MOLECULAR NO DESORPTION FROM CRYSTALLINE-SODIUM NITRATE BY RESONANT EXCITATION OF THE NO3- PI-PI-ASTERISK TRANSITION

We have studied the laser desorption of NO from single crystal sodium nitrate following pulsed 213-nm excitation of the pi* <-- pi(2) absorption band localized on the nitrate anion. The excitation laser flux is maintained at low levels (<2 MW/cm(2)) to obtain product distributions free of secondary interactions following fragment ejection from the crystal surface. 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 upsilon('') = 4 is observed and translational distributions for the upsilon('') = 0-3 levels are determined. Rotational state populations and translational energy distributions are well characterized by thermal distributions at the substrate temperature. A local excitation mechanism for NO desorption following resonant excitation is proposed. Under these experimetnal 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.