Reactions of Si+ with H2O and O2 and SiO+ with H2 and D2

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Abstract
The reaction of Si+ with H2O to form SiOH+ has a measured rate constant of 2.3(−10) cm3 s−1 at 300 K. This is the major loss of Si+ ions in the Earth’s atmosphere above ∼90 km. The three‐body reaction of Si+ with O2 (in He) produces stable SiO2+ ions with a rate constant of 1(−29) cm6 s−1 at 300 K. The endothermic binary reaction of Si+ with O2 to produce SiO+ has been measured from threshold to ∼2 eV. The association of Si+ with O2 is the dominant loss process for Si+ ions below ∼90 km in the Earth’s atmosphere and leads to silicon oxidation to SiO2 since a large fraction of the SiO2+ ions are produced in excited states which charge transfer with O2 before relaxation. The reaction of SiO+ ions with H2 (and D2) is found to have a thermal energy rate constant of 3.2(−10) cm3 s−1 [and 2.0(−10) cm−3 s−1] to produce SiOH+ (and SiOD+). This process has been suggested as a step in SiO production in the interstellar medium. The proton affinity of SiO is found to be 8.1±0.7 eV and the dissociation energy of SiO+ to be 4.8 eV and definitely less than 4.9 eV.