Photodissociation dynamics of CH2BrCl studied using resonance enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry

Abstract

The photodissociation dynamics of CH2BrCl have been studied using resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry. Polarization dependent time-of-flight profiles were collected for a range of wavelengths from 248 to 268 nm, corresponding to the red wing of the absorption spectrum. Forward convolution fits to the data have provided translational energy distributions and anisotropy parameters over the entire wavelength range for both Br(2P3/2) and Br*(2P1/2). The average translational energies for the Br and Br* channels are 20 and 23 kcal/mol, respectively. The measured anisotropy parameters indicate that both channels arise preferentially from a parallel transition and that the relative contribution of this transition increases with decreasing wavelength. Nonadiabatic transitions appear to play a smaller role in CH2BrCl dissociation than in its monohalogenated analogues, specifically CH3Br. We suggest that this difference is the result of the intrinsic Cs symmetry and lower radial velocity of CH2BrCl, and it is discussed in terms of a one-dimensional Landau–Zener model. A C–Br bond dissociation energy of 67.5 kcal/mol in CH2BrCl was also calculated using ab initio methods at the MP2/cc-pVtz//MP2/cc-pVdz level.