MEASUREMENT OF ENHANCED INFRARED-ABSORPTION SPECTRA USING EVAPORATED SILVER FILMS AND APPLICATION TO TRACE ANALYSIS

Infrared absorption of thin organic films is enhanced extremely by undercoating with silver island-films. The fundamental properties and mechanisms of this phenomenon, and application to trace analysis are discussed. The silver films of 5 approximately 20 nm in mass thickness (d(Ag)) were vacuum evaporated on CaF2 plates. Several monolayers of p-nitrobenzoic acid were deposited onto the Ag films. The IR spectra were obtained using conventional transmission method. The symmetric modes of the first monolayer chemisorbed on Ag were enhanced selectively. The absorption intensity increased with increasing d(Ag) and became maximal at approximatry d(Ag) = 10 nm. The enhancement factor for the chemisorbed molecules was estimated to be ca. 300 under the optium conditions. The enhancement was much less on quasi-continuum Ag films of d(Ag) > 13 nm. The effect arises mainly from the amplification of the incident IR field through the collective electron resonances (plasma oscillations) of aggregated Ag islands. Some minor chemical contribution to the enhancement is also proposed. The infrared measurement using this surface-enhanced phenomenon has potential as a new trace analytical technique.