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Characterization of acetonitrile ice irradiated by X-rays employing the PROCODA code: II. Desorption processes

G. A. Carvalho, S. Pilling, S. Gerasimenko

TL;DR

This study uses the PROCODA chemical-evolution framework to quantify desorption processes in X-ray-irradiated acetonitrile ice. By integrating broadband X-ray irradiation experiments (6 eV–2 keV) with a 33-species, 240-reaction network and desorption steps, the authors extract desorbed column densities and intrinsic desorption rates, finding that acetonitrile dominates desorption (≈98% at chemical equilibrium) and that its intrinsic desorption rate is among the highest. The work provides a calibrated desorption yield ($Y≈0.2$ molecules per photon) and a total yield near $0.29$ molecules per photon, yielding insights into gas-phase abundances in environments such as protoplanetary disks and ices on small Solar System bodies. The results offer quantitative inputs for astrochemical models and help interpret observational data, including potential insights for JWST-era studies and the Horsehead Nebula, by connecting laboratory desorption physics to astrophysical photon-flux regimes.

Abstract

In this work, we focus on the study of radiation induced desorption processes that occurred in acetonitrile ice irradiated by broadband X-rays (6 eV to 2 keV) monitored by FTIR spectroscopy at different radiation fluences. In a previous work, we used the PROCODA code to derive the chemical evolution of the ice. Here, we have obtained that the acetonitrile desorbed column density is at least two orders of magnitude larger than the desorbed column densities of daughter or granddaughter molecular species at chemical equilibrium stage. This indicates that total desorption column density is mainly governed by the father molecule, as also previously hypothesized in experimental studies. This occurs basically because the acetonitrile column density is larger than the other ones. In particular, at chemical equilibrium acetonitrile desorption column density represents almost 98\% of the total, while it is close to 1\% for H, CN and CH$_2$, the species with larger molecular desorption percentages at chemical equilibrium. Another derived quantity is what we called intrinsic desorption rate, which is a number per second for individual species. Some of the larger intrinsic desorption rates were: CH$_3$CN ($6.2\times 10^{-6}$), CN ($6.2\times 10^{-6}$), H ($5.7\times 10^{-6}$), CH$_2$ ($5.7\times 10^{-6}$) and C$_2$N$_2$ ($4.4\times 10^{-6}$). These results help to put constrain in astrochemical models and can be also useful to clarify some astronomical radio observations.

Characterization of acetonitrile ice irradiated by X-rays employing the PROCODA code: II. Desorption processes

TL;DR

This study uses the PROCODA chemical-evolution framework to quantify desorption processes in X-ray-irradiated acetonitrile ice. By integrating broadband X-ray irradiation experiments (6 eV–2 keV) with a 33-species, 240-reaction network and desorption steps, the authors extract desorbed column densities and intrinsic desorption rates, finding that acetonitrile dominates desorption (≈98% at chemical equilibrium) and that its intrinsic desorption rate is among the highest. The work provides a calibrated desorption yield ( molecules per photon) and a total yield near molecules per photon, yielding insights into gas-phase abundances in environments such as protoplanetary disks and ices on small Solar System bodies. The results offer quantitative inputs for astrochemical models and help interpret observational data, including potential insights for JWST-era studies and the Horsehead Nebula, by connecting laboratory desorption physics to astrophysical photon-flux regimes.

Abstract

In this work, we focus on the study of radiation induced desorption processes that occurred in acetonitrile ice irradiated by broadband X-rays (6 eV to 2 keV) monitored by FTIR spectroscopy at different radiation fluences. In a previous work, we used the PROCODA code to derive the chemical evolution of the ice. Here, we have obtained that the acetonitrile desorbed column density is at least two orders of magnitude larger than the desorbed column densities of daughter or granddaughter molecular species at chemical equilibrium stage. This indicates that total desorption column density is mainly governed by the father molecule, as also previously hypothesized in experimental studies. This occurs basically because the acetonitrile column density is larger than the other ones. In particular, at chemical equilibrium acetonitrile desorption column density represents almost 98\% of the total, while it is close to 1\% for H, CN and CH, the species with larger molecular desorption percentages at chemical equilibrium. Another derived quantity is what we called intrinsic desorption rate, which is a number per second for individual species. Some of the larger intrinsic desorption rates were: CHCN (), CN (), H (), CH () and CN (). These results help to put constrain in astrochemical models and can be also useful to clarify some astronomical radio observations.
Paper Structure (7 sections, 5 equations, 5 figures, 2 tables)

This paper contains 7 sections, 5 equations, 5 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. Experimental data and the chemical reaction network
  4. Computational details
  5. Results
  6. Astrophysical implications
  7. Conclusions

Figures (5)

  • Figure 1: Concentration as a function of time. In panel a) is visible that the concentration of the father molecule is higher compared to the other species. Panels b) and c) highlight regions where concentration scales are smaller. Concentration of all species are normalized by the initial concentration of CH$_3$CN.
  • Figure 2: Panel a) Molecular desorption induced by X-rays obtained by the best-fit model in the mapping of chemical evolution of acetonitrile irradiated ice. Panel b) Highlight of a region with smaller desorption column density values. In both panels, arrows indicate points where the desorption of a given species surpasses that of another one.
  • Figure 3: Panel a) molecular desorption of all molecular species at chemical equilibrium stage (or equilibrium branching ratio - EBR). Panel b) intrinsic molecular desorption rates at EBR. In both panels, observed species are depicted in blue, while non-observed species are showed in red.
  • Figure 4: Desorption flux as a function of photon flux in the range of 6eV$-$2keV for selected molecular species with higher abundances at chemical equilibrium and high desorption rates. The vertical dotted line marks the broadband X-ray flux of 10$^{14}$ photons cm$^{-2}$ s$^{-1}$, which corresponds to laboratory photon flux used during the ice irradiation experiment. The color filled regions correspond to: typical white dwarf X-ray fluxes at 1 ly distances (green), typical neutron star X-ray fluxes at 2 ly distances (red), typical black hole X-ray fluxes at 3 ly distances (blue), Sun's X-ray fluxes from 1 AU to 40 AU (yellow) and YSO models of X-ray flux at $\sim$$30-40$ AU (gray), see text for more details.
  • Figure 5: Total molecular desorption induced on acetonitrile-rich ices by broadband X-ray bombardment as a function of timescale after reaching chemical equilibrium, see Eq \ref{['des_eq']}. Each curve correspond to a given fixed photon flux related to several astrophysical objects.