Ultrafast Non-Adiabatic Curve Crossing and Energy Transfer in the IBr B0+·Ar van der Waals Complex

Abstract

Photodissociation of the T-shaped IBr B0+·Ar complex initiated by an ultrafast laser pulse is investigated theoretically by integrating the time-dependent Schrödinger equation for wavepacket motion in two dimensions. The fragmentation dynamics are explored through calculations of time-dependent expectation values, electronic state populations and vibrational state probabilities. The focus of this study is on the competition between electronic predissociation of IBr, induced by non-adiabatic transitions between the bound B0+ and repulsive Y0+ excited states, and vibrational predissociation of Ar from the intact diatomic, mediated by vibrational energy redistribution between intramolecular and van der Waals degrees of freedom. This is facilitated by comparative calculations in which non-adiabatic curve crossing is either included or artificially disabled. It is found that molecular dissociation to form ground-state I + Br at energies below the B0+ asymptotic limit takes place over time scales ranging from 200-500 fs for vibrational levels that are strongly coupled to the Y0+ continuum, to longer than 10 ps for the most metastable levels. Sequential loss of vibrational quanta from IBr B0+ into the van der Waals mode over a 2-10 ps time scale is significantly disturbed by dissociation of the diatomic moiety: instead of time-ordered vibrational cooling via consecutively lower-energy levels, a complex pattern of energy transfer evolves during the lifetime of metastable B0+ vibrational levels until the Ar atom is propelled into the van der Waals continuum. The probabilities of final IBr B0+ vibrational levels are determined, therefore, by the strength of non-adiabatic transfer to the molecular dissociative continuum in addition to the dynamical coupling of intramolecular and van der Waals modes.