Collision-induced rotovibrational spectra ofH2-He pairs from first principles

A previous study of the collision-induced dipole moment has treated the ${\mathrm{H}}_2$-He complex as a molecule in self-consistent-field and size-consistent coupled-electron-pair approximation [Meyer and Frommhold, Phys. Rev. A 34, 2771 (1986)]. Based on that work, the vibrational dipole transition elements 〈v‖$A_{\lambda L}$(R,r)‖v’〉 associated with the fundamental band (v=0→v’=1) are obtained as functions of separation R of the collisional pair for the isotropic and anisotropic overlap induction components (λL=01 and 21), and the quadrupole- and hexadecapole-induced parts (λL=23 and 45). From these induced dipole components and Meyer, Hariharan, and Kutzelnigg’s isotropic part of the ab initio potential surface, we compute in the (forbidden) fundamental band of hydrogen the collision-induced absorption spectra of the collisional complex of hydrogen (${\mathrm{H}}_2$) and helium from an exact quantum formalism. Both the shape of the computed spectral profiles and the theoretical absolute intensity agree closely with existing measurements at temperatures from 18 to 300 K. The fact that these spectra, and presumably the analogous overtone and ‘‘hot’’ (v>0) bands of the ${\mathrm{H}}_2$-He complex which are not known from measurements, can be accurately obtained from basic principles is significant for research related to the atmospheres of the giant planets and late-type stars.