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 tw o mechanisms. Since the symmetry of the adsorbed molecule is lower tha n that of the free molecule, relaxed selection rules result. Alternati vely, the large gradient in the normal component of the electromagneti c field near a conducting surface can induce scattering through the di pole-quadrupole polarizability. To assess the relative importance of t hese two mechanisms, the field gradient at the surface of a Ag(111) cr ystal was varied by using different incident laser frequencies and its effect on the relative intensity of a putative quadrupole mode monito red. The intensity of the mode remained constant over the frequency re gion in which the field gradient varied. It was concluded that the fie ld gradient mechanism is not important, at least for molecules physica lly adsorbed on flat surfaces. (C) 1996 American Institute of Physics.