6974
J . Phys. Chem. 1993,97, 6974-6978
ARTICLES Internal Excitation of CH3 Radicals Produced in the Photolysis of Acetone at 193 nm and the Collisional Enhancement of the Infrared Emission Intensity in the v3 Spectral Region Fida Mohammad, Vernon R. Morris, A. Clay Jones, and William M. Jackson' Chemistry Department, University of California, Davis, California 95616 Received: March 9, 1993; In Final Form: April 21, 1993
Time-resolved I R emission spectroscopy has been used to monitor the fluorescence in the C-H stretch region of methyl radicals produced in the 193-nm photolysis of acetone. Spectra collected at 20-cm-l resolution in the u3 spectral region do not exhibit any structure. This indicates that the emission in this region is due to both the u3 fundamental of CH3 and combination bands of the radical which overlap each other. Modes other than ~ ~ ( 0 0 n must O ) contribute to the observed emission in the 3000-3350-cm-l region. Translationally hot methyl radicals are also found to undergo very fast T V energy-transfer processes via collisions with various noble gases, resulting in enhanced infrared emission. The intensity of the enhanced emission is a factor of 4 or 5 times the emission intensity in the absence of the noble gases, suggesting that most of the radicals are formed in other vibrational states. The results are explained by assuming that the CH3 radical is initially produced in a broad range of vibrational states.
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produced in the photolysis of acetone at 266 nm.7 Fifty-two percent of the available energy was found in the relative The dissociation dynamics of the photolysis of acetone has translational energy of the fragments, which is in close agreement been the concern of many groups.1-7 Even so, it is still attracting with the previous results of Hancock and Wilson.5 Since the a great deal of interest because of the availability of high-intensity barrier to dissociation for CH3CO is 72 kJ/mol, the remaining excimer lasers and the many new analytical techniques for energy (57 kJ/mol) is not enough for the excited CH3CO to determining trace quantities of products in specific vibrational, dissociate further into CH3 and C0.B Waits et al. found that a rotational, and translational energy states. Results from expersignificant population of slow CH3 radicals is required to iments using lasers as photolysis sources and sensitive analytical accurately fit their experimental data.7 It was argued that CH3tools have considerably enhanced our knowledge of the primary CO radicals coproduced with slow CH3 radicals would have event of photolysis. However, therearestill questions to be settled sufficient internal energy to further dissociate into a CH3 radical before a full picture emerges about the dissociation dynamics of and a CO molecule. This agrees with Gandini and Hackett's acetone at 193 nm. For example, measurements of the internal observation that one-fifth of the dissociationleads to the production state excitation averaged over the two CH3 radicals do not agree of C0.6 on the vibrational excitation observed in the CH3 fragmex~t.~,~ Final product analyses obtained at 193 and 185 nm are Analysis of IR emission in the u3 spectral region of CH3 has consistent with the following scheme for the dissociation of shown significant excitation in the antisymmetric stretch mode.4a acetone.gJ0 It is still not clear which other modes of the CH3 arecontributing to this emission. It has recently been shown that CH3 radicals CH3COCH3 193 nm- 2CH3 CO, 0.95 (1) produced in the photolysisof acetone at 193 nm carry a substantial amount of translational energy.2 The hot CH3 radicals, upon H CH,COCH3, 0.028 (2) collisions with the bath gas, are expected to modify their nascent vibrational state excitation via T V and V V energy transfer. CH, CH,CO, 0.015 (3) If this is the case, the IR emission which is typically observed after the CH3 radical has experienced a few collisions will not represent the nascent internal state excitation of the radicals. In The absorption cross section of acetone at 193 nm is a factor this paper we report on the IR emission in the u3 spectral region of 20 larger than it is at 266 nma4J1 Signal-to-noise ratios are of CH3 and theeffect of collisionson the intensity of this emission. correspondingly larger, and it is, therefore, easier to characterize Our observations suggest that the CH3 radicals from the 193-nm the quantum state distribution of the photofragments at 193 nm photolysis of acetone are initially produced in a broad range of than it is at 266 nm. Three groups have previously studied the vibrational states and that these initial states of CH3 are changed energy distribution in the photofragments from the photolysis of by T V energy-transfer processes occurring on the 2-3-ps time acetone at 193 nm.24J2 Donaldson and Leone analyzed the IR scale of our IR experiment. Previous results on laser photolysis emission from CO(u= 1-3) and from the C-H stretch region of of