Laser-induced fluorescence spectroscopy of the C 2Σ+−X 2Π1/2 band system of jet-cooled CCN radical

Laser-induced-fluorescence spectra of the C̃ 2Σ+−X̃ 2Πr system of the CCN radical were observed under jet-cooled conditions. The radical was generated in a supersonic expansion by a pulsed electric discharge of methyl cyanide diluted in Ar. Sixteen C̃ 2Σ+(v1,v2l,v3)–X̃ 2Π1/2(0,00,0) vibronic bands with v1=0–3, v2=0–2 (l=0–2), and v3=0–2 were observed, including some very weak vibronically induced parallel-type transitions with v2=1 (l=1). Rotational analyses were performed to give the band origins and the rotational constants for vibrationally excited states of all the three normal modes and their combinations in the C̃ 2Σ+ state. From the determined band origins, the term value and the vibrational parameters in the C̃ 2Σ+ state have been determined: Te=24 627.1(20) cm−1, ω1=1903.1(11) cm−1, ω2=469.88(38) cm−1, ω3=1256.6(40) cm−1, and g22=0.86(11) cm−1. A combination of the present ω2 frequency in the C̃ state with the data on the hot bands reported so far in literature has yielded the vibronic structures in the X̃ 2Πr and à 2Δ states, which are subject to a prominent Renner–Teller effect.