Absorption of 12CH3D at 6–10 μm: Triad ν3, ν5, ν6

Abstract The absorption of 12 CH 3 D at 6–10 μm was recorded under vacuum with a resolution of 0.0054 cm −1 . For the first time the ν 5 band was assigned extensively, allowing the analysis of ν 3 , ν 5 , and ν 6 together within a “triad model,” including a rigorous treatment of all the Coriolis couplings involved. The assignments in the spectral range were anlarged from the original 1000 to the present 3500. The newly assigned lines essentially concerned the ν 5 band and a great number of perturbation-allowed transitions (about 30% of all observations). Ground state energy parameters were refined from 2641 combination differences involving J up to 19 and |Δ K | up to 6; the standard deviation of the fit was 0.0003 cm −1 . A set of 35 triad upper state energy parameters was derived, reproducing all observations with a standard deviation of 0.0076 cm −1 . The three dipole moment derivatives involved in the intensity calculations were estimated theoretically, by taking advantage of the recent accurate determination of the band strengths S 3 and S 4 of 12 CH 4 . Finally, about 6000 triad transitions predicted with linestrengths at least equal to 4 × 10 −25 cm·molecule −1 were tabulated with assignments, wavenumbers, linestrengths, and lower and upper energy levels.