3248
J. Phys. Chem. 1987, 91, 3248-3253
to from the TME cation.15 We observed that the red tail of the absorption spectrum of the TMB cation in 3MHX glasses extends to near 450 nm. The absorption spectrum observed seems to be consistent with the range of wavelength effective for the photoreaction of the TMB cation. The &, values for the difference spectra of the higher n-alkanes (C9-CiI) are observed to increase with C-C chain length (Table I). The result may be compared with the ESR resultIb that the hyperfine coupling constants for n-alkane cations decrease with increasing carbon number. Iwasaki et al. showed that the unpaired electrons in the n-alkane cations delocalize over the entire amolecular chains.lb Increase in A,, values with carbon number is observed in the case of the methyl-branched higher alkanes (Table I ) . The result suggests that the cations of the methylbranched alkanes concerned are the o-delocalized type cations. The cation of 2-methylbutane (2MB) was reported to be the a-localized type.1° The cations of 3MP and 3MHX in Freon-1 13 matrices have been found to be the a-localized type, and the cation of 3-methylheptane (3MHP) in Freon-1 13 matrices has been suggested to be the a-delocalized type.9 The conclusion is derived from the result that the overall splittings of the ESR spectra change as follows: 45.5 X 3 G for 2MB, 47.3 X 3 G for 3MP, 52.7 X 3 G for 3MHX, 76.3 G for 3MHP, 58.3 G for 3MO,48.0 G for 4MN, and 37.5 G for 5MD. The overall splitting for the n-heptane cation was reported to be 30 X 2 Gib and is near that for 3MHP. The,,A values of the nC9, nC10, and nC11 cations produced in y-irradiated CCI, matrices were reported to be 700, 750, and 790 nm, respecti~ely.~~ These values except for nC11 are the same as those of the corresponding cations in Table I. Since these higher in alkanes give broad absorption spectra, the difference of A, the case of n C l l is considered to be within the limit of error. Strobbe and Ceulemans examined absorption spectra of the nC8 and nClO cations produced in various organic matrices.16 They values ranging from 670 to 590 nm for nC8 and from found ,A, 770 to 700 nm for nClO and concluded that the optical absorption of these cations is not due to a charge-transfer band, but to
electronic transition within the ion. In the present ESR study of branched butane cations we observed the same hyperfine structures in both the 3MHX and the Freon-1 13 matrices, indicating no matrix effect on the structure of the cations. Klassen et al. observed a absorption spectrum with A, of 630 nm in a pulse radiolysis study of the 3MO-N20 system at 6 K, and the value spectrum was attributed to the 3 M 0 cation.” The A, observed by them is close to ours. An absorption spectrum with A, of about 640 nm obtained by Louwrier and HamillSa in radiolysis of the 3MP-CC14-4MN system is comparable to our value of 660 nm. It was clearly indicated that IR illumination of y-irradiated 3MXH-HME and 3MP-C02-4MN systems induces recombination between negative and positive charges trapped in the matrices. We observed a decrease of both the charges and could not find an increase of the other paramagnetic species on the charge recombination. Formation of free radicals (RH’ eRH* R’ + H) is often postulated in the reaction. However, the present result gives a negative answer to the free-radical formation due to charge recombination. In y-irradiated 3MP glasses containing C 0 2 and branched alkanes Louwrier and Hamillsa observed that OD increases markedly with carbon number of the solute alkanes. We also observed the same effect (Table I). The areas under the difference ESR spectra (Figure 8A-C) showed a similar tendency, and the ratio of the areas (100:82:33) is considered to indicate more directly the ratio of the cation yields rather than the ratio of OD. The mechanism of positive-charge trapping in organic matrices seems to be unresolved, but the difference in ionization potentials between matrix and solute molecules is thought to be an important factor. The cation yields may possibly be related to the difference in ionization potentials. Louwrier and Hamills, also observed that y-irradiation of 3MHX containing C 0 2and 2-methyldecane gives no absorption band due to the solute cation. In our ESR result the 3MHX-13C02-4MN system gave the weak difference absorption due to the 4MN cation (Figure 8G),which is considered to correspond to a decrease in the difference in ionization potentials.
-
+
-
(15) Nunome, K.; Toriyama, K.; Iwasaki, M. Chem. Phys. Lett. 1984,105, (17) Cygler, J.; Teather, G. G.; Klassen, N. V. J . Phys. Chem. 1983, 87, 455.
414.
(16) Strobbe, M.; Ceulemans, J. J . Mol. Struct. 1980, 61, 255.
