Time-dependent Raman spectra from two types of adsorption sites at Ag electrodes

Time-dependent surface-enhanced Raman scattering (SERS) form thiourea (TU) and ClO4- coadsorbed at Ag electrodes was investigated using a potential st ep. The intensity-time profiles for the different vibrational modes can be classified into two categories according to the response rate of the band i ntensity to the: potential step, i.e. a fast response within 5 s and a slow response up to 5 min. More interestingly, all band intensities show the fa st response characteristics after the electrode was made to undertake a neg ative excursion to the potential of -2.0 V. This phenomenon is explained in terms of co-existence of two types of adsorption sites (SERS active sites) , i.e., neutral Ag atom(s) and partial charged Ag atom(s). It is shown that time-dependent SERS combined the potential step method may be helpful not only for gaining a deeper insight into the SERS mechanism(s), but also for revealing the nature and structural dynamics of adsorption sites formed by strong chemical interaction. (C) 1999 Elsevier Science B.V. All rights rese rved.