The study of the interaction of substances with infrared electromagnetic radiation in the 3–33-μm spectral region. In this spectral region the infrared (IR) radiation interacts with vibrational–rotational energy levels of the substance under study. IR spectroscopy can be used to determine the concentration of the sample under study or to study the spectral characteristics of the sample. In the former case, the absorption of IR radiation is related to the sample concentration through the Beer–Lambert law. In studying spectral features, one obtains information about functional groups, interatomic distances, bond-force constants, and molecular charge distributions. See tunable laser spectroscopy.