A REMPI STUDY OF THE CORRELATION OF INTERNAL EXCITATIONS AND SUBSTRATE-ADSORBATE COUPLING FOR CO MOLECULES DESORBED BY ELECTRON-IMPACT

We compare vibrational, translational and rotational excitations of CO molecules desorbed by electron impact, and the yield of oxygen and ca rbon atoms from electron-induced fragmentation of CO molecules for: (I ) CO monolayers on bare transition metals [Ru(001) and Pt(111)], (2) C O monolayers coadsorbed with well-ordered oxygen atoms; (3) weakly bou nd CO monolayers on epitaxially grown silver films; and (4) CO monolay ers decoupled from the metallic substrate by mono-atomic xenon spacer layers. For all but the last system, we find CO molecules which are vi brationally extremely hot. This is explained by the excitation of stro ngly antibonding multi-electron states which are quenched in the vicin ity of the metal surface before enough translational energy is acquire d by the nuclei to complete dissociation. For CO/Xe/Ag(111), vibration ally hot CO molecules are missing: among the desorbing particles, wher eas strong fragment signals persist. Because of the isolating Xe layer , the substrate-adsorbate coupling is too weak to terminate the dissoc iation reaction which is induced by the electron impact before the rup ture of the molecular bond. (C) 1997 Elsevier Science B.V.