ELECTRIC-FIELD INDUCED CHANGE OF ADSORBATE VIBRATIONAL LINE STRENGTH

Electric field E changes an adsorbate's vibrational line strength S. The dependence of S on E is described by delta-SE = d ln S/dE. At an electrode in an electrochemical cell the dependence is described by delta-S-PHI = d ln S/d-PHI, where PHI is electrode potential. The observed effect for CO on Pd(100) and Pt(110) electrodes in aqueous electrolytes is delta-S-PHI = -0.135 +/- 0.086 and -0.135 +/- 0.041 V-1, respectively. The corresponding delta-SE for CO on Pd(100) is (-4.8 +/- 3.1) X 10(-9) cm/V, consistent with Bagus and Pacchioni's ab initio calculation of -4.3 X 10(-9) cm/V. A semiclassical explanation of delta-SE is given. The semiclassical delta-SE for CO and CN- is approximately proportional to the polarizability derivative (d-alpha/dQ). For free CO oriented along the field direction, the semiclassical delta-SE = (-6.2 +/- 0.8) X 10(-9) cm/V. Three previous ab initio calculations for free CO gave delta-SE = -6.5 X 10(-9), -4.6 X 10(-9), and -3.6 X 10(-9) cm/V. Experimental studies of CO on Ni(100), Pt(110), and Pd(100) indicate that (d-alpha/dQ) of CO on these surfaces is increased relative to (d-alpha/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.