Competition between electronic and vibrational predissociation dynamics of the HeBr2 and NeBr2 van der Waals molecules

Abstract

Direct measurements of the lifetimes of He79 Br2 and Ne79 Br2 B -state vibrational levels 10 ν′ ≤20 have been performed using time-resolved optical pump-probe spectroscopy. The values do not obey the energy gap law for direct vibrational predissociation. For both molecules, the dissociation rate for ′ =11 is much faster than for ν′ =12, and the ν′ =13 rate is also faster than is consistent with the energy gap law. We attribute this unexpected behavior to an electronic predissociation channel. Based on Franck-Condon factors between the Br2 B -state vibrational wave functions and the possible Br-Br product wave functions, we surmise that either the Br23Πg (1g) or (2g) state is responsible for the electronic predissociation. To our knowledge, this is the first time electronic predissociation and direct Δν=-1 vibrational predissociation have been observed to be in competition for a wide range of vibrational levels. As such, this problem deserves a detailed theoretical analysis. © 2010 American Institute of Physics.