Surface-Enhanced Raman Spectroscopy A Charge Transfer Theory

Surface-Enhanced Raman Spectroscopy A Charge Transfer Theory 

We are now in a position to understand how the presence of a metal surface near a molecule can enhance the intensity of Raman scattering from that molecule through a charge transfer process. We imagine that the molecule is weakly bound to the surface at some equilibrium distance along 

In any complete quantum mechanical treatment, we could not consider the energy levels of the molecule to be distinct from those of the metal. We would have to speak of the levels of the molecule-metal system. The correct Hamiltonian for such a system would be written  

We now consider a derivation of a SERS charge transfer enhancement in terms of the polarizability components. If we view the molecule-metal distance to be a normal mode of the combined system and assume that the zero-order wave functions of the molecule or metal have been chosen so as to satisfy the Born-Oppenheimer conditions, then we may follow the Herzberg-Teller theory, adopting Eq. (40) and Eqs. (46)-(51). 

Considering the terms B and C, we now let the intruding states be those of the metal and utilize the letter M to replace S when referring to metal 

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