Very high rotational excitation of CO in a cooled electric discharge through carbon monoxide

Abstract

Infrared emission from CO12 and CO13, excited in the cathode region of a discharge tube immersed in liquid nitrogen, was recorded by Fourier-transform spectrometry at a resolution of 0.005 cm−1. The Δv=1 sequence bands recorded in the 2500–1800 cm−1 spectral interval, indicate the existence of three different rotational populations; (i) molecules in the zero-ground level with Trot≈100 K (responsible for reabsorption of part of the 1–0 emission band); (ii) molecules with Trot≈275 K (maximum intensity for Jmax′≈6 in each band, Tvib≈3000 K for v′=2–4, Tvib≈8600 K for v′=5–13); (iii) molecules with v′ limited to 6, for which R-rotational lines are observed for J′ values between 50 and 120 (Jmax′≈90, non-Boltzmannian population distribution). The full-width at half-maximum (FWHM) of all the observed lines is less than 0.007 cm−1. A Doppler width of 0.005 cm−1 and translational temperature Ttr≈280 K can be deduced. Such high-J levels of the CO molecule had never been observed in the laboratory. In the absorption spectrum of the Sun photosphere, the same lines present FWHM values 5–8 times larger. The best available Dunham coefficients are checked to reproduce the high-J lines wave numbers to at least 0.001 cm−1. Dissociative recombination of the dimer (CO)2+ cation, which is likely to be formed in our experimental conditions, is discussed as a possible mechanism to produce CO fragments with very high rotational excitation, while keeping vibrational excitation limited to v′=6.