Theoretical foundation of electroabsorption spectroscopy: Self-contained derivation of the basic equations with the direction cosine method and the Euler angle method

Self-contained derivation of the electroabsorption (E-A, Stark effect) equations of Liptay and Czekalla, which describe the external electric field effects on absorption spectra of a mobile ensemble of light-absorbing molecules at thermal equilibrium, is reported. Two mathematically different ways for obtaining the ensemble averages, the “direction cosine method” and the “Euler angle method”, are compared. Some examples illustrating the application of E-A equations in the analysis of electroabsorption spectra are presented.