803 cm-I, the C-S first overtone at 1408 cm-I, and the third C-S overtone at 2816 cm-I. There is a weak feature which we attribute to the methyl symmetric stretch fundamental (211) at 2930 cm-'. This disagrees slightly with Thompson and Skerrett's estimate of 2870 cm-I, but they note that their IR absorption data were too congested to fully resolve this feature. The unusual character of the C-S stretch progression requires special comment. Despite the fact that the third overtone is nearly as intense as the first, the second overtone cannot be clearly discerned in our Raman spectrum. It is quite possible that this member of the progression is reduced and slightly
Resonance Raman Spectrum of Methyl Mercaptan shifted by Fermi resonance with the f i s t overtone of the H-C-S bending mode. On the basis of harmonic frequencies, these features would be expected near 2112 and 2120 cm-I, respectively. The possibility of such a resonant interaction is therefore quite strong. Galica and c o - w o r k e r ~have ~~.~ noted ~ missing overtones in CD31due to Fermi resonance with adjacent levels. It is equally possible that what we have identified as the third C-S overtone may be enhanced in intensity due to a Fermi mixing with the quite intense S-H stretch fundamental. This would mean that the pure C-S stretch progression would decay faster with higher overtones than the size of the observed feature would indicate. Studying the Raman excitation profiles of these features would resolve the questions regarding possible Fermi mixing, but the experiments have not been done. In any event, there is no discernible overtone or combination band activity other than the overtone sequences in the C-S stretch. The S-H fundamental is the strongest feature seen in this spectrum. Because of its strength, it seemed reasonable to look for overtones. However, scans near the expected f i s t and second overtones failed to reveal any feature above the background. The overtones could not have escaped detection had their intensity been as much as 20% of the fundamental's intensity despite the increasingly strong fluorescent background in this region. As previously discussed, TOF measurements6 at 193 nm indicate approximately equal propensities for breaking the S-H and C-S bond. Despite the surprising relative strength of the S-H fundamental in our experiments, the absence of overtones in that mode probably indicates that this intensity arises from off-resonant Raman scattering via completely different electronic state(s). The lower energy 1'A'' state with which the 2IA" state interacts nonadiabatically is an unlikely candidate in view of the relative weakness of the X'A" 1'A" transition centered at 227 nm. Furthermore, the lack of evidence of a strong overtone progression in the S-H stretch in the experiments of Keller et ~ 2 1 at . ~ 222 nm also points to the probability that the fundamental owes to nonresonant scattering from a higher state. The absence of overtones other than the C-S stretch in our Raman spectrum makes it difficult to draw any conclusions about the short-time dynamics of the photodissociation process. It is dangerous to formulate conclusions based solely upon fundamentals due to the possibility of non-resonant contributions. On the evidence of the ab initio results, direct photodissociation from the 2'A" state is not possible, and the actual process must proceed through conversion to the 1IA'' state. The picture that emerges, therefore, on the basis of the Raman results is that the earliest dynamics involves motion primarily in the CS coordinate on the 2IA" PES. Subsequent transfer of a substantial part of the wave packet to the 1'A" PES occurs with sufficient displacement to overcome any barrier to scission of the CS bond, resulting in roughly equal branching. This observation does not resolve the curious differences between the translational energy distributions observed by Segall et al. and those reported by Keller et al. The relationship between the Raman spectra reported here and those of Keller et al. deserves some remarks. Our resolution is sufficient to distinguish unambiguously between various vibrational bands. Measurements at different nearby excitation wavelengths (193.5 and 193.7 nm) also unequivocally differentiate between true Raman features (which exhibit a constant shift from the excitation frequency) and fluorescence (which remains fixed in absolute frequency). Consequently, we believe the spectrum shown in Figure 5 is, within its signal-to-noise limitations, an accurate representation of the true Raman
-
J. Phys. Chem., Vol. 99, No. 28, I995 11071 TABLE 2: Band Heads of CS X W cm-1)
- A ' l V Emission (in
V"
0 1 2 3 4
52 157.3
45 486 46 724.2 45 555.2 50 139.1 49 530.6 48 335.8 47 154.1 45 985.3 51 152 49 941.4 48 746.3 47 564.0 46 395.1 51 537.0 50 328.1 49 134.0 47 951.7
spectrum.29 We do not believe a convolution of this spectrum with any reasonable band shape for the laser used by Keller et al. will produce the spectrum they observed. Our conclusions about the dynamics do not differ very much from theirs, however. We suspect that the intensity observed by Keller et al. at 193 nm excitation owed largely to spurious signals from photoproducts gathered over their 100 ns long detector gating. As we demonstrate in our results, it is surprisingly and distressingly easy to generate such unwanted signals. This phenomenon is further discussed in the following section.
