Vibrational predissociation dynamics of the He79Br2 van der Waals molecule near the B state dissociation limit: Binding energies, lifetimes, and implications for the rare gas–halogen potential

Excitation spectra of the He79Br2 complex have been obtained for the B state vibrational levels v′=34–48 using the pump–probe spectroscopic technique. The Δv=−1 channel for vibrational predissociation is closed for levels v′=45 and higher. The B and X state van der Waals binding energies are determined to be 13.5±1.0 and 17.0±1.5 cm−1, respectively. Lifetimes for vibrational predissociation were found to range from 5.3 ps for v′=34 to 1.8 ps for v′=44. For v′=45 the lifetime increases to 3.8 ps due to the closing of the Δv=−1 dissociation channel. The lifetimes are consistent with the momentum gap law using a Morse parameter equal to 1.9±0.1 Å−1. The trend of increasing blue shifts as v′ increases reverses at v′=40. This appears to indicate that the He–Br interaction changes dramatically when the Br–Br separation becomes larger than 4.0 Å. Based on the results presented here, we have reevaluated previous data for the HeI2 molecule, and suggest that the ground-state binding energy be revised to 17.6±1.0 cm−1 from the currently accepted value of 18.8 cm−1.