The dissociation of NO-Ar (Ã) from around threshold to 200 cm ^-1 above threshold

We report an investigation of the dissociation of à state NO-Ar at energies from 23 cm^-1 below the dissociation energy to 200 cm ^-1 above. The NO product rotational distributions show population in states that are not accessible with the energy available for excitation from the NO ground state. This effect is observed at photon energies from below the dissociation energy up to approximately 100 cm^-1 above it. Translational energy distributions, extracted from velocity map images of individual rotational levels of the NO product, reveal contributions from excitation of high energy NO-Ar X̃ states at all the excess energies probed, although this diminishes with increasing photon energy and is quite small at 200 cm^-1, the highest energy studied. These translational energy distributions show that there are contributions arising from population in vibrational levels up to the X̃ state dissociation energy. We propose that the reason such sparsely populated levels contribute to the observed dissociation is a considerable increase in the transition moment, via the Franck-Condon factor associated with these highly excited states, which arises because of the quite different geometries in the NO-Ar X̃ and à states. This effect is likely to arise in other systems with similarly large geometry changes.