INFRARED-SPECTROSCOPY OF MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - THE ARCHETYPICAL CASE OF CARBON-MONOXIDE WATER COADSORPTIONON PT(111)

Infrared reflection-absorption spectroscopic (IRAS) along with work-fu nction measurements are presented for carbon monoxide and deuterated w ater co-dosed onto Pt(lll) at 110 K in ultrahigh vacuum (uhv) in compa rison with coverage- and potential-dependent infrared spectra for the analogous in situ Pt(111)-aqueous electrochemical interface. The twin objectives are to assess the manner and degree to which die former ''u hv electrochemical modeling'' tactic mimics the vibrational spectral p roperties of the latter interface, and to exploit the: additional mole cular structural information obtainable for the former surface configu ration in elucidating solvation effects for the in situ electrochemica l system. Postdosing water onto CO adlayers in uhv yields progressive attenuation of the C-O stretching (upsilon(CO)) IRAS bands associated with atop and twofold bridging coordination on the clean Pt(111) surfa ce, being replaced by bands at lower frequencies that are more charact eristic of the CO coverage (theta(CO))-dependent upsilon(CO) features observed for the electrochemical interface. This ''titration-like'' sp ectral response, indicating that the added water assembles into hydrog en-bound islands which are nonetheless microscopically intermixed with the contiguous CO adlayer regions, is typically complete by water cov erages corresponding to about 2-3 ''equivalent bilayers.'' Water dosag e-dependent spectra in the O-D stretching (upsilon(OD)) region for zer o or low predosed CO coverages display the hallmarks of markedly incre ased hydrogen bonding beyond the bilayer point, whereas such hydrogen bonding is prevalent even at submonolayer water dosages for intermedia te or near-saturated theta(CO) values. The work-function decreases, -D elta Phi, induced by water dosing also are indicative of synergistic w ater/CO interactions, larger -Delta Phi values being obtained in the p resence of low or intermediate predosed CO coverages than in its absen ce. A distinction is drawn between ''specific'' and ''nonspecific'' wa ter co-dosing effects upon the CO adlayer structure; the former involv es adjacent CO/D2O coadsorbate interactions, while the latter is descr ibable most simply by the changes in the surface potential. The theta( CO)-dependent CO site occupancies and upsilon(CO) frequencies for the hydrated uhv and electrochemical Pt(lll) systems are found to be in cl ose concordance, the latter when the interfaces are compared at equiva lent values of the surface potential. The findings thereby support a c lose structural connection between analogous low-temperature hydrated uhv and ambient-temperature electrochemical interfaces.