Molecular treatment of collisions between a Rydberg sodium atom and a rare-gas perturber

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Abstract
An interpretation of the observed population transfer Na(6p)+He(Ne) to Na(5l)(l>or=2)+He(Ne) at thermal energies is presented which improves the previous treatment of Masnou-Seeuws et al. (1982). The model potential calculations are extended at larger internuclear distances for the two Sigma molecular potential curves which display three avoided crossings, the previously studied crossing at 6.7 and two additional crossings at 14.6 and 27 au. The radial coupling matrix element is computed. A comparison with the estimations of the Fermi model is given. A two-channel quantal treatment of the scattering problem is performed. For the NaNe case, the inner crossing is mainly responsible for the transition. For the NaHe case, the crossing at large distances are important and the effect of the rotational coupling with the pi 6p state must be introduced. A three-channel calculation has been performed. The calculated cross sections are 28.2 and 9.5 AA2 for the He and Ne perturbers, the experimental values being 26+or-1.7 and 9.7+or-0.6 AA2.