J . Phys. Chem. 1990, 94, 3031-3039 of the matrix following the photolysis. N F 2 would be observed only if the CI atom could escape the matrix cage, and the fact that no NF2 bands26grow in as a result of the photolysis provides further evidence of the inability of CI to leave the cage. There is no indication of a reaction between the N F 2 and CI photofragments to produce N F CIF. This process would be analogous to mechanism 2 for NFClz and would require some movement of the fragments within the matrix cage. Hence, we have additional support for the theory that such motion is prohibited by the low-temperature matrix environment. Our interpretation of these results is that the state accessed at 230 nm in NF2Cl leads to the production of NF, and CI.
3031
the same dissociative pathways. Photolysis of the NF2C1/Ar matrix did not produce any IR-active products, strongly implying that the state at 230 nm dissociates to NF2 CI and that because of the matrix environment, recombination to NF2CI is an energetically favorable pathway. A potential reaction between the N F 2 and CI photofragments to form N F CIF is apparently impossible in the matrix cage. In NFCI,, excitation at 270 nm resulted in the production of NF, and three mechanisms are proposed for its formation in the matrix. Based on the experimental results, the most likely mechanism is a concerted process in which one photon is absorbed and both N-CI bonds are broken simultaneously leaving NF. If conservation of spin angular momentum is important in NFCI,, then N F may be produced in an excited singlet state. This potential source of NF(alA) or NF(b’Z+) is a very important result with respect to high-energy laser applications.
+
+
+
Conclusion
The experiments reported here were carried out to obtain information concerning the spectroscopy of the fluorochloroamines and to examine the possibility of producing various NX,, NXY, and NX fragments from them photolytically. The UV absorption spectra of NFCI, and NF2CI indicate that dissociative states are accessible in the 200-400-nm range, and these states were probed via photolysis of the low-temperature matrix-isolated species. It is apparent from the data that the excited states produced in NF2Cl and NFCI,, at 230 and 270 nm, respectively, do not follow
Acknowledgment. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the US.Air Force Office of Scientific Research under Grant AFOSR-87-0210. The authors also thank Dr. Robert D. Coombe for many valuable discussions during the course of this work.
The 193-nm Photodissociation of Cyciobutanone: Dynamics of the C2 and C, Channels Karen A. Trentelman; David B. Moss,$Scott H. Kable, and Paul L. Houston* Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853- 1301 (Received: September 14, 1989)
The 193-nm photolysis of cyclobutanone has been investigated by obtaining vacuum-ultraviolet laser-induced fluorescence spectra of the CO photoproduct. The rotational distribution in each vibrational level (0’’= 0-3) was fit well by a sum of two Boltzmann distributionswith average temperatures TI, 200 K and TKSh 3000 K. The high-temperaturecomponent comprised -85% of the total CO yield, independent of vibrational level. The vibrational distribution was also thermal, corresponding to a temperature of 2560 100 K. Doppler profiles of individual rovibronic transitions were fit by Gaussian line shapes corresponding to an average translational temperature of -2500 K. The two rotational distributionswere assigned to CO produced via two reaction pathways. The vibrational, rotational,and translational distributions for the high-temperature channel were found to be consistent with the distributions predicted by a statistical prior calculation for dissociation of cyclobutanone to CO and cyclopropane, the C, channel. The low-temperature channel was assigned to CO resulting from unimolecular decomposition of an excited ketene product, the C2channel. Statistical prior calculations for the ketene plus ethylene channel indicate that only electronically excited ketene should be sufficiently energetic to dissociate to CO and methylene.
-
-
*
I. Introduction Studies of the dissociation dynamics of carbonyl-containing compounds cover a wide range of molecular complexity, from relatively simple systems, such as formaldehydeI4 and ketene,5-6 to considerably more complex molecules, such as a ~ e t o n e , ~ - I ~ 3-~yclopentenone,~~ and, presented here, cyclobutanone. Cyclobutanone is an appealing system to study in that it provides an opportunity to examine a photodissociation event in which the photolysis products must be the result of two bond-breaking events. The dominant features of the electronic spectrum of cyclobutanone are those common to the entire class of ketones.16 The n valence shell excitation lowest energy transition is the T* to the SIstate, which in cyclobutanone has its origin at approximately 30 300 cm-I and extends to 38 000 cm-I. The second electronic absorption band covers the region from 48 530 to 54950 cm-l and has been assigned as predominantly the 3s n Rydberg transition to the S2state.17 The 3s Rydberg state is thought to
-
-
’
Present address: Department of Chemistry, Northwestern University, Evanston, IL 60208. *Present address: Department of Physics, Lynchburg College, Lynchburg, VA 24501.
0022-3654/90/2094-303 1 $02.50/0
be perturbed as a result of mixing with the nearby u* state. The degree of mixing between the 3s Rydberg state and the underlying ( I ) Moore, C . B.; Weisshaar, J. C. Annu. Reo. Pfiys. Cfiem. 1983, 34, 525-555. (2) Bamford, D. J.; Filseth, S. V.; Foltz, M. F.; Hepburn, J. W.; Moore, C . B. J . Cfiem. Pfiys. 1985.82, 3032-3041. (3) Ho, P.; Bamford, D. J.; Buss, R. J.; Lee, Y. T.; Moore, C. B. J . Cfiem. Phys. 1982, 76, 3630. (4) Debarre, D.; Lefebvre, M.; Ptalat, M.; Taran, J.-P. E.; Bamford, D. J.; Moore, C. B. J . Cfiem. Pfiys. 1985.83. 4476-4487. (5) Bitto, H . ; Chen, I.; Moore, C. B. J . Cfiem. Pfiys. 1986, 85, 5101. (6) Nesbitt, D. J.; Petek, H.;Foltz, M. F.; Filseth, S. V.; Bamford, D. J.; Moore, C. B. J . Cfiem. Pfiys. 1985, 83, 223. (7) Brouard, M.; MacPherson, M. T.; Pilling, M. J.; Tulloch, J. M.; Williamson, A. P. Cfiem. Phys. Lett. 1985, 113, 413. (8) Donaldson, D. J.; Leone, S. R. J . Cfiem. Pfiys. 1986, 85, 817. (9) Gaines, C.A.; Donaldson, D. J.; Strickler, S. J.; Vaida, V. J . Pfiys. Cfiem. 1988, 92, 2762. (IO) Donaldson, D. J.; Gaines, G. A.; Vaida, V. J . Pfiys. Cfiem.1988, 92, 2766. ( I I ) Lee, E. K. C.; Lewis, R. S. Adu. Pfiotocfiem. 1980, 12, I . (12) Lightfoot, P. D.; Kirwan, S. P.; Pilling, M. J . J . Pfiys. Cfiem. 1988, 92, 4938. (13) Trentelman, K . A.; Kable, S. H.; Moss, D . B.; Houston, P. L. J . Chem. Pfiys., 1989, 91, 7498.
