Sub-millimeter wave spectroscopy of the C3H radical: Ro-vibrational transitions from ground to the lowest bending state

Linear C3H in its (X2Π) electronic ground state possesses strong Renner-Teller coupling in the two lowest bending modes, ν4 and ν5. The 2Σμ level of the v4 = 1 bending mode is shifted towards lower energies and is supposed to lie only 20.3 cm−1 above the ground state [S. Yamamoto, S. Saito, H. Suzuki, S. Deguchi, N. Kaifu, S. Ishikawa, M. Ohishi, Astrophys. J. 348 (1990) 363]. In the present study, first measurements of ro-vibrational transitions from the 2Π3/2 ground state to the 2Σμ lowest vibrational state were performed using a Terahertz spectrometer equipped with a supersonic jet nozzle. Rotational levels of the 2Π3/2 and v4 = 1(2Σμ) state are close in energy and a crossing of the rotational energy ladders occurs between J = 24.5 and 25.5. A strong vibronic coupling leads to a significant intensity enhancement of 2Π3/2 − 2Σμ ro-vibrational transitions. The search for ro-vibrational transitions was facilitated by measurements on pure rotational transitions in the 2Π1/2, 2Π3/2 and v4 = 1(2Σμ) states, substantially extending the former data set published by Yamamoto et al. Data analysis yields an accurate value for the v4 = 1(2Σμ) energy level which has been found to lie 609.9771(42) GHz or 20.34664(14) cm−1 above the 2Π ground state. Furthermore, the value of the vibronic coupling constant β has been improved significantly and determined as 1231.77(51) MHz. The new set of spectroscopic parameters obtained in the present study permits very reliable frequency predictions into the Terahertz region.