Observation of Delta vertical bar k-l vertical bar=3 transitions in the nu(3) band of H3O+

Although the vibration-rotation spectrum of H3O+ 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. Chern. Phys. Lett. 161, 195-201 (1989)], a near degeneracy exists between some (J, 0)(+) and (J, 3)(-) ground state rotation-inversion levers, 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 Ni fundamental transitions. We have observed 11 forbidden Delta\k - l\ = 3 transitions to the nu(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. (C) 1997 Academic Press.