The anisotropy of the interaction potential of the ${\mathrm{H}}_2$-He collisional complex is treated as a small perturbation in a computation of the zeroth and first spectral moment of the collision-induced rototranslational spectra in the far-infrared region. The moments of the rototranslational spectra are separated into sums of moments of rotational lines, characterized by rotational transitions j→j’ with j≠j’, and translational components (j=j’, for j=0,1,. . .). The quantum expressions thus obtained consist of a leading term computable from the isotropic component of the interaction potential and dipole operator, plus an anisotropic ‘‘correction term’’ accounting for the lowest-order ($P_2$) anisotropy. These are evaluated for temperatures between 77 and 195 K. The results provide a much needed basis for assessing the validity of line-shape computations based on the isotropic potential, distorted-wave, or infinite-order sudden approximations often employed to avoid the enormous cost and complexity of close-coupling computations. The spectroscopic effects of the anisotropy of the ${\mathrm{H}}_2$-He system are briefly discussed.

• Received 15 December 1986

DOI:https://doi.org/10.1103/PhysRevA.35.3679