EXCITED-STATE PHOTOCHEMISTRY OF IODOALKANES

Unimolecular photodissociation of iodoalkanes in the C-3-C-8 size rang e is presented. The photolysis products are photoionized with coherent vacuum ultraviolet radiation and analyzed by time-of-flight mass spec trometry. With 248 nm excitation, the only primary reactions observed are those involving the C-I bond. For primary iodoalkanes, C-I bond cl eavage predominates. For secondary and tertiary iodoalkanes, a competi tive pathway leading to the elimination of hydrogen iodide becomes mor e favorable as the number of beta-hydrogen atoms increases. With 193 n m excitation primary reactions involving C-I, C-C, and in certain case s C-H bond cleavages are observed. As the molecular size increases, C- C bond cleavage occurs farther away from the C-I bond. Ground state (P -2(3/2)) excited state (P-2(1/2)) iodine atoms are produced by C-I bon d cleavage at both wavelengths, but 248 nm excitation shows a much hig her propensity for excited state iodine atom production. The hydrogen iodide produced by 248 nm photodissociation is internally excited sinc e it can be photoionized with radiation 0.7 eV below the ground state ionization energy. These results are consistent with rapid, selective dissociation of the C-I bond at 248 nm which yields nonstatistical ene rgy partitioning among the products. This selectivity persists as the molecular size increases. In contrast, excitation at 193 nm permits bo nd cleavage throughout the molecule and is consistent with a more rand om partitioning of the absorbed energy prior to dissociation.