Phototysb of BrCN between 193 and 266 nm Jeffrey A. Russell,+Ian A. McLaren, William M. Jackson,* and Joshua B. Halpern Department of Chemistry, Howard University, Washington, D C 20059 (Received: January 6, 1986, In Final Form: February 13, 1987)
The quantum state distributions of CN(X2Zt) fragments from the photolysis of BrCN have been measured at several wavelengths between 193 and 266 nm. These distributions show that there is little or no vibrational excitation of the CN fragments for photolysis wavelengths below 230 nm. Above this wavelength, significant population is found in excited vibrational states. An explanation of this behavior is proposed and discussed. These results are shown to be in accord with recent theoretical ideas on excitation to a state where there is strong coupling between the rotational and translational degrees of freedom in the excited state.
Introduction The cyanogen halides have long been a favorite system for those interested in photodis~ociation.~-~ The fragmentation is simple, producing halogen atoms and C N radicals. Both of these species are relatively easy to detect. Quantum state distributions of Present address: Avco-Everett Laboratories, Everett, MA. *Author to whom all correspondence should be sent. Present address: Department of Chemistry, University of California at Davis, Davis, CA 95616.
0022-36541871209 1-3248$01.50/0
ground-state C N radicals can be measured by laser-induced fluorescence (LIF),4 and electronically excited radicals can be characterized by emission spectro~copy.~Halogen atoms can be (1) Okabe, H. The Photochemistry of Small Molecules; Wiley Interscience: New York, 1978. ( 2 ) Ashfold, M. N. R.; MacPherson, M. T.; Simon, J. P. Topics in Current Chemistry; Springer-Verlag: Berlin, 1979; VoI. 86, p I . (3) Okabe, H.; Jackson, W. M. Aduances in Photochemistry; Volman, D., Hammond, G.,Gollnick, K., Eds.; Wiley: New York, 1986; p 1 . (4) Jackson, W. M.; Cody, R. J. J . Chem. Phys. 1974, 61, 4183.
0 1987 American Chemical Society
Photolysis of BrCN monitored by time-of-flight spectroscopy6or, if they are excited, by their IR emi~sion.~ The earliest work was done on ICN,S-7 whose A continuum has a maximum at the fourth harmonic of the Nd:YAG laser, i.e., 266 nm. Later studies were done on the photolysis of BrCN and ClCN using fixed frequency excimer lasers.89 Some experiments have also been performed using pulsed molecular beams to reduce the contributions of vibrationally and rotationally excited parent The original measurements of the nascent C N quantum state distributions from the photolysis of BrCN were carried out at 193 nm by Halpern and Jackson.8 LIF spectra were measured at pressures below a tenth of a milliTorr, with delays between the photolysis and probe lasers of less than 500 ns. The nascent CN quantum state distributions had less than 6% of the population in the vibrational levels higher than u" = 0. The rotational distribution in the U" = 0 level is nonthermal and peaked sharply at about N" = 65, with a cutoff at about N " = 80. Fisher et al.9 studied the photolysis of BrCN at a number of excimer wavelengths and they noted that the population of vi-. brationally excited C N fragments decreases with decreasing photolysis wavelength. Collisions may have affected their work, since at 193 nm 16% of the C N fragments were observed to be vibrationally excited. On the other hand, Halpern and Jackson8 had found less than 6% of the radicals were vibrationally excited. The latter result has been confirmed by Hay and co-workers12 in their studies of energy transfer following collisions of rotationally excited C N radicals produced in the photolysis of BrCN with various gases. CN radicals in rotational levels with N"> 70 were observed to be essentially unrelaxed by collisions, while lower rotational levels relax in one or two collisions. Fisher et al.9 maintained pressures of BrCN of between 75 and 150 mTorr and used a photolysis-probe delay of 40-1 20 ns for P-t products of 9 000-22 500 mTorrm. Halpern and Jackson8 used pressures of BrCN of less than 0.2 mTorr and delays of about 500 ns. The P-t product in the latter case is 100 mTorr=ns,which is considerably lower than the value of Fisher et al.9 At room temperature the conditions of ref 9 would correspond to an average of about 0.08 collisions. The CN fragments are not, however, ' produced with a translational energy that is even close to room temperature. For example, at 193 nm, even considering the possibility of electronic excitation of the bromine atoms, the C N translational energy varies from 1.3 to well over 2 eV. The cross section for conversion of this translational energy to rotational and vibrational energy by collision can be large. This was clear to us in our original work, where changes could be observed in the nascent vibrational