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 Title: SUB-DOPPLER INFRARED SPECTROSCOPY OF JET COOLED CH2I RADICAL: CH2 STRETCH VIBRATIONS AND “CHARGE-SLOSHING” INTENSITY DYNAMICS Author(s): Scrape, Preston G. Contributor(s): Nesbitt, David; Lesko, Daniel; Kortyna, Andrew Subject(s): Radicals Abstract: Iodomethyl radical ($\mathrm{CH_2I}$) is relevant to atmospheric chemistry, especially marine boundary layer dynamics, with recent attention arising from its use as novel precursor for Criegee intermediates ($\mathrm{CH_2OO}$). As a first step towards the spectroscopic investigation of a Criegee intermediate, we have pursued high resolution characterization of the CH$_2$I radical in our slit jet discharge spectrometer. The methyl iodide radical is generated by seeding $\mathrm{CH_2I_2}$ into a Ne/He/H$_2$ mixture in a pulsed slit discharge, produced through either electron dissociative attachment to form iodine anions or hydrogen abstraction of iodine, with subsequent cooling in a supersonic expansion to 16 K. Infrared absorption in the CH symmetric stretch vibrational band is observed at high single-to-noise ratio (S/N = 25:1), yielding a symmetric stretch band origin at $3046.9527 \pm 0.0006 \, \mathrm{cm}^{-1}$. The sub-Doppler rotational structure is fitted to a rigid-rotor Hamiltonian with spin-rotation coupling, generating principal rotational constants and the spin-orbit coupling tensor for the vibrationally excited state. Interestingly, an extensive search for the asymmetric stretch mode yielded null results, despite simple bond-dipole model predictions of three-fold larger absorption intensities for the asymmetric vs.\ symmetric stretch band. We conclude that the asymmetric stretch absorption intensity must be at least a factor of 25 below that of the symmetric stretch. {\em Ab initio} calculations indicate that enhancement of the symmetric vs.\ asymmetric stretch intensity arises from charge sloshing'' motion of electrons in the highly polar carbon-iodine bond of the correct A$_1$ symmetry. 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/2142/100700 DOI: 10.15278/isms.2018.WD11 Other Identifier(s): WD11 Date Available in IDEALS: 2018-08-172018-12-12
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