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

We report electron stimulated desorption (ESD) measurements of O- yiel ds produced by dissociative electron attachment (DEA) to physisorbed C O2. 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 sol ids, we observe four peaks in the O- yield function at incident electr on 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 s tates of CO2-, which dissociate into O-(P-2)+CO(X (1) Sigma(+)), and a re 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 y ields from similar CO2 multilayer solids on Pt(111), also presented he re, suggest that the 11.2 and 15 eV O- peaks are associated with the m anifold 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 phy sisorbed on 20 ML thick rare gas substrate films significant sharp enh ancements (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 energ y. These enhancements are attributed to a coupling of the (electron pl us 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.