Vibrational Stark effect of CO on Ni(100), and CO in the aqueous double layer: Experiment, theory, and models

Abstract

The vibrational Stark effect (VSE), the effect of applied electrostatic field on a molecule’s vibrational frequency, is observed for adsorbates in ultrahigh vacuum (UHV) and in the electrochemical double layer. The VSE has been measured and compared with theory. Models of the double layer have also been compared with experiment. The vibrational Stark tuning rate of c(2×2) CO on Ni(100) at 300 K was measured using IR spectroscopy. The observed Stark tuning rate in terms of the applied E field δ_νE was (5.27±0.27)×10⁻⁷ cm⁻¹/(V/cm). Quantum mechanics has been used to express δ_νE in terms of a molecule’s potential energy function and dipole moment function. The quantum theory confirms the accuracy of δ_νE predicted by a classical model. The theory, with potential energy and dipole moment functions observed for the experimental system, gives δ_νE =(4.7±1.1)×10⁻⁷ cm⁻¹/(V/cm), in agreement with the present measurement. The fully resolved CO vibrational line shape was asymmetric with a 16 cm⁻¹ full width at half‐maximum. The measured IR cross section is consistent with a previous electron energy loss study of c(2×2) CO on Ni(100). Both experiment and theory find that the effect of applied E field on the IR cross section can be neglected in analyzing the electroreflectance spectrum. The Stark tuning rate in terms of the local E field is denoted δ_νe. The relationship between the applied E field, and the local E field acting on the adsorbed CO, was determined using the IR cross section. For CO on Ni(100), δ_νe was (1.42±0.31)×10⁻⁶ cm⁻¹/(V/cm), in good agreement with a previously reported ab initio calculation for CO on Cu(100). In comparing different systems, theory predicts δ_νe to be approximately proportional to the dynamic dipole moment, consistent with the observed trend. The VSE has also been observed previously for CO adsorbed on electrodes in electrochemical cells. The present measurement of δ_νe is compared with the VSE for CO terminally bonded to Pt, Pd, and Au electrodes in aqueous solution. A direct comparison between experiment in UHV and at the double layer can be made for Au. A model of the aqueous double layer proposed by Bockris, Devanathan, and Müller is consistent with observation. An alternative model, with the double layer potential across a CO monolayer, is also consistent with the data.