PHOTOFRAGMENT IMAGING - THE 266-NM PHOTOLYSIS OF CD3I

Internal state and velocity distributions of methyl fragments from the 266-nm photodissociation of CD3I in a supersonic beam are measured by using photofragment imaging. Methyl radicals are state-selectively ionized by using (2+1) resonance-enhanced multiphoton ionization (REMPI) via the 3pz Rydberg state, and the imaging technique records the velocity distribution of the ions. A line-by-line rotational analysis of the 3pz ← X̃ 000 band, including conservation of the parent molecule's ortho:para ratio in the planar fragment molecules, indicates that about 90 cm-1 of rotational excitation about a C2 axis in CD3 results from the photolysis. The shape of the probability distribution for rotational energy excitation upon dissociation is extracted from the analysis of the 3pz ← X̃ 000 band. The photodissociation appears to conserve the initial (low) rotational excitation about the C3 axis of the (cold) parent molecule. From the previously derived 3pz- X̃ Franck-Condon factors the branching ratio for the v = 2/v = 0 levels of the umbrella mode is determined to be 1.15 ± 0.13. Propensity ratios for forming the selected fragment state via the ground-state iodine I(2P3/2) (I) or excited-state iodine I(2P1/2)(I*) channel are obtained from the images. I/I* ratios are obtained while resonant on the 000, 220, 213, 211, 222, 233, and 111 transitions. The I/I* branching ratio is found to increase rapidly with increasing vibrational excitation. For v2 v = 0, 1, 2, and 3 we find I/I* ratios of (0.04 ± 0.02), (0.09 ± 0.03), (0.27 ± 0.08), and (1.4 ± 0.2), respectively. © 1990 American Chemical Society.