The photodissociation of methyl iodide at 229.4 nm: A determination of the fragment recoil anisotropy using energy-selective electron impact ionization and time-of-flight mass spectrometry

Abstract

Energy-selective electron impact ionization of laser-produced photofragments together with time-of-flight mass spectrometry is a general and sensitive means of studying primary photodissociation processes. Low-energy electrons ionize photofragments without the production of background fragment ions from dissociative ionization of the parent molecules, and the time-of-flight mass spectral peak shapes provide direct information on the photofragment recoil anisotropy. In the first application of this combination of techniques, we have studied the photodissociation of methyl iodide at 229.4 nm, the short-wavelength end of the A band, in order to assess the contribution of transitions to the 1Q1 state to the absorption profile. The results presented here show that I(2P1/2) is the primary iodine–atom product and that the transition is largely parallel [β=(1.6±0.1) for I(2P1/2)] at 229.4 nm. These data together with previous photofragmentation results suggest that excitation to the 3Q0 state dominates the entire A band absorption profile.