SERS of pyridine on Ag island films prepared on a sapphire substrate

Abstract

The enhanced Raman signals have been observed from pyridine molecules deposited in ultrahigh vacuum silver island films prepared in situ on sapphire substrates. The coverage of the silver and pyridine was monitored by a quartz crystal microbalance. Experimental results have been obtained which indicate that physical proximity of the molecule to Ag alone is not enough to see the enhanced signal. Furthermore, indications are that the enhanced signals are due to pyridine molecules adsorbed on a variety of ’’active sites.’’ These include both physisorption and chemisorption sites. The physisorption sites are sites from which the pyridine molecules are removed when the substrate is warmed to about 130 K. The pyridine molecules on the chemisorption sites are not readily removed even when the substrate temperature is raised above room temperature. The peak positions of the Raman signal differ slightly for these two types of sites. For example, the strongest line for the physisorbed pyridine is centered around 989 cm−1 and the corresponding line for the chemisorbed pyridine is centered around 996 cm−1.