Photodissociation of bromoform cation at 308, 355, and 610 nm by means of time-of-flight mass spectroscopy and ion velocity imaging

Abstract

The photodissociation dynamics of bromoform cation, CHBr3+, have been studied at 308, 355, and 610 nm by means of time-of-flight mass spectrometry combined with ion velocity imaging. The bromoform cation is produced via vacuum ultraviolet photoionization at 118 nm. The only fragment ions found in the time-of-flight spectra at 355 and 610 nm are CHBr2+ ions. At 308 nm CBr+ and CHBr+ ions are also found in the time-of-flight spectrum in addition to the CHBr2+ ions. These results indicate that there is only one dissociation channel that produces CHBr2++Br at both 355 and 610 nm, while there are two more channels involved at 308 nm. One is a molecular bromine elimination channel forming CHBr++Br2, and the other is a three-body dissociation channel producing CBr++HBr+Br. Translational energy and angular distributions for each channel are deduced from the two-dimensional images of CHBr2+, CHBr+, and CBr+. A soft fragment impulsive model closely reproduces the fraction of the available energy in translation for the atomic Br channel. The absence of Br2 channel at 355 nm may indicate a highly dynamically restricted gateway for the molecular elimination, which requires a specific molecular configuration, i.e., a three-center transition state, before the molecule falls apart along the much simpler atomic elimination channel.