Title: High-resolution infrared spectroscopy of jet cooled CH₂Br radicals: The symmetric CH stretch manifold and absence of nuclear spin cooling

Abstract

Direct laser absorption of a slit supersonic discharge expansion provides the first high-resolution spectroscopic results on the symmetric CH stretch excitation (ν₁) of the bromomethyl (CH₂Br) radical in the ground electronic state. Narrowband (<1 MHz) mid-infrared radiation is produced by difference-frequency generation of two visible laser beams, with the open shell halohydrocarbon radical generated by electron dissociative attachment of CH₂Br₂ in a discharge and rapidly cooled to T_rot = 18 ± 1 K in the subsequent slit-jet supersonic expansion. A rovibrational structure in the radical spectrum is fully resolved, as well as additional splittings due to spin-rotation effects and ⁷⁹Br/⁸¹Br isotopologues in natural abundance. Spectroscopic constants and band origins are determined by fitting the transition frequencies to a non-rigid Watson Hamiltonian, yielding results consistent with a vibrationally averaged planar radical and an unpaired electron in the out-of-plane p_π orbital. Additionally, extensive satellite band structure from a vibrational hot band is observed and analyzed. The hot band data is compared to CFOUR/VPT2 (CCSD(T)cc-pVQZ) ab initio anharmonic predictions of the vibration rotation alpha matrix, which permits unambiguous assignment to CH₂ symmetric-stretch excitation built on the singly excited CH₂ out-of-plane bending mode (ν₁ + ν₄ ← ν₄). Longitudinal cooling of the Doppler widthmore » in the slit-jet expansion geometry also reveals partially resolved hyperfine structure on transitions out of the lowest angular momentum states in excellent agreement with predictions based on microwave studies. High level ab initio MOLPRO calculations [CCSD(T)-f12b/VnZ-f12 (n = 3, 4, CBS)] are also performed with explicitly correlated f12 electron methods for the out-of-plane CH₂ bending mode over the halogen series CH₂X (X = F, Cl, Br, I), which clearly reveals a non-planar geometry for X = F (with a ΔE ≈ 0.3 kcal/mol barrier) and yet planar equilibrium geometries for X = Cl, Br, and I. Finally, a detailed Boltzmann analysis of the transition intensities provides support for negligible collisional equilibration of the entangled H atom nuclear spin states on the few hundred microsecond time scale and high collision densities of a slit supersonic expansion.« less

Authors:

Kortyna, Andrew  ^[1];

• Search DOE PAGES for author "Kortyna, Andrew"
• Search DOE PAGES for ORCID "0000-0002-8911-6864"

Doney, Kirstin  ^[1];

• Search DOE PAGES for author "Doney, Kirstin"
• Search DOE PAGES for ORCID "0000-0002-9829-0346"

Nesbitt, D. J.  ^[2]

• Search DOE PAGES for author "Nesbitt, D. J."
• Search DOE PAGES for ORCID "0000-0001-5365-1120"

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+ Show Author Affiliations

1. Univ. of Colorado, Boulder, CO (United States). JILA; National Inst. of Standards and Technology (NIST), Boulder, CO (United States)
2. Univ. of Colorado, Boulder, CO (United States). JILA. Dept. of Chemistry. Dept. of Physics; National Inst. of Standards and Technology (NIST), Boulder, CO (United States)

Publication Date:

Thu Apr 02 00:00:00 EDT 2020

Research Org.:

Univ. of Colorado, Boulder, CO (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

Contributing Org.:

NSF (CHE, PHYS)

OSTI Identifier:

1801533

Alternate Identifier(s):

OSTI ID: 1608154; OSTI ID: 1924529

Grant/Contract Number:  

SC0002123; FG02-09ER16021

Resource Type:

Accepted Manuscript

Journal Name:

Journal of Chemical Physics

Additional Journal Information:

Journal Volume: 152; Journal Issue: 13; Journal ID: ISSN 0021-9606

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY