Jet emission spectra in the Schumann region: The L2Σ − → A2Π i transition of CN

Abstract

The emission spectrum of the CN radical in the ultraviolet and Schumann regions from approximately 3500 to 1500 Å has been photographed in higher orders of a 10.6-m concave grating vacuum spectrograph. Of the two sources used to generate the spectra one is a radiofrequency discharge in flowing pure HCN, the other a dc discharge in supersonically expanding Ar seeded with HCN. An extensive system of red-degraded double-headed bands appears in the jet emission spectrum between 2000 and 1500 Å. The bands have a rotational temperature of ∼40 K and arise in transitions from eight vibrational levels of a new 2Σ − electronic state, labeled L 2Σ −, to the well-known A 2Π i upper state of the CN red system. Vibrational and rotational constants have been determined; the new state has ω e = 1165.80 and B e = 1.3337 cm −1. Possible reasons for the lack of emission from v = 2 of L 2Σ − are discussed.