Experimental study of the reaction of NO2− ions with CO2 molecules at temperatures and energies relevant to the Martian atmosphere

Abstract

Comprehensive theoretical models of the Martian ionosphere require reliable kinetic data for ion – molecule reactions over a wide range of temperatures. In situ measurements of the negative ions have not yet been performed and so ion chemical modelling is relied upon, which indicate that nitrite anions, NO2−, are important. Thus, in the present work the gas phase reaction of NO2− with carbon dioxide molecules, CO2, has been studied at the temperatures and energies relevant to the Martian atmosphere. The rate coefficient was measured over the temperature range 220 ± 2 K to 334 ± 2 K using a variable temperature selected ion flow tube (VT-SIFT) instrument. Additionally, a commercial Tandem Quadrupole Mass Spectrometer (TQMS) has been exploited to explore the reaction occurring during NO2−/CO2 collisions over the centre-of-mass energies from near-thermal to 6 eV. Two different channels of the reaction were studied. Under the thermal conditions of the VT-SIFT, the reaction is seen to proceed slowly via ternary association ultimately producing CO3– ions, the measured effective ternary rate coefficient in helium being k3 = (3.8 ± 1.5) × 10−30 cm6 s−1 at 220 ± 2 K and k3 = (0.5 ± 0.2) × 10−30 cm6 s−1 at 334 ± 2 K. The effective ternary rate coefficient can be greater in the Martian atmosphere where CO2 is the stabilizing gas. In the TQMS experiment, CO3– production occurs via a binary reaction above a threshold energy of 1.8 ± 0.2 eV. These experimental data can be used to refine the existing ion-chemical models of the Martian atmosphere.