0 1990 American Chemical Society
3032 The Journal of Physical Chemistry, Vol. 94, No. 7 , 1990 u* state is known to be wavelength dependent, although the exact nature of this dependence has not been measured.'8 Several studies of the decomposition of cyclobutanone following excitation of the a* n transition agree on the existence of two mechanistic pathways ( I and 2) to account for the observed product^.'^-^^ Pathway 1 produces CO and vibrationally excited cyclopropane (c-C,H6*); pathway 2 produces ethylene (C2H4)and ketene (CH,CO). C4H6O -+ c - C ~ H , C O (1) C,H,O -+ C2H4 + CH2CO (2)
-
+
Early studies by Schlag and c o - ~ o r k e r on s ~ the ~ ~ energy ~~ distribution in the reaction products have determined that much of the vibrationally excited cyclopropane contains enough internal energy to isomerize to propylene (CH2=CHCH3) unless it is collisionally stabilized. Direct production of propylene as a primary product, however, would require the formation of a trimethylene biradical as an intermediate. Mechanistic studies of the ~ y s t e m l ~produced .~' no direct evidence of production of a biradical intermediate, so the authors concluded that any propylene observed is a secondary product, the result of isomerization of vibrationally excited cyclopropane. I t was further noted in the studies of Schlag and co-workers that although the hot cyclopropane has a broad internal energy distribution, it receives on average a greater fraction of the available energy than would be predicted on a purely statistical basis. This result is interpreted as indicating that for excitation energies accessing the S, state the excess dissociation energy has not equilibrated completely among the decomposition products. By contrast, excitation into the S2 state at 200 and 185 nm produced cyclopropane with a nearly statistical fraction of the available energy. The seminal studies by E. K. C. Lee et al.22-27have addressed the dissociation mechanism, the energies of the intermediates, and intermolecular energy transfer following the photodecomposition. Initial excitation into lower vibrational levels of the S, manifold produces cyclobutanone which may either fluoresce, undergo intersystem crossing to the lowest lying triplet state, T I , or internally convert to high vibrational levels of the ground state, So. The fluorescencequantum yield is only about 0.2%,24and therefore the primary decay channels are nonradiative. Benzene photosensitization s t u d i e ~demonstrated ~~,~~ that the T I state of cyclobutanone decomposes almost exclusively to cyclopropane and CO, while the precursor to the ketene and ethylene products is a singlet state. The ratio of the rate of intersystem crossing to the rate of internal conversion is therefore reflected in the ratio of the yield of C,-containing products (from reaction I , which goes through a triplet state) to the yield of C2-containing products (from reaction 2 , which goes through a singlet state). This C3/C, ratio was observed to undergo a dramatic decrease as a function of increasing excitation energy in the region corresponding to roughly 1000 cm-I above the S, origin (& 3 15 nm) and was accompanied by a sharp decrease in the lifetime of ~ ~ decrease .~~ in lifetime was attributed to the the S, ~ t a t e . The
-
(14) Woodbridge, E. L.; Fletcher, T. R.; Leone, S. R. J . Phys. Chem. 1988, 92, 5387. (15) Sonobe. B. I.; Fletcher, T. R.; Rosenfeld, R. N . J . Am. Chem. SOC. 1984. 106, 4352-4356. (16) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic Press: New York, 175; Chapter 1V.C. (17) Whitlock, R. F.; Duncan, A . B. F. J . Chem. Phys. 1971. 55, 218. (18) Causley, C . G.; Russell, B. R. 3. Chem. Phys. 1980, 7 2 , 2623. (19) Campbell. R . J.; Schlag, E. W.; Ristow, B. W. J . Am. Chem. SOC. 1967, 89, 5098. (20) Campbell, R. J.; Schlag. E. W. J . A m . Chem. SOC.1967, 89, 5103. (21) Klemm, R. F.; Morrison, D. N.; Gilderson, P.; Blades, A. T. Can. J . Chem. 1965. 43, 1934. (22) Denschlag, H. 0.;Lee, E. K. C. J . Am. Chem. SOC.1968, 90, 3628. (23) Lee, N E.; Lee, E. K. C . J . Am. Chem. SOC.1969, 50, 2094. (24) Hemminger. J. C.; Rusbult, C. F.; Lee, E. K. C. J . Am. Chem. SOC. 1971. 93, 1867. (25) Hemminger. J. C.; Lee, E. K . C J . Chem. Phys. 1972, 56, 5284. ( 2 6 ) Tang, K . Y . ; Lee, E. K. C. J . Phys. Chem. 1976, 80, 1833. (27) Shortrldge. R. G.;Yang, W.; Lee, E. K. C. Mol. Photochem. 1969, I. 32.1
Trentelman et al. onset of a new nonradiative process, and the accompanying change in the C3/C2 ratio was inferred to indicate that the new process leads eventually to a dissociation favoring the production of C, products. Lee and co-workers suggested that a predissociative process with a threshold in this region might be responsible for the observed photophysics. Beyond this region, for wavelengths shorter than 3 15 nm, the C3/C2 ratio was observed to be relatively insensitive to wavelength, increasing gradually with increasing excitation energy. Despite the extensive data concerning cyclobutanone decomposition following excitation to S,, only a few studies have investigated dissociation following excitation to the S2state via the 3s n Rydberg transition. Baba et a1.28have observed multiphoton ionization and fragmentation of cyclobutanone at 248 nm (a* n ) and also at 193 nm (3s n). Both wavelengths produced similar mass spectra with strong ion signals at m / e = 70 (parent), 42 (ketene and cyclopropane), and 28 (ethylene and CO). A few measurements of the C3/C2 ratio have been made and these data establish a consistent following excitation S2.19-27 trend over both absorption bands. From this it has been speculated that the second excited electronic state may undergo rapid internal conversion to SI and subsequently follows the same decomposition routes as those observed following excitation via the a* t n transition. In this work we have measured the internal and translational energy distributions of the CO photofragment following excitation of the S2state of cycobutanone at 193 nm. The primary issues we hope to resolve about this process are as follows. Firstly, by what mechanism does the dissociation occur? It has been established that there are two primary dissociation channels for excitation to S, below the predissociation threshold. By observing the photofragment dynamics following excitation to the S2 state, we hope not only to establish the dissociation pathways but also to gain some insight into the nature of the excited-state surfaces on which the dissociation occurs. Secondly, how is the initial excitation energy redistributed among the fragments? This question is closely related to the first in that the mechanism by which the dissociation occurs determines the distribution of energy among the fragments. Once a mechanism has been established, we examine the internal and translational energy of the fragments in light of the predictions of two limiting case models of dissociation dynamics: one treating the dissociation event(s) as impulsive and the other allowing statistical redistribution of energy among the fragments. +
+ -
-
11. Experimental Section
C O fragments from the photolysis of cyclobutanone were detected by using the vacuum-ultraviolet laser-induced fluorescence (vacuum-UV-LIF) apparatus described p r e v i ~ u s l y . ~In~brief, tunable vacuum-UV radiation in the region from 140 to 170 nm was generated by four-wave sum frequency mixing of two visible dye lasers in magnesium vapor. One dye laser was tuned to a two-photon resonant transition in the magnesium, and the other was tuned through a continuum of autoionizing states, producing a coherent beam at the sum frequency 204 + w2. The photolysis source was an ArF excimer laser (I93 nm). The beam was directed straight into the molecular beam chamber and loosely focused with a 40-cm cylindrical lens, producing a spot size of about 1 cm2 and an energy on the order of about 60-70 mJ/pulse in the intersection region with the free jet expansion. The pump and probe lasers were propagated at right angles to each other, perpendicular to the direction of the molecular beam. The sample was a supersonic expansion of a mixture of roomtemperature cyclobutanone (Aldrich Chemical Co., 99%) in helium. The total stagnation pressure behind the 500-pm orifice was approximately 200 kPa ( 2 atm). This corresponds to approximately 4% cyclobutanone in He.30 The cyclobutanone was (28) Baba, M.; Shinohara. H.; Nishi, N.; Hirota. N. Chem. Phys. 1984, 83, 221-233.
