Evanescent Field, Penetration Depth, and Effective Thickness

Evanescent Field, Penetration Depth, and Effective Thickness 

Calculation of the propagation of a plane wave from a medium 1 with higher refractive index (IRE) into a non-absorbing medium 2 with lower refractive index (rarer medium) under conditions of total reflection yields 

Harrick (5) introduced the concept of the effective thickness d, which is the equivalent path in a hypothetical transmission experiment that results in the same absorption signal A — r-mcdc as in the ATR experiment under identical conditions, where A is absorbance e , the molar absorption coefficient and c the concentration of dissolved molecules  

For a Ge IRE (wi = 4) in contact with water ( 2 = 1.33) at an angle of incidence of 6 = 45°, the effective thickness at 1640 cm is 0.44 pm for parallel polarized light (polarized in the plane of incidence of the light) and 0.22 pm for perpendicular-polarized light. For a ZnSe IRE the corresponding values are 1.12 and 0.56 pm. The sensitivity of ATR experiments can be increased by multiple internal reflections. 

---