J . Phys. Chem. 1986, 90,4033-4037
CH3SH Photolysis at 248 nm. Hydrogen Atom Yield and Rate Constant for the H CH3SH Reaction
4033
+
P. H. Wine,* J. M. Nicovich, A. J. Hynes, and J. R. Wells Molecular Sciences Branch, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received: February 18, 1986)
Hydrogen atoms have been detected by time-resolved resonance fluorescence spectroscopy following 248-nm pulsed-laser photolysis of CH3SH. The primary photolysis yield of H atoms has been determined to be 1.08 0.24 by comparing fluorescence signals observed following CH3SH photolysis with those observed following photolysis of 03/Hz mixtures. The kinetics of the H CH3SH reaction have been investigated. Over the temperature range 249-405 K the rate constant is given by the Arrhenius expression k = (3.45 & 0.13) X lo-'' exp[-(845 & 12)/7'l cm3 molecule-' s-'.
+
Introduction In recent studies of H atom production from the OH formic acid reaction' and the reactions of O('D) with HC1 and HBr? we found 248-nm pulsed-laser photolysis of CH3SH (methyl mercaptan) to be a useful source of hydrogen atoms for calibrating resonance fluorescence detection sensitivity. One photolytic parameter that is needed to carry out such a calibration is the quantum yield for H atom production (Q the number of H atoms produced per photon absorbed). A number of studies are reported in the literature where end-product yields of H,, CH4, and CH3SSCH3have been determined following 254-nm photolysis of CH3SH.3-7 In general, these studies support the conclusion that the only primary photochemical process is
+
CH3SH + hv(254 nm)
-
CH3S + H
(la)
although Bridges and White6 did conclude that about 7% of the photodissociation events occurred via the alternate channel
CH3SH + hv(254 nm)
CH3 + SH
(1b)
Channel l b is thought to become more important at shorter wavelengths.6*8 While previous work provides indirect evidence that the hydrogen atom yield from 248-nm photolysis of CH3SH should be near unity, our recent experimentsZseem to represent the first direct observation of H atoms as a primary product of CH3SH photolysis. In the presence of CH3SH, hydrogen atoms are believed to undergo the
H
+ CH3SH
-
CH3S
+ H2
(2a)
A number of relative rate measurements of kz at 298 K are reported in the l i t e r a t ~ r eas~ is ~ one ~ ~ temperature-dependent ~~~'~ relative rate measurement (323-493 K).4 In these studies the relative rates of removal of CH3SH and a reference compound were monitored under conditions where both species were thought to react only with the same steady-state pool of hydrogen atoms. Reference compounds used in the relative rate studies were C2H4 ( e t h ~ l e n e ) ? ~NO,7 ~ . ~ and O2.l0 No direct, real-time kinetic data is available for reaction 2. In this paper we present the results of experiments where H(?3) was monitored by time-resolved resonance fluorescence spec(1) Wine, P. H.; Astalos, R. J.; Mauldin, R. L., I11 J. Phys. Chem. 1985, 89, 2620. (2) Wine, P. H.; Wells, J. R.; Ravishankara, A. R. J. Chem. Phys. 1986, 84, 1349. (3) Skerret, N. P.; Thompson, N. W. Trans. Faraday SOC.1941, 37, 81. (4) Inaba, T.; Darwent, B. deB. J. Phys. Chem. 1960, 64, 1431. (5) (a) Steer, R. P.; Kalra, B. L.; Knight, A. R. J. Phys. Chem. 1967, 7 1 , 783. (b) Steer, R. P.; Knight, A. R. J. Phys. Chem. 1968, 72, 241. (6) Bridges, L.; White, J. M . J. Phys. Chem. 1973, 77, 295. (7) Balla, R. J.; Heicklen, J. Can. J . Chem. 1984, 62, 162. (8) Callear, A. B.; Dickson, D. R. Trans. Faraday SOC.1970, 66, 1987. (9) Kuntz, R. R. J. Phys. Chem. 1967, 71, 3343. (10) Balla, R. J.; Heicklen, J. J . Photochem. 1985, 29, 311.
