Observation of Delta|k - l | = 3 Transitions in the nu3 Band of H3 O+

Although the vibration-rotation spectrum of H3 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 Delta|k - l | not equal 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 nu3 fundamental transitions. We have observed 11 forbidden Delta|k - l | = 3 transitions to the nu3 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.