(29) Burak, 1.; Hepburn, J. W.; Sivakumar, N.; Hall, G. E.; Chawla, G.; Houston. P. L. J . Chem. Phys. 1987, 86, 1258-1268.
Photodissociation Dynamics of Cyclobutanone at 193 nm
The Journal of Physical Chemistry. Vol. 94, No. 7, 1990 3033
f
2-0
band
0-2
band
(d
W
h
4
.rl
2e,
4
d
U
e,
; a) 0 vl
e, k
s 165.5
166.0
-
166.5
167.0
Wavelength (nm)
Figure 1. Vacuum-UV LIF spectra of the A X transition of the CO photofragment following 193-nm dissociation of cyclobutanone. Top: u" = 0, exciting the 2-0 band. Bottom: u" = 2, exciting the 0-2 band. The off-scale bands between 165.6 and 165.8 nm in the 0-2 spectrum are atomic C resonances. Q-branch assignments are indicated in both spectra.
subjected to a freeze-pump-thaw cycle in order to remove most dissolved gaseous impurities. No other purification of the sample was performed. The beam valve was operated at I O Hz with an open pulse duration of 200 ps and produced pressure in the molecular beam chamber of about 2 X Torr with the beam valve operating. For some experiments, a small adapter placed over the nozzle orifice was used to rethermalize the beam, thereby providing a cluster-free, effusive expansion. A rotationally resolved LIF spectrum of C O verified that the resulting sample had a rotational temperature of 300 K . Laser-induced fluorescence was collected with a solar blind photomultiplier tube (PMT), fitted with a 193-nm filter to reduce interference due to scattered light from the pump laser. The probe laser was delayed by 200 ns with respect to the photolysis laser to further discriminate against the 193-nm radiation. The fluorescence signal was processed by a gated integrator/boxcar averager and transferred to a microcomputer-controlled data acquisition system which also scanned the wavelength of the vacuum-UV probe laser. Scans were normalized to both the vacuum-UV power (detected by a second solar blind PMT) and the photolysis laser power (proportional to the scattered light signal at the first PMT). For Doppler profile measurements, etalons were inserted into both dye lasers used for vacuum-UV generation, narrowing the vacuum-UV bandwidth from approximately 0.6 to 0.21 cm-I. The w2 dye laser was pressure tuned with N2, again under microcomputer control. Multiple scans of each transition were summed to improve the signal-to-noise ratio of the Doppler profiles. 111. Results A . Rotational Distribution. The internal energy distribution of the C O fragment was determined from vibrationally and ro(30) The heat of vaporization of cyclobutanone was estimated to be 7800 cal/mol by using Trouton's rule that the molar entropy of vaporization at the standard boiling point is constant at about 21 cal/(K mol). With this heat of vaporization, the vapor pressure of cyclobutanone at 298 K was estimated to be 5 5 Torr.
-
supersonic jet d a t a effusive b e a m d a t a
-
2
1
N
v
\ 0
-
3 1 a
a
a
-
- 0
...,
-
-1 0
500
1000
1500
2000
2500
3000
J(Ji-1)
Figure 2. Boltzmann plot of the u" = 0 CO fragment rotational distribution obtained from both a supersonic free jet expansion (filled symbols) and an effusive molecular beam (open symbols). The solid line is a biexponential fit to the supersonic jet data. The two data sets exhibit similar biexponential distributions.
-
tationally resolved vacuum-UV laser-induced fluorescence excitation spectra of the CO A l I I XIZ+transition for u" = 0-4. Spectra of the 2-0 and 0-2 vibronic transitions, probing population in u" = 0 and 2, respectively, are presented in Figure I . A complete and accurate assignment of all the spectra was facilitated by a tabulation of the rovibrational levels in the perturbed A ' I I state by Field and co-workers." Q-branch assignments from J" = 1 to 50 are shown above each spectrum. Six strong lines protruding through the comb and off-scale in the 0-2 band have (3!) LeFloch, A. C.; Launay, F.; Rostas, J.; Field, R. W.; Brown, C. M.; Yoshino, K. J . Mol Specrrosc. 1987, IZI, 337. Field, R. W. Unpublished results.
3034 The Journal of Physical Chemistry, Vol. 94, No. 7 , I990
Trentelman et al.
