CHARGE-TRANSFER PROCESSES IN SURFACE-ENHANCED RAMAN-SCATTERING - FRANCK-CONDON ACTIVE VIBRATIONS OF PYRAZINE

SERS spectra of pyrazine on silver electrode have been recorded and an alyzed, assuming a charge transfer effect and using selection rules an alogous to those of resonance Raman. With the aim of predicting the ef fect of this mechanism on the selective enhancement of fundamentals, a method has been proposed based on an analysis of the results of geome try optimizations carried out by ab initio calculations. The strongest SERS bands coincide with those assigned to the normal modes connectin g the equilibrium geometries of the neutral molecule and the radical a nion. These results support the presence of a charge transfer process from the metal to the adsorbate in the SERS spectra of pyrazine where a significant enhancement of vibrations 8a, 9a, 1, and 6a can be obser ved. The prediction capability of the analysis proposed here has been checked with published resonance Raman spectra of pyrazine and pyrazin e-d(4). In all of the cases the strongest bands in the spectra are dir ectly related to the largest Delta Q values obtained through the trans formation Delta Q = L(-1)Delta R.