ELECTROSTATIC-FIELD EFFECTS ON ADSORBATE BONDING AND STRUCTURE AT METAL-SURFACES - PARALLELS BETWEEN ELECTROCHEMICAL AND VACUUM-SYSTEMS

The observed effects of varying the surface potential, and hence the accompanying electrostatic fields, upon adsorbate bonding at ordered monocrystalline metals in electrochemical environments are summarized and discussed in comparison with field effects at related metal-vacuum interfaces for the specific case of carbon monoxide. The dependencies of the C-O stretching frequency, nu(CO), upon the electrochemical surface potential, E (and interfacial field, E), as evaluated for CO adlayers at a fixed coverage, theta(CO), and surface binding site are summarized as a function of theta(CO), the metal substrate, and the double-layer ionic environment, in order to illustrate the virtues of electrochemical systems for exploring such interfacial field effects. The increasing preference for multifold versus terminal CO binding commonly observed towards lower potentials (and at larger negative fields) is also noted. These findings are compared with related field-induced effects observed for metal-CO adlayers in vacuum and also for chargeable metal-carbonyl clusters in solution. Variations in field for the former systems are induced by postdosing dipolar or ionizable coadsorbates, or by applying an external electrostatic field. Some physical differences as well as similarities in the electrostatic fields present for such systems are noted. The nu(CO)-E dependencies observed for the different systems are compared briefly with the predictions of theoretical models which account for potential-dependent surface-adsorbate bonding and interfacial field-induced Stark effects.