Adiabatic approximation and non-adiabatic effects for open-shell atoms in an inert solvent: F atoms in solid Kr Original Research Article

The dynamics of P-state F atoms in solid Kr is studied by molecular dynamics simulations in two frameworks: (i) The adiabatic approximation, in which nuclear motion is confined to the lowest adiabatic potential surface of the system; (ii) A method that treats semiclassically non-adiabatic transitions between electronic states in the course of the dynamics. The simulations deal with the spectroscopy of the F atom at different lattice sites, and with orbital reorientation dynamics due to the coupling with lattice vibrations. Also explored is migration of the F atom, following the preparation of an exciplex Kr+2F− which dissociates radiatively in the lattice. Some of the main findings are: (1) p-orbital reorientation dynamics on very short timescales (t ≲ 20 fs) is dominated by non-adiabatic mechanisms. Adiabatically, reorientation effects have timescales of the order of 0.25 ps or longer. (2) Lattice vibrations of particular symmetry types are particularly efficient in inducing p-orbital reorientation. (3) Dissociation of a Kr+2 F− exciplex can result in migration of the F atom into several lattice sites. The F atom spectroscopy for the different sites is different, and can be experimentally distinguished. (4) The migration probabilities of the F atom calculated adiabatically are much greater than the non-adiabatic ones. The results shed light on the coupling between electron orbital and nuclear dynamics for P-state atoms in solids.