Two basic approaches (asymptotic and quasistatic) to the description of light absorption at large frequency detunings are discussed. The approximations underlying these methods are analyzed. It is shown that the main physical features of the problem at hand are related to the crucial role of statistical averaging involving the classical intermolecular interaction potential. The differences between the approaches considered result from the choice of the computational strategy and are not crucial for understanding the physics of the absorption process. As we think, the analysis performed suggests that the problem of the line wing absorption in vibration-rotational molecular spectra is solved on condition that the intermolecular interaction potential is known. The physical features of the line wing absorption are illustrated by several examples treated within the framework of the asymptotic approach.
spectral line shape theory, Moller operators, line wings, semiclassical approach, quasistatic approach, intermolecular interaction potential