CHARGE-TRANSFER EFFECT ON SURFACE-ENHANCED 2ND-HARMONIC GENERATION

Surface-enhanced second-harmonic generation (SESHG) arising from electrochemically roughened silver surface adsorbed molecules has been investigated. The repetitions of electrode potential sweep in the nonfaradaic region were observed to cause irreversible losses in the optical second-harmonic signal. The SESHG intensity profiles obtained in the presence of adsorbates were seen to depend on the electrode potential. We interpret the dependence of the SESHG intensities on the electrode potential as a charge-transfer effect. It is suggested that the resonant excitation of the charge-transfer transition enhances the second-harmonic generation intensities. A charge-transfer calculation is used to fit the data for the effect of electrode potential on the intensities. It is found that the second-harmonic intensity profile predicted in the calculation agrees very well with the experimental values.