Observation of Δ|k − l| = 3 Transitions in the ν3Band of H3O+

Abstract Although the vibration–rotation spectrum of H 3 O + has been studied extensively, there has been no experimental information on the value of the C rotational constant because of the absence of any observed Δ| k − l | ≠ 0 forbidden transitions. However, as pointed out by Verhoeve et al. in their far-infrared spectroscopic work [P. Verhoeve, M. Versluis, J. J. Ter Meulen, W. L. Meerts, and A. Dynamus, Chem. Phys. Lett. 161, 195–201 (1989)], a near degeneracy exists between some ( J , 0) + and ( J , 3) − ground state rotation–inversion levels, which causes sizable shifts in these levels due to off-diagonal centrifugal distortion. These shifts were observed in the ground state combination differences of the ν 3 fundamental transitions. We have observed 11 forbidden Δ| k − l | = 3 transitions to the ν 3 band induced by the mixing in the ground state and involving the J = 7, 9, and 11 levels, and determined the energy difference between different K levels, the C rotational constant, and the off-diagonal centrifugal distortion mixing coefficient.