ELECTRON-STIMULATED DESORPTION OF O- AND METASTABLE CO-ASTERISK FROM PHYSISORBED CO2

We report electron stimulated desorption (ESD) measurements of O- yields produced by dissociative electron attachment (DEA) to physisorbed CO2. The molecules are condensed at about 17-20 K on polycrystalline Pt, either as pure multilayer films, or in submonolayer (ML) quantities onto thick rare gas substrates. For the pure disordered multilayer solids, we observe four peaks in the O- yield function at incident electron energies, E(e), of about 4.1, 8.5, 11.2, and 15 eV. The lowest two are assigned, respectively, to the (2) Pi(u) and (2) Pi(g) resonance states of CO2-, which dissociate into O-(P-2)+CO(X (1) Sigma(+)), and are known to dominate the gas phase DEA O- production cross section for E(e)less than or equal to 20 eV. Measurements of ESD CO* metastable yields from similar CO2 multilayer solids on Pt(111), also presented here, suggest that the 11.2 and 15 eV O- peaks are associated with the manifold of close-lying CO2*(-) states which dissociate into O-(P-2)+CO* (a (3) Pi, a' (3) Sigma(+), or d (3) Delta)). For 0.15 ML of CO2 physisorbed on 20 ML thick rare gas substrate films significant sharp enhancements (fwhm less than or equal to 0.5 eV) are observed in the ESD O- yields at about 0.3-0.4 eV below the lowest substrate exciton energy. These enhancements are attributed to a coupling of the (electron plus exciton) core-excited anion resonances of the rare gas atoms to the dissociative Rydberg anion states of the coadsorbed CO2 at the solid's surface. This is followed by a transfer of the charge and excitation energy to the coadsorbate. (C) 1995 American Institute of Physics.