O2(a1Δg) + Mg, Fe, and Ca: Experimental kinetics and formulation of a weak collision, multiwell master equation with spin-hopping

Abstract

The first excited electronic state of molecular oxygen, O 2 (a 1 g ), is formed in the upper atmosphere by the photolysis of O 3 . Its lifetime is over 70 min above 75 km, so that during the day its concentration is about 30 times greater than that of O 3 . In order to explore its potential reactivity with atmospheric constituents produced by meteoric ablation, the reactions of Mg, Fe, and Ca with O 2 (a) were studied in a fast flow tube, where the metal atoms were produced either by thermal evaporation (Ca and Mg) or by pulsed laser ablation of a metal target (Fe), and detected by laser induced fluorescence spectroscopy. O 2 (a) was produced by bubbling a flow of Cl 2 through chilled alkaline H 2 O 2 , and its absolute concentration determined from its optical emission at 1270 nm (O 2 (a 1The following results were obtained at 296The total uncertainty in these rate coefficients, which mostly arises from the systematic uncertainty in the O 2 (a) concentration, is estimated to be ±40%. Mg + O 2 (a) occurs exclusively by association on the singlet surface, producing MgO 2 ( 1 A 1 ), with a pressure dependent rate coefficient. Fe + O 2 (a), on the other hand, shows pressure independent kinetics. FeO + O is produced with a probability of only ∼0.1%. There is no evidence for an association complex, suggesting that this reaction proceeds mostly by nearresonant electronic energy transfer to Fe(a 5 F) + O 2 (X). The reaction of Ca + O 2 (a) occurs in an intermediate regime with two competing pressure dependent channels: (1) a recombination to produce CaO 2 ( 1 A 1 ), and (2) a singlet/triplet non-adiabatic hopping channel leading to CaO + O( 3 P). In order to interpret the Ca + O 2 (a) results, we utilized density functional theory along with multireference and explicitly correlated CCSD(T)-F12 electronic structure calculations to examine the lowest lying singlet and triplet surfaces. In addition to mapping stationary points, we used a genetic algorithm to locate minimum energy crossing points between the two surfaces. Simulations of the Ca + O 2 (a) kinetics were then carried out using a combination of both standard and non-adiabatic Rice-Ramsperger-Kassel-Marcus (RRKM) theory implemented within a weak collision, multiwell master equation model. In terms of atmospheric significance, only in the case of Ca does reaction with O 2 (a) compete with O 3 during the daytime between 85 and 110 km.

The Mesosphere and Metals: Chemistry and Changes

“… b Rate coefficient is from Helmer and Plane. 316 c Rate coefficient is from Plane et al 115 d Rate coefficients are from Broadley and Plane. 109 e Rate coefficients are from Plane and Rollason.…”

“…Recently, the kinetics of the reactions of O 2 ( 1 Δ g ) with Mg, Fe, and Ca have been studied. 115 This first excited state of O 2 is formed in the upper atmosphere by the photolysis of O 3 . Its lifetime is more than 70 min above 75 km, so that during the daytime the O 2 ( 1 Δ g ) concentration is about 30 times greater than that of O 3 .…”

A new model of meteoric calcium in the mesosphere and lower thermosphere

“…NaHCO 3 , FeOH, and Mg(OH) 2 ), and the dimerization rate coefficient needs to be increased to account for this since Ca ablates in a large excess of these other metals: the elemental ablation ratio of Ca atoms to the sum of Na, Fe, Mg, Si, Al and K atoms is 0.01 (Carrillo-Sánchez et al, 2016). A similar procedure was used to model the K layer, where the dimerization rate coefficient of KHCO 3 was increased by a factor of 270 (Plane et al, 2014).…”

“…The m/z = 40 profile consist primarily of Ca + (Zbinden et al, 1975). Based on recent detailed modelling of Na and Mg chemistry (Plane and Whalley, 2012;Marsh et al, 2013b), significant contributions from NaOH + and MgO + can be ruled out. The 42/40 and 44/40 signal ratios in sporadic E layers have been found to be close to the 44 Ca/ 40 Ca and 42 Ca/ 40 Ca terrestrial isotopic ratios (Herrmann et al, 1978).…”

A new model of meteoric calcium in the mesosphere and lower thermosphere

“…Based on recent detailed modeling of Na and Mg chemistry (Plane and Whalley, 2012;Marsh et al, 2013a) . Figure 4a compares the annual mean Ca and Ca + vertical profiles simulated by WACCM-Ca with the annual mean lidar measurements at Kühlungsborn, and two rocket measurements of Ca + over Red Lake, Canada, which is at a similar latitude.…”