The importance of transverse optic-longitudinal optic (TO-LO) splitting in the interpretation of infrared spectra of thin films is experimentally presented. When infrared spectra are observed at oblique angles of incidence, the shifts in peak position and/or the changes in peak shape may be due to TO-LO splitting. Before one assigns the optical behavior to surface phenomena such as molecular orientation, proper care must be employed. The proposed technique includes optical calculation to extract surface phenomena from TO-LO splitting in the spectra, and it is applied to reflection absorption spectra of perfluoropolyether.
Kiyoshi Yamamoto and Akio Masui, "TO-LO Splitting in Infrared Spectra of Thin Films," Appl. Spectrosc. 50, 759-763 (1996)