The sputtering of radiolytic O2 in ion irradiated H2O-ice
Graphical abstract
Introduction
Planetary surfaces with absent or tenuous atmospheres are irradiated with charged particles. These particles can significantly alter the composition of the surface, as well as erode the surface through processes including sputtering. Sputtering occurs when incoming particles collisionally remove material (nuclear or elastic sputtering), electronically excite and eject material (electronic sputtering), or produce and consequently release radiolytic products [1], [2], [3]. Sputtering by magnetospheric charged particles is responsible for the production of extended atmospheres around the Jovian and Saturnian icy satellites [4], [5], [6]. For this reason, the sputtering of H2O-ice via energetic ions has been extensively studied, particularly for hydrogen, helium, and argon ions [2], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Additionally, electrons have been shown to erode surfaces through sputtering [16], [17], [18], [19]. However, few have quantified the sputtering yields of lower energy heavy ions [7], [8], [10], [14], [15], [20], which are a substantial population of the charged particles within the Jovian magnetospheric plasma [21].
Sputtering is quantified by a term known as the sputtering yield (Y) or the number of ejected molecules, atoms, or ions per incident particle. Nuclear sputtering occurs through billiard ball style collisions resulting in the removal of material [1]. Electronic sputtering occurs through repulsive interactions between atoms when collisional energy is transferred to electronic energy promoting electrons to anti-bonding orbitals [22]. Y is dependent on the projectiles’s nuclear () and electronic stopping cross section (); where is the differential loss in energy per unit path length, and N is the number density of the target. Early studies found that Y varied approximately linearly with for solids, and when is dominant the sputtering yield followed predictions by linear cascade theory [1]. However for insulating ices like H2O, it was later shown that Y S and consideration of exclusively nuclear stopping cross sections results in drastic underestimations of Y [23]. Moreover, in H2O electronic sputtering dominates at larger ion energies (>10 keV) [9]. Lower ion energies (<10 keV) are dominated by nuclear sputtering, or some combination of nuclear and electronic sputtering. Few studies have focused on this transitional region between nuclear and electronic dominated sputtering [7], [10], [14], [20].
Sputtering yields of H2O-ice induced by low energy (0.5–6 keV) H+ and Ne+ at a wide range of temperatures (30–140 K) were first quantified employing a calibrated quadrupole mass filter by Bar-Nun et al. [14]. Their study confirmed a nuclear sputtering mechanism for Ne+ and demonstrated a transition from a nuclear to electronic mechanism for H+ within the energy range studied. Additionally, temperature dependent fluxes of ejected O2 and H2 were identified. Furthermore, Y was observed to be constant at temperatures less than 80 K but increased above 80 K, which is consistent with trends seen for higher (MeV) energy ions and believed to be a result of the increased production of radiolytic O2 and H2 [24], [25]. Soon after, Christensen et al. 1986 quantified sputtering yields for 2–6 keV Ar+, Ne+, N+, He+, and e−, at 78 K. These yields were calculated based on resulting impact crater diameter [7]. They found that sputtering yields of H2O-ice at these lower energies agree fairly well with Sigmund’s linear cascade theory for collisional sputtering. Famá et al. 2008 confirmed a sputtering yield enhancement at temperatures greater than 80 K for low energy Ar+, which they also attributed to the increased production of O2 [10]. Additionally, Famá et al. developed a theoretical model to predict sputtering yields of H2O-ice. This semi-analytical model has been validated using a compilation of data from references [7], [10], [15], [23], [26]. More recently, Teolis et al. 2017 generalized Famá’s model for total sputtering yield, to predict sputtering yields of different ejected species including H2, O2, and H2O [27].
Interestingly, laboratory studies have also shown that the concentration of radiolytic O2 is not constant with depth below the surface ice but reaches a maximum somewhere within the first 100 ML (∼300 Å) below the surface [11], [28]. Examining this surface region in more detail using low energy ions will give direct insight into the concentration profile of radiolytic O2. In addition, it will also test how well theoretical models predict values for O2 sputtered from H2O-ice in this energy range, which is of particular interest to the astronomical community, as ions in this energy range are thought to be the main producer of exospheres around icy satellites [20], [21]. Thus, here we investigate the sputtering yield of H2O-ice induced by 0.5–5 keV Ar+ at temperatures between 40 K and 120 K, using microbalance gravimetry as our analytical technique. We compare our H2O sputtering yields to previous work, as well as to predictions made by the Famá et al. 2008 sputtering model. Additionally, we estimate the sputtered flux of radiolytically produced O2 and compare those estimates to the values predicted by the Teolis et al. 2017 model, giving possible explanations for any observed deviations between the laboratory data and theoretical predictions.
Section snippets
Experimental setup
All sputtering yield measurements were performed in a stainless steel ultra-high vacuum chamber with a base pressure of Torr (Fig. 1); we estimate that the pressure at the sample is significantly lower given that it is protected by a thermal-radiation shield. To prepare our samples, we vapor deposited H2O-ice at 100 K at normal incidence onto an optically flat gold mirror electrode of an Inficon IC6 quartz-crystal microbalance (QCM) as in our previous studies [29]. Under these
Flux and thickness dependence of the total mass loss
A key goal of our study is to determine the effect of sputtering due to low energy ion bombardment at temperatures and energies relevant to extraterrestrial icy surfaces. Thus, we first needed to verify that our results were independent of the ion flux and sample thickness.
To investigate whether we were in a range where the Ar+ flux effected the sputtering yield of H2O-ice, we irradiated a H2O cm−2 sample with 3 keV Ar+ at 80 K with ion fluxes between 0.085 and ions cm−2 s−1.
Conclusions
We report sputtering yields for 0.5–5 keV Ar+ at irradiation temperatures between 40 and 120 K. Below 80 K, our total sputtering yields cluster around the theoretical sputtering yields predicted by the Famá et al. 2008 model and are generally consistent with previous laboratory studies. In addition, we also estimate the sputtering yield of radiolytically produced O2 as a function of energy for temperatures between 40 and 120 K. At 120 K, we find that the O2/H2O sputtered ratio increases nearly
CRediT authorship contribution statement
Patrick D. Tribbett: Conceptualization, Methodology, Software, Investigation, Formal analysis, Writing - original draft. Mark J. Loeffler: Conceptualization, Methodology, Software, Formal analysis, Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by NSF Grant # 1821919. Data can be found in Northern Arizona University's long-term repository (https://openknowledge.nau.edu/5536/).
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