TABLE I: Rotational Temperatures and Relative Vibrational Populations of u" = 0-4 of CO from the 193-nm Photolysis of Cyclobutanone TI,,, K TM,. K vib population, 92
663% 247% 63%
1
2 3 4"
230 f 230 f 210f 160 f
20 20 30
50
2640 f 5050 f 2140f 2820 f 760 f
50 300 90 3000 400
66.3 24.1 6.3 2.2 05
J
2 2%
It
0
I
1
0 5%
f 2.6
--i
f 1.5 f 0.4 f 2.5 f 0.3
" A single rotational temperature was fit to all of the
C"
= 4 data.
been assigned as the lowest energy 3P 3P transition in atomic carbon. Their significance is discussed later. The intensity of each assigned rovibronic transition was measured and converted to relative population (within a single vibrational state) by dividing by the Honl-London factor, multiplying by the rotational degeneracy, and correcting for the fractional A 1 ncharacter of the excited state using the data of Field.3' The population data for u" = 0 are presented in the form of a Boltzmann plot (In [population/degeneracy] vs energy) in Figure 2. While there is no a priori reason to expect that photolysis products should conform to a temperature, this is the case for many systems. In addition, deviations from linearity provide a diagnostic for a nonthermal distribution. Such is the case for the results presented here. The data are not linear but exhibit a marked upward curvature in the low-J region. The h i g h 4 data appear to be linear, indicating that this region can be described by a rotational temperature. The low-J data, in turn, appear to be approaching a second linear region that might be characterized by a second temperature. Figure 2 is typical of the data for all vibrational levels, though the scatter and extent of the data become progressively worse as signal levels decrease with higher CY Given the apparent bimodality of the data, a sum of two Boltzmann distributions with variable relative amplitudes was fit to the data from each vibrational level. The fit to the u " = 0 data is shown as a solid line in Figure 2. For 'u = 0, 1, and 2, the data are described very well by a double-exponential fit. For v"= 3, a good fit is also obtained, although the standard deviation is considerably larger than for the lower vibrational levels.32 The data for C " = 4 do not extend to sufficiently high J to allow a double-exponential fit to the data, so the available low-J data were fit to a single-exponential function for use in obtaining an estimate of the total population in C " = 4. We are unable to determine from these data whether or not a two-temperature distribution similar to those obtained for the lower vibrational levels would be more appropriate. The rotational temperatures obtained from the fits are presented in Table I. Evaluation of the relative amplitude of each component reveals that within each vibrational level approximately 15% of the population is described by the lower rotational temperature. A bimodal distribution can arise from a number of sources, not all inherent to the unimolecular dissociation dynamics of specific interest here. Three potential artifactual causes of the observed bimodal rotational distribution have therefore been investigated: collision-induced rotational/vibrational relaxation, dissociation of cyclobutanone clusters, and secondary photodissociation of nascent photoproducts. The expansion conditions used in these experiments, in conjunction with the measured C O velocity distribution (section IV.C), predict that the nascent C O fragments will undergo on average one or two hard-sphere collisions in the 200 ns between the photolysis and the LIF probe. To investigate whether the observed increase in population at low J could be the result of efficient rotational relaxation, the u" = 0 rotational distribution was remeasured at a probe delay of 500 ns. This should allow more than twice as many collisions to take place. The rotational = distribution within u" = 0 measured after 500-11s delay ( TIOW +-
(32) In spite of the large error in the high-temperature fitting parameters, the consistency of the result with those obtained from the lower levels seems sufficient justification to retain a two-temperature fit.
il Figure 3. Complete measured vibration-rotation distribution in the CO photofragment. (Only Q-branch data are plotted for clarity.) The solid lines are fits of a two-temperature function to the Q-branch data in each vibrational level (except u" = 4, see text). The percentage of the total population in each vibrational level is indicated in the figure. 130 f 50 K; Thigh= 2780 f 140 K) was, however, virtually identical with the distribution measured after 200 ns (T,,, = 230 f 30 K; Thigh = 2640 f 50 K). This agreement indicates that collisions are probably not making a significant contribution to the results. Clustering has been shown in certain cases to affect dramatically the observed product distributions following a dissociation event.33 We therefore explored the u" = 0 rotational distribution under the cluster-free conditions of an effusive molecular beam.I3 The signal-to-noise ratio for this measurement was rather poor,34and data were obtainable only for low rotational levels. Even so, these data were sufficient to perform a comparison to the supersonic beam data and are superimposed in Figure 2. The distributions produced under both conditions exhibit similar behavior, indicating that it is unlikely that clusters are adversely affecting the results. Dissociation of primary photofragments that contain CO (ketene, CH,CO, is the most likely CO-containing fragment), by absorption of a second 193-nm photon, is another possible pathway for production of CO. In these experiments, however, the CO fluorescence intensity was found to vary linearly with the 193-nm laser power, indicating that C O is produced in a single-photon dissociation event. In addition, the molar absorption coefficient of ketene at 193 nm is very small ( < I 50 L/(mol cm))35936compared to that of cyclobutanone (9000 L/(mol ").I6 Photodissociation of ketene is thus not expected to comprise a significant channel to the observed product. B. Vibrational Distribution. The CO vibrational distribution from cyclobutanone photolysis was determined in a manner similar to that described previ~usly.'~The rotational distributions determined above were scaled to one another by using relative intensities obtained from spectra containing bands originating from more than one vibrational level in combination with the known Franck-Condon factors for the A X transition. Figure 3 contains the scaled Q-branch rotational distributions (now plotted as population vs J ) along with their fits. It is pleasing to note that the biexponential fits to the Q-branch data alone are excellent and that the bimodal character of the distribution is clearly visible.
-
(33) See,for example: Sivakumar, N.; Burak, I.; Cheung, W.-Y.; Houston, P. L.; Hepburn, J. W. J . Phys. Chem. 1985, 89, 3609-1 I . (34) The decrease in signal-to-noise ratio is most likely due to the broadening of the absorption transition in the effusive beam, such that a smaller fraction of the sample is resonant with the 193-nm pump laser. This is in contrast to acetone, here the broadening apparently improved the resonance, leading to better signals in the effusive beam. (35) Rabelais, J. W.; McDonald, J. M.; Scherr, V.; McGlynn, S. P. Chem. Reu. 1971, 7 1 , 73-108. (36) Okabe. H. Photochemistry of Small Molecules; Wiley: New York. 1978; pp 309-3 14.
Photodissociation Dynamics of Cyclobutanone at 193 nm
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3035 619 kllmol
MXI
'. '.
'CH,+CO+C,H, -IPm
'.
,T
'. '.
= 2560 f 100
K CHFO ( S , ) + C,H,
P
'. '.
w zca -
'F
c
O
-
Ct12C0(SJ + C& C,H,+CO
'c-h A + CO
0
C, channel
O t
-1
0
2000
4000
E(v")
6000
8000
(cm-')
Figure 4. Boltzmann plot of the vibrational distribution for the CO photofragment. The error bars indicate two standard deviations. The dashed line is a linear fit to the data with a temperature of 2560 f 100
K.
A
1.11
a) v"=O,' Q(2l)
2
k
04-
:data
4
- - fit laser
3
e a
v
0010-
h
08-
-
06-
v)
4
d
04-
v
o.2 0.0
-1.0
C, channel
Figure 6. Energy level diagram and proposed branching ratios for the photodissociation of cyclobutanone a t 193 nm.
