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

Abstract

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 X 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 X 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 FranckCondon factor associated with these highly excited states, which arises because of the quite different geometries in the NO-Ar X and à states. This effect is likely to arise in other systems with similarly large geometry changes.

The binding energies of NO–Rg (Rg = He, Ne, Ar) determined by velocity map imaging

“…3, the measurement uncertainties are contained within the experimental data points. 25) 44 287.0 ± 0.5 87.8 ± 0.5 88.1 ± 0.5 Present work 44 292.7 ± 0.9 93.8 ± 0.9 93.8 ± 0.9 a The authors report an appearance energy for the products rather than a dissociation energy.…”

“…In the cases of NO-He and NO-Ne, the values should also be viewed as lower limits since we cannot discount a similar influence from thermally populated levels for these two complexes. However, in this regard we note that the NO-Ne complex does not display the high energy "tails" seen in the NO translational distributions following dissociation of NO-Ar, 25,34 suggesting that the influence of higher energy, thermally populated levels is significantly diminished in this more shallowly bound system. In the case of NO-He, the well is so shallow that such a tail would be unobservable.…”

“…The apparatus has been described in detail as has the analysis of the images to produce translational energy distributions. 25,26 Those details pertinent to the present series of experiments are as follows. Gas mixtures of 5% NO in the rare gas were used to create the NO-Rg complexes via supersonic expansion.…”

“…The photolysis and probe laser systems are those previously described. 25 The probe laser was delayed by between 120 and 150 ns following the photolysis pulse to separate the probe-generated signal from the background signal arising from NO products ionised by the photolysis pulse.…”

Collisions of electronically excited molecules: differential cross-sections for rotationally inelastic scattering of NO(A2Σ+) with Ar and He

“…Direct spectroscopic investigation of excited state collision complexes is also possible, through the formation of van der Waals ground state complexes, and has been applied to the NO(A)-Ar system [30][31][32]. However, these measurements are only sensitive to those portions of the PES accessible through the Franck-Condon region.…”