PURE ROTATIONAL SPECTRA OF HNCO IN THE FAR-INFRARED - 3 EXCITED BENDING STATES

Pure rotational transitions of the quasilinear molecule HNCO in the ex cited bending states, upsilon(4) = 1, upsilon(5) = 1, and upsilon(5) = 1, were measured with high resolution in the far infrared employing t he Giessen Fourier transform spectrometer. Besides a-type transitions with high rotational quantum numbers, b-type transitions from K-a = 1 <-- 0 up to K-a = 5 <-- 4 could be identified for nu(4) and nu(6) and from K-a = 2 <-- 1 up to K-a = 7 <-- 6 for nu(5). Effective molecular parameters were determined using the linear molecule Hamiltonian to re produce the measured line positions for each subband. Several forbidde n transitions between the in-plane bending vibrations nu(4) and nu(5) and the out-of-plane bending vibration nu(6) could be identified, whic h enabled us to determine precisely the relative positions of the vibr ational energy levels. The strong a-type Coriolis interaction, which c auses the forbidden transitions, have been analyzed with the help of a Pade-formulation to describe the K-a relational structure of the bend ing excited states. The considerably weaker b-type Coriolis interactio n, which shifts the effective rotational parameters, has also been dis cussed. (C) 1996 Academic Press, Inc.