and rotational distributions starting at pressures of the order of 10 mTorr and delays of 0.5 ps. Thus collisions between translationally hot CN radicals and the ambient gas could account for the observed differences in vibrational partitioning between the various groups. If collisions have affected the vibrational partitioning results of Fisher et al., they will have certainly affected the observed rotational distribution. It therefore seemed desirable to determine the nascent CN internal energy distributions at a variety of wavelengths under conditions that are known to produce the unrelaxed product. In the present study we have done that and we confirm the original observation of Fisher et al.9 on the changes in vibrational population with photolysis wavelength. We have also been able to determine nascent rotational distributions at a variety of wavelengths. These are particularly important in view ( 5 ) Ashfold, M. N. R.; Simons, J. P.J. Chem. SOC.,Faraday Trans. 2 1977, 73, 858. Ashfold, M. N. R.; Simons, J. P. J. Chem. SOC.,Faraday Trans. 2 1978, 74, 280. (6) Ljng, J. H.; Wilson, K.R. J. Chem. Phys; 1975, 63, 101. (7) Pitts, W. M.; Baronavski, A. P. Chem. Phys. Lett. 1980, 71, 395. (8) Halpern, J. B.; Jackson, W. M. J. Phys. Chem. 1982,86, 3528. (9) Fisher, W. H.; Eng, R.; Carrington, T.; Dugan, C. H.; Filseth, S. V.; Sadowski, C. M. Chem. Phys. 1984,89,457. (10) Lu, R.; McCrary, V.; Halpern, J. B.; Jackson, W. M. J. Phys. Chem. 1984,88, 3419. (1 1) Nadler, I.; Reisler, H.; Wittig, C. Chem. Phys. Lett. 1984, 103, 451. (12) Hay, S.; Shokoohi, F.; Callister, S.; Wittig, C. Chem. Phys. Lett. 1985, 118, 6.
The Journal of Physical Chemistry, Vol. 91, No. 12, 1987 3249
Figure 1. Sketch of the experimental apparatus. Photolysis light was generated either with an excimer laser using KrF (248 nm) or ArF (193 nm), or doubled and then anti-Stokes Raman-shifted with light from a Nd:YAG pumped dye laser. This UV photolysis beam was sent into a small cell, with a counterpropagating beam of light from a nitrogenpumped dye laser which excited the L I F spectra of nascent C N fragments. The lasers were fired by a computer-controlled data acquisition system which also monitored the LIF signal and the intensities of the two lasers.
of recent theoretical results by SchinkeI3on the photodissociation of triatomic molecules.
Experimental Section Figure 1 is a sketch of the experimental apparatus. Tunable far-UV light was generated by Raman shifting the output of a doubled Nd:YAG pumped dye laser system (Quantel 581C/ TDL-50). Light at 193 and 248 nm was generated by a Questek Model 2200 excimer laser. The CN(X2Z++B2Z+) LIF spectrum was excited by a Molectron UV-400 nitrogen-pumped dye laser. The dye laser output was attenuated to ensure that the LIF signal was not saturated. The two laser beams were counterpropagated through a small vacuum cell. Fluorescence was observed at right angles, through a 10-nm band-pass dielectric filter, by an EM1 98 13 photomultiplier tube. The experiment was controlled by an IBM XT microcomputer-based data acquisition system using an interface constructed in our 1aborat0ry.l~ An experimental sequence consisted of firing the dissociating and probe lasers and triggering a PAR 162/ 164 boxcar integrator system which sampled the LIF signal. The PAR 164 gated integrator was operated in the linear summing mode, rather than the exponential averaging mode normally used in boxcar operations. The gate was triggered both before and after each probe laser pulse, with the output being digitized and reset after each gate pulse, thus providing a base-line signal with each LIF signal. Digitization of these signals was carried out on a shot-to-shot basis and the base line was subtracted from the LIF signal. This procedure eliminates errors due to dc offsets or low-frequency drifts. Shot-to-shot sampling and measurement of the dye laser output energy, using an Evans 4 130 Gated Integrator module, permitted accurate signal normalization. Photolysis laser intensities were observed to remain constant over the course of each run and thus were not used to normalize the LIF signal. After a given number of laser pulses, a stepping motor attached to the dye laser sine drive was advanced, thus scanning the probe laser wavelength to produce an LIF spectrum. BrCN was obtained from Fisher Scientific. It was placed in a cold finger and pumped on for a few minutes before being used. Results Figure 2 shows the LIF spectra for photolysis at 193,202,220, 240, and 248 nm. The spectra characteristic of 266 nm can be found in ref 12. At 193 and 248 nm, the pressure of BrCN was less than 1 mTorr. Otherwise the BrCN pressure was between 10 and 40 mTorr. The time between the photolysis and probe (13) Schinke, R. J. Chem. Phys. 1986,85, 5049. (14) McLaren, I. A., unpublished results.
