Laser ablation of sodium nitrate: NO desorption following excitation of theπ-π* band of the nitrate anion

Abstract

We have studied the laser desorption of NO from single crystal sodium nitrate following pulsed laser excitation of theπ* ← π2 absorption band localized on the nitrate anion. The excitation laser irradiance is maintained at low levels (< 2 MW/cm2) to obtain product distributions free of secondary interactions following desorption from the crystal surface. At low fluence, the photodesorption yield is found to be linear with desorption laser power indicating that single-photon absorption leads to fragment ejection. The desorption yield is enhanced by a factor of at least 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 tov″ = 4 is observed. Rotational state populations and translational energy distributions are well characterized by thermal distributions at the substrate temperature. A local excitation mechanism for NO desorption is proposed. The structural and electronic properties of NaNO3 were calculated using ab initio periodic Hartree-Fock theory to investigate the photochemical mechanisms involved in laser desorption of this material. The results of the electronic structure calculations will be discussed in context with the experimental results.