This article reports on a reexamination of the Herzberg-Teller theory of charge-transfer contributions to the theory of surface enhanced Raman scattering (SERS).
In previous work, the Kramers-Heisenberg-Dirac framework was used to explain many of the observed features in SERS; however, recent experimental and theoretical developments suggest that the theory should be revised to take advantage of the time-dependent picture of Raman scattering. In the current study, results were obtained for molecular adsorption on nanoparticles in both the strong confinement limit and the weak confinement limit. The study found that the Herzberg-Teller contributions to the charge-transfer effect in SERS displayed a resonance at the molecule-to-metal or metal-to-molecule transition while retaining the selection rules associated with normal Raman spectroscopy (i.e., harmonic oscillator, as opposed to Franck-Condon overlaps). The charge-transfer contribution to the enhancement factor scales as Γ−4, where Γ is the homogeneous linewidth of the charge-transfer transition, and thus is extremely sensitive to the magnitude of this parameter. The study found that the Herzberg-Teller coupling term may be associated with the polaron-coupling constant of the surface phonon-electron interaction. A time-dependent expression for the Raman amplitude was developed, and the implications of these results for both metal and semiconductor nanoparticle surfaces are discussed. (publisher abstract modified)