acetone are summarized below. CH3 at a resolution of 30-60 cm-l.4a Even though the spectral resolution employed in their experiment was so low that they Final product analysis and detection of fragments produced during the photodissociation of acetone at 266 nm show that the could not completely deconvolute the IR emission bands in the fragmentation occurs in one step into CH3 and an acetyl radicals5 C-H antisymmetric stretch region of CH3, they concluded that A minor channel consisting of up to 15%of two CH3 radicals and their spectra were consistent with a rotational temperature of 1500 K and a vibrational population distribution of 0.73:0.14: a CO molecule has also been observed.6 Waits et al. measured the translational energy of state-selected CH3(Oo) radicals 0.13 in the u = 1,2,3 states, respectively. The emission intensity
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Internal Excitation of CH3 Radicals
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 6975
vs wavelength data for the CO fragment showed very high rotational excitation in the CO, peaking a t j = 32, which could not be fit to a single rotational temperature. The vibrational distribution of the C O in the u = 1-3 states was extracted from the emission data after thermalizing the rotational excitation with added Ar gas in the photolysis sample of acetone, and it was found that the distribution corresponded to a vibrational temperature of 1200 K. Woodbridge et al. repeated the experiment using time-resolved FTIR spectroscopy with a resolution of less than 1 cm-lU4bSome of the rotational lines in the u = 1-3 vibrational levels of CO were fully resolved and led to a vibrational distribution of this molecule that was described by a single temperature of 2000 K. The rotational structure of these vibrational bands was characterized by a single rotational temperature of 3400 K. The substantial rotational excitation is indicative of a two-step mechanism of dissociation. Trentelman et al. used UV-LIF and MPI-TOF-MS to characterize C O and CH3, respectively.* For the CO, these investigators found rotational and vibrational distributions corresponding to temperatures of 3000 and 2700 K, respectively, which are in agreement with the values obtained by Woodbridge et aL4b Trentelman’s study complemented the previous study by measuring the population of C O in the vibrationless ground state as well as its translational energy.2t4a The velocity of CO reflects the recoil angles of the two CH3 radicals, and as such it is a crucial piece of information for discovering the mechanism of acetone photodissociation at 193nm. Even in Trentelman’s study it was difficult to characterize the internal states of CH3, and it appears that the vibrational excitation observed in the CH3 fragment has been underestimated.2 This underestimation may be due to the predissociative nature of the CH3 radical, which generally accompanies its detection whenever its 3p 2A2” 2p 2AP Rydberg transition is used for this purpose. However, Trentelman et al. found that CH3 radicals carry an average of 44 kJ/mol in translational excitation, which corresponds to rootmean-square (rms) velocity of 2400 m/s for CH3.2 Hall et al., using absorption/gain spectroscopy, monitored the time evolution of CD3 population produced in the photolysis of CD3COCD3 at 193 nm in the u2(u=0-3), v3(u= 0-1) state^.^ The nascent population in all of the detected states only amounted to 15% of the total number of CD3 radicals produced during the photolysis. This led these investigators to conclude that CD3 is produced vibrationally excited and is broadly distributed over many vibrational states without preference for any of these states. Assuming the dissociation dynamics of CD3COCD3 and CH3COCH3do not differ significantly, we would expect, on the basis of the decrease in the density of states in going from CD3 to CH3, that a little more than 15% of the CH3 radicals will be produced in vibrational states in which CD3 radicals were detected as noted above. This value is close to the value of 14% which is obtained for the CH3 u = 0 population on the basis of prior distribution.12 The foregoing discussion suggests that CH3 produced in the photolysis of acetone a t 193 nm could be excited in a broad range of vibrational states including fundamentals, overtones, and combinations. Some of these levels are only a few hundred cm-l from the u = 1 level of the u3 fundamental of CH3 (see Figure 3 of ref 4c) where the IR emission has been monitored. On the other hand, since CH3 has been found to carry 44 kJ/mol of energy in translation, it is quite possible that part of the translational energy is converted into the internal energy of the CH3via collisions with the bath gases. By monitoring the time dependence of the IR emission from this region, the intensity of emission might be enhanced with the addition of noble gases. In this paper, we report a detailed study of the effect of noble gas pressure on the intensity of the I R emission in the C-H stretch region.
