INFRARED-SPECTROSCOPY OF MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - SOME IMPLICATIONS FOR IONIC AND CHEMISORBATE SOLVATION AT ELECTRODE SURFACES

The utility of infrared reflection-absorption spectroscopy (IRAS) for examining structure and bonding for model electrochemical interfaces i n ultrahigh vacuum (UHV) is illustrated, focusing specifically on the solvation of cations and chemisorbed carbon monoxide on Pt(111). These systems were chosen partly in view of the availability of IRAS data ( albeit limited to chemisorbate vibrations) for the corresponding in-si tu metal-solution interfaces, enabling direct spectral comparisons to be made with the ''UHV electrochemical model'' systems. Kelvin probe m easurements of the metal-UHV surface potential changes (Delta phi) att ending alterations in the interfacial composition are also described: these provide the required link to the in-situ electrode potentials as well as yielding additional insight into surface solvation. Variation s in the negative electronic charge density and, correspondingly, in t he cation surface concentration (thereby mimicking charge-induced alte rations in the electrode potential below the potential of-zero charge) are achieved by potassium atom dosage onto Pt(111). Of the solvents s elected for discussion here - deuterated water, methanol, and acetonit rile - the first two exhibit readily detectable vibrational bands whic h provide information on the ionic solvation structure. Progressively dosing these solvents onto Pt(111) in the presence of low potassium co verages yields marked alterations in the solvent vibrational bands whi ch can be understood in terms of sequential cation solvation. Comparis on between these spectra for methanol with analogous data for sequenti al methanol solvation of gas-phase alkali cations enables the influenc e of the interfacial environment to be assessed. The effects of solvat ing chemisorbed CO are illustrated for acetonitrile; the markedly larg er shifts in CO frequencies and binding sites for dilute CO adlayers c an be accounted for in terms of short-range coadsorbate interactions i n addition to longer-range Stark effects. The differing degrees of sel ective solvation of cations versus CO for water and acetonitrile are a lso explored. While the former solvent removes all specific effects of K+ upon CO, these short-range interactions partly remain upon acetoni trile solvation. The close connections between the UHV-based findings and the behavior of the in-situ electrochemical systems are also discu ssed.