Photodissociation of the dibromomethane cation at 355 nm by means of ion velocity imaging

The photodissociation dynamics of the dibromomethane cation, CH2Br2+, have been studied by means of ion velocity imaging and time-of-flight mass spectroscopy methods at 355 nm. The dibromomethane cation is produced through the direct ionization of the neutral molecule with a pulsed 118 nm laser. The translational energy distribution shows that the CH2Br+ fragment is formed in highly vibrationally excited states with two distinguished dissociation channels following a parallel excitation from 2b2 to 3b2 of the parent ion. The broad fast speed distribution is fit with two Gaussian functions, from which a branching ratio of Br*(2P1/2) to Br(2P3/2) is determined as 2.2:1. The sharp peak with very slow speed was modeled with a Boltzmann distribution with a temperature of 300 K. This channel contributes ∼4.5% to the reaction and is proposed to proceed on the ground state surface following internal conversion. Ab initio calculations for both the parent and the fragment ions have been performed that strongly support the proposed dissociation mechanisms.