Photodissociation of Methyl Iodide via Selected Vibrational Levels of the B̃ ((2)E3/2)6s Rydberg State.

Abstract

We have determined the I (2)P3/2 and (2)P1/2 branching fractions following the photodissociation of methyl iodide (CH3I) via a number of vibronic bands associated with the B̃ ((2)E3/2)6s Rydberg state at excitation wavelengths between 201.2 and 192.7 nm. Vacuum ultraviolet light at 118.2 nm was used to ionize both the product iodine atoms and the methyl radical cofragments, and velocity map ion imaging was used to determine the product translational energy distributions and angular distributions. The known relative photoionization cross sections for I (2)P3/2 and (2)P1/2 at 118.2 nm were used to determine the corresponding branching fractions. The results extend our earlier work at 193 nm by Xu et al. (J. Chem. Phys. 2013, 139, 214310), and complement the closely related work of González et al. (J. Chem. Phys. 2011, 135, 021102). We find that for most of the excited vibronic levels of the B̃ state studied, the I (2)P3/2 branching ratio is small, but nonzero, and that this channel is associated with internally excited CH3 radicals. The results are discussed in relation to the recent theoretical results of Alekseyev et al. (J. Chem. Phys. 2011, 134, 044303).