Navegando por Assunto "Solid state: volatile"

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    Mapping the evolution of pure CO2 ices irradiated by ions, UV, and electrons using the upgraded PROCODA code (employing an effective rate constant ordering by thermochemistry data)
    (Elsevier) Pilling, Sergio; Rocha, Will Robson Monteiro; Carvalho, Geanderson Araújo; Abreu, Heitor Avelino de
    Chemical reactions and desorption processes are being triggered by incoming ionizing radiation over astrophysical ices in cold space environments. The quantification of these processes is crucial to achieve a detailed understanding of the underlying chemistry occurring within the ice. With this goal, we have upgraded the PROCODA code (Pilling et al. 2022a) which solves a system of coupled differential equations and describes the evolution of the molecular abundances under processing by radiation, now including an effective rate constant (ERCs) ordering by employing thermochemistry data taken from literature. This methodology helps to identify the most important reactions within the reaction network and therefore decreases the degeneracy of the solutions and enhancing the accuracy of the calculations. Here, we described the chemical evolution of four irradiated pure CO2 considering 11 different chemical species, 100 reaction routes and 11 radiation-induced desorption processes. The best-fit models provide the effective rate constants, several desorption parameters, as well as, the characterization of the chemical equilibrium (CE) phase. A comparison with previous code version was given and indicates that the ordering of rate constants by thermochemistry data is more important when more energy is deposited in the ice. The current work present more realistic values for the effective rate constants and a better characterization of the CE phase, such data can be used to refine astrochemical models to better describe cold space environments in the presence of incoming ionizing radiation field such molecular clouds and protoplanetary regions and the surface of comets and frozen moons and planets.
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    Simulation of CH3OH ice UV photolysis under laboratory conditions
    (EDP Sciences) Rocha, Will Robson Monteiro; Woitke, Peter; Pilling, Sergio; Thi, Wing-Fai; Jørgensen, Jes Kristian; Kristensen, Lars Egstrøm; Perotti, Giulia; Kamp, Inga
    Context. Methanol is the most complex molecule that is securely identified in interstellar ices. It is a key chemical species for understanding chemical complexity in astrophysical environments. Important aspects of the methanol ice photochemistry are still unclear, such as the branching ratios and photodissociation cross sections at different temperatures and irradiation fluxes. Aims. This work aims at a quantitative agreement between laboratory experiments and astrochemical modelling of the CH3OH ice UV photolysis. Ultimately, this work allows us to better understand which processes govern the methanol ice photochemistry present in laboratory experiments. Methods. We used the code ProDiMo to simulate the radiation fields, pressures, and pumping efficiencies characteristic of laboratory measurements. The simulations started with simple chemistry consisting only of methanol ice and helium to mimic the residual gas in the experimental chamber. A surface chemical network enlarged by photodissociation reactions was used to study the chemical reactions within the ice. Additionally, different surface chemistry parameters such as surface competition, tunnelling, thermal diffusion, and reactive desorption were adopted to check those that reproduce the experimental results. Results. The chemical models with the code ProDiMo that include surface chemistry parameters can reproduce the methanol ice destruction via UV photodissociation at temperatures of 20, 30, 50, and 70 K as observed in the experiments. We also note that the results are sensitive to different branching ratios after photolysis and to the mechanisms of reactive desorption. In the simulations of a molecular cloud at 20 K, we observed an increase in the methanol gas abundance of one order of magnitude, with a similar decrease in the solid-phase abundance. Conclusions. Comprehensive astrochemical models provide new insights into laboratory experiments as the quantitative understanding of the processes that govern the reactions within the ice. Ultimately, these insights can help us to better interpret astronomical observations.