IN HONOR OF PROF. DR. HANS B ÜRGER FOR HIS MANY VALUABLE CONTRIBUTIONS TO SCIENCE
The υ 6 = 2 vibrational state of the main isotopomer of trifluorosilane, 28 SiHF 3 , has been investigated in the centimeter-and millimeter-wave ranges. Rotational spectra following the J = 1, k = l = 0 selection rule have been measured up to J = 24 and K = 23 and for both values of |l|. Two types of direct l-type resonance transitions induced by the ( l = k = ±2) interaction could be observed by means of waveguide Fourier transform microwave spectroscopy in the range 8-26 GHz: 252 transitions following the J = 0, l = k = ±2 selection rule covering values of J = 7-39 and G = |k -l| from 1 to 18 and 90 transitions following the J = 0, l = k = ±4 selection rule covering values of J = 17-52 and G from 1 to 4. Due to the strong (2,2) resonance, 18 A 1 -A 2 splittings of the k = l = ±2 states from J = 36-53 could also be observed. Accidental near-degeneracies lead to strong perturbations due to (kl) = ±3 interactions, enabling the observation of perturbation-allowed transitions with selection rules k = ±3(l = ±2) ↔ ±4(±2), ±2(±2), A + ↔ ±1(∓2), A -and ±1(0) ↔ ±6(±2). In a multiple-fit analysis the experimental data have been refined using five reduced forms of the effective Hamiltonian as proposed by Sarka and Harder [J. Mol. Spectrosc. 197, 254-261 (1999)]. Parameters up to seventh order have been determined including the axial rotational constant C for both values of |l| and the vibrational separation of the |l| = 0 and 2 states. The unitary equivalence of the determined parameter sets has been demonstrated up to fifth order. Differences of the rotational constants in the various parameter sets have been explained by the theory of reduction. Sign relations of the fitted parameters and general features of the direct l-type resonance spectrum in a υ t = 2 level are discussed.