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The utilization of infrared reflection-absorption spectroscopy for the in situ molecular characterization of monocrystalline metal-solution interfaces is outlined in comparison with the behavior of metal surfaces in ultrahigh vacuum (uhv) and illustrated for the adsorption of carbon monoxide on low-index platinum and rhodium surfaces in aqueous media. The effects of altering the electrode potential on the C-O stretching frequencies (vm) and terminal/bridging binding-site geometries are discussed in relation to similar spectral changes induced at metal-uhv interfaces by alterations in the local electric field and by the addition of dipolar and ionizable adsorbates. Quantitative links between these electrochemical and uhv-based phenomena are established by utilizing a common surface-potential scale. Interpretation of the various potential-induced spectral shifts is outlined in terms of alterations in the local electrostatic field and in the adsorbate-surface coordinate binding; the need to incorporate the latter phenomenon in order to account for the present experimental results is emphasized. The inherently coupled nature of the solvent dipolar and free charge contributions to the electrochemical surface potential is pointed out. The role of water and hydrogen coadsorption in the electrochemical systems is explored both on the basis of in situ infrared data and from related spectral observations at metal-uhv interfaces. Alterations in CO binding induced by metal coadsorption are briefly noted. The prospects for utilizing infrared spectroscopy to interconnect more generally the structural properties of related metal-solution and metal-uhv interfaces are discussed, along with the value of electrochemical systems for exploring fundamental issues relevant to both types of surfaces.

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