SUB-DOPPLER INFRARED SPECTROSCOPY OF JET COOLED CH2Br RADICAL: CH2 STRETCH VIBRATIONS

The recent use of bromomethyl radical as a novel precursor for producing the simplest Criegee intermediates ($\mathrm{CH_2OO}$) motivates our high-resolution investigation of $\mathrm{CH_2Br}$. Cold $\mathrm{CH_2Br}$ (rotational temperature of 20 K) is generated by seeding $\mathrm{CH_2Br_2}$ into a Ne/He pulsed supersonic slit-jet discharge expansion. Sub-Doppler infrared absorption in the CH$_2$ symmetric stretch band is fully resolved at high single-to-noise ratio, yielding band origins measured at $3052.9781 \pm 0.0004 \, \mathrm{cm}^{-1}$ for CH$_2 \,\! ^{79}$Br and $3052.9775 \pm 0.0004 \, \mathrm{cm}^{-1}$ for CH$_2 \,\! ^{81}$Br, respectively. The rotationally resolved structure is fitted to a non-rigid rotor Hamiltonian with spin-rotation coupling, furnishing rotational constants and the spin-orbit coupling tensor for the vibrationally excited state. The results are consistent with a vibrationally averaged planar $\pi$ radical with unpaired electron spin density in a partially filled p$_{\pi}$-orbital on the central carbon atom. These measurements complete the set of high-resolution investigations of the symmetric-stretch mode for mono-halogen substituted methyl radicals (CH3X, X = F, Cl, Br, I). We use this opportunity to examine trends in molecular properties with variation of the substituent halogen atom, for example, regarding vibrationally averaged geometries and symmetric stretch band origins. A combination band is also analyzed, likely originating from the lowest-energy, out-of-plane vibrational bending mode. Evidence of hyperfine structure is observed as well. Interestingly, despite high signal to noise on the symmetric CH stretch vibrational mode, we find no evidence for the asymmetric CH stretch spectrum at our current sensitivities, which we interpret as resulting from strong enhancement of the symmetric stretch band by vibrationally mediated "charge-sloshing" dynamics in the C-Br bond