troscopy following 248-nm pulsed-laser photolysis of CH3SH. By comparing fluorescence signals with those obtained following photolysis of OJHZ mixtures, we determined the quantum yield for H atom production from CH3SH photolysis. Also, the absolute rate constant for reaction 2 was measured over the temperature range 249-405 K. Experimental Section The experimental apparatus was identical with the one employed previously in our laboratory for time-resolved detection of H(2S).'q2 A brief review of its operation is given below. A Pyrex, jacketed reactor with an internal volume of 150 cm3 was used in all experiments. The reactor was maintained at a constant temperature by circulating ethylene glycol or methanol from a thermostated bath through the outer jacket. A copperconstantan thermocouple with a stainless steel jacket was injected into the reaction zone through a vacuum seal, thus allowing measurement of the gas temperature under the precise pressure and flow-rate conditions of the experiment. The source of 248 nm radiation was a KrF excimer laser (20-11s pulse width, up to 800 mJ per pulse). A hydrogen resonance lamp situated perpendicular to the photolysis laser excited resonance fluorescence in the photolytically produced atoms. Resonance radiation (121.6 nm) was coupled out of the lamp through a MgF, window and into the reactor through a MgF2 lens. The region between the lamp and the reactor was evacuated. Fluorescence was collected at 90° to both the resonance lamp and photolysis laser by a second MgF, lens and imaged onto the photocathode of a solar-blind photomultiplier. The region between the reactor and the photomultiplier was purged with zero-grade air to prevent detection of impurity emission of 0 and N resonance radiation. Signals were obtained with photon-counting techniques in conjunction with multichannel scaling. For each decay rate measured in the rate constant determinations, sufficient flashes were averaged to obtain a well-defined temporal profile for at least three l / e times. For each temporal profile measured in the quantum yield determinations, sufficient flashes were averaged to allow accurate determination of the signal level at t = 0 (Le., immediately after the excimer laser fired). In order to avoid accumulation of photolysis or reaction products, all experiments were carried out under "slow-flow" conditions. The linear flow rate through the reactor was in the range 3-6 cm s-', and the laser repetition rate was 5 Hz. Since photolysis was perpendicular to the direction of flow and the photolysis beam was collimated to be 1 cm in diameter as it traversed the reactor, the mixture in the reaction zone was replenished every 1-2 laser shots. CH3SH, 03,and Hz were flowed from 12-L bulbs containing dilute mixtures in inert buffer gas (He, Ar, or N2);these mixtures were premixed with additional buffer gas before entering the reactor. Concentrations of each component in the photolysis mixtures were determined from measurements of the appropriate mass flow rates and the total pressure. The fraction of CH3SH in the 12-L storage bulb was
0022-3654/86/2090-4033$01 .50/0 0 1986 American Chemical Society
Wine et al.
4034 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 TABLE I: Results of the H Atom Ounntum Yield Experiments’ LO319
10” molecule cm-3 2.98 0 0 2.77 3.18 0 0 2.83 3.38 0 0 3.15 4.24 0 0 4.68 4.31 0 0 3.65 3.14 0 2.99 0 0 3.10
[CH3SHl,
10” molecule cm-3 0 19.35 11.9 0 0 10.19 4.94 0 0 3.55 6.64 0 0 5.32 2.38 0 0 2.25 4.89 0 0 4.42 0 1.05 2.48 0
L, mJ/cm2 0.573 29.92 29.45 0.563 0.533 30.60 28.98 0.575 0.510 27.02 29.18 0.510 0.441 18.10 18.06 0.506 0.476 17.29 17.61 0.419 0.352 26.48 0.336 26.56 26.19 0.332
CP 1.12 1.95 1.60 1.12 1.12 1.51 1.20 1.12 1.12 1.14 1.29 1.12 1.12 1.21 1.08 1.12 1.12 1.07 1.19 1.12 1.12 1.17 1.12 1.02 1.08 1.12
S
XP
4080 18900 14750 3890 3070 14280 10000 4210 5090 9590 12700 4580 6250 9950 5500 6800 6150 5060 8200 5510 4820 11240 4100 3700 7450 3970
2.478 2.122 2.245 2.591 1.845 2.305 2.794 2.683 3.062 3.799 2.819 2.957 3.466 4.168 4.606 2.978 3.109 4.632 3.777 3.736 4.522 3.745 4.232 4.511 4.129 4.000
ab 0.837 0.886 1.108 1.234 1.262 0.937 1.294 1.430 1.353 1.011 0.856 1.096 1.003
‘All experiments were carried out at 298 k 2 K with reaction mixtures containing photolyte (0, or CH3SH),2 Torr H2,and 50 Torr He. C,, S, and Xp are defined in the text. b@(geometricmean) = 1.08 & 0.18.
