New Investigations of the AΔ-XΠ Band System in the CH Radical and a New Reduction of the Vibration-Rotation Spectrum of CH from the ATMOS Spectra

Seven bands of the A²Δ-X²Π system of the CH radical have been photographed in emission from a Geissler tube using conventional spectroscopic techniques. Under high resolution and using Th lines as standards, as well as an interferometric comparator equipped with a photoelectric scanning device, the 0-0, 1-1, 2-2, 0-1, and 1-2 bands have been rephotographed and the 3-3 and 2-3 bands, with a total number of 144 lines, have been recorded for the first time. In the previously reported (J. Mol. Spectrosc.134, 305, 1989; 147, 16, 1991) evaluation of the ATMOS spectrum of CH for determining the molecular parameters we found total interparameter correlations between some of them, due to the absence of high-J lines of the P and Q branches. With the help of the precise constants obtained from our 0-0 band measurements (12 branches with J_max = 24.5, f = 181, σ = 0.0025 cm⁻¹) of the A²Δ-X²Π transition, we have been able to remove the correlations and obtain more accurate molecular parameters for the X²Π ground state. These constants have subsequently been used to derive new exact molecular parameters for the A²Δ , ν = 1, 2, and 3 levels of CH. The Λ-doubling constants in the A²Δ state were obtained for the first time. The complex vibrational analysis has been carried out to determine the equilibrium molecular constants. Also, RKR potentials and r-centroids have been calculated for both the combining states, as well as Franck-Condon factors for the A-X system.