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 3S)+He(2 3P) at the multi-reference configuration interaction level down to internuclear separations of 3.5 a0. 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 D2 line, associated with the He 2 3S + 2 3P2 dissociation limit, are assigned. The agreement is less satisfactory with the levels observed to the red of the He D1 line, although vibrational levels have been found in the outer ultra-long-range well of a double-well potential with the He 2 3P1 dissociation limit. A semiclassical analysis is presented to explain the limited sensitivity of the near-dissociation binding energies to the short-range potential.

The shape and broadening of He lines affects radiative transport in dense, He-rich, stellar atmospheres. At wavelengths inaccessible for direct observation, we rely on theoretical calculations of self-broadening to support stellar structure and spectral modeling. In this work, we examine lines of He due to 1s-2p and 2p-3s transitions. The line profiles are analyzed in terms of a unified theory of spectral line broadening using ab initio potential energies that have been recently determined. For temperatures up to 20 000 K, the linear dependence of width and shift on gas density and the non-linear dependence on temperature of the Lorentzian core of the resonance line are described. Beyond the conventional symmetrical Lorentzian core, we show that they are asymmetrical and have significant additional contributions on the short wavelength side. This blue asymmetry is a consequence of maxima in the corresponding He 2 potential energy difference curves at short and intermediate internuclear distance. Over a limited range of density and temperature, laboratory measurements in the visible and near infrared can be used to validate the potentials that underlie the spectral line profile theory, which is useful for modeling spectra over the extreme ranges of temperature and density encountered in stellar and planetary atmospheres.

The emission spectrum produced at 300 K in dense plasmas of helium excited by a corona discharge shows an asymmetric atomic line at 706 nm that may be associated with quasimolecular states of He2. We present here the first calculations of He-He collisional profiles of this line done in a unified line shape semi-classical theory using ab initio molecular potentials. The excellent agreement between experimental and theoretical determinations of the near wing of the line profiles establishes the accuracy of the interaction potentials. The strong pressure dependence of the collision profiles can then be used as a diagnostic of the physical parameters of the source.

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