Observation of singlet–triplet anticrossings in 4He2

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Abstract
Anticrossings between excited singlet and triplet levels of the He2 molecule have been observed. Measurements have been made on anticrossings which take place between the f (3d) 3Δu and F (3d) 1Δu states both in the v=0, N=2,3,4 and v=1, N=2 vibrational and rotational levels. Analysis of these results yields a new and more precise value for the excitation energy, T0, from the a (2s) 3Σ to the A (2s) 1Σ state. This result, when combined with available optical data, fixes the absolute energy of all He2 excited states. The present experiment establishes that the steady state populations of the 3Δ levels are greater than the 1Δ levels under a variety of excitation conditions. Also, collisional transfer may take place before radiation giving rise to a given anticrossing signal appearing on optical emission lines of several nearby rotational levels.