Knowledge Bank

\maketitle Precise measurements on rovibrational transitions of the$\nu 4$($\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma \mu$) CCH bending mode of (X$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$) C$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$H 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 C$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ 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 ($\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma \mu$) component of the $\nu 4$ bending mode towards lower energies and into the submillimeter region. Pure rotational transitions in the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 1/2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 3/2$ ground state levels and in the \textit{v}$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$ = 1 ($\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma \mu$) excited vibration state were recorded, extending earlier data published by Yamamoto \textit{et al.} } 348, 363-369 (1990)} In addition, some rovibrational transitions between the ground state and the \textit{v}$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$ = 1 ($\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma \mu$) state have been identified. Thus the vibrational energy of the $\nu 4$($\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma \mu$) state was determined precisely as 609977.1(42) MHz or 20.34664(14) cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. Furthermore, the Coriolis coupling constant between the ground and $\nu 4$($\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma \mu$) state has also been determined with greatly improved precision.