Exit channel dynamics in the ultraviolet photodissociation of the NO dimer: (NO)2→NO(A 2Σ+)+NO(X 2Π)

Abstract

The correlated angular and product rotational state distributions obtained in the 221.67 nm photodissociation of (NO)2 yielding NO(A 2Σ+)+NO(X 2Π) have been examined in the molecular beam using the velocity map ion imaging technique. The translational energy and angular distributions of selected rotational states of NO(A 2Σ+) products in N=0, 5, 6 for which the maximum energies available to the NO(X 2Π) products are 202.5, 142.5, and 118.5 cm−1, respectively, have been measured. The recoil anisotropy parameter of the photofragments, βeff, is 1.2±0.1, less than that previously measured at 213 nm (1.36±0.05). The correlated product state distributions near dissociation threshold agree with the predictions of phase space theory. These experimental results, as well as those obtained previously at 213 nm, are compared to statistical calculations, including v⋅J correlations. Application of the β-ET correlation model to the 213 nm results indicates that [NO(A,N),NO(X,J)] pairs with high NO(X,J) rotational levels are produced preferentially via planar dissociation, in contrast to the statistical expectation of the v⋅J correlation, which reveals no preference for planar dissociation. A mechanism involving vibrational predissociation with restricted intramolecular vibrational energy redistribution can explain both the observed scalar and vector properties. Specifically, the low frequency torsional (out-of-plane) mode does not couple efficiently to the other modes, especially at higher excess energies when the dissociation is rapid. On the other hand, the long-range attraction between NO(A) and NO(X), which is revealed both in the photodissociation dynamics of the dimer and in the quenching of NO(A) by NO(X), encourages long-range mode couplings and can explain the largely statistical rotational state distributions observed near threshold. From images obtained near threshold, the bond energy of the NO dimer in the ground state is determined to be 710±10 cm−1, in good agreement with previous results.