Dynamical u ↔ g electronic state transitions in matrix photochemistry: Photodissociation of F₂ in solid Ar

The photodissociation of F₂ in a lattice model of 255 argon atoms is studied using Tully's semiclassical 'surface hopping' approach. The DIM method is used to model the 36 relevant potential energy surfaces and the nonadiabatic couplings between them. The photoexcitation is modeled by vertical promotion of the F₂ into the ¹Π_u state. It is found that early dynamics following excitation leads to rapid build-up of the g states population, and around t ≈ 50 fs the (total) g and u populations are roughly equal. Matrix-induced nonadiabatic transitions between u and g states cause the effect, and are briefly discussed.