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 electro lytes on single-crystalline Pt(111)- and Pt(110)-electrode surfaces. F or clean and well-ordered Pt(111) electrodes, a single vibrational ban d between 2080 and 2150 cm(-1) with a nonlinear frequency dependence o n the potential is observed and assigned to the C=N stretching vibrati on of chemisorbed cyanide. A second band between 2145 and 2150 cm(-1) with very weak potential dependence appears on a surface which was sub jected to oxidation-reduction cycles and is attributed to cyanide asso ciated with a microscopically disordered surface. This assignment is s upported by preliminary results for a Pt(110) single-crystal electrode . On a well-ordered (110) surface a single and potential-dependent cya nide vibration between 2070 and 2112 cm(-1) is observed. After oxidati on of the cyanide and readsorption, this band is replaced by a higher frequency band at 2144 cm(-1) which is essentially not potential-depen dent. Occasionally, additional vibrational bands at lower frequencies not reported in corresponding IR studies are observed on Pt(111).