On the role of electromagnetic field gradients in surface Raman scattering by molecules adsorbed on single crystal metal surfaces

Bands normally forbidden in the vibrational Raman spectra of molecules appear with some intensity when they are adsorbed on solid surfaces. The origin of this intensity can, in principle, be accounted for by two mechanisms. Since the symmetry of the adsorbed molecule is lower than that of the free molecule, relaxed selection rules result. Alternatively, the large gradient in the normal component of the electromagnetic field near a conducting surface can induce scattering through the dipole–quadrupole polarizability. To assess the relative importance of these two mechanisms, the field gradient at the surface of a Ag(111) crystal was varied by using different incident laser frequencies and its effect on the relative intensity of a putative quadrupole mode monitored. The intensity of the mode remained constant over the frequency region in which the field gradient varied. It was concluded that the field gradient mechanism is not important, at least for molecules physically adsorbed on flat surfaces.