INFRARED-SPECTROSCOPY OF MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - ROLES OF SOLVATION IN THE VIBRATIONAL STARK-EFFECT

The effects of dosing various solvents along with potassium upon the i nfrared reflection-absorption spectra (IRAS) for saturated chemisorbed CO adlayers on Pt(111) at 90 K in ultrahigh vacuum (UHV) have been ex amined with the objective of assessing the roles of interfacial solvat ion upon the vibrational Stark effect (i.e., the adsorbate frequency-e lectrode potential dependence) for this archetypical Pt/CO electrochem ical system. The solvents overlayers chosen-water, methanol, acetonitr ile, acetone, and ammonia-used also in our earlier ''UHV electrochemic al modeling'' studies, span a range of polarity and other solvating pr operties. Potassium dosage, with coverages theta(K) less than or equal to 0.08, yields cations with the electron transferred Co the metal su rface and is therefore analogous to electrochemical double-layer charg ing. Kelvin-probe measurements of the work-function changes, Delta Phi , attending the addition of cations and solvent were undertaken so to evaluate the desired relationship between shifts in C-O vibrational fr equencies, Delta nu(CO), and the overall surface potential. Complete s olvation of the CO adlayer, usually requiring 1-2 solvent monolayers o n top of the chemisorbate, induced substantial and solvent-sensitive d ownshifts of the atop nu(CO) frequency and the work function (up to 50 cm(-1) and 1.5 eV, respectively). Addition of solvent to saturated CO adlayers with predosed K+ yielded profound changes in the nu(CO) spec tra. First, low solvent dosages, corresponding to solvent/cation stoic hiometries greater than or equal to 3, essentially eliminated the mark edly red-shifted nu(CO) band component due to short-range K+-CO intera ctions, apparently as a result of primary cation solvation. Additional solvent dosing red-shifted further the atop and bridging nu(CO) bands that are characteristic of longer-range cation-CO interactions, final ly yielding the simple linear Delta nu(CO)-Delta Phi, dependence famil iar for in-situ electrochemical interfaces. In contrast to tile additi on of solvent to cation-free interfaces, the nu(CO) frequency shifts i nduced by progressive cation addition in the presence of large solvent dosages are relatively insensitive to the nature of the solvent. Furt hermore, the Delta nu(CO)-Delta Phi dependences are essentially indepe ndent of the solvent for dosages, theta(s) similar to 2 monolayers, s ufficient to complete the nu(CO) spectral changes. This last finding i ndicates that much of the potential drop fails across the CO adlayer u nder these conditions, the solvent acting as a dielectric in screening the cation charge. Such simple solvent-independent Delta nu(CO)-Delta Phi behavior, which contrasts the complex spectral patterns seen upon alkalimetal addition in the absence of solvent, is in good agreement with the Delta nu(CO)-electrode potential dependences observed by in-s itu IRAS at the corresponding electrochemical interfaces. The likely m ultifaceted robs of the solvating medium in influencing the nature as well as magnitude of the electrochemical Stark effect are discussed in light of these findings.