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Author(s):Coudert, L. H.
Subject(s):Mini-symposium: Large Amplitude Motions
Abstract:Quasi-linear molecules display a large amplitude bending mode allowing them to sample their linear configuration. This leads to a strong coupling between the overall rotation and the bending mode and to a singularity in their Hamiltonian. Quasi-linearity has been extensively studied in bent light triatomic molecules like water molecule and the amidogen (NH$_2$) and methylene (CH$_2$) radicals as they display a low barrier to linearity ranging from 12\,000~cm$^{-1}$ for NH$_2$ to less than 2000~cm$^{-1}$ for CH$_2$.\footnotetext[1]{Jungen, Hallin, \& Merer, {\em Molec.\ Phys.}~{\bf 40} (1980) 25; Bunker, Jensen, Kraemer, \& Beardsworth, {\em J.\ Chem.\ Phys.}~{\bf 85} (1986) 3724; Partridge \& Schwenke, {\em J.\ Chem.\ Phys.}~{\bf 106} (1997) 4618}$^a$ Their rovibrational energy levels can be computed with almost spectroscopic accuracy using variational approaches or, if a higher accuracy is needed, with reduced dimensionality models such as the effective Bending-Rotation approach,\footnotetext[2]{Coudert, {\em Mol.\ Phys.}~{\bf 96} (1999) 941}$^b$ already applied to treat the anomalous centrifugal distortion of the water molecule\footnotetext[3]{Yu, Pearson, Drouin, Martin-Drumel, Pirali, Vervloet, Coudert, M\"uller, \& Br\"unken, {\em J.\ Mol.\ Spectrosc.}~{\bf 279} (2012) 16}$^c$ and the amidogen radical.\footnotetext[4]{TD03, Martin-Drumel, Pirali, \& Coudert, 72nd ISMS, Urbana-Champaign, June 19--23, 2017}$^d$ In this talk, the Bending-Rotation approach$^b$ will be tentatively applied to the fitting of high-resolution data pertaining to the vibronic ground state of the methylene radical. The data from previous experimental investigations will be considered in the analysis. Due to the very low barrier to linearity$^a$ of CH$_2$, its rovibrational energy levels are somewhere in between those of a bent molecule and those of a linear molecule leading to anomalous centrifugal distortion.\footnotetext[5]{Br\"unken, M\"uller, Lewen, \& Giesen, {\em J.\ Chem.\ Phys.}~{\bf 123} (2005) 164315}$^e$ Its rotational energy cannot be computed using the standard approaches developed for semi-rigid molecules. The present analysis will, therefore, provide us with a test for the Bending-Rotation approach.$^b$ In the talk, we will try to see how well the anomalous centrifugal distortion is taken into account and we will try to retrieve the dependence of the spin-rotation coupling on the bending angle.
Issue Date:22-Jun-20
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:APS
Date Available in IDEALS:2020-06-26

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