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

Infrared reflection-absorption spectroscopic (IRAS) along with work-function measurements are presented for carbon monoxide and deuterated water co-dosed onto Pt(lll) at 110 K in ultrahigh vacuum (uhv) in comparison 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 ''uhv electrochemical modeling'' tactic mimics the vibrational spectral properties of the latter interface, and to exploit the: additional molecular structural information obtainable for the former surface configuration in elucidating solvation effects for the in situ electrochemical 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) surface, being replaced by bands at lower frequencies that are more characteristic of the CO coverage (theta(CO))-dependent upsilon(CO) features observed for the electrochemical interface. This ''titration-like'' spectral response, indicating that the added water assembles into hydrogen-bound islands which are nonetheless microscopically intermixed with the contiguous CO adlayer regions, is typically complete by water coverages corresponding to about 2-3 ''equivalent bilayers.'' Water dosage-dependent spectra in the O-D stretching (upsilon(OD)) region for zero or low predosed CO coverages display the hallmarks of markedly increased hydrogen bonding beyond the bilayer point, whereas such hydrogen bonding is prevalent even at submonolayer water dosages for intermediate or near-saturated theta(CO) values. The work-function decreases, -Delta Phi, induced by water dosing also are indicative of synergistic water/CO interactions, larger -Delta Phi values being obtained in the presence of low or intermediate predosed CO coverages than in its absence. A distinction is drawn between ''specific'' and ''nonspecific'' water co-dosing effects upon the CO adlayer structure; the former involves adjacent CO/D2O coadsorbate interactions, while the latter is describable 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 close concordance, the latter when the interfaces are compared at equivalent values of the surface potential. The findings thereby support a close structural connection between analogous low-temperature hydrated uhv and ambient-temperature electrochemical interfaces.