Optical Detection of Microwave Transitions Between Excited Electronic States of CN and the Identification of the Transitions Involved

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Abstract
Thirteen microwave transitions in the frequency range from 8800 to 9900 MHz have been observed between excited electronic states of CN. These correspond to all allowed transitions, ΔF=0, ±1, in the K′=4 perturbation complex between the three hyperfine levels of the unperturbed component of the Λ doublet of the AΠ3322 (v=10) level and the three hyperfine levels in each of the perturbed and the unperturbed components of the spin doublet of the BΣ+2 (v=0) level. The identification of all allowed transitions permits a unique determination of nine of the 12 hyperfine energy levels of this perturbation complex. The measured energy separation in this level of the Σ2 state is interpreted in terms of the electronic structure of the CN molecule in the following paper. The experiment is the first microwave measurement of the fine and hyperfine structure of an excited electronic state of a molecule and also is the first accurate measurement of the hyperfine structure of a Σ2 state. CN was produced predominantly in the metastable AΠ2 state by a chemical reaction when methylene chloride was added to a nitrogen afterglow. Resonant microwave pumping from the Π state increased the population of the three hyperfine levels of each Σ state by 0.1 to 5%. The population change was detected by measuring an increase in the intensity of the BΣ+2→XΣ+2 (0,0) violet band of CN near 3875 Å. A rigid-sphere collision diameter of 4.4 Å was calculated from linewidth measurements of the microwave Π→Σ transition. Collisional population of rotational levels adjacent to K′=4 also was observed.