Relative Raman scattering cross sections for chemisorbed and physisorbed pyridine molecules adsorbed on surface-enhanced Raman-scattering-active Ag films in ultrahigh vacuum: Local-field effects in the low-coverage regime

We have studied the Raman scattering from pyridine adsorbed on silver films evaporated on cleaved mica (100 K) in ultrahigh vacuum. These silver films have only a small number of chemisorption sites (∼2%). The chemisorption sites are filled preferentially during initial stages of adsorption, followed by filling of the physisorption sites. Both physisorbed and chemisorbed pyridine exhibit strongly enhanced Raman scattering. At one monolayer coverage the average scattering enhancement is a factor-of-55 larger for the chemisorbed molecules. When pyridine-$d_5$ molecules are used to saturate the chemisorption sites, subsequent adsorption of pyridine-$h_5$ results in partial displacement of the pyridine-$d_5$ to physisorbed sites. This displacement phenomenon allows us to measure the ratio of the Raman cross sections for chemisorbed and physisorbed pyridine in the neighborhood of chemisorption sites. This ratio is less than 4. Combining this measurement with the factor-of-55-larger overall average enhancement for chemisorbed molecules compared to phys- isorbed molecules, we conclude that molecules (including those just physisorbed) in the neighborhood of the chemisorption site exhibit an extra enhancement. We have also analyzed the coverage dependence of the Raman intensity and found it to be well described by a simple depolarization model. The average optical polarizability in this model is ∼10 A${̊}^3$. This value is essentially the same as the value reported for pyridine adsorbed on annealed surface-enhanced Raman-scattering-inactive silver films.