HeBr2 B,vÄ8]X,v9Ä0 excitation spectrum: Ab initio prediction and spectroscopic manifestation of a linear isomer

The B←X rovibronic excitation spectrum of the HeBr2 van der Waals complex is calculated using an ab initio potential energy surface for the ground electronic state. The coupled-cluster single double triple calculations predict double-minimum topology ~linear and T-shaped wells! for the X-state potential with a low isomerization barrier. The two lowest vibrational levels, assigned to T-shaped and linear isomers using the localization patterns of the corresponding wave functions, are almost degenerated and lie slightly above the isomerization barrier. This indicates that T-shaped and linear isomers can coexist even at low temperatures and give rise to two separated bands in the excitation spectrum. The main band of the B←X excitation spectrum is assigned to transitions from the T-shaped isomer, whereas the very good agreement between the observed and calculated spectrum, using the ab initio X-state potential, demonstrates that the unassigned secondary band corresponds to excitation of the linear isomer of the HeBr2(X) complex. The complete assignment of the spectrum in terms of individual rovibronic transitions is presented. © 2002 American Institute of Physics. @DOI: 10.1063/1.1502638#