Vibrational stark effect of adsorbates at electrochemical interfaces

Abstract

Electrochemical experiments find that an adsorbate's vibrational frequency ν varies as a function of electrode potential φ. This has been attributed both to the effect of φ on the molecule's bonding and to the vibrational Stark effect - the molecule's interaction with local electrostatic field F. Both theories are shown to describe the same effect. In application, however, they involve different approximations. For CO on Pt, (dν/dφ) with aqueous electrolyte is the same as (Δν/ΔΦ) in vacuum, where ΔΦ is the coadsorbate-induced change of the metal's work function. This is explained in terms of CO's bonding. With a nonaqueous electrolyte, the effect of cation size on (dν/dφ) is explained by the vibrational Stark effect. At low CO coverage in vacuum, a semiclassical theory accurately predicts (dν/dF). Ab initio calculations of (dν/dφ) are also discussed. For CO on two surfaces, Pt(111) and Pt(335), both (dν/dφ) with aqueous electrolyte and (dν/dF) in vacuum have been measured. If local F is assumed to be screened the same by the metal's electrons in electrolyte and vacuum, a comparison gives (dF/dφ) = 0.59 ± 0.07 and ∼ 0.85 Å-1 for Pt(111) and Pt(335), respectively. This is significantly different from the (dF/dφ) ∼ 0.28 Å-1 predicted by simple models of the double layer.