This article reports on the extension of a previous theoretical expression of SERS to include the possibility of coupling the molecular levels with the continuum of levels in the conduction band in the metal. The project modified the expression for excitation profiles to account for interference between nearby conduction band levels, producing an asymmetric Fano line shape.
Both charge-transfer and molecular transitions were considered, since both can contribute to the SERS enhancement. The theory was tested by comparison with the observed profiles in pyridine, p-aminothiopehnol (PATP), and an azo dye (DABS). The experimental profiles are clearly asymmetric, dipping into the continuum on one side and approaching the continuum asymptotically on the other. In each case, an excellent fit of the Fano profile was obtained. (publisher abstract modified)