Ion Fragment Imaging of the Ion-Pair Photodissociation of CH3Cl, CH3Br, C2H5Cl, and C2H5Br at 118 nm

The ion-pair photodissociation R + + X - (R = CH 3, C 2 H 5 and X = Cl, Br) from their respective alkyl halides has been studied using ion photofragment imaging spectroscopy at 118.27 nm. From the structure of the R + images a fragment total translational energy distribution and an anisotropy parameter for angular distribution were derived. The anisotropy parameter β was 1.5 ± 0.4 (CH 3 Cl), 1.6 ± 0.3 (CH 3 Br), 1.9 ± 0.3 (C 2 H 5 Cl), and 1.7 ± 0.2 (C 2 H 5 Br), respectively. The fraction of total energy release going into translation was found to be relatively small, 13−21% for the molecules studied. The translational energy distributions for the methyl halides are well represented by Boltzmann distributions with temperatures of 1000 K, whereas those for the ethyl halides by Gaussian distributions peaking at 5−6 kcal mol -1 .