--
3250 The Journal of Physical Chemistry, Vol. 91, No. 12, 1987
Russell et al.
P(0,O) P(0,'"
N'
w
30 50 30 50
60
6o N'
P(l,l)
rn7l-r
40 50 60
I
P(2,21 1
0
20
"
I
30
R (0,O)
40 Delay = 100ns
P = 20 mtorr 220 NM PHOTOLYSIS
.,
202 NM PHOTOLYSIS
Figure 3c
242nm
F i g u r e 36
Dtloy = 80 ns
Delay = 80 ns .
.
ROTATIONAL OUANTUM NUMBER 193 NM PHOTOLYSIS
Figure 3 e
Delay = 80 ns
1: I
r*
1
I
20
'0
40 60 80 ROTATIONAL QUANTUM NUMBER
Figure 3. Relative populations of the CN(X2Z+)vs. rotational quantum number N"obtained at several photolysis wavelengths. At 248 and 242 nm the populations for the u" = 0 and 1 levels are shown.
Delay = 100 ns
5
x i 0 4 9
Figure 2. Experimentally observed LIF spectra of nascent CN radical fragments following the photolysis of BrCN at several wavelengths. The R branch is shown only for the 248-nm spectrum.
lasers was 80 ns, with a maximum jitter of less than 10 ns so our P-t product varies between 80 and 3200 mTorrm. At each wavelength the pressure was decreased until the spectra that were observed did not change. We have also measured spectra following photolysis of BrCN at 230 nm and at several wavelengths between 242 and 240 nm using UV light generated by Raman shifting the output of a doubled Nd:YAG pumped dye laser. The spectra show that the C N is always formed rotationally hot, but significant vibrational excitation occurs only above 240 nm. The C N rotational populations in each vibrational level at the different photolysis wavelengths are shown in Figure 3. A smooth rotational distribution is observed at photolysis wavelengths below 230 nm. Above this wavelength, the observed rotational distribution is bimodal. The bimodal nature of the distribution is clearer at 248 nm than at 242 nm. At those photolysis wavelengths where the CN rotational distribution becomes bimodal there is also an increase in the observed population in excited vibrational levels. There are no published far-UV absorption spectra of BrCN. King and Richardson display only a schematic representation of the ~pectrum.'~Figure 4 shows the absorption spectrum of BrCN measured in a Beckman DU-5 spectrophotometer which has a resolution of I nm. The spectra were taken by placing crystals of BrCN in a I-cm cell and allowing the vapor to equilibrate. The absorption cross section was calculated from the absorbance by using the known vapor pressure of BrCN.16 The heats of formation of BrCN, CN(X2Z+), and Br(2P312) are 46.1 f 1.5, 101 & I, and 28.18 f 0.02 kcal/mol, respecti~ely.~ Thus the thermochemical limit for dissociation is 29 060 900 cm-l or 344 f 10 nm. The Br atoms can also be produced in the *PI,,, state, which lies 3678 cm-1 above the ground state.
*
(15) King, G . W.; Richardson, A. W. J . Mol. Spectrosc. 1966, 21, 339. (16) Handbook of Chemistry and Physics, 62nd ed.; Chemical Rubber
Co.: Boca Raton, FL, 1981; p D-174.
0 190
IO
240
nanometers
Figure 4. Absorption spectrum of BrCN at room temperature in a I -cm cell as measured by a DU-4 spectrophotometer.
The results obtained in this study can be used to calculate the average fraction of available energy that appears in the rotational, cf,), vibrational, and translational, energy of the C N fragment. Each of these quantities is calculated by using the following relationships:
u,),
G)
vt),
C([N(U)/NTlE(U)/E,",lI 1 c
[N(U,N'?/NTI
s o t ) E
E(N'? / E a v a ~ l !
