MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - SOLVENT EFFECTS UPON COVERAGE-DEPENDENT INFRARED-SPECTRA OF CARBON-MONOXIDE ON PLATINUM(111)

The influences of the solvating medium on a model electrochemical chem isorbate system-carbon monoxide on Pt(111)-have been explored by means of the infrared spectral responses in the C-O stretching (nu(CO)) reg ion to the dosage of various solvents onto CO adsorbed on Pt(111) in u ltrahigh vacuum at 100 K. Measurements were made for a range of preads orbed CO coverages (theta(CO)) as a function of the solvent dosage. Th e molecules selected-methanol, acetonitrile, acetone, benzene, and amm onia-span a range df solvating and dielectric properties. At low CO co verages (theta(CO) less than or equal to 0.25), even submonolayer solv ent dosages induced in most cases a near-complete shift in the CO bind ing geometry from atop to doubly bridging coordination, as seen from a displacement of the sharp nu(CO) band at 2090-2095 cm(-1) by a marked ly weaker feature at ca. 1790-1820 cm(-1). Additional (multilayer) sol vent dosing yielded essentially no further spectral changes, other tha n small additional frequency downshifts. Submonolayer ammonia dosage t riggered more substantial frequency decreases, yielding nu(CO) bands a t ca. 1640 and 1530-1565 cm(-1). At saturated (or near-saturated) CO c overages (theta(CO) similar to 0.65), solvent dosages yield only milde r less than or equal to (50 cm(-1)) frequency downshifts in the atop a nd bridging nu(CO) features that are present in the absence of solvent . The marked nu(CO) spectral changes occurring at low (and intermediat e) theta(CO) values are interpreted in terms of CO-solvent coadsorptio n, involving short-range dipolar interactions and ''through-metal'' ch arge polarization. The marked solvent-induced attenuation in the vco b and intensities seen at low theta(CO) are discussed in terms of dielec tric screening. The conventional dipole-coupling treatment is able to describe approximately the nature and solvent-dependent magnitude of t he effect, including the severe intensity attenuation observed with th e polarizable solvent benzene. The milder nu(CO) frequency downshifts observed at higher solvent dosages, as well as for high CO coverages, are consistent with Stark-tuning effects exerted by overlayer (rather than coadsorbed) solvent. Comparisons are made with in-situ infrared s pectra for corresponding electrochemical interfaces. Attention is call ed to the value of this infrared-based ''UHV electrochemical modeling' ' approach for elucidating interfacial solvation effects.