Fig. 1

Low J lines in the Q branch of the [18.6]3.5 – X(1)4.5 0-0 band of UF showing the perturbation in the Q(7.5) to Q(10.5) transitions. The inset shows the same pattern in the R(7.5), Q(8.5), and P(9.5) lines indicating that the perturbation is in the upper state.