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 chemisorbate system-carbon monoxide on Pt(111)-have been explored by means of the infrared spectral responses in the C-O stretching (nu(CO)) region to the dosage of various solvents onto CO adsorbed on Pt(111) in ultrahigh vacuum at 100 K. Measurements were made for a range of preadsorbed CO coverages (theta(CO)) as a function of the solvent dosage. The molecules selected-methanol, acetonitrile, acetone, benzene, and ammonia-span a range df solvating and dielectric properties. At low CO coverages (theta(CO) less than or equal to 0.25), even submonolayer solvent dosages induced in most cases a near-complete shift in the CO binding 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 markedly weaker feature at ca. 1790-1820 cm(-1). Additional (multilayer) solvent dosing yielded essentially no further spectral changes, other than small additional frequency downshifts. Submonolayer ammonia dosage triggered more substantial frequency decreases, yielding nu(CO) bands at ca. 1640 and 1530-1565 cm(-1). At saturated (or near-saturated) CO coverages (theta(CO) similar to 0.65), solvent dosages yield only milder less than or equal to (50 cm(-1)) frequency downshifts in the atop and bridging nu(CO) features that are present in the absence of solvent. The marked nu(CO) spectral changes occurring at low (and intermediate) theta(CO) values are interpreted in terms of CO-solvent coadsorption, involving short-range dipolar interactions and ''through-metal'' charge polarization. The marked solvent-induced attenuation in the vco band intensities seen at low theta(CO) are discussed in terms of dielectric screening. The conventional dipole-coupling treatment is able to describe approximately the nature and solvent-dependent magnitude of the effect, including the severe intensity attenuation observed with the 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 spectra for corresponding electrochemical interfaces. Attention is called to the value of this infrared-based ''UHV electrochemical modeling'' approach for elucidating interfacial solvation effects.