The A 2Π–X 2Σ+ transition of HC2 isolated in solid argon

Fourier transform absorption spectra have been obtained between 700 and 7900 cm−1 at a resolution of 0.2 cm−1 for Ar:C2H2 samples codeposited at 12 K with a beam of argon atoms that had been excited in a microwave discharge. Detailed isotopic substitution studies have confirmed that the predominant product species is HC2, which contributes not only the absorptions previously assigned to its two stretching fundamentals but also several weaker absorptions in the 2000–3600 cm−1 spectral region and a prominent, complicated pattern of absorptions between 3600 and 7800 cm−1. The previous assignment of the 3611 cm−1 HC2 absorption as the CH-stretching fundamental is reviewed, and the assignment of an absorption at 2104 cm−1 as ν2+ν3 of ground-state HC2 is discussed. The near infrared absorption band system has been assigned to the à 2Π–X̃ 2Σ+ transition of HC2, extensively perturbed by interaction with high vibrational levels of the ground state. The position of the transition origin could not be definitively established. The previous assignment of gas-phase absorptions of HC2 at 3786, 4012, and 4108 cm−1 to ground-state combination bands has been confirmed, and carbon-isotopic data have been obtained for these bands. Because all of the HC2 absorptions observed in an argon matrix must originate in the X̃(000) state and because the observed matrix shifts are relatively small, these studies may provide a map to facilitate more detailed gas-phase studies.