Production of I*(2P1/2) in the ultraviolet photodissociation of α -branched alkyl iodides

Abstract

Photodissociation dynamics of a series of α-branched alkyl iodides at excitation wavelengths of 222, 266, and ∼305 nm has been investigated by measuring the quantum yield (φ*) of I*(2P1/2) 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 α-branched homologs. On the contrary, I(2P3/2) is the major product at ∼305 nm for all the iodides. Assuming that I* originates from the 3Q0 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 3Q0 and 1Q1 surfaces for these two molecules has been estimated. At ∼305 nm, simultaneous excitation to the 3Q0 and 3Q1 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 α-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.