checked frequently by UV photometry a t 202.6 nm (Zn’ line) or 228.8 nm (Cd line); absorption cross sections were taken to cm2 a t 228.8 be 8.71 X lo-’* cm2 at 202.6 nmll and 6.70 X nm.I2 O3was monitored in situ in the slow-flow system at 253.7 nm (Hg line) with a 2-m absorption cell plumbed in series with the reactor; the O3absorption cross section was taken to be 1.147 X lo-’’ cm2 at 253.7 11131.~~ The gases used in this study had the following stated minimum purities: Ar, He, N2, and H2.at 99.999%, O2at 99.99%, and CH$H at 99.5%. All gases were used as supplied except CH3SH, which was degassed repeatedly a t 77 K before use. O3 was prepared by passing U H P O2through a commercial ozonator and was stored on silica gel a t 197 K. Before use it was degassed at 77 K to remove 02.
Results and Discussion Primary Photolysis Yield of Hydrogen Atoms. The atom yield experiments were all carried out at ambient temperature (298 f 2 K) and all employed 50 Torr of helium as the buffer gas. The yield was determined by comparing H atom fluorescence signals obtained from the photolysis of CH3SH/H2/He mixtures with those obtained from the photolysis of 03/H2/He mixtures. Photolysis of 03/H2/He mixtures leads to H atom production via the following mechanism: O3 + hv(248 nm) O(lD) + 02(a’Ag) (3a)
- + +-OZ(x3z,-) + - +
o(~P) O(lD) H2
OH
H
(3b) (4a)
o ( 3 ~ ) H~ (4b) We have previously dernon~trated’~ that the O(lD) yield from 248-nm photolysis of O3is 0.91 f 0.03 and the H atom yield from reaction 4 is 0.98 f 0.02. Hence, the overall quantum yield for H atom production via reactions 3 and 4 is 0.89 f 0.05. A typical 03/H2/Hemixture contained 1 mTorr 03, 2 Torr H2, and 50 Torr ~~~~~~
He. The rate constant for reaction 4 is 1 X cm3 molecule-’ s-’,15 so H atom production was complete in less than 1 ps. Deactivation of O(’D) by O2and/or He was negligible. Reaction of O(3P) with H2 is negligibly slow under the experimental conditions employed.I6 Examination of the dependence of fluorescence intensity on ozone concentration demonstrated that, at O3 concentrations of less than 1014 molecule cm3, the resonance fluorescence signal was not degraded through absorption of Lyman a radiation by O3 or via quenching of fluorescence by 03. Degradation of fluorescence signal by H2was also very small and did not have to be accounted for in the data analysis because equal amounts of H2 were used in the O3 photolysis experiments and CH3SH photolysis experiments. Experiments were carried out on a time scale (50 ps per channel) where the H(2S) appearance was essentially instantaneous but the H(2S) decay was temporally resolved. The signal at time t’(a time shortly after the laser fired when H(%) formation had gone to completion but no appreciable decay had occurred) could be determined accurately and represented H(2S) produced via either reaction 1 or reactions 3 and 4. Yield determinations involved back-to-back experiments where the signal at t = t’was determined for an 03/H2/He mixture and then for one or two different CH3SH/H2/He mixtures and then again for an 03/ H2/He mixture. All signals were normalized for laser power and photolyte concentration. In the case of CH3SH/H2/Hemixtures, signals also had to be corrected for fluorescence intensity degradation due to absorption of Lyman cr radiation by CH3SH;the fluorescence sensitivity calibration curve is shown in Figure 1. The results of the yield experiments are summarized in Table I. Correction factors that were used to scale the fluorescence signals were obtained from Figure 1 for the CH3SH/H2/He mixtures; for the 03/H2/He mixtures a correction factor of 1.12 was used to account for the fact that the H atom yield from reactions 3 and 4 is 0.89, not unity. To compute quantum yields from the data, absorption cross sections for O3 and CH3SH a t
~
(1 1) Wine, P. H.; Thompson, R. J.; Semmes, D. H. In?.J. Chem. Kine?.
