The Beer-Lambert law in the infrared: derivation from wave mechanics and application to solids

The Beer-Lambert law is directly deduced from wave mechanics for a parallel radiation. The formal derivation outlines some aspects which are important for quantitative works on the infrared intensities: the meaning of band areas and integrated absorption coefficients, the spectrograph resolution, the isotope effect and the influence of anisotropy. It is shown systematically how to calculate a valuable absorption coefficient for bands due to independent molecular groups in solids. In order to take account of the orientation effects, the coefficient used is an integrated parallel absorption coefficient, which is directly proportional to the concentration of molecular groups responsible for the band. The effect of light polarization by the spectrograph and the sample is considered.