0.5 cm-l. The molecular velocity distribution was obtained by deconvoluting the laser line shape (Gaussian, fwhm = 0.21 cm-') from the fitted Gaussian line shape. The deconvolution of two Gaussians is straightforward, the resultant being another Gaussian with a line width given [(Aufit)*- (AuI,,)z]'/2. The deconvoluted line widths (typically 0.43-cm-' fwhm) are used to calculate the translational energy of the C O fragment from the standard equations relating Doppler width, temperature, and translational energy.38 A Doppler line width of 0.43 cm-I corresponds to an average C O translational energy of about 2300 cm-' and to an rms velocity of 1400 m/s. D. Carbon Production. Atomic carbon resonances were observed within the C O 0-2 band (Figure I ) . These comprise six individual transitions, assigned as belonging to the lowest energy 3P 3Pband. The relative population of the three ground-state angular momentum components (3P0,,,2)has been measured and shows that they are distributed statistically according to their degeneracy. The production of carbon relative to C O was determined from the measured line intensities in combination with relative line strengths estimated from tabulated radiative lifetimes of CO and C (9.9 and 2.4 ns, re~pectively~~). A single CO(u",J'? line can be related to the total C O production from the previous population data; the C lines are similarly related to the total C production by their statistical weights. We consequently estimate that C production is only -0.2% of the total C O production-a minor channel that is most likely the result of absorption of multiple 193-nm photons by either cyclobutanone or one of the fragments. (A more detailed description of the calculation of the carbon population may be found in refs 40 and 41.)
-
02-
t:
177,
-0.5
0.0
0.5
1.0
Doppler shift (cm-') Figure 5. Doppler profiles of selected CO rovibronic transitions, plotted as intensity vs shift from line center in cm-l. A best-fit Gaussian line shape is plotted for each transition, with the width indicated in the figure. Fits do not account for convolution of the Gaussian laser line (also shown).
The relative population in each vibrational level was obtained by summing over the rotational distribution. The fitted curves were summed for each J " u p to the value of J"corresponding to the maximum available energy. The vibrational distribution thus obtained is given in Table I. A Boltzmann plot of the vibrational population vs energy is presented in Figure 4. The data are described well by a linear fit, corresponding to a thermal vibrational distribution with a temperature of 2560 f 100 K. C. Doppler Profiles. Doppler line shapes of selected rovibronic transitions were recorded in order to measure the translational energy carried away by the C O and also to determine whether any vector correlation in the photodissociation dynamics can be observed. Profiles of Q(21) and R(26) from u"= 0 are presented in Figure 5. The profiles are fit well by Gaussian line shapes, and there is no distinctive difference between the line shapes of the Q lines and those of the R and P lines, as would be expected for line shapes reflecting vector ~orrelation.~'The full width at half-maximum (fwhm) for each of these lines is approximately (37) Houston, P. L. J . Phys. Chem. 1987, 91, 5388-5397.
IV. Discussion A . Decomposition Pathways. In this set of experiments we have measured the rotation, vibrational, and translational energy distributions in the C O fragment produced from the 193-nm photolysis of cyclobutanone. The rotational distributions in each vibrational level clearly indicate the presence of two distinct populations, one rotationally hot (Tro, 2500 K) and the other rotationally cold (Tmt 250 K), with relative populations of 85% and 15%, respectively. Previous studies of the gas-phase photolysis of cyclobutanone, excited to either the SI or Sz electronic state, support the existence of two parallel dissociation mechanisms, one proceeding on a cyclobutanone triplet surface and leading to CO and a three-carbon product, mostly cyclopropane, plus possibly some propylene (the C3 channel), and the other proceeding on
-
-
(38) Mitchell, A. C. G.; Zemansky, M. W. Resonance Radiation and Excited Atoms; Macmillan: New York, 1934; p 99. (39) Moore, C. E. Atomic Energy Leoefs; U S . National Bureau of Standards: Washington, DC, 1971; Vol. 1, NSRDS-NBS35. Field, R. W.; d'Azy, 0. Benoist; Lavolle, M.; Lopez-Delgarno, R.; Tramer, A. J . Chem. Phys. 1983, 78, 2838. (40) Trentelman, K. A. Ph.D. thesis, Cornell University, 1989. (41) Straws, C. E.; Kable, S. H.; Burak, I.; Houston, P. L. Manuscript in preparation.
3036 The Journal of Physical Chemistry, Vol. 94, No. 7, I990 T A B L E 11: Comparison of Observed/Inferred and Calculated Energy Distributions in C O and Cyclopropane Fragments of Cyclobutanone Excited at 193 nm
energy, kJ/mol
fragment
(degree of freedom) experimental
simple
prior
co 26 12 28
rot
vib trans c-CIH,
}
rol
vib trans
total
21 19 26
0-44'
22 40
506
42&o'
20
26
18
592
592
59 1
} 460-504'
4i;501
'The range indicates the inability of the simple model to partition rotational energy between the two fragments on the basis of conservation of angular momentum, due to the wide range of orbital angular momentum available to the receding fragments. a cyclobutanone singlet surface and leading to ethylene and ketene (the C2 channel):
C4H6O*
-
CdH60(T,)
-
C-C,H6 (or C3H6)
+ co
(1)
In contrast to many photodissociation reactions, all of the photoproducts from cyclobutanone are closed-shell species. Much of the energy required to sever bonds in the parent is thus recovered as bonds form in the products and is therefore available to the various product degrees of freedom. An energy level diagram for this system (Figure 6) shows that, for 193-nm excitation, there is in fact sufficient excess energy to permit additional unimolecular chemistry within the fragments. In addition to the possibility of cyclopropane-propylene isomerization, there is sufficient energy for the production of ethylene and methylene from the C3product C-CjH6 (or C3H6)
-
-
C2H4
CH2
(3)
and for the dissociation of ketene to C O and methylene CH2CO
CO + CH2
(4)
T o model this system successfully, we must first determine which of the two observed populations corresponds to each channel. Toward that end, we should note that although the final products of reactions I , 3 and 2, 4 are identical (CO, ethylene, and methylene), the energy available to the CO fragment from each reaction pathway is not. Specifically, it can be seen from the energy level diagram in Figure 6 that the C, channel (with CO as a primary product) has 592 kJ/mol available to partition between C O and cyclopropane, while the C2 channel (where CO is a secondary product) has only 207 kJ/mol to partition between CO, CH2,and C2H4. Therefore, on energetic grounds alone we might associate the CO high-Trotpopulation with the C3 channel. Dynamical arguments also support the assignment of the high-T,,, population to the C, channel. The nearly linear C= C=O geometry of ketene would be expected to provide little rotational excitation to a departing CO fragment, as has been observed by Moore and c o - w o r k e r ~ By . ~ ~contrast, ~ direct production of CO from cyclobutanone in anything other than a completely symmetric process with a planar geometry could impart substantial rotational energy to the departing CO. We therefore assign the high-T,,, population as the product of reaction 1 and the low-T,, population as the product of reaction 2 followed by reaction 4. In the remainder of the Discussion, we will examine each pathway in turn and consider what conservation laws and simple models of the dissociation process can tell us about the dynamics of each channel. B. Dynamics of Primary CO Production. The application of conservation laws to this two-body dissociation is relatively straightforward. From the experimental measurements it is possible to calculate the average energy in each of the CO degrees of freedom. The average C O translational energy and conservation of linear momentum will predict the translational energy of the
Trentelman et al.