(A) (B)
L.N
Cr,,,") = 1 - cr,, - U") (C) In eq A the sum is over the observed vibrational levels, N(u)/NT is the fraction of the molecules in a given u level, E(u) is the energy of the u level, and Eavallis the available energy. The E,,,,, is defined as the energy of the photon minus the heat of dissociation into the two fragments. In eq B the sum is over the observed rotational levels in each of the observed vibrational levels. The quantity N(v,N")/N, is the fraction of the observed radicals in u,N" level. These calculations are summarized in Table I. This table shows that at all wavelengths most of the energy appears as translational
The Journal of Physical Chemistry, Vol. 91, No. 12, 1987 3251
Photolysis of BrCN TABLE I: Energy Partitioning in the BrCN Photolysis” highest (most populated) wavelength, nm N” E,,,, cm-’ 248 242 220 202 193
62 66 68 77 84
(49) (52) (51) (56) (62)
Br(ZP,,*)
7421 (4655) 8402 (5236) 8915 (5039) 11411 (6065) 13566 (7421)
“Statistical: v) = 0.43; cf,) = 0.29;
cf,)
(f” )
0.55 0.61 0.70 0.70 0.67
0.03 0.05 =O 20 =O
W2P,/2) (fr
)
0.41 0.34 0.30 0.30 0.33
(f,)
0.34 0.45 0.61 0.64 0.61
(f” ) 0.05 0.07 =O =O =O
cf,) 0.61 0.48 0.39 0.36 0.39
Ur)= 0.29.
Photolysis Wavelength (nm)
I
266 @-
246 242
220
$2
A- Hiphest obaerved rotational energy
’5 /
”-I
163
$
Peak 01 the rotational dl(ltributlon curve
/A
/’
10,000 Available Energy, Ea
20,000
(cm-’)
Figure 5. Plots of the most populated and the highest observed rotational
state as a function of the available energy. The photolysis wavelength is indicated at the top of the figure. The point for 266 nm was taken from
ref 11. The bars represent estimates of error from the data. The dots refer to the peaks of the observed rotational distribution curve and the crosses the energy of the highest observed rotational states in the u” = 0 level. There is a discontinuity at abwt 230 nm. All of the points above 242 nm can be made to fall on the same straight line if the available energy is associated with the production of Br(ZP3/z)atoms while below 230 nm it is associated with the production of Br(*PII2). This is illustrated by the encircled crosses and dots. The dotted line then extends the original points. recoil energy between the two fragments. The fraction of the excess energy that appears in translation increases with increasing amounts of available energy, but the amount of energy that appears as rotation between the two fragments is relatively constant. The average fractional available energy distributed in each of the three modes for an imaginary “statistical triatomic””J8 are also included in Table I. The results show that the amount of available energy that appears in the vibration is far from the statistical expectation. Considerably more energy appears in the translational and rotational degrees of freedom than would be predicted by this idealized statistical model. A clue to the change that is occurring in the photolysis mechanism above 240 nm can be obtained by plotting, as in Figure 5, the total available energy following photolysis against the energy of the highest observed rotational level, as well as against the energy of the most populated rotational level. The results of Wittig’s group at 266 nmll are also included in this plot. In Figure 5, separate straight lines are obtained for each of these quantities in the two different photolysis wavelength regions. The shift to a new set of straight lines occurs at about 240 nm. All of the points can be forced to fall on the same line by assuming that photolysis at wavelengths above 2 4 0 nm results only in the formation of Br(2P3/2),while at wavelengths below this only Br(2P,/2)is formed. It is remarkable that both of these plots are linear because the C N radicals that are observed at the highest rotational energies in the photolysis at 248 and 266 nm can, on energetic grounds, only be produced with a B I - ( ~ P , / atom. ~) The radicals that are produced with the energy of the most populated rotational level (17) Bersohn, R. J . Phys. Chem. 1984, 88, 5145. (18) Levine, R. D.: Kinsey, J. L. Atom Molecule Collision Theory; Bernstein, R. B., Ed.: Plenum: New York, 1979; p 693.
Figure 6. Scaled rotational distributions from Figure 2. The scaling method is described in the Discussion. Briefly this involved decreasing the available energy by the electronicenergy of Br(2P,Iz)where suggested by the results of Figure 3.
