Photolysis of CH3CN Ices by Soft X‑rays: Implications for the Chemistry of Astrophysical Ices at the Surroundings of X‑ray Sources

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 124; no. 41; pp. 8574 - 8584
• Main Authors: Carvalho, G. A, Pilling, S
• Format: Journal Article
• Language: English
• Published: American Chemical Society 15.10.2020
• Subjects: A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.0c06229

In this work, broad-band soft X-ray (6–2000 eV) was employed to irradiate frozen acetonitrile CH3CN, at the temperature 13 K, with different photon fluences up to 1.5 × 1018 photons cm–2. Here, acetonitrile is considered as a representative complex organic molecule (COM) present in astrophysical water-rich ices. The experiments were conduced at the Brazilian synchrotron facility (LNLS/CNPEM) employing infrared spectroscopy (FTIR) to monitor chemical changes induced by radiation. The effective destruction cross section of acetonitrile and effective formation cross section for daughter species formed inside the ice were obtained. The identified radiation products were HCN, CH4, H2CCNH, and CH3NC showing that fragmentation and rearrangement contribute to acetonitrile destruction. Chemical equilibrium and molecular abundances at this stage were determined, which also includes the abundance estimates of unknown molecules, produced but not directly detected, in the ice. The chemical equilibrium was reached at fluence around 1.5 × 1018 photons cm–2. Time scales for ices, at hypothetical snow line distances, to reach chemical equilibrium around compact objects, young stellar objects, and O/B stars and inside solar system were given. Among the obtained results are the time scales for reaching chemical equilibrium around different astronomical strong X-ray emitters, e.g., 14 days (for the Sun at 5 AU), 41 and 82 days (for O/B stars at 5 AU), 109–1011 years (for white dwarfs at 1 LY), 450 years (for Crab pulsar at 2.25 LY), around 107 years (for Vela pulsar at 2.25 LY), and 7.5 × 106 years (for Sagittarius A* at 3 LY).