1984, 16, 1623. (12)Wine, P. H.; Kreutter, N. M.; Gump, C. A.; Ravishankara, A. R. J. Phys. Chem. 1981,85, 2660. (13) Hearn,A. G. Proc. Phys. Soc. 1961, 78, 932. (14) Wine, P. H.; Ravishankara, A. R. Chem. Phys. 1982, 69, 365.
(15)DeMore, W. B.; Margitan, J. J.; Molina, M. J.; Watson, R. T.; Golden, D. M.; Hampson, R. R.; Kurylo,M. J.; Howard, C. J.; Ravishankara, A. R. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling Jet Propulsion Laboratory, California Institute of Technology, 1985,JPL publication No.85-37. (16) Schatz, G.C. J . Chem. Phys. 1985,83, 5677 and references therein.
CH3SH Photolysis at 248 nm
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4035
CCHISHl(1014cm-5)
Figure 1. Resonance fluorescence signal intensity as a function of CH$H concentration in the reactor.
248 nm must be known. The O3cross section was taken to be 1.08 X cm2l5 while the CH3SHcross section was measured during the course of the investigation to be 3.00 X cm2-in good agreement with a published value.17 In order to minimize the correction for absorption of Lyman a radiation by CH3SH, all experiments were carried out with [CH3SH] C 2 X 1014 molecule cm3. Also, in order to carry out in situ measurement of 0, it was necessary to employ photolysis mixtures with [O,] > 2 X lot3molecule cm3. For the above reasons, much larger laser photon fluences were required in the CH3SH photolysis experiments than in the O3photolysis experiments. It was verified in a separate set of experiments that, at constant CH3SHconcentration, the resonance fluorescence signal increased linearly as a function of laser photon fluence up to a fluence of 30 mJ/cm2. To compute quantum yields from the experimental data, we define the parameter Xp as follows:
xp= 1 0 - 7 s c p / ~ p ~ ~ ~ ~
(5)
Callear and Dickson8 observed CH3, CH3S,and SH following flash photolysis of CH3SH at 195 nm and interpreted their results in terms of two primary processes, channels la and lb; they estimated that channel l a accounted for -63% of the photodissociation events. All other studies of CH3SH p h o t ~ l y s i s ~employed -~ continuous wave photolysis end-product analysis techniques. Bridges and White photolyzed CH3SH at 214 and 254 nm with n-butane added to cool translationally hot H atoms. On the basis of observed yields of H2, CH4, and CH3SSCH3in the high [nbutane] limit, they concluded that the quantum yield for process l a was 0.75 at 214 nm and 0.93 at 254 nm; quantum yields for process l b were deduced to be 0.25 at 214 nm and 0.07 at 254 nm. Other investigators have studied CH3SH photolysis only at 254 nm.3-537 Their observations have generally been interpreted as indicating that channel l a is the only primary process. Our results confirm the interpretation of previous end-product analysis studies that the dominant primary process in the 250-nm region is one that involves formation of hydrogen atoms. Our results do not, however, rule out a small quantum yield for a primary process that does not produce H atoms. Direct, quantitative detection of other possible primary products such as SH, CH3S, CHzS, S, ... would be helpful in this regard. Rate Constant for the H CH3SHReaction. The rate constant measurements employed argon as the buffer gas at a total pressure of 100 Torr. To measure k2 it is desirable to establish experimental conditions where the H atom temporal profile is governed by the following processes:
+
-
CH3SH hv(248 nm)
H
-
H
+ CH3SH
H
+ CH3S
(la)
products
(2)
loss by diffusion from the detector field of view and reaction with background impurities (7)
Then, if [CH3SH] >> [HI (pseudo-first-order conditions), simple first-order kinetics are obeyed In ([H],/[H]J = (k2[CH3SH] + k7)t
