D. BEHARAND RICHARD W. FESSENDEN
2752 levels must be mainly isomerization or possibly dissociation. One is forced to conclude that primary yields of isomers must be manyfold greater than the yields shown in Table VIII. Even simple assumptions account easily for factors of 3 to 5 but these are not adequate in cases where fluorescent and presumably also triplet-state yields approach zero. One of the most interesting questions raised by the present work relates to the ratio of rates of (19) and (21). In the case of benzene, Ishikawae" showed that excitation at 253.7 nm gave biacetyl decomposition corresponding to somewhat longer wavelengths but that no evidence for formation of 'BiAI could be found. Recent work by NakamuraZ0 and this research indicate that perhaps 30-40% of the collisions of lYI0 with biacetyl lead to 'BiAI and the rest to lBiAII. The theory of this phenomenon is not evident. Since emission resulting from collisions between triplet xylene molecules and biacetyl is too intense to give reasonable triplet-state yields at some incident wavelengths, it is necessary to conclude either that the
emitter is a complex or that for some reason the triplet biacetyl is formed in a state of higher emissivity than is produced by crossover from 'BiAI. This interesting phenomenon needs a careful study. Finally the quenching of singlet xylene emission by oxygen proceeds with a relatively high cross section, a phenomenon observed previously for benzene. 9 , 4 2 This matter is under investigation in other laboratories and will not be discussed in detail here. Presumably, singlet oxygen molecules are formed by collision between excited singlet aromatics and ground-state triplet oxygen. 42
Acknowbdgment. The authors wish to thank Drs.
I
