Reflection-absorption infrared spectra RAIR

Reflection-absorption infrared spectra (RAIRS) are in general recorded with P-polarized light at different incident radiation angles in the range 30-80°, by using commercial variable angle accessory mounted on the IR optical bench. 

Whereas ATR spectroscopy is most commonly applied in obtaining infrared absorption spectra of opaque materials, reflection-absorption infrared spectroscopy (RAIRS) is usually used to obtain the absorption spectrum of a thin layer of material adsorbed on an opaque metal surface. An example would be carbon monoxide adsorbed on copper. The metal surface may be either in the form of a film or, of greaf imporfance in fhe sfudy of cafalysfs, one of fhe parficular crysfal faces of fhe mefal. 

This technique is used to study thin (down to submonolayer) films adsorbed on reflective substrates such as metals. Experimentally it involves measuring the change in the reflectance spectrum of the substrate that accompanies thin film formation. Various acronyms for the technique are used infrared reflection—absorption spectroscopy (IRRAS, IRAS) and reflection—absorption infrared spectroscopy (RAIRS). The Basics of IRRAS spectra are described in Chapter 5.2. 

Observation of absorption bands due to LO phonons in RAIR spectra of thin, silica-like films deposited onto reflecting substrates demonstrates an important difference between RAIR and transmission spectra. Berreman has shown that absorption bands related to transverse optical (TO) phonons are observed in transmission infrared spectra of thin films obtained at normal incidence [17]. However, bands related to LO phonons are observed in transmission spectra of the same films obtained at non-normal incidence and in RAIR spectra. Thus, it is possible for RAIR and transmission spectra of thin films of some materials to appear very different for reasons that are purely optical in nature. For example, when the transmission infrared spectrum of a thin, silica-like film on a KBr disc was obtained at normal incidence, bands due to TO phonons were observed near 1060,790,and450cm [18]. 

As an example, both monofunctional and multifunctional polymeric mercapto-esters were deposited onto optically smooth silicon wafers coated with vapor-deposited copper. The copper had been oxidized to Cu20, as verified by XPS. Infrared reflectance (RAIRS) at 81° (4 cm-1 resolution, 2000 scans) using an MCT detector yielded information on both the nature and the durability of the mercaptoester bond to the metal oxide film. A 16 cm l shift (1740— 1724 cm-1) was observed in the carbonyl absorption of stearyl thioglycolate (STG) deposited onto the Cu20 mirror. The absorption spectrum of the carbonyl region is illustrated in Fig. 11, both for the pure STG and the reacted monolayer. 

---