Enhancement of the infrared absorption of methanol adsorbed on silver island films

Infrared absorption spectra for physisorbed methanol at 90 K on silver films of mass thicknesses 4-20 nm, deposited in UHV onto Ge substrate surfaces at 473 K, have been measured at normal incidence of radiation. The results show that the absorption intensity of the methanol C-O stretch mode depends on the mass thickness of silver film and methanol exposure. The absorption intensity obtained with an 8 nm thick silver film is about 60 times larger than the intensity observed without silver. Independent of silver thickness, the intensity is a maximum at an exposure less than 1 L, but a substantial enhancement (5-10 times) is observed at 5 L. This aspect makes it likely that the enhancement is due to an electromagnetic field enhancement caused by the collective electron resonances of the silver islands. The absorption enhancement features are correlated with the morphologies of the silver films.