where S is the number of fluorescence signal counts at t = t’per 50 ps channel per 100 laser shots, Cpis the signal correction factor (see above text), L is the laser photon fluence in units of mJ/cm2, [PI is the photolyte (0, or CH3SH) concentration in units of molecules per cm3, and up is the photolyte absorption cross section at 248 nm in units of cm2. The quantum yield, 9,is obtained from the relationship
k’t
(8)
The bimolecular rate constant, k2, is determined from the slope o t a k’vs. [CH,SH] plot. Observation of H temporal profiles that are exponential (i.e., obey eq 8), a linear dependence of k’ on [CH3SH], and invariance of k’ to variations in laser photon fluence serve as strong evidence that the only processes that affect the H atom time history are reactions 1, 2, and 7. Initial experiments were carried out at 298 K, where H atom temporal profiles were measured as a function of laser photon = xCH3SH/X03 (6) fluence and CH3SH concentration. Some typical data are shown in Figure 2. At relatively high laser powers, nonexponential H where Ro3is the average of measurements made immediately before and immediately after the measurement of X C H ~The ~ ~ . atom decays were observed-the disappearance rate was more rapid at short times after the photolysis flash than at longer times. geometric mean of the 13 determinations of 9 made from the data As the photolysis laser power was lowered, H atom decays became in Table I gives the value 1.08 i 0.18 where the error is 1u and exponential with rates approximately equal to those observed at represents only the precision of the results. When possible syslong time in the experiments at high laser power. The above tematic errors in determinations of photolyte concentrations, observations suggest that at high laser power, a radical-radical absorption cross sections, relative laser fluences, and signal correaction competes with CH3SH for H atom removal. A likely rection factors are taken into account, the overall accuracy of the candidate is the reaction quantum yield determination is estimated to be *22%, Le., 9 f l a = 1.08 f 0.24. H + CH3S -+ H2 CHzS (9a) There are a number of energetically allowed channels for CH3SH photolysis a t 248 nm M CH3SH (9b) CH3SH hu(248 nm) CH3S + H (la) Contributions from the radical-radical reaction were found to be CH, + SH (1b) negligible a t laser fluences of 1 2 mJ/cm2. All reported kinetic data for reaction 2 were obtained with laser photon fluences of CH2SH + H (IC) 2500. CHI + S (14 The data used to determine k2 as a function of temperature are summarized in Table I1 and k’vs. [CH3SH] plots are shown CHZS + H2 (le) in Figure 3. Errors quoted in Table I1 for individual k2, deCH, H2S (If) terminations are l a and refer only to the prbcision of the k’vs. [CH3SH] data. The absolute accuracy of the results is limited by both precision and uncertainties in the determination of CH,SH (17) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, concentrations. We estimate that the absolute accuracy of a 1966.
’
+
---
-
+
-
+
4036
Wine et al.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
I
1L
I
I
0
3
6
/
371K
I
e
tlmehs)
ob
Figure 2. Dependence of the H atom temporal profile on laser photon Jluence at two different CH3SH concentrations. T = 298 K, P = 100 Torr Ar. Laser photon fluence in units of millijoules per cm2: (a) 14; (b) 0.5; (c) 6; (d) 0.7. CH3SH concentration in units of lOI4 molecules per cm3: (a,b) 0.371; (c,d) 4.50. TABLE II: Summary of Kinetic Data for the H + C H 3 H Reaction' no. of range [CH3SH], range 1012k2, T, K exptb loL4molecule cm-3 k', s-' cm3 molecule-' s-lC 249 7 0.312-9.41 156-1240 1.18 i 0.01 6 0.447-7.12 190-1250 1.60 i 0.02 217 298 5 0.371-4.50 161-996 2.01 i 0.03 0.151-5.53 159-1700 2.85 f 0.03 338 6 0.144-5.17 169-1940 3.53 i 0.05 6 371 405 6 0.314-5.04 269-2300 4.31 i 0.08
'
'
'
'
'
6
'
'
'
'
'
10
CCHaWl ( 1O"CIN~)
Figure 3. Plots of k'vs. CH3SH concentration at six temperatures. Solid lines are obtained from linear least-squares analyses and give the bimolecular rate constants listed in Table 11.
