Reaction kinetics of muonium with the halogen gases (F sub 2 , Cl sub 2 , and Br sub 2 )

Journal Article · Wed Nov 15 04:00:00 EST 1989 · Journal of Chemical Physics; (USA)

DOI:https://doi.org/10.1063/1.457435· OSTI ID:5367620

Gonzalez, A C; Reid, I D; Garner, D M; Senba, M; Fleming, D G; Arseneau, D J; Kempton, J R ^[1]

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1. TRIUMF and Department of Chemistry, University of British Columbia, 4004 Wesbrook Mall, UBC Campus, Vancouver, British Columbia V6T 2A3, Canada (CA)

Bimolecular rate constants for the thermal chemical reactions of muonium (Mu) with the halogen gases---Mu+X{sub 2}{r arrow}MuX+X---are reported over the temperature ranges from 500 down to 100, 160, and 200 K for X{sub 2}=F{sub 2},Cl{sub 2}, and Br{sub 2}, respectively. The Arrhenius plots for both the chlorine and fluorine reactions show positive activation energies {ital E}{sub {ital a}} over the whole temperature ranges studied, but which decrease to near zero at low temperature, indicative of the dominant role played by quantum tunneling of the ultralight muonium atom. In the case of Mu+F{sub 2}, the bimolecular rate constant {ital k}({ital T}) is essentially independent of temperature below 150 K, likely the first observation of Wigner threshold tunneling in gas phase (H atom) kinetics. A similar trend is seen in the Mu+Cl{sub 2} reaction. The Br{sub 2} data exhibit an apparent negative activation energy ({ital E}{sub {ital a}}=({minus}0.095{plus minus}0.020) kcal mol{sup {minus}1}), constant over the temperature range of {approximately}200--400 K, but which decreases at higher temperatures, indicative of a highly attractive potential energy surface. This result is consistent with the energy dependence in the reactive cross section found some years ago in the atomic beam data of Hepburn {ital et} {ital al}. (J. Chem. Phys. {bold 69}, 4311 (1978)). In comparing the present Mu data with the corresponding H atom kinetic data, it is found that Mu invariably reacts considerably faster than H at all temperatures, but particularly so at low temperatures in the cases of F{sub 2} and Cl{sub 2}. The current transition state calculations of Steckler, Garrett, and Truhlar (Hyperfine Interact. {bold 32}, 779 (986)) for Mu+X{sub 2} account reasonably well for the rate constants for F{sub 2} and Cl{sub 2} near room temperature, but their calculated value for Mu+Br{sub 2} is much too high.

• OSTI ID: 5367620
• Journal Information: Journal of Chemical Physics; (USA), Journal Name: Journal of Chemical Physics; (USA) Vol. 91:10; ISSN JCPSA; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
400700* -- Radiochemistry & Nuclear Chemistry
640303 -- Atomic
Molecular & Chemical Physics-- Positronium
Muonium
& Muonic & Mesic Atoms & Molecules
74 ATOMIC AND MOLECULAR PHYSICS
BROMINE
CHEMICAL REACTION KINETICS
CHLORINE
ELEMENTS
FLUORINE
HALOGENS
HIGH TEMPERATURE
KINETICS
LOW TEMPERATURE
MEDIUM TEMPERATURE
MUONIUM
NONMETALS
REACTION KINETICS