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Quantum yields of acetyl radicals resulting from the photodissociation of 0.1-1% acetone in air were studied in the wavelength range 280-330 nm and at total pressures ranging from 130 to 1000 hPa. Through the addition of small amounts of NO 2 to the gas mixtures, acetyl peroxy radicals were scavenged to form peroxyacetyl nitrate (PAN). The PAN quantum yields obeyed a Stern-Volmer type pressure dependence at all wavelengths studied. The experimental data were combined with results from the literature to derive the rate of photodissociation from the excited singlet state of acetone and the competing intersystem crossing rate populating the triplet state, both relative to that of pressure quenching, as a function of energy above the dissociation threshold. The rate of photodissociation was found to increase with excitation energy, whereas that of intersystem crossing shows a maximum at intermediate energies. Previously reported quantum yields for CO formation were analyzed to allow comparison with acetyl radical production.

Page 9436. The quantum yield for the photodissociation of acetone in air is subject to a Stern-Volmer type pressure quenching, which increases with increasing wavelength in the spectral region 280-337 nm. 1,2 By combining our own data with those of Gierczak et al., 1 we were able to show 2 that the quenching coefficient k M /k D , that is, the ratio of the rate coefficients for collisional quenching by carrier gas molecules M and dissociation of the photoexcited acetone molecule declines exponentially with excitation energy above the dissociation threshold. In the semilogarithmic plot chosen to demonstrate this behavior, all the existing data fell reasonably well on a straight line except our point at 330 nm. The experiments had utilized the yield of peroxy acetyl nitrate (PAN)sderived from CH 3 CO + O 2 in the presence of NO 2 as scavengersto determine the photodissociation quantum yield. At 330 nm, in contrast to experiments at other wavelengths, the degree of NO 2 photodissociation relative to that of acetone was undesirably high, because of the low absorption cross section of acetone. Photodissociation of NO 2 leads to side reactions, for example, the reaction of NO with CH 3 O 2 (formed from CH 3 + O 2 ), which is followed by further reactions that ultimately generate OH radicals. Therefore, the experimental data at 330 nm had required corrections to account for the additional reactions. Corrections were made by means of computer calculations based on a rather extensive reaction mechanism given in our paper. We have recently recognized that the mechanism is incomplete, in that it disregarded the reaction of acetone with NO 3 radicals, which also are a byproduct of NO 2 photolysisThe rate coefficient for this reaction, k c ) 8.5 × 10 -18 cm 3 molecule -1 s -1 , is fairly small, 3 and for this reason we had omitted the reaction. Moreover, most of the NO 3 radicals associate with NO 2 to form N 2 O 5 , and the equilibrium set up between NO 3 and N 2 O 5 favors the latter species. The only loss reaction included previously for NO 3 radicals was that with NO. Owing to the rather high concentration of acetone, however, reaction c must be included in the mechanism. Consequently, it was added to the list of reactions and new calculations were made. Also included in the mechanism was the formation of acetonyl peroxy nitrate by the reaction sequence where reaction d was assumed to be rapid, and for the forward and reverse pathways of reaction e the known rate coefficients k e ) 6.4 × 10 -12 cm 3 molecule -1 s -1 and k -e ) 10 s -1 were used. 4,5 The results of the calculations show that reaction c has indeed an effect on the dissociation quantum yield and its pressure dependence, which determines the quenching coefficient k M / k D . On the other hand, the (temporary) formation of acetonyl peroxy nitrate has no significant influence, because the comparatively long irradiation times favor decomposition. The calculations lead to the following corrected linear correlation for the inverse (corrected) quantum yield at 330...

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