Production of I∗(²P_½) 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∗(²P_½) 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(²P_3/2) 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.