Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 10, Issue 34, Pages 5287-5298Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/b805356a
Keywords
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Funding
- NERC [NE/B00015X/1]
- University of Leeds
- Natural Environment Research Council [NE/B00015X/2, NE/B00015X/1] Funding Source: researchfish
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A series of gas-phase reactions involving molecular Ca-containing ions was studied by the pulsed laser ablation of a calcite target to produce Ca(+) in a fast flow of He, followed by the addition of reagents downstream and detection of ions by quadrupole mass spectrometry. Most of the reactions that were studied are important for describing the chemistry of meteor-ablated calcium in the earth's upper atmosphere. The following rate coefficients were measured: k(CaO(+) + O -> Ca(+) + O(2)) = (4.2 +/- 2.8) x 10(-11) at 197 K and (6.3 +/- 3.0) x 10(-11) at 294 K; k(CaO(+) + CO -> Ca(+) + CO(2), 294 K) = (2.8 +/- 1.5) x 10(-10); k(Ca(+)center dot CO(2) + O(2) -> CaO(2)(+) + CO(2), 294 K) = (1.2 +/- 0.5) x 10(-10); k(Ca(+)center dot CO(2) + H(2)O -> Ca(+)center dot H(2)O + CO(2)) = (13.0 +/- 4.0) x 10(-10); and k(Ca(+)center dot H(2)O + O(2) -> CaO(2)(+) + H(2)O, 294 K) = (4.0 +/- 2.5) x 10(-10) cm(3) molecule(-1) s(-1). The quoted uncertainties are a combination of the 1 sigma standard errors in the kinetic data and the systematic errors in the models used to extract the rate coefficients. Rate coefficients were also obtained for the following recombination (also termed association) reactions in He bath gas: k(Ca(+)center dot CO(2) + CO(2) -> Ca(+)center dot(CO(2))(2) 294 K) = (2.6 +/- 1.0) x 10(-29); k(Ca(+)center dot H(2)O + H(2)O -> Ca(+)center dot(H(2)O)(2)) = (1.6 +/- 1.1) x 10(-27); and k(CaO(2)(+) + O2 -> CaO(2)(+)center dot O(2)) < 1 x 10(-31) cm(6) molecule(-2) s(-1). These recombination rate coefficients, as well as those for the ligand-switching reactions listed above, were then interpreted using a combination of high level quantum chemistry calculations and RRKM theory using an inverse Laplace transform solution of the master equation. The surprisingly slow reaction between CaO(+) and O was explained using quantum chemistry calculations on the lowest 2A', (2)A '' and (4)A '' potential energy surfaces. These calculations indicate that reaction mostly occurs on the (2)A' surface, leading to production of Ca(+)((2)S) + O(2)((1)Delta(g)). The importance of this reaction for controlling the lifetime of Ca(+) in the upper mesosphere and lower thermosphere is then discussed.
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