Infrared spectroscopy of carbo-ions. IV. TheA 2Πu–X 2Σ+gelectronic transition of C−2

The infrared spectrum of the A 2Πu←X 2∑+g electronic transition of C−2 has been observed under high resolution and analyzed. Three bands (v′←v)=(0,0), (1,1) and (0,1) have been observed; the first two bands were observed by using the difference laser frequency system in the frequency range of 3960–3780 cm−1 and the last by using a diode laser in the frequency range of 2210–2120 cm−1. A gas mixture of 50 mTorr of acetylene and 7 Torr of He was used for the ac discharge in an air-cooled and a water-cooled multiple inlet–outlet discharge tube. The simplicity of the optimum gas mixture suggested that C−2 is produced directly by simple dissociative electron attachment of acetylene. Altogether 103 absorption lines have been observed and accurately measured. Most of them are P, Q, and R form branches of allowed F1↔F1, F2↔F2 transitions although some forbidden F1↔F2 transitions and O and S transitions have also been measured. The hot bands (1,1) and (0,1) have been observed with intensity which is less than that for the (0,0) by a factor of only 2 or 3, in spite of the fact that the Franck–Condon factors of these transitions are lower than that for the (0,0) by 2.0 and 3.2, respectively, indicating that the vibrational temperature of C−2 in the He plasma is extremely high. All observed transitions are simultaneously fit to the formula for a 2Πu←2∑+g transition. The formulation of Brown and Watson has been used for the Hamiltonian for the 2Πu state. Molecular constants for C−2 in the two electronic states have been determined from the least-squares fitting. The constants are compared with those of other species isoelectronic to C−2. A short discussion is given about the astrophysical implication of the spectrum.