The 4f states of He2: A new spectrum of He2 in the near infrared

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Abstract
A new spectrum of the He2 molecule has been discovered in the region 5100–5900 cm−1. It is identified as the 4f–3d Rydberg transition of He2 with a weak contribution of 4d–3p in the same region. The uncoupling of l from the internuclear axis in both the triplet and the singlet system is almost complete in the 4f complex; the appropriate quantum number is N+ corresponding to the rotational angular momentum of the He+2 core. The electronic fine structure in each N+ level depends on the dipole polarizability α of the core and its anisotropy as well as on the quadrupole moment Q2 and higher moments Q4,... of the core. Conversely, from the observed structure some of the quantities characterizing the core have been determined, e.g., α=1/3(2α⊥+α∥) =2.82 ±0.02, Q2=1.39±0.13, Q4=0.99±0.14, all in atomic units. The detailed rotational analysis of the spectrum shows that widespread perturbations occur in the triplet 4f levels, the largest affecting the N=6,N+=3 and N=9,N+=7 levels which are shifted towards higher energies by 1 and 2 cm−1, respectively. The perturbations are clearly caused by Δl=2 interaction with corresponding levels of 4pπ for which Orth and Ginter have found the opposite perturbations. There are no similar perturbations in the corresponding singlet levels.

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