Characterization of the ‘surface enhanced Raman scattering’ by use of Raman-spectroscopic and quantum mechanical investigations of simple cluster compounds

Abstract

This paper presents a characterization of the ‘Surface Enhanced Raman Scattering’ by use of Raman spectroscopic and quantum mechanical investigations of simple cluster compounds. It is shown that similarities exist between metal cluster complexes and molecules adsorbed on metal surfaces. The line intensities in Raman spectra of metal clusters are determined by means of a modified CNDO/2 method (1,2). Perturbation terms of an external electric field are inserted into the Hartree-Fock operator used in the CNDO/2 theory(1,2) to yield the induced dipole moment as a function of the normal co-ordinate q, from which the polarizability change upon q is obtained. The results for Ni 3CO, Ni 7CO, Ni 9CO, Ni 10CO and Ni 13CO are discussed. In this manner we determine the number of metal atoms a cluster must contain before its properties are approximately indistinguishable from those of the bulk metal. Calculated intensities of the CO stretching vibrations of metal clusters agree well with experimental intensities obtained from Raman spectra of CO molecules adsorbed on Ni surfaces. In this manner we have obtained an approach for the description of ‘Surface Enhanced Raman Scattering’.