Blueshifts of the B←X excitation spectra of He79Br2 using a DIM-based potential

A first-order diatomics-in-molecule potential energy surface (PES) for the excited B state of the HeBr2 Van der Waals complex is implemented for the calculation of its B←X vibronic spectra at high vibrational excitations v of the Br2(B) fragment. The parameters of the PES, which should describe true He–Br interactions, are determined empirically. It is found that three-body interactions included in the model markedly change the topological properties of the PES with respect to a simple pairwise one. In particular, they are responsible for the decrease of the frequency shift at v>33 observed experimentally. Better agreement with the experimental spectrum is also attained at very high vibrational excitations where the effects of intramolecular vibrational relaxation are essential.