l-type doubling of the kl = −1 levels in the microwave spectrum of the C4v molecule BrF5

Measurements are reported for the J 3 → 4, 4 → 5, and 5 → 6 transitions of BrF₅ in the excited vibrational states v₅(B₁) = 1 and v₉(E) = 1. These two states are nearly degenerate and an unusually strong Coriolis interaction perturbs both excited state spectra. New expressions are obtained for the E-species absorption frequencies which are valid in a strong Coriolis resonance situation. The analysis of the E-state spectrum has provided the first experimental observation of the doubling of the kl = −1 levels predicted for molecules with C_4v symmetry.