Thickness dependence of infra-red absorption enhancement for methanol on evaporated silver films

The infra-red absorption intensity for the C-O stretch mode of methanol physisorbed at 90 K on 0.5-4-nm mass thickness silver films, deposited in UHV on to Ge substrates at 296 K, has been measured at normal incidence of radiation. The absorption intensity observed as a function of methanol exposure is normalized with respect to the corresponding intensity observed without silver to obtain an estimate of the absorption enhancement by the presence of silver. At any thickness of silver, the enhancement is at a maximum at an a exposure of less than 0.5 L; for silver films at 1 and 2 nm, the absorption intensity is more than 20 times larger than that observed without silver, though a substantial enhancement is observed at larger exposures. The exposure dependence is rather different on 3- and 4-nm-thickness silver films that exhibit an enhancement of ca. 10 at maximum. On the basis of the surface morphology of the silver films, the observed enhancement features are consistent with the view that collective electron resonances are involved in the absorption enhancement.