Characterization of acetonitrile ice irradiated by X-rays employing the procoda code – I. Effective rate constants and abundances at chemical equilibrium

Abstract

ABSTRACT In this work, the chemical evolution of pure acetonitrile ice at 13 K irradiated with broad-band soft X-rays (from 6 eV to 2 keV) is determined by using a computational methodology (procoda code) to best fit the experimental data. To simulate the chemical evolution of the acetonitrile ice under an astrophysical analogous situation, the code employs 273 reaction rates involving 33 molecular species (5 species observed in the experiment and 28 non-observed or unknown). The considered reaction network describes 240 chemical reactions (including dissociation, bimolecular, and termolecular rates) and 33 individual desorption rates. The summed desorption yield was determined to be 0.23 molecules per photon, in agreement with previous estimates. Average values for dissociation, bimolecular, and termolecular effective rate constants were determined as 2.3 × 10−3 s−1, 9.7 × 10−26 cm3 molecule−1 s−1, and 3.2 × 10−47 cm6 molecule−2 s−1, respectively. Some branching ratios within reaction groups were also determined. Molecular abundances at chemical equilibrium were obtained, such as CH3CN (67.5 per cent), H (10.6 per cent), CN (6.7 per cent), CH2 (6.4 per cent), CH (2.5 per cent), CH3 (1.2 per cent), CH4 (1.1 per cent), C2N2 (0.8 per cent), HCN (0.8 per cent), and CH3NC (0.6 per cent). The results of this work can be employed in future astrochemical models to map chemical evolution embedded species in astrophysical regions in the presence of an ionizing radiation field.