P. Kambhampati et al., ON THE CHEMICAL MECHANISM OF SURFACE-ENHANCED RAMAN-SCATTERING - EXPERIMENT AND THEORY, The Journal of chemical physics, 108(12), 1998, pp. 5013-5026
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.