The electronic and vibrational energies of two double-welled 3Σu+ states of He2

The potential energy curves for the 3Σ+u states of He2 which correspond to the interactions of He 1s2 1S with He 1s2s 3S and He 1s2s 3S with He 1s2s 3S are obtained from second-order configuration interaction calculations which themselves are based on a multiconfiguration self-consistent field calculation. The basis set includes functions obtained by optimizing the dipole, quadrupole, and octapole polarizability of the 3S atom and the dipole and quadrupole polarizabilities of the ground 1S atom. For the lowest 3Σ+u state we estimate the dissociation energy to be De =15 740±60 cm−1; we agree to within about 4 cm−1 with the observed vibrational energy level differences, and agree nicely with the position and depth of the short-range minimum deduced from spectra. We also obtain a potential hump at intermediate separations whose main features agree very well with those obtained from scattering experiments. The potential curve exhibits a van der Waals minimum about 3.82 cm−1 deep at about 11.6 a0. The excited 3Σ+u state has a deep metastable well at small internuclear separations as well as a well in the intermediate region which is about 3430 cm−1 deep.