Kinetic Study of the Reactions of CaO with H2O, CO2, O2, and O3: Implications for Calcium Chemistry in the Mesosphere

Abstract

CaO(X1Σ+) plays a central role in the atmospheric chemistry of meteor-ablated calcium. A series of CaO reactions was studied by the pulsed photodissociation at 193.3 nm of calcium acetyl acetonate [Ca(C5H7O2)2] vapor, producing CaO in an excess of reactant and N2 bath gas. CaO was monitored by time-resolved nonresonant LIF, by pumping the CaO(B1Π − X1Σ+) transition at 385.9 nm and detecting B1Π − A1Σ+ emission at λ > 693 nm. The recombination reactions of CaO with H2O, CO2, and O2 were found to be in the falloff region over the experimental pressure range (2−12 Torr). The data were fitted by RRKM theory combined with ab initio quantum calculations on Ca(OH)2, CaCO3 and CaO3, yielding the following results (180−600 K and 0−103 Torr). For CaO + H2O, log (krec,0/cm6 molecule-2 s-1) = −23.39 + 1.41 log T − 0.751 log2 T, krec,∞= 7.02 × 10-10 exp(−38.4/T) cm3 molecule-1 s-1, Fc = 0.31. For CaO + CO2: log (krec,0/cm6 molecule-2 s-1) = −36.14 + 9.24 log T − 2.19 log2 T, krec,∞= 7.97 × 10-10 exp(−190/T) cm3 molec...