Principles, Theory, and Practice of Internal Reflection Spectroscopy
Ultimately, the origin of internal reflection spectroscopy (IRS) is rooted in the existence of an evanescent wave in a lower index of refraction medium in contact with an optically more dense medium in which a propagating wave of radiation undergoes total internal reflection. Thus, a treatment of the theory of IRS must begin with a description of the properties of this evanescent field. The foregoing idealized picture of total internal reflection yields a convenient description of the properties of the evanescent field. However, the assumption that the rarer medium is nonabsorbing is clearly unrealistic, since absorption of energy must occur if a measurement is to be made. The need for the sample to interact with the internal reflection element in IRS adds another dimension that complicates the production of high quality spectra relative to transmission spectroscopy.