REINVESTIGATING THE $\tilde{A}-\tilde{X}$ SPECTRUM OF ETHYNYL RADICAL: NEW INSIGHTS INTO THE SPECTROSCOPY OF $^2\Pi-^2\Sigma^+$ VIBRONIC BANDS

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2009

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Ohio State University

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The ethynyl radical, C$_2$H, is an important reactive intermediate both in combustion processes, as it is readily formed in an acetylene (C$_2$H$_2$) flame, and in the chemistry of the interstellar medium, where it is suspected to be involved in the formation of longer carbon chain species (C$_n$H). We have recently interrogated several of the vibronic transitions to the low-lying excited $\tilde{A}$ state from the vibrationless level of the ground electronic state via high resolution infrared spectroscopy. This was done using direct absorption laser spectroscopy in a slit-jet discharge supersonic expansion of C$_2$H$_2$ diluted in a Neon/Helium gas mixture. In comparing our spectra with those already published using magnetic rotation spectroscopy \textbf{123}, 486~(1987).}$^,$ \textbf{82}, 3479~(1985).}, we find discrepancies between rovibronic frequencies in the 0$_0^0$ band at 3600~cm$^{-1}$. The inconsistency is localized in the excited state, by comparison of 2-line combination differences with mm-wave measurements of the ground state by Thaddeus and coworkers \textbf{127}, 114320~(2007).}. Calculating the $^2\Pi$ energy levels using both Hund's case (a) and (b) basis sets and revisiting the analysis in the aforementioned work, we have determined that the discrepancies arise from a parity mislabeling of the lambda-doubled excited states. The improved low J signal intensities and resolution of satellite transitions that are observable under sub-Doppler, jet-cooled conditions complement the previous data and permit refinement of the rotational, spin-rotational, and lambda-doubling constants.

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W.~B.~Yan, C.~B.~Dane, D.~Zeitz, J.~L.~Hall and R.~F.~Curl, \textit{J.~Mol.~Spectrosc.R.~F.~Curl, P.~G.~Carrick and A.~J.~Merer, \textit{J.~Chem.~Phys.T.~C.~Killian, C.~A.~Gottlieb and P.~Thaddeus, \textit{J.~Chem.~Phys.
Author Institution: JILA, National Institute of Standards and Technology and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado; 80309; Department of Chemistry and Rice Quantum Institute, Rice University, Houston, Texas 77005

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