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Experimental Section
The experimental setup used for these time-resolved I R emission studies has been previously described so that only a brief description of the changes that have been made will be given.13 To increase the collection efficiency of IR emission from the photolysis region, two gold-coated I R mirrors are used in the Welsh configuration.14 The mirrors are each 14 cm in diameter and cut in half in the vertical plane to give a set of four half mirrors. Each pair of mirrors has a vertical slot of 2-3 mm. The emission after it is collected by the mirrors exits through the slot between the front pair of mirrors (the one closer to the spectrometer). The radius of curvature of the mirrors is 50 cm. They are housed in the reaction cell in such a way that the separation between the coated surfaces coincides with their radius of curvature. The new reaction cell is made of aluminum, is 61 cm long, and has an internal diameter of 16.5 cm. A typical run of the experiment is as follows: Spectra grade acetone is degassed and passed through the reaction cell a t 10-40 mTorr. The flow rate is adjusted so that each laser shot photolyzes a fresh sample of acetone. Noble gases from 0 to 400 mTorr are added to the photolysis sample of acetone before it enters the reaction cell to ensure complete mixing. The pressure of the system is measured with an MKS Baratron (range0-1000 mTorr). An ArFexcimer laser (Lambda-Physik, EMG 103) with an energy between 40 and 60 mJ/pulse is used for photolysis in all the experiments. The laser is run at 28 Hz by enslaving it to the chopper of the spectrometer. To this end, a 14-Hz signal taken from the chopper’s motor is used after doubling its frequency. Transient IR emission from the C-H stretch region of CH3 is collected by the Welsh optics and sent to the monochromator. An EG&G liquid N2 I R cooled InSb detector is used to detect the IR emission after it has passed through the exit slit of the monochromator. A cold, band-pass filter installed between the window and active element of the detector intercepts thermal radiation from the room a t wavelengths longer than 2850 cm-1, which increases the signal-to-noise ratio by a factor of 6. The current produced in the detector when the emitting photons strike its surface is fed into a current-to-voltage preamplifier which has an input impedance of 1 MQ. The preamp output is amplified in a second stage amplifier with a gain of 160 and eventually fed into a 12-bit digitizer. The digitized signal is stored in a PC computer, which is interfaced to the experiment. Noise from the laser firing and from the thermal background at room temperature is removed when the chopper blade blocks the field of view on alternate laser shots, and the signals of these shots are subtracted from each other in the computer. An average of 500-1000 laser shots is required for a reasonable signal-to-noise ratio. The spectrometer is parked a t a desired wavelength, and the signal is averaged and collected over a predetermined number of shots. The computer then steps the spectrometer to a new wavelength, and the process is repeated. Spectra as a function of time are obtained by sorting these data. Results and Discussion
Figure 1 shows two plots of I R emission intensity vs delay time in the C-H stretch region of the CH3 fragment. The plot with filled triangles is due to 20 mTorr of neat acetone whereas the plot with filled circles has 311 mTorr of Ar added to the 20 mTorr of acetone. The rise time for both of the plots is detector limited ( T N 1-2 ps), and the maximum intensity occurs at 5 ps after the laser pulse. The addition of 300 mTorr of Ar causes the maximum of the emission intensity to increase by a factor of more than 5 . A similar behavior was observed when He was used as the bath gas. Plots of the maxima of the emission intensity as a function of the noble gas pressure are shown in Figure 2. As can be seen from these plots, the intensity first rises and then levels off as noble gas pressure is increased. As the figure shows, the rise of the signal levels off at a higher H e pressure than Ar
6976 The Journal of Physical Chemistry, Vol. 97, No, 27, 199'3
Mohammad et al.
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Figure 1. Time decay curves of the IR emission intensity vs time delay. The emission was monitored at 3240 cm-I at a resolution of 65 cm-l. The A refer to 20 mTorr of neat acetone, while the 0 to 20 mTorr of acetone and 3 11 mTorr of Ar.
