Infrared spectroscopy is an analytical technique useful for identification and characterization of known or unknown compounds. An IR spectrum enables to reveal clearly the presence of some functional groups and can serve as a “spectroscopic identity card” for a molecule or a material. Two Fourier-Transform Infra-red spectrophotometers are available in the GMI, an ATI Mattson spectrophotometer (KBr beamsplitter: energy range 4000-400 cm-1) and a Digilab Excalibur 3000 spectrophotometer (CsI beamsplitter: energy range 4500-250 cm-1). Access to low energies is of particular interest to study the metal-ligand vibrations.
Moreover, it is possible to use an ATR accessory (Single Reflexion Attenuated Total Reflectance) with these spectrophotometers. The principle consists in realising a close contact between a small crystal (ZnSe, Ge, Si diamond) and the sample. The IR beam then propagates in the crystal; if the refractive index of the crystal is superior to the one of the sample, then the beam undergoes total reflexions at the interface crystal/sample. Only one wave, namely the evanescent wave, emerges from the crystal and is absorbed by the sample. This evanescent wave is responsible for the observed infrared spectrum. Penetration depth of the evanescent wave is around 1 to 2 µm. The ATR technique thus provides information about the surface of the sample. This is of particular interest for the analysis of pure samples (without dilution into KBr) since the risk of saturation of the peaks is very low. Moreover, especially at low energies, the resolution is generally better than for a “classical” transmission spectrum. ATR is a very reproducible technique, particularly adapted to quantitative analysis of pure powders and especially of liquids, pasty compounds and films. L’ATR est une technique très reproductible qui se prête notamment très bien à l’analyse quantitative et qui est particulièrement adaptée pour l’analyse de poudres pures, et surtout de liquides, de produits pâteux et de films. It is possible to realise measurements under controlled applied pressure. The ATR accessory available at the DCMI is a ZnSe ATR (energy range 4000-650 cm-1).
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