MODEL ELECTROCHEMICAL INTERFACES IN ULTRA-HIGH-VACUUM - SOLVENT-INDUCED SURFACE-POTENTIAL PROFILES ON PT(111) FROM WORK-FUNCTION MEASUREMENTS AND INFRARED STARK EFFECTS

The influence of various solvents upon the interfacial-potential profi le on Pt(111) has been investigated by means of work-function changes and infrared frequency Stark shifts attending sequential-molecular dos ing in ultra-high vacuum (UHV) at a suitably low temperature (ca. 100 K) with the primary objective of assessing the role of surface solvati on in related electrochemical systems. The solvents examined - dichlor omethane, benzene, acetone, acetonitrile, methanol, and ammonia - span a range of polarity and other solvating properties. These species wer e dosed onto both clean acid GO-saturated Pt(111), the Stark shifts be ing evaluated for the C-O stretching mode of terminally co-ordinated c arbon monoxide. Marked decreases (greater than or equal to 1 eV) in th e work function, Phi, and hence in the surface potential, phi, are obs erved on the addition of most solvents onto clean Pt(111). Milder yet still substantial Phi decreases are also observed for solvent dosage u pon CO-saturated Pt(111). These latter Phi changes correlate approxima tely with the observed nu(CO) frequency downshifts, suggesting that th e latter property is also sensitive to the solvent-induced electrostat ic interfacial field. The functional form of both the Phi decreases an d the corresponding nu(CO) frequency downshifts induced by solvent dos age provide insight into the dosage-dependent potential profile and it s relationship to both the monolayer and multilayer solvent structure. The present findings are also briefly compared with corresponding nu( CO)(t) - Phi data obtained for potassium atom dosing, where the surfac e potential is altered instead by varying the surface electronic charg e in the presence of a given solvent. The underlying factors responsib le for the surprisingly large solvent-induced surface potential shifts are discussed in detail, and the likely importance of the surface ele ctronic charge distribution as well as solvent dipole orientation and adsorbate-metal charge sharing is pointed out.