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Figure 2. Plots of IR emission intensity maxima occurring at 5 ps after the laser pulse as function of the noble gas pressure. The IR emission was monitored at 3240 cm-I at a resolution of 65 cm-I. The A refer to Ar added as the quenching gas, while the 0 refer to He. The solid lines are third-order polynomial fits to the experimental points.
pressure. Furthermore, the total increase in the signal intensity compared to neat acetone is greater for Ar than it is for He. Methyl radicals are produced with an rms velocity of 2.4 km/s, and as such, they might fly out of the viewing zone of the spectrometer on the time scale of 2-3 MUS, which is of the order of our detection time. Addition of a noble gas might prevent this flight and hence cause the signal intensity to increase. We have measured this initial intensity as function of the acetone pressure and found that it increases linearly with the acetone pressure (see Figure 3). Since radical flight out of the viewing zone decreases as the pressure is increased, the observed linear dependence of intensity on acetone pressure indicates that radical flight out of the viewing region is not important in our case. This fact and the fact that we are collecting IR emission by using optics in the Welsh configuration clearly indicate that radical flight from the viewing region is not important. To avoid contamination of the CH3 emission by secondary emission from acetone and ethane (see Figure 5, emission profile at 40 MUS),we monitor the IR emission by parking our spectrometer at 3240 cm-I and observing a bandwidth of 60-70 cm-1. The collapse of higher rotational states into this bandwidth and/or the cascading of higher vibrational states into this region due to the addition of a noble gas on the order of the time scale of our detection could also explain the observed increase in the signal. Figure 4 shows two plots of emission intensity as function of frequency (see caption for experimental conditions). The filled triangles represent the emission profile when 28 mTorr of neat acetone is used. The filled circles represent emission intensity obtained when 340 mTorr of Ar gas is added to 28 mTorr of acetone. Comparing the areas under these plots, it is clear that
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Figure 4. Plots of maximum IR intensity which occur 5 ps after the laser pulse as function of frequency for 65-cm-I resolution. The A refer to 28 mTorr of neat acetone, while the 0 refer to a mixture of 28 mTorr of acetone and 335 mTorr of Ar. Note: a different set of InSb IR detector (Santa Barbara Research Associates) and preamp was used to obtain these results. Also, no cold filter was installed on the detector.
the addition of Ar gas results in an overall increase in the concentration of the species emitting in this region and rules out the possibility of intensity shifts across the frequency spectrum as a significant source of the observed increase in the emission intensity. In the following paragraphs, we propose a mechanism for the change in the intensity of IR emission when acetone is photolyzed in the presence of noble gases. For this purpose, we first summarize the essential features of the dissociation processes which are generally believed to occur in the photolysis of acetone at 193 nm. The absorption of a 193-nm photon by acetone is accompanied by a Rydberg transition in which an electron from a nonbonding orbital on the oxygen atom is promoted to the 3s orbital on the same oxygen atom.Is This excites the acetone molecule to the Sz surface which has been shown to have unique properties.16 Vaida and colleagues have found that cluster formation between acetone molecules prolongs the lifetime of acetone on this surface by a factor of 5 and reduces the cross section for dissociation.17b It was argued that the S2 surface is quasi-bound and is coupled to a mixed surface (S1,TIJ by the CCO bending and out-of-plane skeletal motion of the acetone molecule.l7* Since cluster formation affects these motions, they are less effective in promoting coupling between Sz and (S1,Tl)surfaces. It is generally agreed that the dissociation of acetone at 193 nm proceeds in two steps on the above mixed surface {S1,T,).2 This acetone surface correlates to CH3 and CH3CO products via an energy barrier of 56 kJ/mol.** One CH3 radical splits first from the molecule on the predissociative surface {Sl,Tl], leaving
Internal Excitation of CH3 Radicals
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 6977
behind a highly internally excited acetyl radical. The lifetime
oftheexcitedacetylradicalsisnotknown. However itisestimated to be on the order of 1 P S . ~Hence, further dissociation of the excited CH3CO radical follows on the time scale of a few rotational periods of the radical. Dissociation of CH3CO to CH3 and CO has also been found to have a barrier of 72 kJ/mol.8 Thus, both of the dissociation steps have to overcome barriers before dissociation can occur. For molecules the size of acetone and acetyl radical, these barriers and the recently surmised predissociative nature of the potential surface will delay the dissociation long enough for the available energy to be distributed over all of the modes of the excited m01ecule.I~ It is clear from the above discussion that CH3 fragments produced in either of these steps can be internally excited. This picture is consistent with the conclusions of Hall et al.3 If the recoil of the fragments takes place from the top of the barriers, we would expect that part of the barrier energy is used to translationally excite the fragments. Translational excitation of the fragments and low internal excitation of CH3 radicals have been taken as evidence that the dissociation of acetone at 193 nm follows an impulsive model. Translational excitation of the fragments, as described above, can be due to the conversion of the energy of the barrier height rather than due to repulsive potentials alone. The similar translational excitation of the fragments in the photolysis of acetone at two very different energies available at 266 and 193 nm supports this ~ i e w . 