Electrochemical Stark tuning

Potential-dependent frequencies in spectra of adsorbates in electrochemical interfaces are commonly observed. Thus so-called Stark tuning rates, Qv/QE, of 30cm have been reported for adsorbed CO on platinum [65, 111] (Fig.59) and adsorbed CN on silver [109, 111, 162]. Even higher values were found for sulfate species adsorbed on polycrystalline platinum (100 cm V [36, 38]) or on single crystal Pt(lll) [141, 143] (120cm V ). In some cases, as for adsorbed tetra-cyanoethylene [163] and anthracene [164], vibrational features which are forbidden by the surface selection rule become active under the influence of the electric field at the interface. 

It is very interesting that the Stark tuning rate is coverage-dependent for adsorbed CO, in electrochemical media [55-57] as well as in vacuum [172]. It is found that at lower degrees of coverage the Stark tuning rate increases significantly. This depen-... 

In what follows, we propose a phenomenological model of the chemisorbed radical-anion standing in the electrochemical double-layer. We shall hence detail the reasons why this chemisorbed radical anion is intrinsically unstable but most probably has a finite lifetime on the polarized metallic surface. We outline the procedure through which we expect that an order of magnitude of the lifetime of the chemisorbed radical-anion may be evaluated numerically via this model. The model potential felt by the radical-anion as it is formed on the polarized electrode is described as the sum of three terms, for which a parametrisation is proposed. One of these terms is meant to include both the surrounding solvent and the repulsion by the polarized electrode, thanks to a mean, locally uniform, electric field. In the present paper, the intensity of this uniform electric field is calibrated on the basis of a conparison between experimental Stark-Tuning shifts for CO chemisorbed on palladium surfaces in solution and Density Functional Theory calculations of field-induced vibrational shifts for CO chemisorbed on palladium clusters. The shape of the resulting model potential is then discussed. 

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