An Arrhenius plot is shown in Figure 4. From a linear leastsquares analysis of the In k2vs. 1/ T data, we obtain the Arrhenius expression (units are cm3 molecule-' s-')
where the quoted errors are l a and represent precision only (aA = Aq,,). The observed Arrhenius parameters suggest that reaction 2 proceeds via a direct hydrogen abstraction mechanism. Hence, it is expected that k2 should be independent of pressure. The pressure dependence of k2 was investigated briefly by carrying out one set of experiments in 760 Torr N2. Resonance fluorescence detection sensitivity was significantly degraded at high N2pressures, so experiments had to be done under conditions where contributions from radical-radical reactions were not totally negligible. It was clear from these experiments, however, that within error limits of about 20%, k2 is the same in 760 Torr N2 as in 100 Torr Ar. All previous values of k2 reported in the literature have been derived from competitive kinetics studies; they are summarized in Table 111. Comparison of our direct measurements with previous measurements is complicated by the fact that the competing reactions employed in the earlier studies all have bimolecular rate constants that depend on total pressure and buffer gas identity. The reaction H + C2H4 products (11)
k2 = (3.45 f 0.13) X lo-" exp[-(845 f 12)/Z7
is thought to be near its high-pressure limit in 760 Torr argon
aArgon was used as the buffer gas at a pressure of 100 Torr. bExperiment is the determination of one pseudo-first-order rate constant. CErrorsare l a and represent precision only.
typical k2 determination is *lo%. Over the temperature range investigated, the temperature dependence of k, is described very well by the Arrhenius equation k2(T) = A exp(-E,,,/RT)
TABLE 111: Comparison of Our H investigators ref
+ CH$H
(10)
-
Kinetic Data with Other Studies Reported in the Literature" T, K P, torr M
competing reactant
Inaba and Darwent
4
C2H4
Kuntz Steer and Knight Balla and Heicklen Balla and Heicklen this work
9 5b 7
C2H4
10
0 2
C2H4 NO
323 393 493 297 d 296 296 249 217 298 338 37 1 405
150-300 180 120-500 25-250 5200 20-300 20-300 100
C2H4 C2H4 C2H4
NO 0 2
k, 1k,.6 1.IC 2.0' 2.2e 1.7' 2.32e 626 Torr 935 Torr
Ar
k,'
f f f
1. I t 2.3g
f
l.gh 1.18 1.60 2.01 2.85 3.53 4.31
"All results other than ones were obtained by competitive kinetic techniques. k,, is the bimolecular rate constant for the reaction of H with the competing reactant. CUnitsare cm3 molecule-' s-I. dNot specified in paper but probably 297 f 2 K. eReported to be pressure independent over the specified pressure range. f k c , not known accurately under the specified conditions. 'Calculated by assuming a value of 1 X cm3 molecule" s-l for the H + C2H4rate constant. hCalculated by assuming a value of 5.5 X om6 molecule-2 s-' for the H + O2 O2 rate constant and third-order behavior for P I300 Torr.
+
J . Phys. Chem. 1986, 90,4037-4043
1
' " 2.5
'
"
" " 3.0
'
"
~
3.5
4.0
1000/T (K)
+ CH3SH reaction. The solid line is obtained from a linear least-squares analysis. Figure 4. Arrhenius plot for the H
-
at 298 K'*J9 with kll 1 X 10-l2cm3molecule-' 8';if we assume this value for k, (rate constant for the competing reaction), then the results of Kuntzg and those of Steer and KnightSbagree with our 298 K rate constant within &20%. One measurement of Balla and Heicklen'O employed the competing reaction (18) Lee, J. H.; Michael, J. V.;Payne, W. A.; Stief, L. J. J. Chem. Phys. 1978, 68, 1817. (19) Nicovich, J. M.; Ravishankara, A. R., to be published.
4037
Under the assumption that O2has the same third-body efficiency as N2, then klz is known'5 to be 5.5 X [O,] cm3 molecule-' s-I. Using this value to convert Balla and Heicklen's relative measurement to an absolute rate constant gives k2 = 1.8 X cm3 molecule-I s-l-in excellent agreement with our 298 K result. The only temperature dependence data reported previously is that of Inaba and Darwent.4 They concluded that the activation energy for reaction 2 is slightly larger than the activation energy for reaction 11; i.e., the ratio of k2/kll increased slightly with increasing temperature over the range 323-493 K. Recent direct measurements suggest that, over the temperature and pressure ranges of Inaba and Darwent's study, reaction 11 is in the fall-off region between third and second order.lg Inaba and Darwent, however, observed k2/kll to be independent of total pressure a t both 323 and 493 K. Hence, quantitative comparison with Inaba and Darwent's results does not seem warranted. It is worth noting that Lee et a1.I8 report an activation energy for reaction 11 in 760 Torr Ar of 2.1 kcal/mol; their experiments covered the temperature range 198-320 K. We report an activation energy for reaction 2 of 1.7 kcal/mol (249-405 K). Hence, our direct measurements coupled with those of Lee et al. predict that k2/kll should decrease slightly with increasing temperature. Acknowledgment. This work was supported by NASA through Subcontract No. 9548 14 from the Jet Propulsion Laboratory and by NSF through Grant No. ATM-82-17232. Registry No. CH3SH, 74-93-1; 03,10028-15-6;H2, 1333-74-0; H, 12385-13-6.
