Helium-induced electronic transitions in photo-excited Ba⁺-He_n exciplexes

The possibility for helium-induced electronic transitions in a photo-excited atom is investigated using Ba⁺ excited to the 6p ²P state as a prototypical example. A diabatization scheme has been designed to obtain the necessary potential energy surfaces and couplings for complexes of Ba⁺ with an arbitrary number of helium atoms. It involves computing new He-Ba⁺ electronic wave functions and expanding them in determinants of the non-interacting complex. The 6p ²P ← 6s ²S photodissociation spectrum of He⋯Ba⁺ calculated with this model shows very weak coupling for a single He atom. However, several electronic relaxation mechanisms are identified, which could potentially explain the expulsion of barium ions from helium nanodroplets observed experimentally upon Ba⁺ photoexcitation. For instance, an avoided crossing in the ring-shaped He₇Ba⁺ structure is shown to provide an efficient pathway for fine structure relaxation. Symmetry breaking by either helium density fluctuations or vibrations can also induce efficient relaxation in these systems, e.g., bending vibrations in the linear He₂Ba⁺ excimer. The identified relaxation mechanisms can provide insight into helium-induced non-adiabatic transitions observed in other systems.