A collaborative theoretical and experimental study of the structure and electronic excitation spectrum of the BAr and BAr2 complexes

We report the investigation of the 3s<--2p transition in the BAr2 cluster. In a supersonic expansion of B atoms entrained in Ar, at high beam source backing pressures we observe several features in the fluorescence excitation spectrum which cannot be assigned to the BAr diatom. Using BAr(X,B) potential energy curves which reproduce our experimental observations on this molecule and an Ar-Ar interaction potential, we employ a pairwise additive model, along with variational and diffusion Monte Carlo treatments of the nuclear motion, to determine the lowest vibrational state of the BAr2, cluster. A subsequent simulation of the fluorescence excitation spectrum reproduces nearly quantitatively the strongest feature in our experimental spectrum not assignable to BAr. Because of the barrier in the BAr(B (2) Sigma(+)) potential energy curve, the 3s<--2p transition in the BAr2 is predicted to have an asymmetric profile, as is found experimentally. (C) 1997 American Institute of Physics.