A new absorption band which was below the original ultra-violet band was detected via the magnetic circular dichroism of VK center absorption in 7 alkali halide crystals. This new band was attributed to the transition, u  u, which became dipole allowed as a result of a loss of inversion symmetry. This decreased symmetry was attributed to a slow oscillation, along the VK axis, which resulted in a momentarily frozen geometry of the species at the classical turning point. An approximate estimation of the coupling between the even and odd parity molecular orbitals of  and  symmetry was obtained in the case of KBr. The results suggested that the coupling between even and odd parity states was not negligible.

U.Rogulis, K.S.Song, J.M.Spaeth: Journal of Physics - Condensed Matter, 1995, 7[39], 7699-708