Surface Selection Rules in SERS

The directional properties of the local field electromagnetic enhancement have been used to deduce the orientation of small molecules adsorbed on Ag colloids and electrodes. Moskovits and Suh and Creighton used these properties to deduce surface selection rules, which essentially are the modifications of band intensities due to the normal and tangential enhancement components of the local field of a molecule near a metal surface. Here the 

Before we illustrate these surface selection rules according to the formulation of Moskovits and Suh, it should be pointed out that attempts to apply the treatment to SERS spectra from cold-deposited silver films were not as successful. Also, in this treatment, the surface is modeled as an individual metal sphere however, SERS-active colloids are large aggregates with randomly adhering spheres,and the surface of a SERS electrode is more like a set of prolate hemispheroids connected by a conducting plane. However, a small-sphere model is a good first-order approximation. 

To illustrate these surface selection rules, recall from expression (69) that the SERS intensity is proportional to 

One might then expect that three classes of vibrational modes would coupled into different products of the enhancements factors as 

For a molecule with C2 symmetry, such as phthalazine or pyridine, the irreducible representations are 

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