On the reflectivity discontinuity generated by critical adsorption at the liquid-vapour interface

The reflectivity discontinuity arising from the concentration profile at critical adsorption at the liquid-vapour interface is interpreted in terms of the spectral properties of a potential well. When the well is soft enough at the top, the spectrum of bound states becomes continuous and reflectivity resonances caused by the potential well become a discontinuity. In order to apply this property to the true interaction potential between radiation and sample, we use a theorem of Abeles relating the reflectivity of a potential function to the reflectivity of a corresponding symmetric potential well.