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

The photodissociation dynamics of CH2BrCl have been studied using resonance -enhanced multiphoton ionization with time-of-flight mass spectrometry. Pol arization dependent time-of-flight profiles were collected for a range of w avelengths from 248 to 268 nm, corresponding to the red wing of the absorpt ion spectrum. Forward convolution fits to the data have provided translatio nal energy distributions and anisotropy parameters over the entire waveleng th range for both Br(P-2(3/2)) and Br*(P-2(1/2)). 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 prefe rentially from a parallel transition and that the relative contribution of this transition increases with decreasing wavelength. Nonadiabatic transiti ons appear to play a smaller role in CH2BrCl dissociation than in its monoh alogenated analogues, specifically CH3Br. We suggest that this difference i s the result of the intrinsic C-s symmetry and lower radial velocity of CH2 BrCl, 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 calcu lated using ab initio methods at the MP2/cc-pVtz//MP2/cc-pVdz level. (C) 19 99 American Institute of Physics. [S0021-9606(99)00937-X].