Photofragmentation Dynamics of Carbdn Suboxide Brad R. Weiner and Robert N. Rosenfeld*' Department of Chemistry, University of California, Davis, California 95616 (Received: February 20, 1986)
Nascent CO photofragment vibrational energy distributions from the UV photolysis of C302 are measured by time-resolved carbon monoxide laser absorption spectroscopy. Photodissociation of C3O2 at 193 and 249 nm produces CO(u=O-5) and CO(v=0-3), respectively. The CO photofragment is rotationally excited to some extent, in both cases. A statistical model for energy disposal fits the experimentally determined CO vibrational energy distributions for 193- and 249-nm photolyses. A Franck-Condon model for vibrational energy partitioning cannot reproduce the experimental observations. Our data suggest that following abso tion of a W photon C3O2 internally converts to the electronic ground state and then dissociates, yielding C,O(E'A) and CO(%Z+). The photofragmentationdynamics of C302are compared with those of the "quasi-linear" polyatomics, CH2C0 and NCNO.
fntroduction Information about the dissociation dynamics of polyatomic molecules is implicit in the energy distribution of the nascent fragments? For molecules that undergo direct photodissociation, the dynamics of collisionless fragmentation are often dominated by the shape of the potential surface accessed by the absorption of a p h ~ t o n . ~Data obtained by monitoring energy disposal to the photofragments' degrees of freedom have proven to be a valuable dynamical With increasing molecular complexity, molecular rovibratiunal state densities increase, which can in turn greatly enhance the rate of nonradiative transitions. Various intramolecular relaxation processes can thus m r in competition with dissociation from optically prepared states. By probing the (1) Fellow of the Alfred P. Sloan Foundation (1985-1987). (2) Lcvine, R. D.; Bernstein, R. B. Molecular Reaction Dynamics; Oxford University Press: New York, 1974. (3) Bersohn, R. J . Phys. Chem. 1984,88, 5145. (4) Leone, S.R. Adu. Chem. Phys. 1982, 50,255. ( 5 ) Imre, D.; Kinsey, J. L.; Sinha, A.; Krenos, J. J. Phys. Chem. 1984,88,
3956.
(6) Simons, J. P. J. P h p . Chem. 1984.88, 1287. (7) Shapiro, M.; Bersohn, R. Annu. Reo. Phys. Chem. 1982, 33, 409.
partitioning of excess energy among the rovibrational modes of the photofragments, one can obtain information about the primary photophysical and photochemical events following photoactivation. Such data are important because currently available models for polyatomic photodissociation dynamics7 are limited in scope, at best. Carbon suboxide, C3O2, is an interesting test case in developing models for photodissociation dynamics, in part because it may represent an intermediate point between the small (nonstatistical) and large (statistical) molecule limits. The molecule has been shown to be linear.8-9 Useful comparisons can be drawn between the photodecarbonylation of C302 and similar "quasi-linear" species, e.g., ketene, nitrosyl cyanide, and isocyanic acid.1° In this paper, we report data on the energy disposal to the carbon monoxide fragment following excimer laser photolysis of carbon suboxide at 193 and 249 nm. Time-resolved C O laser (8) Livingston, R. L.;Rao, C. N. R. J . Am. Chem. SOC.1959, 81, 285. (9) Lafferty, W. J.; Maki, A. G.; Plyler, E. K. J . Chem. Phys. 1964, 40, 24. (10) Okabe, H. Photochemistry of Small Molecules; Wiley-Interscience: New York, 1978.
0022-3654/86/2090-4037$01.50/00 1986 American Chemical Society
