LONG-RANGE INTERACTIONS IN H-HE - AB-INITIO POTENTIAL, HYPERFINE PRESSURE SHIFT AND COLLISION-INDUCED ABSORPTION IN THE INFRARED

The collisional complex H-He, with both atoms in their electronic ground-states, is treated as a molecule in self-consistent field (SCF) and multi-reference configuration interaction (MR-Cl) calculations to determine interaction energy, dipole moment and spin density as function of internuclear separation. A basis set tailored for long-range interactions was used and the basis set superposition errors were controlled. The resulting functions are analyzed and presented in analytical form, in terms of exchange and damped dispersion contributions. For all three properties there is full agreement with the accurately known long-range coefficients, but the dipole moment function shows rather large overlap effects even at large distances which obscure higher-order dispersion coefficients. The well depth of 22.56 muEh is significantly deeper than most recent ab initio calculations and model potentials have suggested, but our number corroborates existing semiempirical values. Likewise, the calculated spin density variations are more pronounced than recent work has suggested. The resulting hyperfine pressure shift of H atoms in a helium buffer gas is in very good agreement with experiment, except for temperatures of the order of 1 K. Infrared absorption continua associated with the induced dipole moment are evaluated for their astrophysical interest.