ELECTRONIC-STRUCTURE OF SODIUM-NITRATE - INVESTIGATIONS OF LASER-DESORPTION MECHANISMS

This theoretical study uses ab initio quantum mechanical methods to in vestigate the electronic properties of ground and excited state sodium nitrate. We calculated electronic properties of the crystalline mater ial for bulk, clean, and defected surfaces. The results of these calcu lations are used to explain the photoexcitation/desorption mechanism a nd support the conclusions of an earlier experimental investigation of the laser desorption of NO from single-crystal sodium nitrate. Ab ini tio periodic Hartree-Fock (PHF) theory was used to investigate the ''m olecular-ionic'' character of crystalline sodium nitrate. The calculat ions indicate that the electronic structure of the bulk and cleavage s urface are virtually identical (i.e., no shift in the resonant absorpt ion profile is expected). This finding is consistent with the experime ntal results on the wavelength dependence of NO desorption yields for crystalline NaNO3. However, changes in the absorption manifold are fou nd to accompany the removal of external nitrate oxygens (producing sur face nitrite groups). The presence of these chemical defects causes st ates to appear in the band gap producing a red shift in the absorption band. Complete active space self-consistent field (CASSCF) calculatio ns on NO3-, NO3, and NO2-, support the ''local excitation model'' of t he pi<-- pi(2) transition. These calculations also indicate that the transition energies of the nitrate ion are unaffected by the presence of the surrounding ions. The photoexcitation/dissociation mechanism of the nitrate ions in the crystal, however, differs from gas-phase proc esses in that the neutral photodetachment channel, observed in the gas phase, is energetically inaccessible in the solid due to the stabiliz ation of ionic species (relative to neutrals) by the crystalline field .