The direct l-type resonance spectrum of CF3CCH in the vibrational state ν10 = 2

Abstract The direct l -type resonance spectrum of CF 3 CCH in the vibrational state ν 10  = 2 has been measured by means of waveguide microwave Fourier transform spectroscopy in the range 8–26 GHz. Two types of direct l -type resonance transitions induced by the (Δ k  = ±2, Δ l  = ±2) interaction could be observed: 262 transitions following the Δ J  = 0, Δ k  = Δ l  = 2 selection rule covering values of J  = 17–64 and G  = ∣ k  −  l ∣ from 2 to 15, and 75 transitions following the Δ J  = 0, Δ k  = Δ l  = 4 selection rule covering values of J  = 44–70 and G up to 3. The strong (2, 2) resonance furthermore allowed the observation of A 1 –A 2 splittings of the k  =  l  = ±2 states from J  = 63–70. The transitions with G  = 3 showed splittings due to the (4, −2) and (0, 6) interactions. The corresponding energy level systems and part of the Hamiltonian matrix are discussed. Strong perturbations due to Δ( k  −  l ) = 3 interactions coupling the states k  = ±1, l  = ±2 and k  = ±4, l  = ±2 made possible the observation of perturbation-allowed transitions with selection rule k  = ±1, l  =± 2 ↔  k  = 0, l  = ±2. Additionally, the J  = 2–1 and 3–2 rotational transitions have been measured. A multiple fitting analysis has been performed in which the experimental data have been fitted using five reduced forms of the effective Hamiltonian as proposed by Sarka and Harder [J. Mol. Spectrosc. 197 (1999) 254]. Parameters up to sixth order have been determined including the axial rotational constant A for both values of ∣ l ∣ and the unitary equivalence of the determined parameter sets has been demonstrated.