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Abstract:
Much work over the past several decades has demonstrated that the vibrational frequencies of adsorbates on metal surfaces in ultra-high vacuum (UHV) shift with changing surface coverage. Extensive theoretical efforts have shown that this shift can be quantitatively understood as the result of dipole-dipole interaction. A similar phenomenon is known from spectro-electrochemistry (the so-called electrochemical Vibrational Stark Effect (VSE)): the vibrational frequency of the adsorbate is observed to change as a function of applied external potential. Typically this potential-dependent change in adsorbate frequency is assumed to be linear (or piece-wise linear) with applied bias and thus changes in slope are assigned to changes in adsorbate structure or chemical reactions. Here we measure the change in spectral response of the (bi)sulfate anion adsorbed on a Pt(111) working electrode with changes in applied bias and quantitatively interpret the results using a microscopic model of the VSE that explicitly accounts for the effect of dipole-dipole interaction. Using this model we show that changes in external bias that cause a linear change in molecular polarizability can induce a nonlinear change in the center frequency and intensity of the spectral response of the adsorbed (bi)sulfate: the nonlinearity of the VSE depends both on the local field at potential of zero charge and also the relative change induced by external bias. These results imply that essentially all spectroelectrochemical studies that have attributed chemical significance to changes in slope of spectral parameters with applied bias should be revisited, as potential dependent dipole-dipole interaction can induce such changes independent of any structural or chemical change.
