The short-range mechanism of surface enhanced raman scattering

Abstract

In addition to the electromagnetic influences which increase the local field at the surface of suitably rough free-electron-like metals, molecule and site specific electronic resonances are also frequently operative. This leads to enhanced scattering beyond that caused by the electromagnetic mechanism alone. The electronic resonances involve charge-transfer excitation between members of an active complex M w Ag x Cl y (H 2O) z which involves the probe molecule M and, in the electrochemical environment, the constituents of the solution. These complexes allow the resonance to be communicated to Raman transitions in all members of the complex. The stability of the complexes are sensitive to changes in the environment, such as the applied voltage and to the presence of foreign metal ions, through induced changes in y, the number of halide or pseudo-halide ions in the complex. Through analysis of the vibrational signature of the probe molecule under various environmental influences we are able to observe subtle changes in the character of the bonding between that molecule and the rest of the complex. These changes can also be interpreted as indirect results of changes in the value of y.