NO angular distributions in the photodissociation of (NO)2 at 213 nm:: Deviations from axial recoil

Angular distributions of selected rotational states of NO(A (2)Sigma(+),nu=0) products obtained in the 213 nm photodissociation of (NO)(2) have been determined in a molecular beam by using the photofragment ion imaging technique. Specifically, images of NO(A,nu=0) products in N=0, 11, and 19 have been recorded, for which the maximum energies available to the NO(X (2)Pi) products are 2038, 1774, and 1278 cm(-1), respectively. The recoil anisotropy parameter of the photofragments, beta(eff), decreases significantly at low center-of-mass translational energies from its maximum value of 1.36+/-0.05, and depends strongly on the rotational angular momentum of the photoproducts. This behavior is described well by a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. For each of the observed NO(A) N states, highly rotating NO(X) levels are produced via planar dissociation, and the angular momenta are established at an interfragment separation of about 2.6 Angstrom. For most of the center-of-mass translational energy range, both corotating and counterrotating fragments are produced, but at the lowest energies, only the latter are allowed. The correlated rotational energy distributions exhibit deviations from the behavior predicted by phase space theory, suggesting that exit-channel dynamics beyond the transition state influences the product state distributions. In this study, a new method for image reconstruction is employed, which gives accurate angular distributions throughout the image plane. (C) 2002 American Institute of Physics.