Electric field induced change of adsorbate vibrational line strength

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Abstract
Electric field E changes an adsorbate’s vibrational line strength S. The dependence of S on E is described by δSE=d ln S/dE. At an electrode in an electrochemical cell the dependence is described by δSΦ=d ln S/dΦ, where Φ is electrode potential. The observed effect for CO on Pd(100) and Pt(110) electrodes in aqueous electrolytes is δSΦ=−0.135±0.086 and −0.135±0.041 V−1, respectively. The corresponding δSE for CO on Pd(100) is (−4.8±3.1)×10−9 cm/V, consistent with Bagus and Pacchioni’s ab initio calculation of −4.3×10−9 cm/V. A semiclassical explanation of δSE is given. The semiclassical δSE for CO and CN− is approximately proportional to the polarizability derivative (dα/dQ). For free CO oriented along the field direction, the semiclassical δSE=(−6.2±0.8)×10−9 cm/V. Three previous ab initio calculations for free CO gave δSE=−6.5×10−9, −4.6×10−9, and −3.6×10−9 cm/V. Experimental studies of CO on Ni(100), Pt(110), and Pd(100) indicate that (dα/dQ) of CO on these surfaces is increased relative to (dα/dQ) of free CO by factors of 6.0±5.5, 2.6±1.8, and 1.7±0.6, respectively. These values are consistent with Raman studies of CO on Ni(100) and on Pd films. Electrochemical studies of CO and CN− at polycrystalline electrodes are also discussed.

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