C3 fragment. The remainder of the available energy can then be assigned to the rotational and vibrational degrees of freedom of the C3 fragment.42 These energies, calculated below, are summarized in Table I I . The average amount of rotational energy carried away by the C O fragment is given by ER(CO) = Cc,kT,,, the sum of the average rotational energy in each vibrational level. Here c, is the fractional population in L'" = n (from the vibrational distribution), and T, is the rotational temperature in o f f = n. From the data in Table I, we calculate an average rotational energy of 26 kJ/mol for the high-Tro,channel. The c,, can also be used to calculate , which the average vibrational energy as Ev(CO) = ~ n c , , u f 'for we obtain a value of 12 kJ/mol. Finally, from the Doppler profiles (all of which were taken for J values that are predominantly due to the high-T,,, channel), we determine the average C O translational energy to be approximately 28 kJ/mol. From conservation of linear momentum, the average translational energy of the C3 fragment is calculated to be 20 kJ/mol. Given the above values and the total available energy of 592 kJ/mol, the sum of the cyclopropane vibrational and rotational energy is estimated to be 506 kJ/mol. (Note that this is well above the 372 kJ/mol needed to dissociate cyclopropane to ethylene and methylene.) As in our analysis of photofragment energy distributions produced in the photolysis of acetone,', the results obtained here will be compared with the predictions of models for two limiting cases of the dissociation dynamics. The first is an adaptation of the impulsive model of Busch and Wilson,43generalized from triatomics to larger polyatomics. The second limiting case is a statistical distribution of photofragment energies, which will be described in more detail below. 1 . fmpulsiue Model. An impulsive model of the dissociation has been found not to agree with the experimental observables. The model assumes that all of the available energy from the dissociation is localized in the dissociating bond(s) and is initially partitioned between the atoms forming the bond(s) strictly on the basis of conservation of linear momentum. This energy is then redistributed into the fragment degrees of freedom on the basis of conservation of linear and angular m o m e n t ~ m . Each ' ~ carbon atom in each of the two breaking C-C bonds of cyclobutanone will thus initially receive half of the energy from that bond. The energy ultimately carried away by the carbonyl carbon (and thence by the C O fragment) will depend on the delay and orientation between the two impulses and their relative magnitudes. Although the final products from this channel will be C O and cyclopropane, the impulsive breaking of two C-C bonds will result initially in the formation of CO and a trimethylene biradical. The total available energy to be used in this calculation is therefore probably best represented by the difference between the photon energy and the energy required to produce C O and trimethylene without including the added stabilization introduced by formation of the cyclopropane molecule. We estimate the stabilization energy as being half the dissociation energy of cyclopropane to methylene and ethylene, 372/2 = 186 kJ/mol. The available energy is therefore reduced from 592 to 406 kJ/mol for this channel. To proceed with the calculation, the first issue to be addressed is the distribution of the available energy between the two dissociating bonds. Given that the bonds are equivalent, it seems appropriate to divide the available energy equally, 203 kJ/mol per bond. The second issue to consider is the effect of energy redistribution on the predicted C O fragment energy distribution. If no energy redistribution is allowed, then the components of the two impulses perpendicular to the carbonyl bond cancel exactly and the CO will emerge rotationally cold, in contradiction with our supposition that this C3 channel is associated with the high CO rotational temperature. (For a planar dissociation there would be no rotation. (42) In all of the following calculations, the identity the C3 fragment is assumed to be cyclopropane. The differences between cyclopropane and propylene in these calculations is presumed to be minor. ( 4 3 ) Busch, G. E.; Wilson, K . R. J . Chem. Phys. 1972, 56. 3626.
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3031
Photodissociation Dynamics of Cyclobutanone at 193 nm
....,.
-1.2
,
,
, ,
I
I
, . , , ,
, ,
!.
: b k1.0 -
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, ,
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:
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a l . 20.2 1 d
.
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'
.-k 1 . 2 :.
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.
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.
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0.8 0.6
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.
A 0.4
-
: . 2 0.2 1 1
:
-
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: n
c (
0.0 *
Assuming that the angle between the carbonyl and the ring is -30°,25the predicted energy in C O rotation is on the order of 15 kJ/mol, still too low by a factor of 2.) On the other hand, if we assume that all of the impulse of the first broken bond is absorbed into internal degrees of freedom of the opened ring, the carbonyl carbon then receives an impulse only from the dissociation of the second bond. (Since the molecule has not yet fragmented, there can be no translational energy imparted in the initial center-of-mass frame.) Within the impulsive framework presented here, this represents the minimum energy that can appear in the carbonyl fragment. Carrying through the calculation for this minimum-energy process, the energy predicted for the CO fragment is 203/2 ;= 101 kJ/mol. using eqs 1 1 and 12 from ref 13 to partition this energy between the translational and internal degrees of freedom of the C O fragment yields
Etrans = (12/28)E(CO) = 43 kJ/mol
(5)
Erol+vib = [ I - (12/28)]E(CO) = 58 kJ/mol
(6)
In each case the calculated energy exceeds the energy observed in the C O fragment by at least 50%. If some of the redistributed energy is allowed to accumulate in the second dissociating bond, the overestimation simply becomes worse. On the other hand, partitioning less than half of the available energy into the second bond seems to violate the spirit of the impulsive model. We are therefore led to conclude that, unlike acetone, the dissociation dynamics of cyclobutanone are definitely not impulsive. 2. Statistical Models. The other limiting case to examine involves the assumption that the available energy is statistically distributed among all of the degrees of freedom of the parent molecule prior to dissociation. A simple estimate of the average energy carried away by the fragments is obtained by dividing the available energy by the number of vibrational degrees of freedom of the parent and assigning that amount of energy to each fragment degree of freedomm (Some of the parent degrees of freedom will carry over into relative rotation and translation of the fragments.) For the case of cyclobutanone producing cyclopropane and CO, the parent has 27 vibrational degrees of freedom, cyclopropane has 2 I , and CO has I . Two of the remaining five will contribute to rotation and three to translation. Thus, CO should receive
I
*
'
I
I
'
*
"
(1 /27)592 = 22 kJ/mol in vibration, cyclopropane should receive (21 /27)592 = 460 kJ/mol, rotations should account for 44 kJ/mol,
and translation should account for the remaining 66 kJ/mol. the translational energy can be partitioned between the two fragments on the basis of conservation of linear momentum, 40 kJ/mol for CO and 26 kJ/mol for cyclopropane. There is, however, no simple way to partition the rotational energy. For comparison with the experimental results, we indicate only that the sum of the rotational energies in CO and cyclopropane in this model is 44 kJ/mol. The agreement of this simple model with the experimentally measured and inferred results is surprisingly good (see Table 11). Particularly, the model correctly predicts that the bulk of the excess energy should reside in the cyclopropane, with only modest amounts in the other degrees of freedom. We are interested not only in the average energy in each degree of freedom but also in the distribution of energies expected within those degrees of freedom. Statistical prior distributions have therefore been calculated for all of the fragment degrees of freedom. The calculation treated the CO vibrations and rotations quantum mechanically and used a direct count for these, but the cyclopropane rotational and vibrational energies were treated as classical variables, since a direct count of cyclopropane vibrational states would be prohibitive at these energies. The density of vibrational states in cyclopropane was calculated from the Whitten-Rabinovitch formula,44using the (harmonic) frequencies tabulated by H e r ~ b e r g . The ~ ~ statistical prior calculations serve as a check on the simpler model and give a qualitative idea of the distribution of energies that might be expected in each of the fragment degrees of freedom. The average energies are recorded in the last column of Table 11. As in the predictions of the simple statistical model, most of the available energy is contained in cyclopropane vibrational degrees of freedom. The average energy predicted by this model for each degree of freedom is in excellent agreement with the observed value in every case with the exception of the CO vibrational energy. Both statistical models overestimate by a factor of 2 the vibrational energy of CO; this may reflect a combination (44) Whitten, G . Z.; Rabinovitch, B. S. J . Chem. f h y s . 1963, 38, 2466. See also: Forst, W. Chem. Reu. 1971, 71, 339. (45) Herzberg, G . Molecular Spectra and Molecular Structure: I I . Infrared and Raman Spectra of folyatomic Molecules; D. Van Nostrand: Princeton, NJ, 1945; pp 326, 352.
