Alkali halide photofragment spectra. II. Alkali bromide bond energies and excited state symmetries at 266 nm

Photofragment spectroscopy studies of the five alkali bromides at 266 nm show exclusive production of ground state 2P3/2 Br atoms in the photodissociation of LiBr and mixtures of ground and excited 2P1/2Br* atoms in the other bromides. Br*/Br branching ratios are 3.0, 2.5, 5.7, and 0.8 for NaBr, KBr, RbBr, and CsBr, respectively. From measured alkali atom recoil time-of-flight distributions the following bond energies (D00, kcal mol−1) are determined (all uncertainties ±1 kcal mol−1): LiBr-99.2; NaBr-85.9; KBr-89.7; RbBr-90.1; CsBr-92.1. The recoil angular distributions indicate that the parallel and perpendicular transitions leading to Br and Br* atoms have approximately equal intensities for the four heavier bromides, while for LiBr 29% of the dipole strength to the single exit channel Li+Br is parallel polarized. These results are discussed in the context of simple one-electron charge-transfer model for the optical excitations, and are compared with similar studies on the alkali iodides.