ATOMIC AND MOLECULAR PHOTOSTIMULATED DESORPTION FROM COMPLEX IONIC-CRYSTALS

Calcium carbonate and sodium nitrate are isostructural crystals in whi ch the covalently bonded polyatomic anions are also isoelectronic, how ever, the chemical properties of these crystals are distinctly differe nt. In this study, we report results involving the photostimulated des orption (PSD) of neutral CO and O(P-3) products from geologic calcite (CaCO3) using low laser fluence at 193 nm. Product states are probed u sing (2+1) resonance enhanced multiphoton ionization for both neutrals . The CO products display a narrow angular distribution normal to the surface and a translational energy characterized, by a temperature (T = 110K) significantly lower than that of the substrate (T = 295K).(1) Atomic oxygen products display both broader angular and kinetic energy distributions than that of the CO fragments. Emission of O(P-3) is se en at angles greater than 30 degrees with respect to the surface norma l. In contrast, recent PSD studies of product NO from single crystal s odium nitrate (NaNO3) show product distributions with both thermal and hyper-thermal components.(2) Projected density of state plots compute d using periodic Hartree-Fock theory indicate a strong overlap in the metal and carbonate bands of the low lying excited states in CaCO3, wh ile NaNO3 was found to have a well-separated nitrate band in the excit ed state below the mixed metal/nitrate bands. The differences in the e lectronic structure of these materials may account for differences in the observed product distributions in the PSD of calcite and sodium ni trate.