Production of I*(P-2(1/2)) in the ultraviolet photodissociation of Cu-branched alkyl iodides

Photodissociation dynamics of a series of alpha-branched alkyl iodides at excitation wavelengths of 222, 266, and similar to 305 nm has been investigated by measuring the quantum yield (phi*) of I*(P-2(1/2)) production. I* is found to be the major product at 222 nm and 266 nm from methyl and ethyl iodides but not from the higher alpha-branched homologs. On the contrary, I(P-2(3/2)) is the major product at similar to 305 nm for all the iodides. Assuming that I* originates from the (3)Q(0) state over the entire A-band, production of both I and I* in methyl and ethyl iodides at 222 and 266 nm is explained by invoking the curve-crossing mechanism in the upper state. The crossing probability (P) between the (3)Q(0) and le, surfaces for these two molecules has been estimated. At similar to 305 nm, simultaneous excitation to the (3)Q(0) and (3)Q(1) states remains a distinct possibility. For higher branched (i.e., i-propyl and t-butyl) alkyl iodides, the mechanism for I* production is qualitatively different from that of unbranched iodides. Coupling of alpha-carbon bending vibrational modes with the C-I bond excitation as well as the actual time spent in the excited state surfaces in i-propyl and t-butyl iodides seem to be the reasons for altering the dynamics of dissociation drastically in comparison with that of methyl iodide. (C) 1996 American Institute of Physics.