PHOTOCHEMISTRY AND DYNAMICS OF C6H6-O-2 CLUSTERS AT 226 NM

The photochemistry and dynamics of small C6H6-O-2 clusters were studie d in a supersonic expansion using 226 nm laser excitation and multipho ton ionization probes. We were able to detect a strong signal due to O (P-3(2)) when mixed clusters were present in the expansion but no O at om fragments could be observed in the absence of benzene in the expans ion mixture. Photofragmentation of O-2 in the unique environment of th e cluster is enhanced by at least three orders of magnitude compared t o the isolated oxygen molecule. The kinetic energy release of the O(P- 3(2)) was determined with a time-of-flight method and found to be rela tively small and characterized by a completely isotropic spatial distr ibution. The fine structure population of the O(P-3(j)) was also exami ned and the resultant branching fractions, P-2,P-1,P-0 = 0.68 +/- 0.03 , 0.26+/-0.06, 0.06+/-0.01, are similar to those obtained for photodis sociation of isolated O-2 by other workers. We also find that photoche mical production of oxygen containing products, such as C6H6O, becomes feasible in larger cluster species due to solvent cage effects which trap the recoiling O atom fragments. The observed dynamics can be attr ibuted to either excitation of the supramolecular C6H6-O-2 charge-tran sfer state, or localized excitation of a perturbed transition in O-2. The net effect of cluster absorption is to greatly enhance a chemical pathway that is only weakly observed in the separated molecules, simil ar to the behavior that has recently been described for the C6H6-I-2 c omplex. (C) 1997 American Institute of Physics.