A study of the reactions of Ni+ and NiO+ ions relevant to planetary upper atmospheres

Abstract

The reactions between Ni⁺(²D) and O₃, O₂, N₂, CO₂ and H₂O were studied at 294 K using the pulsed laser ablation at 532 nm of a nickel metal target in a fast flow tube, with mass spectrometric detection of Ni⁺ and NiO⁺. The rate coefficient for the reaction of Ni⁺ with O₃ is k(294 K) = (9.7 ± 2.1) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹; the reaction proceeds at the ion-permanent dipole enhanced Langevin capture rate with a predicted T^−0.16 dependence. Electronic structure theory calculations were combined with Rice–Ramsperger–Kassel–Markus theory to extrapolate the measured recombination rate coefficients to the temperature and pressure conditions of planetary upper atmospheres. The following low-pressure limiting rate coefficients were obtained for T = 120–400 K and He bath gas (in cm⁶ molecule⁻² s⁻¹, uncertainty ±σ at 180 K): log₁₀(k, Ni⁺ + N₂) = −27.5009 + 1.0667log₁₀(T) − 0.74741(log₁₀(T))², σ = 29%; log₁₀(k, Ni⁺ + O₂) = −27.8098 + 1.3065log₁₀(T) − 0.81136(log₁₀(T))², σ = 32%; log₁₀(k, Ni⁺ + CO₂) = −29.805 + 4.2282log₁₀(T) − 1.4303(log₁₀(T))², σ = 28%; log₁₀(k, Ni⁺ + H₂O) = −24.318 + 0.20448log₁₀(T) − 0.66676(log₁₀(T))², σ = 28%). Other rate coefficients measured (at 294 K, in cm³ molecule⁻¹ s⁻¹) were: k(NiO⁺ + O) = (1.7 ± 1.2) × 10⁻¹⁰; k(NiO⁺ + CO) = (7.4 ± 1.3) × 10⁻¹¹; k(NiO⁺ + O₃) = (2.7 ± 1.0) × 10⁻¹⁰ with (29 ± 21)% forming Ni⁺ as opposed to NiO₂⁺; k(NiO₂⁺ + O₃) = (2.9 ± 1.4) × 10⁻¹⁰, with (16 ± 9)% forming NiO⁺ as opposed to ONiO₂⁺; and k(Ni⁺·N₂ + O) = (7 ± 4) × 10⁻¹². The chemistry of Ni⁺ and NiO⁺ in the upper atmospheres of Earth and Mars is then discussed.