Internal Reflection Spectroscopy (IRS)

Internal Reflection Spectrocopy is based on the reflection of the light in the inner face of an optical element in contact with the sample. This optical element must be a suitable prism and light should be irradiated exceeding the critical angle. Because of the characteristics of the evancescent wave, the only requisite for recording the spectra of a sample is to place it in contact with or in close proximity to the reflecting surface, without any further sample preparation, reducing the complexity and time expenses of the analysis.
It is a nondestructive technique valid for solid, liquid and powders, but not for gases. The spectra for IRS are best understood because Fresnel´s effective thickness equations give physical insight into the interaction of the evanescent wave (penetrating electro-magnetic field) with the sample. These equations clearly show the effect of the involved parameters in the spectrum.
For thin films, IRS spectra are the same as transmission spectra, while for thick films the absorption bands are more intense at longer wavelengths. Therefore, as the angle of incidence approaches the critical angle, the bands are displaced to longer wavelengths (lower wave numbers). Only very close to the critical angle dispersion type spectra are observed.
This technique is also referred to as ATR (Attenuated Total Reflectance) or MIR (Multiple Internal Reflectance) and for a certain configuration of the optical element is highly dependent on the samples´ film thickness and the incidence angle of the beam.

