Near infrared absorptions of CH4/He plasma: the Phillips band system of C2

Zero-background laser absorption spectroscopy using concentration modulation has been applied to study the near infrared absorption spectrum of the discharge of CH4 and He at a ratio of about 1:170. In the 10 300–14 250 cm−1 region, a total of 13 vibronic bands have been observed with 11 of them assigned to the very weak Phillips band system of C2. Combining these transitions with those observed in emission by Douay et al. [J. Mol. Spectrosc. 131 (1988) 250], a set of spectroscopic constants with high accuracy have been obtained.