Acetonitrile and [D3]acetonitrile in the vicinal region of a planar AgX fiber contain linear dipole-dipole linked oligomers as shown by 1) comparison of infrared band intensity ratios in the gaseous and condensed phases and 2) remarkable plots of absorbance (C--N stretch) versus time during evaporation from an AgX planar fiber element. The plots (CH3CN 2252 cm(-1), CD3CN 2262 cm(-1)) reveal the presence of octamers, hexamers, tetramers, and dimers along with some heptamer, trimer, and monomer structures. A novel isotope effect arises from the somewhat smaller size of the CD3CN resulting in an increase in the CN band intensity. The organized oligomers may be termed pseudocrystals and are the main components responsible for absorption intensity in the infrared spectrum of acetonitrile, on the AgX planar fiber or in an IR cell.

We have discovered surface-enhanced infrared absorption (SEIRA) on a planar silver halide surface, the first time this effect has been observed on a non metallic substrate. The enhancement is not due to the presence of metallic silver on the surface as ruled out by X-ray photoelectron spectroscopy (XPS). In addition, the fibers show 10-fold spectral amplification due to the increased coupling of evanescent waves through an increased number of internal reflections to the samples on the surface of the thin planar element. A special cell for the fiber permits deposition of small quantities of solution followed by slow evaporation of the solvent with dry nitrogen and allows measurements to be made on much less than a monolayer of sample. The combination of easy access to the sample and high sensitivity promises many useful applications to biological, chemical, and physical problems in the mesoscopic and nanoscopic domains.

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Surface-enhanced infrared absorption (SEIRA), recently discovered for trypsin on a planar silver halide surface, is now demonstrated for the small organic molecule, p-nitrobenzoic acid (pNBA). Evaporation of pNBA solutions in acetonitrile yields the acid dimer (confirmed by the unique IR signature of carboxylic acid dimers). The absorption changes for pNBA with quantity exhibit two regimes: "enhanced" and "regular". A plot of absorbance versus quantity reveals how SEIRA changes with distance from the surface. The surface orientation of pNBA has the long and short axes (unit cell) parallel to the surface (packing density/carbonyl peaks). "Slice" spectra differentiate between surface pNBA and other layers. Most of the numerous previous reports on pNBA actually refer to the p-nitrobenzoate anion probably generated through reaction of surface silver oxide with the acid.

Surface-enhanced infrared absorption spectroscopy (SEIRA) of methanol, ethanol, 1-propanol, and 2-propanol in thin films on planar silver halide (AgX) fibers under slow N(2) flow using 1 sec scans reveals structure in absorbance-time plots. The absorption intensities show extra enhancements (3x) in the absorbance (O--H stretch) ascribed to oligomers present at the AgX surface (molecule enhanced, thus MOSEIRA). This is above those due to amplification (40x, 20 reflections) and enhancement (30x, image dipoles or surface phonon polaritons). In the case of ethanol an excellent initial pentamer spectrum evolves over 8-10 min to a mixture of pentamer, tetramer, and trimer spectra that within another minute forms small oligomers and monomers. We use a new type of cell for infrared spectroscopy containing an AgX planar fiber. The optical configuration leads to a vicinal region at the surface defined by evanescent waves. Within this region are surface-induced organized species such as ethanol oligomers. The planar AgX fiber supports 20 reflections and transmits light over a wide visible-infrared wavelength range. Short scan times permit the study of volatile substrates or solvents, including the effects of solvent polarity.