A THEORY OF RAMAN-SCATTERING DUE TO DIPOLE ACTIVE VIBRATIONAL-MODES OF MOLECULES ADSORBED ON SMALL METAL PARTICLES

The theory of intense Raman scattering, generated by dipole active vibrational modes of molecules adsorbed on small metal particles, is reformulated here to make use of our recently developed approach to calculating the polarizability of small metal spheres and to include terms missed in our earlier development. The approach is based on the local-dielectric-response approximation and requires a knowledge of the conduction electron density profile at the metal/ambient interface. The modulation of the polarizability of a metal particle, due to the quadrupolar field generated by a phonon mode of the adlayer, is calculated using an integration procedure that incorporates both electron density and induced charge density profiles determined by Lang and Kohn for jellium. Scattering intensities for both Raman and dipole active modes of CO and CO2 on small silver particles are calculated and compared. The results show that the ratio of the scattering intensity for a Raman active molecular mode to that for a dipole active mode increases rapidly with the particle radius, being comparable for the two stretching modes of CO2 on Ag particles of radius 25-50 angstrom.