2 ,If~ dissociation had occurred because of the repulsive potential alone, one would expect that the translational energy would increase as the available energy increases. Low internal excitation in CH3 is also not tenable in light of results from Hall et al. and our results, which are discussed below.’ The foregoing discussion suggests that the dissociation of acetone does not follow an impulsive model as has been previously To summarize, then, it is not surprising to see that CH3 obtains a significant amount of vibrational excitation from the changes in the potential energy that are occurring during the dissociation of acetone when it absorbs a 193-nm photon. The CH3 radicals produced as a result of these changes are expected to be excited in a broad range of vibrational states as has been concluded by Hall et al.3 The available energy in the photolysis of acetone at 193 nm after the two C-C bonds are broken is 18 000 cm-I (22 kJ/mol). If this energy is statistically distributed among the 24 modes of vibrations of acetone during dissociation, 5000 cm-1 of this energy will go into internal excitation of each CH3 radical. There are 64 vibrational states of the CH3 radical available to share this energy. Since we are monitoring IR emission from the C-H antisymmetric stretch region of CH3, most of these states cannot be seen. Some of these states, however, lie only a few hundred cm-1 from thislevel: with theC-H antisymmetricstretch mode excited (see Figure 3 of ref 4c). A collision between a translationally hot CH3 radical and a noble gas atom can convert part of methyl’s translational energy into its own internal energy. This effectively pumps some of the darker states into the brighter ones from which we are monitoring I R emission. Quantitatively meaningful calculations of the T V energy-transfer dynamics are very tedious. Qualitatively, the efficiencies of T V energy transfer are generally facilitated by the following factors: (a) Higher relative velocity V, between the colliding partners and a short range a of the interaction potential. Under these conditions the duration of the collision T~ = a / V , is small. In our case, CH3 has been shown to be produced with a broad range of velocities, the rms value of which lies at 2.4 km/s. The range a of the interaction potential for collisions between a CH3 radical and a noble gas atom is not known. However, small values for a can be assumed here because we do not expect any long-range interactions to dominate the character of the potential.
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(b) Small values of the energy, AE, to be transferred from T to V. CH3 has many vibrational states which are within 100-600 cm-1 of the C-H stretch region of the radical (see Figure 3 of ref 4c). Small values of AE will ensure that the oscillator time r, = h/AE is not much different from T ~ the , duration of collision, and will ensure that the adiabaticity parameter, q = T ~ / T , remains , small.19 (c) A short-range repulsive potential. As discussed above, no long-range attractive interactions between the CH3 radicals and the noble gas atoms are expected so that only short-range interactions should occur. Steeper interaction potentials are helpful because inelastic T V energy-transfer processes are very sensitive functions of the slopes of these potentials.20 Recently, it has been argued that short-range attractive components of the interaction potentials make the repulsive potential even steeper and as a result increase the probability of T V transfer.21 (d) A priori internal excitation in the molecule undergoing T V exchanges is another factor which facilitates T V transfer.22 CH3 radical, as we believe, is produced with substantial vibrational excitation which will make it even easier for the CH3 to undergo T V exchange. Physically, we can imagine that a noble gas atom strikes an H atom of the CH3 radical head-on along the H-CH2 bond. The actual T V energy transfer probability will depend on the phase and velocity of the vibrating hydrogen atom in the H-CH2 bond at the time of collision. The phase and velocity of the hydrogen atom in question are, in turn, defined by the superposition of all of the vibrational motions of the CH3 radical. The energy required for T V conversion which would pump a dark CH3 into a brighter CH3 is available as the relative translational energy between the colliders. The measured rms velocity of 2.4 km/s of CH3 corresponds to a center-of-mass collision energy of 32.4 kJ/mol (2700 cm-I) with an Ar atom and 11.7 kJ/mol (975 cm-I) with a He atom. These values are more than sufficient to meet the energy requirements for the noted transitions. Since more energy is available in the collisions of CH3 radicals with Ar atoms than in its collisions with H e atoms and since Ar atoms are bulkier and more polarizable than He atoms, it is expected that Ar gas should be more efficient in the T V energy conversion processes than He gas. These expectations are borne out experimentally as shown in Figure 2. Hall et al. in their study of the photodissociation of CD3COCD3 at 193 nm concluded that transfer of the methyl’s kinetic energy into its internal energy due to collisions is ~ n i m p o r t a n t .In ~ that study they parked on the Y Z ( l+O) absorption and monitored the rise of the concentration of CD3 radicals in the ground vibrational state resulting from the cascading of CD3 