The possibility for helium-induced electronic transitions in a photo-excited atom is investigated using Ba excited to the 6p P 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 P ← 6sS 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 HeBa 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 HeBa excimer. The identified relaxation mechanisms can provide insight into helium-induced non-adiabatic transitions observed in other systems.

Double ionization has been well studied in atoms and diatomic molecules. However, polyatomic molecules, with more complicated electronic structure, have not been studied as extensively.We use few cycle intense ultrafast laser pulses and coincidence velocity map imaging to investigate strong field double ionization in molecules such as CH 2 IBr and 1,3-Cyclohexadiene. By using a time stamping camera to make vector momentum measurements of electrons and ions, we are able to distinguish between multiple double ionization channels. Different double ionization channels which result in different fragment ion pairs show different electron correlation patterns, indicating that the double ionization dynamics are influenced by the orbitals from which the electrons are removed.