SPECTROSCOPIC INVESTIGATION OF ADSORBED CYANIDE AND THIOCYANATE IONS ON PLATINUM BY SUM-FREQUENCY GENERATION

The vibrational properties of cyanide and thiocyanate ions adsorbed on a platinum electrode from a sodium perchlorate supporting electrolyte solution have been investigated with the aid of a method of nonlinear optics, viz., sum frequency generation (SFG). Two bands corresponding to stretching vibrations of both CN- ions and SCN- ions have been obs erved on the vibrational spectra. The first band, which appears at 20 70 cm-1 at a potential of -1 V in the case of both ions and whose posi tion is strongly dependent on the potential, has been assigned to adso rbed CN- and SCN- ions. The second band appears at -0.2 V when the pot ential is scanned in the positive direction. It is in antiphase to the first band and is located at 2150 cm-1 in the case of CN- and at 2140 cm-1 in the case of SCN-. Its position is weakly dependent on the pot ential. This finding has been attributed to the adsorption of CN- by m eans of the carbon atom and the adsorption of SCN- by means of the sul fur atom. Both bands are present in the spectra, if the electrode pote ntial is found in the range between 1 and 0.5 V during scanning, where a cyanide or thiocyanate bilayer forms on the electrode surface. The great difference in the degree of variation of the positions of the ba nd with the potential for these two bands corresponds to the great dif ference in the adsorption processes: substances which are attached to a surface by means of the nitrogen atom are more tightly chemisorbed o n a platinum electrode. Thus, SFG can serve as a powerful method for i nvestigating the vibrational properties of adsorbed substances at vari ous potentials.