Molecular NO Desorption from Crystalline Sodium Nitrate by Resonant Excitation of the NO3- .pi.-.pi. Transition

Abstract

We have studied the laser desorption of NO from single crystal sodium nitrate following pulsed 213-nm excitation of the {pi}{sup *} {yields} {pi}{sub 2} absorption band localized on the nitrate anion. At low fluence, the NO photodesorption yield is found to be linear with desorption laser power indicating that single-photon absorption events lead to fragment ejection. The desorption yield is enhanced by roughly a factor of 1000 for resonant excitation (213 nm) over nonresonant excitation (266 nm) on a per-photon basis. We determine the relative vibrational, rotational, and translational energy distributions of the neutral NO photoproducts. Significant population in vibrational levels up to v{double_prime} = 4 is observed and translational distributions for the v{double_prime} = 0-3 levels are determined. A local excitation mechanism for NO desorption following resonant excitation is proposed. Under these experimental conditions the resonant desorption process (213 nm) is dominated by the photochemistry of the surface nitrate ions. A model for the absorption/dissociation mechanism is proposed that differs from that reported for gas phase ions in that it accounts for the stabilization of the ions due to the crystalline field. The role of exciton migration following resonant excitation is also discussed. 42 refs., 7 figs.