Three-body components of collision-induced absorption

Expressions for the three-body coefficients are derived by virial expansion of the zeroth and first moments of collision-induced spectra. For the ternary supermolecular complexes He-Ar-Ar, He-He-Ar, ${\mathrm{H}}_2$-He-He, ${\mathrm{H}}_2$-${\mathrm{H}}_2$-He, ${\mathrm{H}}_2$-${\mathrm{H}}_2$-${\mathrm{H}}_2$, and ${\mathrm{H}}_2$-Ar-Ar, these are evaluated numerically from first principles at a few temperatures between 50 and 300 K, using the most dependable induced-dipole and intermolecular potential functions available. Both rototranslational and rotovibrational bands are considered. Only the pairwise-additive contributions to the three-body induced-dipole moments are considered. Quantum corrections of the Wigner-Kirkwood type are given and found to be important. Strong three-body spectral components are predicted at the lower temperatures. The computed three-body spectral moments are compared with available laboratory measurements in the near- and far-infrared regions. At temperatures above 50 K, measured ternary moments are found to be consistently greater than calculated ones, especially at the higher temperatures. The excess may be due to a positive ternary spectral component of collision-induced absorption arising from nonadditive induced-dipole components of molecular triplets.