The near degenerate ν3 and ν6 states of HSiD3 revealing Δ(k-I)=0, ±3, and ±6 interactions

The infrared spectrum of HSiD3 has been recorded with a resolution of 3.3 x 10(-3) cm(-1) in the 600-775 cm(-1) region. The fundamentals nu(3) (A(1)). nu(0) = 675.501 cm(-1), and nu(6) (E), nu(0) = 682.687 cm(-1) have been measured and rotationally assigned up to J' = 30. Owing to the closeness of the band centers and the strength of the x-, gamma-Coriolis interaction, zeta(36)(gamma) = 0.62, perturbations both of positions and intensities are strong. Since HSiD, is an oblate top near the spherical top limit, the rotational energy level structure is such that numerous avoided crossings with Delta(k - l) = +/-3 and +/-6 are called forth by the Coriolis resonance. These give rise to "forbidden" transitions, 209 of which with J " and K " less than or equal to 30, and Delta K up to +/-6, were observed. These enabled a significant improvement of the coefficients of the K-dependent ground state energy terms, C-0 = 1.433 1268(12) cm(-1). D-K(0) = 2.851 6(57) x 10(-6) cm(-1), and H-K(0) = - 1.728(85) X 10(-10) cm(-1). Parameters of the v(3) = 1 and nu(6) = 1 excited states were determined with two models corresponding to two different effective Hamiltonians. The model strictly obeying the constraints proposed by E. I. Lobodenko, O. N. Sulakskina, V. I. Perevalov, and V. G. Tyuterev (J. Mol. Spectrosc. 126, 159-170 (1987)) requires 45 adjustable parameters and fits the data with J greater than or equal to 22 only poorly while, with an model employing 44 free parameters, all data up to J' = 30 were fitted with sigma = 1.72 x 10(-4) cm(-1). A positive sign of the Coriolis intensity perturbation was found, and the transition moment ratio was determined, \M-6:M-3\ = 0.70(5). (C) 2000 Academic Press.