Photodissociation via weak intersystem crossing: Incoherent versus coherent excited-state nonadiabatic dynamics in N2

Vacuum ultraviolet (UV) light at a well-defined wavelength excites N2 to different vibronic levels of the singlet electronic states that strongly interact through nonadiabatic coupling. Each discrete vibronic state acts as an isolated resonance: it is weakly coupled to a dissociative continuum of triplet states via a weak spin-orbit coupling. Here, we seek to compare this decay to that of a coherent superposition of bound singlets pumped by a broad in energy - ultrafast pulse. Despite the strong intersinglets and intertriplets nonadiabatic couplings, the coherent set of states decays as a mixture of the isolated vibronic states with essentially their individual lifetimes as determined separately in a sharp wavelength excitation. The vibrational quantum number of the vibronic states is a nearly good one when the spin-orbit coupling is as weak as is the case for N2. Numerically converged dynamical computations valid for longer times show that for nonrotating molecules the individual vibronic resonances overlap and interfere only upon an artificially order of magnitude increase of the strength of the spin-orbit coupling. The resonances strongly overlap only at an even stronger coupling to the dissociative continuum.