SUM-FREQUENCY GENERATION AT ELECTROCHEMICAL INTERFACES - CYANIDE VIBRATIONS ON PT(111) AND PT(110)

We use optical sum-frequency generation to investigate the stretching vibrations of cyanide (CN-) molecules chemisorbed from aqueous electrolytes on single-crystalline Pt(111)- and Pt(110)-electrode surfaces. For clean and well-ordered Pt(111) electrodes, a single vibrational band between 2080 and 2150 cm(-1) with a nonlinear frequency dependence on the potential is observed and assigned to the C=N stretching vibration of chemisorbed cyanide. A second band between 2145 and 2150 cm(-1) with very weak potential dependence appears on a surface which was subjected to oxidation-reduction cycles and is attributed to cyanide associated with a microscopically disordered surface. This assignment is supported by preliminary results for a Pt(110) single-crystal electrode. On a well-ordered (110) surface a single and potential-dependent cyanide vibration between 2070 and 2112 cm(-1) is observed. After oxidation of the cyanide and readsorption, this band is replaced by a higher frequency band at 2144 cm(-1) which is essentially not potential-dependent. Occasionally, additional vibrational bands at lower frequencies not reported in corresponding IR studies are observed on Pt(111).