Quantum dynamics of Ne–Br2 vibrational predissociation: The role of continuum resonances as doorway states

Abstract

Wave-packet simulations of the Ne-Br2(B,upsilon') vibrational predissociation dynamics in the range upsilon' = 16-29 are reported. The aim is to interpret recent time-dependent pump-probe experiments [Cabrera et al., J. Chem. Phys. 123, 054311 (2005)]. Good agreement is found between the calculated and the experimental lifetimes corresponding to decay of the Ne-Br2(B,upsilon') initial state and to appearance of Br2(B,upsilon<upsilon') products. The simulations show that up to upsilon' approximately 22 the dynamics is dominated by direct predissociation, while for higher upsilon' levels an indirect intramolecular vibrational relaxation mechanism of dissociation becomes increasingly important. Such a mechanism occurs via coupling of the initial state in the upsilon' vibrational manifold to nearly degenerate resonances embedded in the continuum of the lower upsilon<upsilon' manifolds, which act as intermediate doorway states to dissociation. The role of the intermediate resonances manifests itself in multiexponential behavior and oscillations in the time-dependent population curves associated with the initial complex state, the final product states, and the Ne-Br2(B,upsilon<upsilon') intermediate complexes. Analysis of the Ne-Br2(B,upsilon<upsilon') intermediate population shows that the continuum resonances are supported by centrifugal barriers involving excitation of the internal rotation of the complex. We find that the coupling between the intermediate state resonances and the continuum product state wave functions extend to Ne-Br2 distances greater than 15 angstroms. In the light of the results, a structure of the spectrum of continuum resonances is suggested and discussed.