Internal reflection spectroscopy is discussed for a nonuniform distribution of an absorbing species. Although some distributions cannot experimentally be distinguished from others, if one assumes a distribution, information about depth and concentration can be obtained by making measurements at several angles of incidence. A practical method is proposed and demonstrated for obtaining thickness and concentration for an assumed step profile.
Harland G. Tompkins, "The Physical Basis for Analysis of the Depth of Absorbing Species Using Internal Reflection Spectroscopy," Appl. Spectrosc. 28, 335-341 (1974)