The Chemical Effects in SERS

The central problem in understanding the chemical contribution to the overall enhancement reduces to the calculation of the polarizability of the adsorbate metal surface complex. The mixing of molecular states with metal states may bring about a large effective polarizability. The polarizability a depends on the normal mode amplitude of the molecular vibration Q. In the image field theory of SERS the effective polarizability aeff(Q) depends on the distance between the point molecule and the metal surface and on and e . In order to obtain Raman enhancement the distance between molecule and metal is in the range of 1.5 to 4 A. 

Recently, Ueba presented a calculation of the Raman scattering polarizability of an adsorbed molecule on a metal surface, where charge transfer excitation participates as an intermediate state of the Raman process. A typical estimation of pyridine chemisorbed on Ag through the N lone pair electron shows an enhancement of about 1(P. This seems to indicate that the charge transfer mechanism (CT) operates in conjtmction with the EM effect. 

Another contribution from charge transfer transitions to the enhanced Raman scattering was treated by Persson In this theory the factor in the Raman enhancement is estimated to 30. 

A different approach was described by Otto et al. and Burstein et al. Their Adatom Model postulates an extra Raman enhancement mechanism for adsorbates at active sites of atomic scale roughness . The adatom acts as an element of small or atomic scale roughness causing localired breakdown in the selection rules for electron-hole pair excitation. The model is based on a Raman scattering mechanism by charge transfer (cf. Fig. 7). An electron on the metal side is excited from 

The transition polarizibility a s (Raman scattering matrix element) can be expressed in terms of  

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