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 Title: EXPERIMENTAL AND THEORETICAL INVESTIGATIONS OF THE THRESHOLD PHOTOELECTRON SPECTRUM OF THE CH2 RADICAL Author(s): Coudert, L. H. Contributor(s): Alcaraz, C.; Garcia, G. A.; Loison, J.-C.; Holzmeier, F.; Gans, B. Subject(s): Mini-symposium: New Ways of Understanding Molecular Spectra Abstract: The methylene cation \ce{CH2+} is spectroscopically poorly characterized as it is difficult to produce in large amounts. It is subject to the Renner-Teller effect giving rise to ground $\widetilde{X}^+\, ^2A_1$ and excited $\widetilde{A}^+\, ^2B_1$ electronic states. Photoelectron spectroscopy of the methylene radical \ce{CH2} allows us to gain information about both \ce{CH2} and its cation. The former is also theoretically challenging as it is a very non-rigid species characterized by a barrier to linearity of less than 2000~cm$^{-1}$ in its ground $\widetilde{X}\, ^3B_1$ electronic state. The first photoelectron spectra of \ce{CH2} were investigated using pulsed-field-ionization zero-kinetic-energy spectroscopy.\footnote{\label{merkt}Willitsch {\em et al.,} {\em J.\ Chem.\ Phys.}~{\bf 117} (2002) 1939; and Willitsch \& Merkt, {\em ibid.}~{\bf 118} (2003) 2235} A rotationally resolved spectrum containing $\widetilde{X}^+\, ^2A_1 \leftarrow \widetilde{X}\, ^3B_1$ transitions was recorded from 83600 to 84070~cm$^{-1}$ and analyzed in terms of \ce{CH2+} rotational constants. The threshold photoelectron spectrum of \ce{CH2} has been recorded from 9.8 to 12~eV (79040 to 96800~cm$^{-1}$) using a recently developed flow tube reactor\footnote{Garcia {\em et al.,} {\em J. Chem. Phys.}~{\bf 142} (2015) 164201} and VUV synchrotron radiation. This new spectrum spans a larger energy range than the previous ones,\textsuperscript{$a$} but with less resolution. It displays narrow and broad features due respectively to the $\widetilde{X}^+\, ^2A_1 \leftarrow \widetilde{X}\, ^3B_1$ and $\widetilde{A}^+\, ^2B_1 \leftarrow \widetilde{X}\, ^3B_1$ ionizing transitions. Using new {\em ab initio} potential energy surfaces and available ones,\footnote{Jensen \& Bunker, {\em J.\ Chem.\ Phys.}~{\bf 89} (1988) 1327; and Jensen, Brumm, Kraemer \& Bunker, {\em J.\ Molec.\ Spectrsoc.}~{\bf 172} (1995) 194} the photoelectron spectrum is currently being computed using two models. The first one accounts for the large amplitude bending mode and the rotation only; the second one, also accounts for the stretching modes. The experimental and theoretical spectra will be discussed in the paper. Issue Date: 06/20/18 Publisher: International Symposium on Molecular Spectroscopy Citation Info: APS Genre: Conference Paper / Presentation Type: Text Language: English URI: http://hdl.handle.net/2142/100563 DOI: 10.15278/isms.2018.WB07 Other Identifier(s): WB07 Date Available in IDEALS: 2018-08-172018-12-12
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