Dynamics of Mg⁺ + H₂O + He: Capture, Collisional Stabilization and Collision-Induced Dissociation

Abstract

A laser flash photolysis technique and quasi-classical trajectory (QCT) calculations have been used to determine the rate coefficients for the title process. The experimental high-pressure-limiting rate coefficient is 7.0 x 10(-11) cm(3) s(-1) at T = 300 K, which compares with the computed QCT value for the Mg(+) + H(2)O capture rate of 2.75 +/- 0.08 x 10(-9) cm(3) s(-1) at the same temperature. The 39-fold difference between the experimental and simulation results is explained by further QCT calculations for the He + Mg(+).H(2)O* collision process. In particular, our simulation results indicate that collision-induced dissociation (CID) of the Mg(+).H(2)O* excited adduct is very likely compared with collisional stabilization (CS), which is an order of magnitude less likely. Including the relative rates of CID and CS in the calculation and assuming that those Mg(+).H(2)O* complexes that perform only one inner turning point in the dissociation coordinate are unlikely to be stabilized by CS, the computed rate coefficient compares well with the high-pressure experimental value.

A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

“…Reaction (2) showed no measurable reaction at 300 K; however, a slow reaction was observed at 190 K over the pressure range 5-15 Torr, yielding k 2 (190 K) = (1.56 AE 0.10) Â 10 À31 cm 6 molecule À2 s À1 . 19 Unexpectedly, k rec (Mg + + N 2 O) exhibited both a pressure dependence and a negative temperature dependence (Fig. 3 illustrates plots of k rec for reactions (3) and (4) as a function of [He] and temperature, yielding k 3 (190-299 K) = [(3.53 AE 0.27) Â 10 À31 ] (T/ 300 K) (À2.55AE0.14) and k 4 (190-403 K) = (7.05 AE 1.14) Â 10 À30 (T/300 K) (À1.86AE0.03) cm 6 molecule À2 s À1 .…”

“…37 We have applied this formalism previously to recombination reactions of metallic species, 27,29 so only a brief description is given here. reaction (5) 19 ). The internal energy of this adduct was divided into a contiguous set of grains (width 30 cm À1 ), each containing a bundle of rovibrational states.…”

“…Following the pioneering work at the NOAA Aeronomy Laboratory in the 1960s, formation of MgO + via reaction (1) is known to be rapid, and should dominate Mg + removal above 90 km in the terrestrial atmosphere. Reaction (5) is comparatively fast, 19 but H 2 O is a trace constituent and so this reaction should not be competitive. 15 In contrast, reaction (4) dominates in the CO 2 -rich Martian atmosphere.…”

“…If reactions (17) (19) is not competitive with reaction (18). If reactions (17) (19) is not competitive with reaction (18).…”

“…If reactions (17) (19) is not competitive with reaction (18). 22 We have recently reported studies of reactions (3), (8), (17) and (18), 17 and reaction (5) 19 using the pulsed laser photolysis/laser induced fluorescence (PLP-LIF) and fast flow tube/mass spectrometer (FT-MS) techniques. 15,16 Reactions (2)- (5) have been studied at 294 K using a selected-ion flow tube (SIFT) apparatus; however, in all cases no reaction was observed.…”