Sub-millimeter wave spectroscopy of the C 3H radical: Ro-vibrational transitions from ground to the lowest bending state

Abstract

Linear C 3H in its (X 2 Π) electronic ground state possesses strong Renner–Teller coupling in the two lowest bending modes, ν 4 and ν 5. The 2 Σ μ level of the v 4 = 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 2 Π 3/2 ground state to the 2 Σ μ lowest vibrational state were performed using a Terahertz spectrometer equipped with a supersonic jet nozzle. Rotational levels of the 2 Π 3/2 and v 4 = 1( 2 Σ μ ) 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 2 Π 3/2 − 2 Σ μ ro-vibrational transitions. The search for ro-vibrational transitions was facilitated by measurements on pure rotational transitions in the 2 Π 1/2, 2 Π 3/2 and v 4 = 1( 2 Σ μ ) states, substantially extending the former data set published by Yamamoto et al. Data analysis yields an accurate value for the v 4 = 1( 2 Σ μ ) energy level which has been found to lie 609.9771(42) GHz or 20.34664(14) cm −1 above the 2 Π 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.