  Raman signals from molecules adsorbed on a noble metal surface are enhanced
by many orders of magnitude due to the plasmon resonances of the substrate.
Additionally, the enhanced spectra are modified compared to the spectra of neat
molecules: many vibrational frequencies are shifted and relative intensities
undergo significant changes upon attachment to the metal. With the goal of
devising an effective scheme for separating the electromagnetic and chemical
effects, we explore the origin of the Raman spectra modification of
benzenethiol adsorbed on nanostructured gold surfaces. The spectral
modifications are attributed to the frequency dependence of the electromagnetic
enhancement and to the effect of chemical binding. The latter contribution can
be reproduced computationally using molecule-metal cluster models. We present
evidence that the effect of chemical binding is mostly due to changes in the
electronic structure of the molecule rather than to the fixed orientation of
molecules relative to the substrate.
