INFRARED-SPECTROSCOPY OF MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - ROLES OF ADSORBATE AND CATION DOUBLE-LAYER HYDRATION IN THEPT(111) CARBON-MONOXIDE AQUEOUS SYSTEM

Infrared reflection-absorption spectroscopic (IRAS) measurements are r eported for carbon monoxide and deuterated water codosed along with po tassium atoms onto Pt(111) at 95 K in ultrahigh vacuum (uhv) with the objective of elucidating the nature and roles of adsorbate and cation hydration on the electrode potential-dependent structure and bonding f or the analogous Pt(111)/CO aqueous electrochemical interface. This uh v-based ternary coadsorption system was chosen in view of the availabi lity of in-situ IRAS data for the electrode-solution interface at nega tive electronic charges, thereby enabling the validity of the ''uhv el ectrochemical modeling approach'' to be directly assessed. Varying the potassium dosage in the uhv system is analogous to charging the elect rochemical double layer since adsorbed alkali cations are formed along with the metal electronic charge. Variations in the metal-uhv surface potential attending alterations in the interfacial composition were e valuated with a Kelvin probe: besides yielding additional insight into surface solvation, the measurements provide the required link to the in-situ electrode potential scale. Indeed, decreasing the surface pote ntial by progressively increasing the K+ coverage at high water dosage s yields potential-dependent C-O stretching (v(CO)) bands for adsorbed CO that closely mimic corresponding IRAS data for the in-situ electro chemical interface. Infrared spectra in the O-D stretching (v(OD)) reg ion for D2O as well as the v(CO) bands are reported as a function of C O, D2O, and K+ coverage in order to explore how the combined and contr olled presence of the ions and solvent acts to modify the structure an d bonding of the electrostatic double layer and the chemisorbed CO. Th e solvent itself is found to exhibit a crucial effect upon the chemiso rbate at low CO coverages, whereas the solvated cations exert the larg est influence upon saturated CO adlayers. The addition of only small ( ca. 0.1 ML) water dosages is observed to attenuate severely the otherw ise dominant short-range K+/CO interactions as gleaned from the v(CO) spectra. Such primary hydrated cations, with stoichiometries K+-(D2O)( n), where n less than or equal to 5, exhibit characteristic v(OD) spec tra, which in the presence of only low (or zero) CO coverages are cons istent with a preferential water orientation with the hydrogens tilted toward (and H-bonding with) the metal surface.