Ultraviolet photodissociation of bromoform at 234 and 267 nm by means of ion velocity imaging

Abstract

The photochemistry of bromoform is of considerable importance to understanding the impact of short-lived halogen species on bromine chemistry in the atmosphere. In the present work, the products of the ultraviolet photodissociation of bromoform at 234 and 267 nm are determined by time-of-flight mass spectrometry and velocity ion imaging. Both ground Br (2P3/2) and spin–orbit excited Br (2P1/2) atoms are found to be formed via resonance-enhanced multiphoton ionization detection. Radical products are detected via vacuum ultraviolet photoionization at 118 nm. The results indicate that there is a primary molecule bromine elimination channel consisting of CHBr+Br2. The quantum yields for atomic Br and molecular Br2 elimination channels are determined from the time-of-flight spectra to be 0.74 and 0.26 at 234 nm, respectively. At 267 nm, they are 0.84 and 0.16, respectively. Energy and angular distributions are deduced from the 2D images of Br, CHBr, and CHBr2. The direct studies described in this paper on the photodissociation of bromoform suggest that the current atmospheric photochemical models that do not anticipate the formation of Br2 need to be reinvestigated to determine their implications for atmospheric bromine chemistry.