3038 The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 of dynamical effects that discourage excitation of the carbonyl stretch relative to the lower frequency bending motions of the cyclopropane. A more critical test of the prior calculation is the distribution of energy it predicts for the various product degrees of freedom. The calculated rovibrational distribution for CO u” = 0-3 is plotted in Figure 7a and is seen to be similar to spirit to the experimental results (cf. Figure 3). The calculated rotational distributions are very nearly Boltzmann and can be described by an average rotational temperature of 3300 K (vs the average experimental value of 3200 K ) . The vibrational distribution is also described well by a temperature of 3550 K, about 50% larger than the experimental value of 2560 K, as would be expected based on the average energies. The distribution of cyclopropane vibrational energies (Figure 7b) is calculated to extend from less than 200 kJ/mol up to nearly the limit of available energy (592 kJ/mol) with a mean energy of 442 kJ/mol and a fwhm of 142 kJ/mol. Nearly 90% of the cyclopropane fragments are expected to have sufficient energy to undergo further unimolecular decomposition to ethylene and (triplet) methylene.46 The translational energy distribution for the CO fragment is shown in Figure 7c. It is peaked at 10 kJ/mol with a mean of 26 kJ/mol. This distribution has been transformed into a Doppler profile and convoluted with a Gaussian laser line of 0.2 I-cm-I fwhm for direct comparison with the experimental profile. The result, shown in Figure 7d, is a profile with a fwhm of -0.5 cm-I, the same width and general shape as the measured profile. Figure 7 demonstrates that the prior distribution predicts both the average photofragment energies and their distributions quite accurately. In summary, the major channel for cyclobutanone photodissociation at 193 nm, characterized by a high CO rotational temperature, is consistent with a statistical dissociation yielding cyclopropane and CO, with most of the excess energy residing in the cyclopropane fragment, much of which can be expected to further decompose to ethylene and methylene. C. Dynamics of Secondary CO Production. It is unfortunately, much more difficult to obtain information about the minor channel. Doppler profiles of the low-J states would contain significant contributions from both the high- and low-temperature populations. Furthermore, even if one could measure the lowtemperature channel exclusively, a measurement of the translational energy of one of the primary fragments would be needed to determine the energy release in the secondary step yielding CO. Conservation of angular momentum is similarly thwarted by the wide range of available energies and impact parameters in the first step. There is, however, some important information at our disposal. Specifically,we know that of the 542 kJ/mol available in the initial decomposition to ketene and ethylene, at least 360 kJ/mol (assuming singlet methylene production46) must be channeled into the ketene fragment in order for it to further decompose to methylene and CO. This is a rather stringent limitation, and it should be possible to make some inferences about the dynamics given that constraint. In particular, for an impulsive model, given that all atoms receiving an impulse are equivalent, it is expected that the impulse would deliver equal amounts of the available energy to the two fragments, on the order of 270 kJ/mol. The mass ratio of the carbon atoms to the ketene fragment then suggests that less than half this energy would appear as internal energy, probably on the order of 100 kJ/mol, the rest being partioned into translation. Since ketene dissociation requires more than 3 times this internal energy, it seems that the C, channel cannot be described by a purely impulsive mechanism.
Trentelman et al. 10
I
I
,
I
,
.
I
I
,
,
I
.
,
,
,
,
,
,
,
,
,
,
.
,
,
,
,
,
,
Ketene Internal Energy (vib only) (kJ/mol)
‘fl
l . O ~ , , , , , . , , , , , , , , , , , , , , , , , , ~ ,
,
,
,
I
-
(46) Although it is known that singlet and triplet ketene correlate with CO plus singlet and triplet methylene, respectively,s16 the electronic state of methylene produced by cyclopropane dissociation in this system is unknown. For simplicity, and because the Cp channel is associated with triplet cyclobutanone. we assume that ground-state (triplet) methylene is the product of cyclopropane dissociation.
Figure 8. Statistical prior distributions for internal energy of ground-state (top) and excited-state (bottom) ketene produced in coincidence with ethylene for a total available energy of 545 kJ/mol. Less than 1% of the ground-state ketene lies above the 360 kJ/mol ketene dissociation energy, while approximately 30% of the excited-state ketene falls above this energy.
