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 simulations. The EEL spectra of pyromellitic dianhydride (PMDA) adsorbed on Cu(100) and Cu(lll) are reported. Simulations of the surface-enhanced Raman spectra and electron energy loss spectra (EELS) of pyromellitic dianhydride adsorbed on Cu(100) and Cu(lll) are reported. The surface enhanced Raman spectra [J. Chem. Sec. Faraday Trans. 92, 4775 (1996)] and the EEL spectra are shown to be sensitive to crystal face. The relevant excited state observed in the EEL spectrum is not intrinsic to molecular PMDA, but results from chemisorption. The Raman spectra are sensitive to the incident laser polarization on both the (100) and (111) surfaces but in different ways. These observations are shown to be a result of the excited state potential energy surface having different shape, and the respective transition dipole moment having a different orientation on the two crystal faces. The nuclear coordinate dependence of the electronic transition dipole moment produces mode selective, polarization dependent Raman scattering cross sections. Based upon this observation we conclude that the transition dipole moment function that couples the ground electronic state to the resonant excited electronic state is also sensitive to the structure of the metal surface. (C) 1998 American Institute of Physics.