Correlated Product Distributions in the Photodissociation of à State NO-CH4 and NO-N2 van der Waals Complexes.

This paper reports correlated product distributions for dissociation of the van der Waals complexes NO-CH4 and NO-N2 on their à state surfaces, providing detailed data sets against which calculations can be benchmarked. NO-CH4 dissociation strongly favors small changes in the CH4 angular momentum, with ΔJ = 0 and 1 providing the bulk of the products. Conversely, the associated NO products show little constraint in terms of the rotational angular momentum transfer, with the full range of energetically accessible angular momentum states populated, although the distributions show minima. The lack of angular momentum transfer to methane accompanied by broad, structured, angular momentum transfer to NO gives the NO-CH4 dissociation some qualitative similarities to NO-Rg complex dissociation. In contrast, for NO-N2, the cluster of highest probability products corresponds to high N2 angular momentum and low NO angular momentum, with a sharp drop in the probability for populating the highest energetically accessible J states. For both the NO and N2 products, there appears to be a constraint limiting angular momentum transfer at the highest energetically accessible rotational states. Both complexes show product distributions that include a component attributed to excitation from warm complexes, which provides insight into their internal energies. Interestingly, for NO-N2, the 44,475 cm-1 photolysis translational energy release distribution for N = 8 extends to energies beyond those accessible from the highest bound X̃ states. This indicates either that there are long-lived (>100 µs) states above the X̃ state binding energy or that there is another mechanism that also contributes to this distribution.

Rotational and angular distributions of NO products from NO-Rg (Rg = He, Ne, Ar) complex photodissociation.

We present the results of an investigation into the rotational and angular distributions of the NO à state fragment following photodissociation of the NO-He, NO-Ne, and NO-Ar van der Waals complexes excited via the à ← X̃ transition. For each complex, the dissociation is probed for several values of Ea, the available energy above the dissociation threshold. For NO-He, the Ea values probed were 59, 172, and 273 cm(-1); for NO-Ne they were 50 and 166 cm(-1); and for NO-Ar they were 44, 94, 194, and 423 cm(-1). The NO à state rotational distributions arising from NO-He are cold, with most products in low angular momentum states. NO-Ne leads to broader NO rotational distributions but they do not extend to the maximum possible given the energy available. In the case of NO-Ar, the distributions extend to the maximum allowed at that energy and show the unusual shapes associated with the rotational rainbow effect reported in previous studies. This is the only complex for which a rotational rainbow effect is observed at the chosen Ea values. Product angular distributions have also been measured for the NO à photodissociation product for the three complexes. NO-He produces nearly isotropic fragments, but the anisotropy parameter, ß, for NO-Ne and NO-Ar photofragments shows a surprising change in sign from negative to positive as Ea increases within the unstructured excitation profile. Franck-Condon selection of a broader distribution of geometries including more linear geometries at lower excitation energies and more T-shaped geometries at higher energies can account for the changing recoil anisotropy. Two-dimensional wavepacket calculations are reported to model the rotational state distributions and the bound-continuum absorption spectra.

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

We report velocity map imaging measurements of the binding energies, D(0), of NO-Rg (Rg = He, Ne, Ar) complexes. The X state binding energies determined are 3.0 ± 1.8, 28.6 ± 1.7, and 93.5 ± 0.9 cm(-1) for NO-He, -Ne, and -Ar, respectively. These values compare reasonably well with ab initio calculations. Because the Ã-X transitions were unable to be observed for NO-He and NO-Ne, values for the binding energies in the à state of these complexes have not been determined. Based on our X state value and the reported Ã-X origin band position, the à state binding energy for NO-Ar was determined to be 50.6 ± 0.9 cm(-1).

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

We report an investigation of the dissociation of A 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 Franck-Condon factor associated with these highly excited states, which arises because of the quite different geometries in the NO-Ar X and A states. This effect is likely to arise in other systems with similarly large geometry changes.