could have been formed with either ground or excited Br atoms, yet merely by adjusting the available energy both plots can be made to follow a straight line. This suggests that below 240 nm the main channel for photodissociation is the production of Br(2p1,2). The transition from the production of ground- and excited-state Br atoms is not as clean as the above plots suggest. The rotational distributions in Figure 3 are clearly bimodal in this region so that at least between 2 4 2 and 248 nm both processes are occurring. The observed rotational state distributions are plotted in Figure 6 as a function of the ratio of rotational energy to the total available energy. The total available energy was chosen with the idea that the branching between ground- and excited-state Br atoms changes at the shorter wavelengths. A bimodal rotational distribution is observed at 248 and 2 4 2 nm, which correspcnds to the production of the 2P1i2state. If one subtracts the scaled distribution at 193 nm from those at 242 and 248 nm and then shifts the remainder to account proportionally for the 3678 cm-l of electronic energy in excited bromine, one is left with a second, overlying rotational distribution corresponding to the production of CN radicals and Br(2P1/2)atoms. In addition, if it is assumed that CN(U”=1) which are produced at 2 4 2 and 248 nm are associated only with the production of Br(2P3j2),then the rotational distribution of CN(u”= 1) are close to the high end of the CN(u”=O) bimodal rotational distribution, as illustrated in Figure 3, a and b. This indicates that most of the energy needed for vibrational excitation came from the translational degrees of freedom. The arguments given above suggest that the observed distributions are consistent with the following photolysis mechanism at wavelengths longer than 242 nm BrCN + hv Br(2P3/,) + CN(o”=O) (1)
-
3252
The Journal of Physical Chemistry, Vol. 91, No. 12, 1987 BrCN
+ hu
BrCN
+ hu
-
-
+ CN(c”=I) Br(2Pl,2) + CN(u”=O) BI(~P,,,)
(2) (3)
In the wavelength region between 193 and 242 nm the mechanism can be represented primarily by reaction 3 . Future experiments are needed to prove this mechanism and directly determine the branching between ground- and excited-state Br atom. These experiments are currently underway. The observed rotational distributions can be used to calculate a distribution of impact parameters. Impact parameters, b, are , and the rotational energy, related to the available energy, EaValI E R via, b=
[(mCN/mBr-CN)(ER/Ea\ail
(4)
where m , is the reduced mass of the molecule x and re the separation between the bromine atom and the center of mass of the C N radical. If the distribution of CN products as a function of is the same for all of the measured photolysis the ratio Er/Eavall wavelengths, the distribution of impact parameters must also be the same. The fragments are spinning apart at the same distance and with the same orientation of the bromine atom and the CN fragment. In other words, as far as the angular motion is concerned, the potential energy surface upon which the dissociation takes place is the same over this entire range of photolysis wavelengths. Referring to Figure 6, the value of b at the most populated rotational level is 0.042 nm, and the value for the maximum rotational level is 0.061 nm. This corresponds to an average Br-C-N bond angle of 70’ and a maximum bond angle slightly greater than 90’. The latter is probably just an expression of the fact that the forces between the separating particles push on the electron clouds of the fragments and not on the nuclei.
Discussion The general hope of the experimental dynamicist is to measure a distribution that will give insight into the excited potential energy surface of the molecule under investigation. It is clear that there is not a direct inversion from the observed distribution to the excited-state potential surface. Thus in the absence of a full ab initio calculation of the particular quantum mechanical photolysis problem, one is always confronted by the choice of the proper potential energy surface upon which to calculate trajectories leading to the final product state distributions. Recently there have been two theoretical g r o ~ p s ’ ~ ,that ’ ~ - have ~ ~ reported on studies that are directly concerned with the problem of inversion from an experimental distribution to an understanding of the excited-state potential surface. Waite et al.’9320have used classical trajectory calculations with analytical potentials for the ground and excited state to predict the rotational distributions for the CN photolytic product of ICN and ClCN. The ClCN calculations were the most sophisticated since they used an ab initio excited-state potential surface. The ClCN results of Waite and Dunlap20 give a theoretical rotational distribution that is in reasonable agreement with experiment,* which suggests that their assumption of strong coupling between the radial motion and the angular motion is a reasonable approximation of the real situation. The theory of Waite and Dunlap does not predict anything about the vibrational distribution. Shinke and his co-workers21~22 have performed theoretical calculations at three levels of accuracy in an effort to determine how strong coupling between the translational motion of the fragments and the internal motion affects the observed internal energy distribution of the fragments. When they used their classical methods, along with the excited-state surface of Waite and D ~ n l a pto, ~compute ~ the internal energy distribution of the (19) Waite, B. A.; Helvajian, H.; Dunlap, B. I.; Baronavski, A. P. Chem. Phys. Lett. 1984, l I 1 , 544. (20) Waite, B. A,; Dunlap, B. I . J . Chem. Phys. 1986, 84, 1391. (21) Hennig, S.; Engel, V.: Schinke, R. J . Chem. Phys. 1986, 84, 5444. (22) Schinke, R.; Engel, V . Faraday Discuss. Chem. Soc. 1986,82, paper 11.
