Photoassociation of He2revisited

A new theoretical study for the photoassociation of metastable helium is presented, based on fully ab initio short-range potentials. Optimization of the exponents of the Gaussian orbitals of a smaller basis set than employed previously allowed the calculation of potentials dissociating to He(2 3 S)+He(2 3 P) at the multi-reference configuration interaction level down to internuclear separations of 3.5 a 0 . The good agreement with observed near-dissociation levels obtained previously using extrapolated short-range potentials is confirmed and almost all levels observed to the red of the He D 2 line, associated with the He 2 3 S + 2 3 P 2 dissociation limit, are assigned. The agreement is less satisfactory with the levels observed to the red of the He D 1 line, although vibrational levels have been found in the outer ultra-long-range well of a double-well potential with the He 2 3 P 1 dissociation limit. A semiclassical analysis is presented to explain the limited sensitivity of the near-dissociation binding energies to the short-range potential.