3105
J. Phys. Chem. 1982, 86, 3105-3114
reduces to eq 24. At 3000 A Hanson and Lee find kf N kp
(4p/$f)(kf/kisc)(kp + kd(Tlq + ~Q(A~H)(AcH)) (24)
5.0 X lo5 s-l; we estimated in this study that kist = 4.0 X lo8 s-l; Gill et al. report k, kd(T1~)N 4.4 X lo4 s-l and k Q ( A a ) N 2.3 X 10' L mol-' s-'. Thus,for the 3130-A data, k, = (4p/4f)(55 + 2.88 X 104(AcH))s-l. In a similar fashion we may derive from 3340-A data, k, = (+,/&) (75.86 + 3.97 X 104(AcH));here (AcH) is in mol L-l. We have used the 4f/$pratios reported by Parmenter and Noyes together with these relations to estimate k values from data at both 3130 and 3340 A (see Table VIIf). The data are reasonably consistent, and insofar as the assumptions made are correct, we find k,.= (2.0 f 0.4) X lo2s-l. The Parmenter and Noyes relation derived from 4, in pure acetaldehyde photolyses at 3340 A, 1/4, N 630 + 5.3PAcH(torr),gives further evidence of the magnitude of k,. Applying the conventional triplet formation and quenching mechanism of Table VI, we anticipate that the intercept in the above function should equal (k,. + kd(T10)/4,&, and with the slope N ~ Q ( A ~ H ) / & ~ , . Taking the recent estimate of k, + kd(T 0 ) from Gill et al. and 4isc these data &e k, 70 s-l and ~Q(A,) N 1.0 for 3340 N 7 X lo6 L mol-'s-l. Both estimates are about a factor of 3 lower than the seemingly more accurate estimates of k, N 2.0 X lo2 s-l derived here, and kQ(AcH)N 2.3 X lo' L mol-l s-l from Gill et al. However, these results also support the view that k, is much smaller than the value of k + kd(Tlo) (4.4 f 0.5) X lo4 s-', measured by Gill et al. b e suggest that the phosphorescence decay constant is controlled largely by the rate constant for triplet decomposition. If this interpretation is correct, then k, + kd(T1O) should show a marked temperature dependence
+
which reflects the high E, which we speculate must be associated with kd(T1O). We conclude that, if only the vibrationally relaxed triplet forms the HCO(O,O,O) radicals for the conditions of Gill et al., then the observed appearance time is consistent with their origin in Tio. Obviously further quantitative work on HCO appearance times at different temperatures is called for to test the current hypothesis. Process 11, unimportant at 3000 A, becomes a major primary process at the shorter wavelengths. It probably originates from higher vibrational levels of the excited singlet state as suggested by many Our data show that process III originates from a state different from process I. This observation and the apparent increased importance of H2 as a product at shorter wavelengths suggest that process I11 also originates from the higher vibrational levels of the excited singlet state. All kinetic information on the primary processes derived here and in previous work and our speculations on the mechanism of CH3CH0 photochemistry at 3000 8, are summarized in Table VI. Further studies of the wavelength dependence of the primary processes in acetaldehyde are underway and will appear in a subsequent publication. Acknowledgment. This work was supported by the US Environmental Protection Agency under the research grant R-806479-03 and the US-Israeli Binational Science Foundation grant No. 2509/81. We are grateful to the Mound Laboratories of the Monsanto Chemical Co. for the professional leave granted to Dr. Kershner which enabled him to participate in this research effort. We thank Dr. Charles S. Kan for his help with computer simulations in the tests of mechanisms and for the preparation of azomethane used in the actinometry.
Wavelength Dependence of the Primary Processes in Acetaldehyde Photolysis Abraham HorowRz' and Jack 0. Calvert' Department of Chemistry, The Ohio State Universny, Columbus, Ohio 43210 (Received: December 23, 1981)
The quantum yields of the gaseous products CO, CHI, and H2were determined in photolyses of acetaldehyde and its mixtures with O2 and C02with excitation at 2900,3000,3130,3200, and 3312 A and 25 f 2 "C. The results are interpreted in terms of the occurrence of three primary photodecomposition modes: CH3CHO*+ CH3 HCO (I); CH,CHO* CHI + CO (11);CH3CHO* H + CH3C0 (111). The quantum yields of these 411,c#J~~I, respectively) at low pressures are as follows: 2900 A, 0.887,0.063,0.076;300G A, 0.930, processes 0,011, 0.059; 3130 A, 0.924,0.018,0.055;3200 A, 0.470,0.079,0.00;3312 A, 0.051, 0.035,O.OO. The values are sensitive to pressure of added gases with the precursor of I less affected than that for I1 and 111. Present evidence suggests that I originates from a vibrationally excited-triplet state; I1 and I11 become increasingly important at the short wavelengths and may arise from the high vibrational levels of the first excited singlet. The present data, coupled with other published information, are used to estimate rate constants for the primary photodecomposition processes for acetaldehyde in the sunlight-irradiated, lower troposphere.
-
+
Introduction In our recent study of the photochemistry of acetaldehyde at 3000 A2 new evidence was presented for the nature and extent of the occurrence of three primary processes in acetaldehyde photolysis: CH3CHO* CHQ+ HCO (1) +
(1) Permanent address: Soreq Nuclear Research Center, Y a m , Israel.
0022-3654/82/2086-3105$01.25/0
-
CH,CHO* CH&HO*
-- + + CH4
H
CO
CH3CO
(11) (111)
We interpreted our results to suggest that the important process I occurs from a vibrationally rich, first excited triplet state of acetaldehyde. Processes I1 and I11 were much less important at 3000 A, and their origin was a state with different quenching properties, presumably the higher vibrational levels of the excited singlet state. A quanti0 1982 American Chemical Society
3108
Horowitz and Calvert
The Journal of Physical Chemisrry, Vol. 86, No. 16, 1982
TABLE I: Quantum Yiela Data from the 3130-A Photolyses of Pure Acetaldehyde and Its Mixtures with Oxygen and Carbon Dioxide" product quantum yields reactant press., torr CH4 cob -0, CO-CH, 2.74 0.297 (0.28) 0.489 (0.47) 1.65 (1.7) 2.71, 0.0479 (0.048) 0.273 (0.29) 0.484 (0.47) 1.77 (1.7) 5.25 0.0418 (0.041) 0.283 (0.29) 0.446 (0.43) 1.58 (1.5) 7.27 0.0360 (0.036) 0.291 (0.29) 0.443 (0.40) 1.52 (1.4) 13.2 0.0217 (0.022) 0.256 (0.28) 0.335 (0.30) 1.31 (1.1) 15.8 0.0149 (0.016) 0.220 (0.28) 0.331 (0.26) 1.51 (0.9) 17.7 0.0129 (0.011) 0.209 (0.27) 0.272 (0.22) 1.30 (0.8) 19.6 0.0083 (0.008) 0.181 (0.26) 0.234 (0.18) 1.29 (0.7) 20.2 0.193 (0.25) 0.247 (0.17) 1.28 (0.7) 1.50 2.32 0.041 (0.02) 0.853 (0.84) 5.92 21 (42) 1.53 3.10 4 5 (42) 2.80 7.06 36 (46) 5.00 11.7 4 2 (50) 4.82 13.1 0.029 (0.012) 0.551 (0.61) 8.14 1 9 (50) 0.574 (0.54) 9.45 > 2 9 (49) 5.00 15.6 6 9 (60) 2.80 340 7.23
