Rotational perturbation between two electronically excited states of 4HeD

The rotational structure of the D ²Σ⁺ →A²Σ⁺ transition of ⁴HeD is highly perturbed, and this is caused by the near degeneracy of the D, ν=0 and C, ν=3 vibrational levels. A perturbation analysis is presented which yields the spectroscopic constants for both of the perturbing levels, yielding the first experimental information on a vibrationally excited state of HeH.