Understanding the Role of the Evanescent Field in Attenuated Total Reflection (ATR) Spectroscopy

In attenuated total reflection (ATR) spectroscopy, the presence of an evanescent field penetrating the sample is generally considered crucial. However, according to wave optics, this evanescent field vanishes when the rarer medium is absorbing, and the attenuation of total reflection results from transmission into this medium. While the evanescent field may not play a significant role in this scenario, a closer examination of the relevant relationships reveals that the system's properties vary smoothly with both the angle of incidence and the imaginary part of the dielectric function. This effect can be further illustrated by comparing electric field maps and spectra for semi-infinite rarer media with those for rarer media composed of layers with thicknesses on the order of the wavelength. In the latter case, ATR spectra can be recorded well below the critical angle, where no evanescent field exists. If the layer is vacuum and the underlying semi-infinite medium is assumed to have the same refractive index but is weakly absorbing, tunneling and frustrated total reflection can be observed. Reflecting on our results, we can now define the critical angle in the presence of absorption as the point at which the real and imaginary parts of the perpendicular component of the wavevector become equal. Overall, we conclude that evanescent waves play little to no significant role. Any deviation from total reflection can be attributed to transmission through the ATR crystal-medium interface.