A statistical model, however, is subject to a similar limitation. Specifically, since ethylene and ketene are of similar vibrational complexity ( I 2 and 9 vibrational modes, respectively, each will receive a similar fraction of the available energy, somewhat less than half of the total. Simply dividing the available energy by the 27 parent degrees of freedom would allocate (9/27)542 = 181 kJ/mol to ketene vibrations, not nearly enough to allow dissociation to C O and methylene. A prior distribution (Figure 8, top, using Herzberg’s frequencies for ethylene4j and those of Moore and Pimente14’ for ketene) indicates that less than 1% of the ketene should have sufficient energy to dissociate. It is possible to circumvent this limitation, however, by assuming the production of excited-state (SI)ketene. In this view, the energy of the excited state (approximately 227 kJ/mol) is localized in the ketene, and the remaining 315 kJ/mol is statistically distributed between the other ketene and ethylene degrees of freedom. For the simpler statistical model, the ketene receives an average of 105 kJ/mol of vibrational energy, which, when combined with the electronic energy, gives a total ketene internal energy of 332 kJ/mol, an am.ount comparable to the dissociation energy (see Figure 6). A prior distribution (Figure 8, bottom)” indicates that -30% of the ketene produced in SI will have a total energy in excess of the dissociation of the dissociation limit. It thus seems likely that the CO characterized by TI,, is produced by the dissociation of ketene which is initially produced in an excited electronic state. D. Potential Surfaces and Branching Ratios. The preceding sections form the basis for an assignment of the observed bimodal rotational distribution in the CO fragment to two independent reactions starting from the S2surface of cyclobutanone. The major channel is characterized by a high Trotand is consistent with a statistical dissociation of cyclobutanone to cyclopropane and CO. The minor channel is characterized by a low Trotand has been assigned to unimolecular dissociation of an electronically excited (47) Moore, C. B.; Pimentel, G. C. J . Chem. Phys. 1963, 38, 2816. (48) S , ketene frequencies are taken from the calculations of Allen and Schaefer (Allen, W.D.; Schaefer 111, H. F. J . Chem. Phys. 1986,84, 2212). The calculated frequencies were reduced by 10% to reflect the typical overestimation observed in the ground-state calculation.
Photodissociation Dynamics of Cyclobutanone at 193 nm ketene primary fragment, produced in coincidence with ethylene. What remains is to determine (or at least suggest) the sequence of events that occurs on the relevant potential surfaces of cyclobutanone and the products with which they correlate. It has previously been argued that the similarity of the SI and S2 states with regard to their photochemistry suggests that the first step should be internal conversion of S2to S1.'9*27 There are three mechanisms by which the SI state can subsequently decay: intersystem crossing to TI,internal conversion to So, and a third channel which has been labeled "predissociation" by Hemminger and Lee.25*49These authors have noted the onset of a fast decay process with a threshold of -1000 cm-' in the SI state of cyclobutanone. Both C3 and C2 products appear below this threshold, but the C3/C2ratio drops from -7 to -0.5 in this region, after which it slowly increases as mentioned previously. Hemminger and Lee attribute this drop in C3/C2to the onset of predissociation from SI cyclobutanone, but they do not comment on the particular surface that might be causing this predissociation. The destinations of the TI and So states are relatively clear. TI dissociates to give almost exclusively cyclopropane and CO, while the dominant products from So are ethylene and ketene, the latter presumably in its ground electronic state. We note, however, that the region of interest in SI cyclobutanone falls about -40 kJ/mol above the energy of ethylene plus SI ketene, suggesting that the predissociation might be associated with a state that correlates to excited-state rather than ground-state ketene. Below the threshold, the C3/C2 ratio would reflect only the competition between internal convention and intersystem crossing. Above the threshold, the predissociation to ethylene plus excited-state ketene could provide an additional contribution to the C2 products and thus account for the observed decline in the C3/C2 ratio. This proposition provides the mechanism for channeling sufficient energy into the primary ketene fragment to allow for its subsequent dissociation to methylene and the rotationally cold C O that we observe. Competition between three channels is also consistent with the observed branching ratio between the C2 and C 3 channels. As mentioned in the Introduction, a monotonic trend in the C3/C2 ratio is observed for wavelengths shorter than 315 nm, C3/C2 increasing slowly with increasing energy. Extrapolating the results of Denschlag and Lee to an excitation wavelength of 193 nm, we predict that the C3/C2 ratio is on the order of 1.3, corresponding to roughly 57% of the products appearing in the C 3 channel. The observed ratio of Thigh/T1ow was 85/15, which is not in agreement with the extrapolation. The predicted C3/C2ratio of 1.3 can be made to be consistent with the observed Thigh/Tlow ratio, however, if roughly 30% of the S2 cyclobutanone dissociates to yield SI ketene and if, in agreement with our statistical calculation, (49) Hemminger, J . C.; Lee, E. K . C. J . Chem. Phys. 1971, 54, 1405.
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3039 30% of the S, ketene dissociates to yield methylene and (low rotational temperature) CO. This scenario would suggest that only a minor amount (13%) of the S2cyclobutanone dissociates to So ketene and C2H4. V. Conclusions The nascent distribution of C O photofragments produced in the 193-nm photolysis of cyclobutanone consists of two populations with rotational temperatures that differ by more than an order of magnitude. The predominant component (85%) is characterized by a high rotational temperature (Trot 3000 K) and has a vibrational and translational energy distribution that is consistent with a statistical partitioning of energy between two photofragments, C O and cyclopropane. The minor component ( 1 5%) is characterized by a low rotational temperature (Trot 200 K) and has been associated with the dissociation of an excited ketene photofragment in a process that also yields ethylene. Statistical prior distributions for the ketene and ethylene photofragments suggest that ground-state ketene will not be sufficient energetic to dissociate but that SI ketene will be produced with a distribution of internal energy such that a substantial fraction will be able to dissociate. In summary, the following mechanism is proposed for the dissociation of S2 cyclobutanone excited at 193 nm:
-
-
C4H60(S2)
- +
-
C4H60(SI)
57%
C4H6O(Sl)
30%
C~H~O(SI)
C4H60(TI)
C2H4
-
CH2CO(Sl)
-
C2H4
C,H~O(SI)
c-C3H6
-
(7)
+ co(Thigh)
(8)
30%
+ CH2 + co(Tlow) (9)
C ~ & , O ( S O ) C2H4 + CH,CO(SO)
(10)
The C4H60(SI)intermediate is presumed to survive long enough to allow substantial vibrational randomization prior to dissociation via any of the three channels, and calculations have indicated that the observed product state distributions are generally consistent with a statistical partitioning of energy among the product degrees of freedom. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-86-17062. K.A.T. gratefully acknowledges the Procter & Gamble Co. and the Dow Chemical Co. for graduate fellowships. This work was done in cooperation with the research program of Yehuda Haas under a US.-Israel Binational Science Foundation grant. We are grateful to Prof. B. K. Carpenter for helpful discussions. K.A.T., D.B.M., and S.H.K. also thank Ginny Lee Wagner for providing inspiration and a pleasant environment for discussion. Registry No. CO, 630-08-0;c-C3H6,75-19-4; C2H,, 74-85-1; C H 2 C 0 , 463-51-4; cyclobutanone, 1191-95-3.
