SITE-SPECIFIC BRANCHING RATIOS FOR H ATOM PRODUCTION RESULTING FROM THE 193-NM AND 248-NM PHOTOLYSIS OF 1-IODOPROPANE

This work systematically explores "site-specific" H atom production in 1-iodopropane photolysis experiments conducted under collisionless conditions. H and D atoms are used as labels to investigate the carbon site at which C-H (or C-D) bond cleavage occurs in the following series of selectively deuterated 1-iodopropanes: ICD2CH2CH3, ICH2CD2CH3, and ICH2CH2CD3. By measuring relative H/D ratios via two-photon (121.6 + 364.7 nm) ionization, competition among alpha-, beta-, and gamma-positions with respect to carbon-hydrogen bond cleavage is quantitatively compared. For excimer laser photolysis at 193 nm, no dominant site is identified. In contrast, 248-nm excimer laser photolysis clearly results in preferential bond cleavage at the beta-carbon site, the estimated contributions being 0.2 +/- 0.1, 0.7 +/- 0.1, and 0.1 +/- 0.1 for the alpha, beta, and gamma-sites, respectively. Power dependence studies suggest that a two-photon mechanism contributes significantly to this site-specific behavior. A critical step appears to be 248-nm photon absorption by an intermediate, quite possibly the propyl radical.