Kinetic Study of the Reactions of AlO with H2O and H2; Precursors to Stellar Dust Formation

Abstract

AlO is relatively abundant around oxygen-rich Asymptotic Giant Branch (AGB) stars, where it can react with major gas-phase species such as H2 and H2O to form AlOH. These Al-containing species are the likely precursors of refractory alumina nanoparticles, which may provide the nuclei for dust formation. In the present study, the kinetics of these AlO reactions were measured from 295 to 780 K using the pulsed laser photolysis of Al(C5H7O2)3, with time-resolved laser induced fluorescence detection of AlO. The experimental results were interpreted using quantum chemistry calculations and a Master Equation solver for reactions with multiple energy wells. For the recombination reaction AlO + H2O (+ N2) → Al(OH)2, log10(krec,0/cm6 molecule–2 s–1) = −32.9185 + 8.80276 log10(T) – 2.4723(log10(T))2; log10(krec,∞/cm3 molecule–1 s–1) = −19.4686 + 7.62037 log10(T) – 1.47214(log10(T))2; Fc = 0.28 (uncertainty ±13% from 295 to 760 K). For the bimolecular reactions, k(AlO + H2O → AlOH + OH) = (3.89 ± 0.47) × 10–10 exp(−(1295 ± 150)/T) and k(AlO + H2 → AlOH + H) = (5.37 ± 0.52) × 10–13 (T/300)(2.77 ± 0.19) exp(−(2190 ± 110)/T) cm3 molecule–1 s–1. Rate coefficients for Al + H2O → AlOH + H, AlOH + H → AlO + H2 or Al + H2O, and the absorption cross sections of AlOH and AlO were calculated theoretically. Al chemistry around an O-rich AGB star was then investigated using a β-trajectory model, which predicts that AlOH is the major gas-phase Al species beyond two stellar radii and shows that the relative AlO abundance is very sensitive to the AlOH photolysis rate.