Correlated electron–nuclear motion during non-adiabatic transitions in LiH and its isotopomers

Abstract

Exciting molecules with a short strong few cycle IR pulse triggers coherent electronic–nuclear dynamics on multiple electronic states. This creates a new type of initial state where non-adiabatic transfer takes place between already populated electronic states. We explore this new feature using accurate quantal simulations of the ultrafast laser-induced dissociative dynamics in LiH-LiD-LiT series of isotopomers. In this novel type of initial state the non-adiabatic amplitude exchange between the coupled electronic states strongly depends on isotopic composition. The effective non-adiabatic coupling strength is modulated by two counteracting factors. On one hand, the 1/mass factor makes the non-Born–Oppenheimer coupling weaker for the heavier masses. On the other hand, the transfer is modulated by the momentum coupling of the two non-stationary nuclear wave packets on the two electronic states. The magnitude of this coupling tends to be larger for the heavier isotopes and varies in time as the wave packet dynamics unfold. The correlation in motion of the nuclear wave packets in R- and k-space is then critical. The direction of population transfer changes in time according to the evolution of the coherences built during the short fs excitation. In addition, heavier isotopomers move slower through the non-adiabatic interaction allowing a longer effective duration for the transfer. As a result, an initially very moderate isotope effect in the immediate post excitation population rapidly grows when the wave packets transverse the non-adiabatic coupling region. We compare and contrast with similar time dependent effect of the mass in the non-adiabatic transfer in N2 where the optically accessible singlet electronic states are all bound and the couplings are quite different.