Far infrared absorption by pairs of nonpolar molecules

Recent progress in the field of binary collision induced spectra of nonpolar gases and mixtures in the far infrared (FIR) region of the spectrum includes accurate measurements of a variety of molecular systems and temperatures, and rigorous quantum calculations. The latter are based on the isotropic potential approximation and either on ab initio induced dipole data obtained with highly correlated wavefunctions, or on the classical multipole induction model. The contributions of both free pairs of molecules in collisional interaction, and bound pairs (van der Waals molecules), are accounted for in equilibrium proportions. The effects of the anisotropy of the intermolecular interaction potential on the spectra are also being understood in quantitative terms. On an absolute intensity scale, the agreement of theory with the laboratory measurements is typically well within the uncertainties of the measurements if all theoretical dimer features are flattened by convolution with with an instrumental profile of 10 or 20 cm^–1 width; certain dimer features have been seen in the FIR spectra of the atmospheres of the outer planets and their big moons. For astrophysical and other applications, the results of the quantum computations have been cast into simple analytical expressions which reproduce collision induced spectra accurately as function of frequency and temperature on computers of small capacity in seconds for a selection of molecular systems.