V. Multiphoton Processes in the Photodissociation of Methyl Mercaptan As indicated in the previous section, our initial efforts to observe Raman scattering by methyl mercaptan were swamped by intense fluorescence that could only come from multiphoton processes. Carbon atom lines at 193.1 nm (IF"' ID) and 247.9 nm ({Po IS) were by far the largest fluorescent features seen. The latter feature appeared from the fourth order during our fifth order scans of the grating; thus, its wavelength appeared to be located at 198.3 nm. Both these carbon features had been previously noted by Sausa et ~ 1 . ~in' their studies of the photodissociation of a number of hydrocarbons by 193 nm light. These atomic features are quite sharp in both emission and excitation and thus can be put "off stage" by working at excitation wavelengths several line widths away from resonance. In addition to the carbon atom signal, an increase in the incident energy from 0.2 to 0.5 rdpulse brought on fluorescence from the CS molecule which completely swamped the methyl mercaptan Raman signal. Figure 6 shows the CS peaks obtained at this higher laser power. Because of poor shot-to-shot stability of the laser intensity, we could not obtain definitive power dependences for the features. These CS emission peaks, which result from the A"Z+ X12+transition, were first seen by Bell and co-workers.22 Their emission bands are listed in Table 2. The CS emission is characterized by broad features that are strongly degraded to the red, with the origins lying close to the band heads. The tall dark lines in Figure 6 indicate the origins obtained from their data for a number of CS transitions originating from v' = 2. The short lines correspond to v' = 5 features. The CS fluorescence in our experiments is prompt on the time scale of the laser pulse. The radiative lifetime of the A' excited state CS has been estimated by Dornhofer and co-workersZ3to be
-
-
-
15 ns.
The appearance of both fluorescent carbon atoms and CS* molecules in methyl mercaptan points to some interesting photochemistry that has yet to be quantified. Figure 7 is a chart of possible branches of photoproducts leading to the observed fluorescent species. Underlined species are those that have been observed by other research groups studying UV photolysis. There are six energetically possible pathways for the one-photon photodissociation of CH3SH. Each arrow in Figure 7 corresponds to a 193 nm photon (approximately52 OOO cm-I energy).
Stevens et al.
11072 J. Phys. Chem., Vol. 99, No. 28, 1995 4.0
I
.-g fi.a 2.0 'I s =
I
~P- \
L 4
0.0
51500.0
.O
4850 Emitted Photon Energy ("1)
m.0
4
-
47500.0
Figure 6. CS fluorescence spectrum measured in this laboratory. The tall lines correspond to the band heads of features from the v' = 2 state. The shorter lines correspond to the v' = 5 state.
ciation can be very small and still produce the observed levels of CS* fluorescence. Photoproduction of CH2S in the first step leaves 40 000 cm-' of excess energy to be partitioned into vibrational, translational, and rotational energy of the fragments. If as much as 16 000 cm-' of that energy goes into internal excitation of the CHzS, absorption of a second 193 nm photon could lead to the formation of HZ and CS*(A''Z+). Another two-photon pathway would be the unlikely but energetically feasible formation of ground electronic state CS by the first photon, followed by excitation to the A' state by the broad band background of our tunable 193 nm laser. However, the fact that we do not detect any appreciable excitation wavelength dependence for CS* emission indicates that this mechanism in unlikely to be of more than minor importance. Furthermore, the intensity of the observed CS* emission increases with the intensity of the seed pulse entering the ArF amplifier. As the seed pulse grows stronger, the number of ArF exciplex molecules that can emit broad band is reduced. Thus, if the CS* fluorescence were being driven by the broadband background, its intensity would be expected to be anticorrelated with the strength of the seed pulse. Our proposed pathway to CS emission is summarized as follows: CH3SH
+ hv - H,CSt k,
-
CHS +H2
Underlinedtype indites an expenmentally
(EintL 16 000 cm-')
(other products)
+H
(1 - y I )
+
H~CS+ hv + k, CS*(V' = 2 or v' = 5 )
with 193.1
CH2 +
s
(32'877)
w,)
,cl* Anelogout to Formbhyde. 80,000 cml is left over 8s product excitatlm. 56,WO cm-1 rewired tor
,
-
(other products)
CS*(v' = 2 or v' = 5 )
(yl)
k3
CS
(y2)
(1 - y 2 )
+ hv
yl and y2 are the branching ratios for the production of CH2St and CS*. The corresponding kinetic scheme is: d[CH,S]/dt = -k,[CH,Sl
(1)
d[CH,S']/df = y,k,[CH3S] - k2[CH2S']
(2)
d[CS*]/dt = y2k,[CH,St] - k3[CS*]
(3)
cs + 2H
(48,860)
CH +SH (7>31,373)
2s +2H
I
(24,059)
CH2 + HZS (32,088)
Here, kl and k2 are the rate constants for products of the photon flux and an appropriate absorption cross section; y~ and y2 are the (presumably small) branching ratios defined above, and k3 is the reciprocal of the radiative lifetime of CS*. If we assume a constant photon flux for the duration of the pulse, these equations may be combined and integrated to yield a timedependent population for CS*:
Figure 7. Pathways of photolysis for methyl mercaptan. Each arrow corresponds to a 5 2 OOO cm-I photon. All values are taken from ref 10.
