SHI irradiated H2CO and CO2 interstellar analog ices

Pure H2CO ice irradiation experiments were carried out at the Grand Accélérateur National d’Ions Lourds (GANIL, Caen, 153 France) on the IRRSUD beamline with the IGLIAS setup between May 14 and 16, 2021. The vacuum chamber was cryocooled down to a temperature of about 10 K, and held at a pressure of ∼ 5 × 10−10 mbar. Infrared spectra were collected with a Bruker Vertex 70v Fourier transform infrared spectrometer (FTIR), operating with a HgCdTe-detector and a 1 cm−1 spectral resolution. The angle between the spectrometer beam and the sample surface was 12◦ . Thin films with thicknesses of ∼ 0.53 μm were deposited at normal incidence with a needle placed at around 15 mm from a ZnS window. The pressure during injection was typically 10−7 mbar. The deposition rate was adjusted to get a good optical quality, and the sample thickness was estimated using  the band strengths of Bouilloud et al. (2015) and the interference fringes from the infrared spectra. H2CO was produced from the thermal decomposition of paraformaldehyde (Sigma Aldrich, purity 95%), heated at 100◦C in a homemade oven maintained under secondary vacuum (∼ 10−7 mbar). The gaseous species sputtered in the chamber were detected with a MKS Microvision 2 quadrupole mass spectrometer (QMS). The branching ratios of the species’ fragments were evaluated during their injection. Irradiations were conducted with a 86Kr18+ beam of 0.86 MeV/u (74 MeV) corresponding to an electronic stopping power of 2830 eV(1015 molecules.cm−2)−1 for an H2CO ice of 0.81 g cm−3 (Bouilloud et al. 2015), as computed with the SRIM-2013 code2 (Ziegler et al. 2010). The flux was 5 × 108 ions.cm−2 .s−1 , and the fluence reached at the end of the experiment was 4.36 × 1012 ions cm−2 . Infrared spectra monitoring the ice composition were taken every two minutes starting from the beginning of the irra- diation. The QMS continously scanned masses from 0-80 u, with  a scan duration of ∼ 15 s. 

Additionally, a control experiment under the same conditions was carried out in which we irradiated a ∼ 0.81μm CO2 (Air Liquide, purity ≥ 99.998%) ice film with the same beam, corresponding to an electronic stopping power of 3290 eV (1015 molecules.cm−2 )−1 , also computed with SRIM-2013, for a CO2 density of 1.0 g.cm3 (Satorre et al. 2008). For this  experiment, the flux varied between 1.1 × 109 and 2.1 × 109   ions.cm−2.s−1 throughout the irradiation and the final fluence was 1013 ions.cm−2 .