ON THE CHEMICAL MECHANISM OF SURFACE-ENHANCED RAMAN-SCATTERING - EXPERIMENT AND THEORY

We have investigated the chemical mechanism of surface enhanced Raman scattering (SERS) on an atomically smooth metal surface using electron energy loss spectroscopy (EELS) and molecular spectroscopy simulation s. The EEL spectra of pyromellitic dianhydride (PMDA) adsorbed on Cu(1 00) and Cu(lll) are reported. Simulations of the surface-enhanced Rama n spectra and electron energy loss spectra (EELS) of pyromellitic dian hydride adsorbed on Cu(100) and Cu(lll) are reported. The surface enha nced Raman spectra [J. Chem. Sec. Faraday Trans. 92, 4775 (1996)] and the EEL spectra are shown to be sensitive to crystal face. The relevan t excited state observed in the EEL spectrum is not intrinsic to molec ular PMDA, but results from chemisorption. The Raman spectra are sensi tive to the incident laser polarization on both the (100) and (111) su rfaces but in different ways. These observations are shown to be a res ult of the excited state potential energy surface having different sha pe, and the respective transition dipole moment having a different ori entation on the two crystal faces. The nuclear coordinate dependence o f the electronic transition dipole moment produces mode selective, pol arization dependent Raman scattering cross sections. Based upon this o bservation we conclude that the transition dipole moment function that couples the ground electronic state to the resonant excited electroni c state is also sensitive to the structure of the metal surface. (C) 1 998 American Institute of Physics.