M. CARIS, T. F. GIESEN, H. S. P. MÜLLER, F. LEWEN, S. SCHLEMMER, I. PHYSIKALISCHES INSTITUT, UNIVERSITY OF COLOGNE, ZÜLPICHER STR. 77, 50937 KÖLN, GERMANY; C. DUAN, COLLEGE OF PHYSICAL SCIENCE AND TECHNOLOGY, HUAZHONG NORMAL UNIVERSITY, WUHAN 430079, P.R. CHINA; K. M. T. YAMADA, AIST, TSUKUBA-WEST, 16-1 ONOGAWA, TSUKUBA, 305-8569 JAPAN.
Precise measurements on rovibrational transitions of the 4(2 ^µ) CCH bending mode of (X2 ) C3H have been performed using the Cologne Supersonic Jet Spectrometer for Terahertz Applications (SuJeSTA). The molecules were formed in a supersonic jet expansion when a gas mixture of C2H2 and CO diluted in He was introduced to an electrical discharge nozzle. Radiation of a backward wave oscillator (BWO) probes the molecular beam 40 mm downstream and spectra in the frequency range 360-600 GHz were recorded.
A strong Renner-Teller coupling effect shifts the (2 ^µ) component of the 4 bending mode towards lower energies and into the submillimeter region. Pure rotational transitions in the 2 1/2 and 2 3/2 ground state levels and in the v 4~=~1 (2 ^µ) excited vibration state were recorded, extending earlier data published by Yamamoto et al. In addition, some rovibrational transitions between the ground state and the v 4~=~1 (2 ^µ) state have been identified. Thus the vibrational energy of the 4(2 ^µ) state was determined precisely as 609977.1(42)~MHz or 20.34664(14)~cm-1. Furthermore, the Coriolis coupling constant between the ground and 4(2 ^µ) state has also been determined with greatly improved precision.