Electric Field Effect on Condensed-Phase Molecular Systems. III. The Origin of the Field-Induced Change in the Vibrational Frequency of Adsorbed CO on Pt(111)

摘要

The effect of an external electric field. oil the C-O stretch frequency, v(C-O), of carbon monoxide was studied for CO in different environments of condensed molecular films: (I) chemisorbed CO on Pt(111) covered with amorphous solid water (ASW), (II) CO trapped in an ASW matrix, (III) chemisorbed CO on Pt(111) covered with solid Ar, and (IV) CO trapped in a solid Ar matrix. Changes in nu(C-O) of these samples under an electric field were measured to investigate the Stark frequency shift and the effect of metal-adsorbate charge transfer on the frequency change. The electric field was applied up to 4.3 x 10(8) V.m(-1) using the ice film capacitor method. Reflection absorption infrared spectroscopy was used to monitor the spectral changes of the nu(C-O) band. The Stark shift was measured from the nu(C-O) change of isolated CO in the ASW matrix:under the field. The effect of Metal-adsorbate charge transfer was estimated for chemisorbed CO by measuring the nu(C-O) shift under the field and subtracting the electrostatic Stark effect. The electrostatic Static effect appeared with a Stark tuning rate of Delta mu = 0.64 +/- 0.04 cm(-1)/(10(8) V-m(-1)) for CO in the ASW matrix. The charge transfer effect on the frequency change had a sensitivity factor of Delta(nu) over bar/sigma approximate to 200 cm(-1)/C.m(-2) for chemisorbed CO on Pt(111), where sigma is the excess charge density of the Pt surface. From these observations, we suggest that in electrochemical experiments, where nu(C-O) of CO adsorbates on the electrode surface changes with the electrode bias potential, the frequency shift may result predominantly from the metal-adsorbate charge transfer rather than the electrostatic Stark shift.