Excitation Profiles and the Continuum in SERS: Identification of Fano Line Shapes

We extend a previous theoretical expression of SERS to include the possibility of coupling of the molecular levels with the continuum of levels in the conduction band in the metal. We modify 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 are considered since both can contribute to the SERS enhancement. We test the theory 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 is obtained.