${\mathrm{O}}_2^{+}$ Production Coming from ${\mathrm{CO}}_2$ Single-Event Electron Impact

In ${\mathrm{CO}}_2$-rich atmospheres that are always exposed to ionizing radiation (e.g., Venus and Mars), every fragmentation process can significantly impact the inventory of moieties present in these environments. Nevertheless, the production of ${\mathrm{O}}_2^{+}$ ions as a direct result of ${\mathrm{CO}}_2$ fragmentation has never been quantified so far. Since molecular oxygen is considered as a potential trace of living organisms, nonbiotic pathways for its production must be known. In this work, ${\mathrm{O}}_2^{+}$ coming from ${\mathrm{CO}}_2$ fragmentation by electron impact is unambiguously identified and measured in absolute scale.

Figure 1

Ratio between the collected ${\mathrm{O}}_2^{+}$ for a time $t$ and for the minimum time $t_0$ for all delay times. Blue squares: ${\mathrm{O}}_2^{+}$ produced by ${\mathrm{O}}_2$ ionization; red circles: ${\mathrm{O}}_2^{+}$ produced by ${\mathrm{CO}}_2$ fragmentation; black lines: theoretical curve for Maxwell-Boltzmann kinetic energy distribution and Gaussian kinetic energy distribution centered at 1.4 eV, for ionization of ${\mathrm{O}}_2$ and ${\mathrm{CO}}_2$, respectively, as illustrated in the plot. Uncertainties are estimated to be 10% and 7% for ${\mathrm{CO}}_2$ and ${\mathrm{O}}_2$ fragmentation data, respectively.

Figure 2

Absolute cross sections for ${\mathrm{O}}_2^{+}$ (red circles) and ${\mathrm{CO}}_2^{2+}$ (blue squares) over electron energy. Solid lines, as indicated: ${\mathrm{O}}_2^{+}$, BEB calculations for ${\mathrm{O}}_2^{+}$ production, combining inner valence shell single ionization [$\mathrm{IVS}({\mathrm{O}}_2^{+})$] and direct double ionization [$\mathrm{D}({\mathrm{O}}_2^{+})$]; ${\mathrm{CO}}_2^{2+}$, BEB calculations for ${\mathrm{CO}}_2^{2+}$, also combining inner valence shell single ionization and direct double ionization (curves not shown here).

Figure 3

Ratio between ${\mathrm{CO}}_2^{2+}$ and ${\mathrm{CO}}_2^{+}$ production over electron impact energy. Red circles: present work; green up triangles: Tian and Vidal [31]; blue squares: Lindsay and Mangan [19, 30]; orange stars: King and Price [53]; pink side triangle: En-Liang et al. [54]; black down triangles: Bhatt et al. (2010) [55]; orange lozenge: Bhatt et al. (2011) [56].