radicals produced initially in the vibrationally excited states. They found a ratio of the CD3 radicals initially produced in the vibrationally excited states to those produced in the u = 0 state to be 94:6. The predominance of the initially produced excited radicals even after argon is added suggests that collisions with the added Argas will only reshuffle these radicals in the excited states so that the ratio of excited to ground state radicals will not be significantly changed by collisions. As a consequence, the sought for effect on the intensity of the absorption signal due to collisions will not be observable. One additional possibility which might contribute to the observed increase in the signal intensity is worth mentioning. This possibility is suggested by the observation that an increase in the pressure of pure acetone in the range 10-200 mTorr has no effect on the size of the signal other than the fact that more of the acetone molecules are photolyzed and the signal increases linearly with acetone pressure (see Figure 3). Hot CH3 radicals excited in dark states might, because of translational excitation, have an acetone quenching rate that is much larger than the quenching rate constant for bright methyl. When a noble gas is added, those hot CH3 radicals experiencing their first one or two
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6970 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 4500
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Figure 5. Plots of IR emission intensity as function of frequency a t delay times of (0) 3, (A) 5, ( 0 )8, and (m) 40 ws. The IR emission from a premixed sample of 40 mTorr of acetone and 300 mTorr of Ar was monitored at a resolution of 20 cm-I. Note: the time intervals indicated refer to the firing of the laser as zero time and the rise time of the emission is detector limited. The plot at 40 ws shows the emission from excited C2H6 produced in the associative recombination of CHI radicals. (A paper detailing C2H6 emission from low-pressure 193-nm photolysis of acetone is in preparation.)
collisions with the noble gas atoms will be pumped to the brighter state(s) and translationally cooled. Those dark methyl radicals that first collide with acetone molecules will react or be translationally quenched and unavailable for emission. We have used very low pressures to try to observe a rise in the signal which is not detector limited. Such a rise was not seen even when 10 mTorr each of acetone and Ar was used. It seems that the first one or two collisions taking place on the time scale shorter than our detection time of 2-3 ps play a dominant role in these phenomena compared to subsequent collisions. This kind of behavior is typical of hot ~pecies.2~ We believe a combination of these factors is responsible for the observed increase in the signal. In the present experiment we cannot separate the effects of these factors. Figure 5 shows emission intensityversus frequency in the C-H stretch region of the CH3 radical. The plots refer to the emission profiles at different times which are detector limited, after the firing of the laser. Even though these profiles were recorded with 20-cm-1 resolution, no structure is observed in these plots. The assumed anharmonicity in the u3 antisymmetric C-H stretch mode is approximately 60 ~ m - * One . ~ ~would expect that the 20-cm-l resolution that was used would be sufficient to allow us to observe some structure if the emission was due only to the u3 = 1 0, 2 1, and 3 2 transitions as concluded in a previous ~ t u d y . ~ a Absence of structure in the observed spectrum indicates not only that the emission is arising from transitions in the u3 manifold but also that mixed-mode transitions must be contributing to the emission.
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Conclusion In the present work we have presented evidence that the CH3 radicals produced in the 193-nm photodissociation of acetone are formed highly vibrationally excited. This vibrational excitation must reside in levels near the u = 1 level of the u3 mode since it is efficiently converted to mixed modes of this level via collision with rare gases. It is also clear that the populations of these levels are not statistical since the higher-energy levels appear to have a higher population than the lower levels. Experiments that
could accurately map these levels would reveal a great deal about the potential energy surface responsible for the dissociation. The experiments reported here do not address this explicitly but do give a hint to the rich structure that is probably present. I R emission monitored at 20-cm-I resolution in the C-H stretch region of CH3 produced in the 193-nm photolysis of acetone shows no structure whatsoever, indicating not only that this emission arises from the u3 = 1-.0,2+1, and 3+2 transitions of CH3 but also that other states are contributing to the emission as well. Moreover, addition of noble gases increases the maximum intensity of this emission 4-5-fold. The actual increase is a function of the type and amount of the added noble gas. It is believed that CH3 radicals are produced vibrationally excited in a broad range of states. Some of these states may be close in energy to the up antisymmetric stretch region where IR emission was monitored in these experiments. These initially produced dark states of the CH3 radicals are pumped to the u3 region by T V energy conversion processes caused by collisions of the translationally hot CH3 radicals with the noble gas atoms. The absence of collisional enhancement of the I R emission intensity in the case of neat acetone suggests that CH3 radicals with vibrational excitation as indicated above are quenched more efficiently by acetone when they are translationally hot.