Russell et al. C N fragments that result from the photodissociation of ClCN, they obtained even better agreement with experiment.22 In light of these results and the similarity between the observed CN rotational distributions obtained in the photolysis of ClCN and BrCN, we will use the methods of Shinke to qualitatively discuss the present results. More quantative discussion will have to await a b initio energy surfaces for BrCN. Vibrational Distribution. The spectra in Figure 3 clearly show that the population of vibrationally excited C N radicals is small below 242 nm, contrary to the results of Fisher et aL9 The most likely reason for this difference is the lower P-t product in the present work, which minimizes the effect of collisions on the distributions. At 242 nm and above, the vibrational population of excited C N radicals is much higher, in agreement with their results and the results of Nadler et al.” In the present work the is found to be 0.24, 0.41, and -0 ratio of the N(u”=l)/N(u”=O) at 248, 242, and 220 nm, respectively. By comparison, Fisher et al.9 measured a ratio of 0.33 and 0.21 at 249 and 222 nm which is higher than our ratio at comparable wavelengths. This discrepancy can probably be explained by their higher P-t product resulting in the formation of some additional vibrational excitation of the fragments by collisions. Both studies show that even when vibrational excitation is observed the vibrational population is much higher in the u” = 0 level than it is in the higher vibrational levels. When this happens, according to the “vibrational reflection principle”,2’ the coupling between vibrational and translational motion in the excited state is small. If the coupling between vibrational and translational motion is small, as the observed vibrational population of CN suggests, then one can conclude that final state interactions are small and are unlikely to result in vibrational excitation of the products. How can we explain the observed vibrational excitation of the product at the longer wavelengths? Nadler et al.Il observed vibrational excitation of the product at 266 nm, but the absorption spectra indicate that there is very little absorption at this wavelength. This result as well as the results reported in this paper suggest vibrational excitation is only observed in the wavelength region where the absorption is weak. In expansion-cooled BrCN,23 the amount of vibrational excitation remains the same in comparison with products obtained from room temperature samples.” BrCN has a symmetric stretch frequency that is only 580 cm-’ above the ground vibrational level so, at 300 K, -4% of the molecules are in this level and ~ 1 . 3 % are in the 110 level at 948 cm-’. In the expansion-cooled samples these levels will remain virtually unchanged since there are generally not enough collisions to cool these higher vibrational levels. If, the Franck-Condon factors are higher for a transition from one of these excited vibrational states to the excited electronic state, then, in the weak absorption region, a higher proportion of these molecules will be excited. The vibrational excitation that is observed in the C N radical is probably due to photodissociation of the 100 or the 110 modes. The relative importance of these modes compared to the 000 mode decreases with decreasing wavelength as excitation of the 000 mode becomes more Franck-Condon allowed. We conclude that the CN(u”=l) that is observed is due to photodissociation of BrCN(100) and/or BrCN(1 lo), while most of the CN(u”=O) comes from photodissociation of BrCN(000). Rotational Distribution. The theoretical work of Waite et al.19.20and S h i ~ ~ k eclearly ’ ~ . ~show ~ that the inverted rotational distributions that are observed in ClCN are due to strong coupling between rotation and translation in the excited state. Since the rotational distribution that is observed for the C N fragment from BrCN is very similar to that from CICN, it is likely that strong coupling also exists in this m ~ l e c u l e . ’Both ~ molecules have weak halogen-carbon bonds and similar reduced masses and one would expect that both molecules have bent excited states. Thus it is not surprising that the C N fragments have similar rotational distributions. The observed CN rotational distributions in Figure 2 are in qualitative agreement with the calculations of S ~ h i n k e . ’He ~ was (23) Shokoohi, F.; Hay, S.; Wittig, C . Chem. Phys. Lett. 1984, 110, 1
J . Phys. Chem. 1987, 91, 3253-3259 able to show that, with the same coupling parameter, the rotational distribution of the fragment would broaden and shift to higher N”va1ues as the available energy increases. The same type of effect is seen in the experimental distributions in Figure 2. The rotational distributions for the v” = 1 also exhibit the same type of behavior as the v” = 0 level. They in fact have similar impact parameters indicating that the same type of anisotropy is involved in the excited-state potential function. It appears that the difference that is observed in the vibrational energy is due to excitation of a slightly different region of the potential energy surface. If our interpretation of the branching between groundand excited-state atoms is correct, then even when the molecule dissociates to give Br atoms in the excited state the fractional energy that appears in rotation is about the same as it is when ground-state atoms are produced. All of these suggest that the anisotropy in the excited-state potential curve is the controlling factor for these rotational distributions. It also may mean that any differences that are observed is just the result of forming the excited molecule on slightly different parts of the excited potential surface. Given the potential curves of the ground and excited state one might expect to get quantitative agreement between theory and experiment.
