Abstract
The reactivity, energetics and dynamics of the bimolecular reactions between Ar2+ and O2 have been studied using a position sensitive coincidence methodology at a collision energy of 4.4 eV. Four bimolecular reaction channels generating pairs of product ions are observed, forming: Ar+ + O2+, Ar+ + O+, ArO+ + O+ and O+ + O+. The formation of Ar+ + O2+ is a minor channel, involving forward scattering, and generates O2+ in its ground electronic state. This single electron transfer process is expected to be facile by Landau-Zener arguments, but the intensity of this channel is low because the electron transfer pathways involve multi-electron processes. The formation of Ar+ + O+ + O, is the most intense channel following interactions of Ar2+ with O2, in agreement with previous experiments. Many different combinations of Ar2+ and product electronic states contribute to the product flux in this channel. Major dissociation pathways of the nascent O2+* ion involve the ion's first and second dissociation limits. Unusually, the experimental results clearly show the involvement of a short-lived collision complex [ArO2]2+ in this channel. The formation of O+ and ArO+ involves direct abstraction of O- from O2 by Ar2+. There is scant evidence of the involvement of a collision complex in this bond forming pathway. The ArO+ product appears to be formed in the first excited electronic state (2Π). The formation of O+ + O+ results from dissociative double electron transfer via an O22+ intermediate. The exoergicity of the dissociation of the nascent O22+ intermediate is in good agreement with previous work investigating the unimolecular dissociation of this dication.
Highlights
Planetary ionospheres are composed of a variety of atomic and molecular species which can be ionised by absorption of energetic photons and by collisional processes
This paper reports an investigation of the reactivity of Ar2+ and O2, at collision energies of 2.7 and 4.4 eV in the centre-of-mass frame, using position-sensitive coincidence mass spectrometry (PSCO-MS)
D) was determined to have a broad maximum centred at B8.5 eV with a full width at half maximum (FWHM) from 6.3–11.6 eV. This kinetic energy release (KER) is in good agreement with that determined by Lundqvist et al.[75]
Summary
Planetary ionospheres are composed of a variety of atomic and molecular species which can be ionised by absorption of energetic photons and by collisional processes. Atmospheres, the formation of the Ar2+ dication is likely, as recognised by Thissen et al.[12] The bimolecular reactivity of Ar2+ was one of the first dicationic collision systems to be investigated, as beams of Ar2+ are relatively easy to generate using electron ionisation.[28,29,30,31] In most of these early investigations of Ar2+-neutral collisions, only the dominant single-electron transfer (SET) and double-electron transfer (DET) channels were observed These early experiments were usually carried out at high laboratory translational energies (0.1–20 keV) and involved rare gases or simple molecules as the neutral collision partner (e.g. He, H2, N2, CO2, C2H6, C6H6).[29,30,32,33,34] More recent experiments, at lower collision energies (o100 eV), led to the observation of bond-forming chemistry following the interactions of Ar2+ with various neutral species, revealing, for example, the formation of Ar–O, Ar–N and Ar–C bonds.[35,36,37,38,39,40] the bimolecular reactivity of rare gas dications is recognized as an effective route to forming these unusual chemical bonds. We find strong evidence of complex formation ([ArO2]2+) in the dynamics of the dissociative SET channel; a reaction that typically occurs via longrange electron transfer
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