Observation of the Δv = −4 vibronic sequence of the C2 Swan system

In this Letter, we report the near infrared spectrum of C2 generated in AC hollow-cathode discharges of ethylene/helium mixtures using high resolution laser absorption spectroscopy with concentration modulation. The high sensitivity achieved by this zero-background technique allows the first detection of the very weak Δv = −4 sequence of the d 3 Π g ← a 3 Π u system (Swan system). The detailed analysis of the spectrum between 13 750 and 14 130 cm−1 revealed that the (4, 8) band was heavily perturbed. On the other hand, the (5, 9) band was unperturbed to yield accurate molecular constants from least-squares fitting of the observed transitions.