Conclusions The nascent quantum state distributions of C N fragments produced in the photolysis of BrCN between 193 and 248 nm have been measured. The rotational distribution of the C N fragment broadens and shifts to higher energies with increases in available energy, which is in accord with recent theoretical prediction^,'^ based upon an excited-state potential surface that has a large coupling coefficient between the translational and rotational motion. At the lower energies and longer wavelengths, the observed rotational distribution for CN(v”=O) is bimodal, indicating that there may be a shift in the photolysis mechanism when the
3253
wavelength of photolysis is changed. We have postulated that this bimodality is due to the simultaneous production of Br(2P1,2) and Br(2P312)atoms though it may be due to other effects. Houston and his group have shown that in the 266-nm photodissociation of ICN a bimodal rotational distribution can be obtained via nonadiabatic interactions on a single surface.24 At the other wavelengths in the A continuum, where photodissociation has been studied, they needed at least three surfaces to reproduce all of the experimental data.25 It is clear from their work and the work reported here that additional studies will have to be done on BrCN to obtain a full understanding of the photodissociation process. The impact parameters for the photodissociation process were derived from the observed rotational distribution. The impact parameters are the same for all photolysis wavelengths between 190 and 266 nm if the bimodal distributions are identified with the production of Br atoms in the 2P,,2 and 2P3,2states. Vibrational excitation, contrary to rotation, decreases with available energy. The observed distribution indicates that there is weak coupling between vibrational and translational motion. The observed vibrational excitation has been explained in terms excitation of the hot bands of BrCN, namely those molecules in the 100 and/or the 110 state.
Acknowledgment. This work was supported by the Office of Naval Research. J.A.R. was supported by NASA under grant No. NAGW-446. I.A.M. acknowledges the support of the SOH I 0 Corp. J.H. was partially supported by N S F grant CHE8219255. We thank the referees and R:Schinke for their comments. (24) Goldfield, E. M.; Houston, P. L.; Ezra, G. S. J . Chem. Phys. 1986, 84, 3 120. ( 2 5 ) Marinelli, W. J.; Sivakumar, N.; Houston, P. L. J . Phys. Chem. 1984, 88, 6685.
Collisional Quenching of CH,O(A*A,) Paul J. Wantuck,* Richard C. Oldenborg, Steven L. Baughcum, and Kenneth R. Winn Chemistry Division, Los Alamos National Laboratory, Los Alamos. New Mexico 87545 (Received: September 10, 1986; In Final Form: December 29, 1986)
Room temperature rate constants for collisional quenching of CH30(AZA1)have been measured for 20 collision partners. Methoxy radicals are produced by 248-nm photolysis of C H 3 0 N 0 and the CH30(AZA1)is produced by exciting the A-X transition using a pulsed dye laser. Rate constants are determined by monitoring the time-resolved fluorescence signal as a function of quenching gas pressure. The experimental quenching cross sections are best correlated in terms of the CH,O-collision partner interaction well depth. Reasonable correlation is observed for 15 quenching gases, but several exceptions are apparent. A similar trend in collision partner quenching efficiency for OH(AZZf)and CH30(A2AI)is noted and discussed.
I. Introduction The methoxy radical (CH30) is known to be an important intermediate species in combustion reaction systems. In addition, photochemical reactions involving C H 3 0 are of importance to chemical processes in the atmosphere. Accurate, direct measurements of the reaction rate constants of methoxy radicals with various atmospheric and combustion system species are of particular interest and are generally not available. The reactions of NO,3 and NO: have been studied by the laC H 3 0 with 0 2 , 1 3 z
( I ) Gutman, D.; Sanders, N.; Butler, J. E. J. Phys. Chem. 1982,86, 66. ( 2 ) Lorenz, K.; Rhasa, D.; Zellner, R.; Fritz, B. Ber. Bunsen-Ges Phys. Chem. 1985, 89, 341. (3) Sanders, N.; Butler, J. E.; Pasternack, L. R.; McDonald, J. R. Chem. Phys. 1980, 48, 203.
0022-3654/87/2091-3253$01 S O / O
ser-induced fluorescence (LIF) technique. In all cases, the methoxy concentration was monitored by the fluorescence emission from the radical following excitation of A2A,-X2E vibronic transitions with a dye laser beam probe. The LIF technique represents a highly sensitive, state-selective method of monitoring the concentration of the methoxy radical. The sensitivity of the technique is a function of the radiative lifetime of the excited state, the quantum efficiency for fluorescence, and the rate at which the excited state is quenched by various collision partners. Many room temperature reactions involving the methoxy radical proceed at relatively slow rates and measurement of these reaction rates with an LIF technique requires high concentrations of reactant. (4) McCaulley, J. A.; Anderson, S . M.; Jeffries, J. B.; Kaufman, F. Chem. Phys. Left. 1985, 115, 180.
0 1987 American Chemical Society