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Acknowledgment. The authors gratefully acknowledge the support of N S F Grant 9008095and NASA Planetary Astronomy Program under Grant NAGW 1144. Vernon R. Morris also thanks the University of California Presidential Postdoctoral Fellowship. References and Notes (1) Lee, E. K. C. A h . Photochem. 1980, 12, 1.
(2) Trentelman, K. A.; Kable, S. H.; Moss, B. D.; Houston, P. L. J . Chem. Phys. 1909, 91,7498. (3) Hall, G. E.; Bout, D. V.; Sears, T. J. J . Chem. Phys. 1991,944182. (4) (a) Donaldson, D. J.; Leone, S. R. J . Chem. Phys. 1986,85,817. (b) Woodbridge, E. L.; Fletcher, T. R.; Leone, S. R. J. Phys. Chem. 1988, 92, 5387. (c) Donaldson, D. J.; Leone, S. R. J . Phys. Chem. 1987, 91, 3128. (5) Hancock, G.; Wilson, K. R. In Proceedings, Fourth International Symposium on Molecular Beams, Cannes, France, 1973. (6) Gandini, A.; Hackett, P. A. J . Am. Chem. Soc. 1977, 99, 6195. (7) Waits, L. D.; Horwitz, R. J.; Guest, J. A. Chem. Phys. 1991, 155, 149. (8) Watkins, K. W.; Word, W. Int. J. Chem. Kine?. 1974, 6, 855. (9) Light, P. D.; Kirwan, S. P.; Pilling, M. J. J . Phys. Chem. 1988, 92,
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(10) Potzinger, P.; Buenau, G. Von. Ber. Bunsen-Ges. Phys. Chem. 1968, 72. 195. (1 1) Calvert, J. G.; Pitts, J. N. Photochemistry; Benjamin Cummings: Menlo Park, 1966; Chapter 5. (12) Strauss, C. E. M.; Houston, P . L. J. Phys. Chem. 1990, 94, 8751. (13) Copeland,L. R.; Mohammad, F.; Zahedi, M.; Volman, D. H.; Jackson, W. M. J . Chem. Phvs. 1992. 98. 5817. (14) (a) Welsh, H . L.; Cumming, C.; Stanbury, E. J. J. Opt. Soc. Am. 1951,41,712. (b) Welsh, H. L.; Stanbury, E. J.; Romanko, J.; Feldman, J. J . Opt. Soc. Am. 1955, 45, 338. (15) Hess, B.; Bruna, P. J.; Buenker, R. J.; Peyerimhoff, S . D. Chem. Phys. 1976, 18, 267. (16) McDiarmid, R. J . Chem. Phys. 1991, 95, 1530. (17) (a) Gaines, G. A.; Donaldson, D. J.; Strickler, S. J.; Vaida, V. J . Phys. Chem. 1988.92.2762. (b) Donaldson, D. J.; Gaines, G. A.; Vaida, V. J. Phys. Chem. 1988,92, 2766. (18) Copeland, R. A.; Crosley, D. R. Chem. Phys. Lett. 1985,115,362. (19) Stevens, B. Collisional Actiuation in Gases;Pergamon: Oxford, 1967; Chapter 2. (20) Murrell, J. N.; Bosanac, S. D. Chem. Sot. Reo. 1992, 21, 17. (21) Yang, X.;Wodtke, A. M. Int. Reu. Phys. Chem. 1993,12. 123. (22) Levine, R. D.; Berstein, R. B. Molecular Reaction Dynamics and Chemical Reactivity; Oxford University Press: Oxford, 1987; Chapter 6. (23) Willard, J. E. Annu. Rev. Phys. Chem. 1955,6, 141.