The cost in energy to go from the previous step is listed below each photoproduct. Thus, the net excess energy can be calculated by counting up the number of 52 000 cm-' photons used and subtracting the cost to make each photoproduct. As the diagram shows, the shortest path to CS*(A') from CH3SH requires two photons. This pathway starts with the formation of a presumably minor photoproduct CH2S (thioformaldehyde) following absorption of one photon. We will show below that the branching ratio to this product in the primary photodisso-
(4) The sensitivity of our detection scheme can be calibrated using N? Rayleigh ~ c a t t e r i n g . ~This ~ . ~allows ~ a determination of the integral of k3[CS*] over the time of the detector gate. The rate coefficient kl is determined by our absorption data. Its value is 1.4 x lo7 s-l. The literature value of the radiative lifetime gives a value of 6.7 x lo8 s-I for k3. Thus, the, unknown quantities in this scheme are the rate coefficient k2 and the product of the two branching ratios y1y2.
Resonance Raman Spectrum of Methyl Mercaptan
This mechanism can be tested for plausibility by determining whether there are plausible combinations of k2 and 7172 that are compatible with the observed number of detected CS* photons. For k2 assumed to be %lOkl, the branching ratio product tums out to be on the order of For k2 k l , y 1 y 2 x and for k2 % O.lkl, yly2 All these values for the branching ratio product are comfortably small; hence, we conclude that the mechanism is plausible. Similarly, one can devise a pathway to account for photoemission by excited carbon atoms. Figure 7 also shows the possibilities for forming the emitting C* species. We propose a mechanism starting with the formation of a methyl radical by the first photon. The second and third photons strip away the hydrogens (either as H2 or as atomic H), leaving a bare C atom in the ID state. This carbon atom can then be resonantly pumped by the broad-band component of the ArF laser to the ‘POstate, completing an overall four-photon process. Although neither the branching ratios nor the VUV absorption cross sections have been measured for the products along. this pathway, all the intermediate products have been seen as dissociation pathways at 193 nm by different experim e n t ~ . ~ . ~ . ~ ~Without . * ~ - * *vacuum-W absorption cross sections and branching ratios for every product in such a many-step process, the kind of simple kinetic study applied to CS becomes intractable for carbon. Nevertheless, given the short lifetime (2.7 ns) of the C* emitting state and the fact that once a carbon atom has emitted a 247 nm photon it can no longer be resonantly reexcited by a 193 nm photon, one can determine the amount of excited carbon produced during the 10 ns pulse. This analysis would imply that only 1 in -lo7 dissociating mercaptan molecules needs to produce C* in order to achieve the carbon atom signal seen on the PMT. VI. Conclusion
In summary, we have measured a dissociative resonant Raman spectrum for methyl mercaptan and found it to be quite different from a previous low-resolution ~pectrum.4.~The appearance of strong fundamentals for H-C-S bending at 1060 cm-I and S-H stretching at 2597 cm-I in our spectrum prevents convolution with any reasonable broad-band envelope to obtain a result that reproduces the earlier spectrum. The inherent low resolution from broad-band excitation, the relatively long 100 ns gate, the lack of tunability, and the higher power of the previous experiment make contamination of their spectrum a definite possibility. As we have shown, it is quite easy to generate fluorescence from multiphoton processes at 193 nm. Multiphoton processes in which one or more of the steps are saturated can appear to be linear in laser power. Unambiguous determination of Raman features requires an excitation source that is tunable and of narrow bandwidth. The only mode which seems to have an overtone progression in our spectrum is the C-S stretch. TOF measurements by other groups at 193 nm indicate approximately equal propensities for breaking the S-H and C-S b ~ n d . ~Despite .~ the surprising relative strength of the S-H fundamental in our experiments, the absence of overtones in that mode probably indicates that this intensity arises from off-resonant Raman scattering via completely different electronic state(s). The lack of evidence of a strong overtone progression in the S-H stretch in the experiments of Keller et aL4 at 222 nm also points to the probability that the fundamental owes to nonresonant scattering from a higher state. On the evidence of the ab initio results, direct photodissociation from the 2’A” state is not possible, and the actual process must proceed through conversion to the 1‘A” state. The picture
J. Phys. Chem., Vol. 99, No. 28, 1995 11073 that emerges, therefore, on the basis of the Raman results is that the earliest dynamics involve motion primarily in the CS coordinate on the 2IA” PES. Subsequent transfer of a substantial part of the wave packet to the 1’A” PES occurs with sufficient displacement to overcome any barrier to scission of the CS bond, resulting in roughly equal branching. This observation does not resolve the curious differences between the translational energy distributions observed by Segall et al.’ and those reported by Keller et aL4 Acknowledgment. We thank the Fannie and John Hertz Foundation, the National Science Foundation Grants CHE8910975 and CHE-9220278, the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Robert A. Welch Foundation. We thank Dr. P. B. Kelly, Dr. B. R. Johnson, Ms. N. Turner, and Dr. B. Y. Chang for their help and discussion.
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