Laboratory measurements to study the sputtering of Hermean surface analogues under He ion impact

Rocky bodies in space without a protective atmosphere like Mercury are subject to harsh conditions, including the bombardment by solar wind ions. This leads to the ejection of particles along with alteration of the surface properties like composition and morphology. These ion-induced sputter processes contribute to the formation of the Hermean exosphere [1]. Therefore, understanding the involved fundamental ion-solid interaction processes is important for the modelling of exosphere creation. From a computational standpoint, those impacts are usually investigated using simulation codes based on the Binary Collision Approximation (BCA) like SRIM [2] or SDTrimSP [3]. However, past studies revealed that especially for compound targets relevant as Hermean regolith analogues, simulations have to be adapted by means of modified input parameters to reproduce experimental results [4]. Thus, laboratory measurements are strongly needed to ensure that valid inputs enter the exosphere modelling.

A well-proven method for such investigations of sputtering is the Quartz Crystal Microbalance (QCM). It allows to measure sputter yields of thin films deposited onto a quartz resonator with high precision in real time and in situ [5]. Expanding on this technique, we use a setup in which we place a second QCM in the vicinity of the irradiated target, facing the centre of particle emission [6]. Ejecta that stick to its surface result in a mass accumulation, which is resolved through the piezoelectric properties of the quartz resonator. By moving this second catcher-QCM in an arc around the target, the angular emission characteristic of sputtered particles can be probed. The experimental realisation is sketched in figure 1. The setup also allows for experiments with targets that cannot be deposited onto a resonator or whose surface properties change during the deposition process. Particularly, the above-mentioned films used in, e.g. [4], are vitreous and smooth. We therefore extended our studies to pellets from ground and pressed mineral specimens that retain some roughness for a more realistic regolith analogue [7]. Through comparison of these measured angular distributions with reference samples, also total sputter yields can be determined for targets whose yields cannot be measured with a single QCM.

Figure 1: Sketch of the setup. A target is irradiated with an ion beam under an angle of incidence α. The angular distribution of ejecta (blue shaded area) is probed with the catcher-QCM by varying the angle α_C.

We initially used a 2 keV Ar⁺ ion beam to irradiate both an enstatite (MgSiO₃) film and a pellet under 60° and 45° incidence with respect to the sample surface normal. This choice of projectile has a high sputter yield and produces sufficient signal at the catcher-QCM to allow for a proof of principle. After initial hardships, modified sample preparation routines made reproducible quantification of the obtained data possible. We attribute differences in shape and magnitude of the sputtered particle angular distributions between the sample types to the different roughnesses of the sample configurations. Geometric considerations alone are sufficient to describe the qualitative behaviour of our results [8]. Simulation results to visualise the impact of surface roughness on the sputter yield as a function of incidence angle are given in figure 2. For a solar wind relevant projectile species, the same measurements were also carried out with a 4 keV He⁺ ion beam. This presentation will include the results from both the He and the Ar irradiation experiments.

Figure 2: Sputter yields Y for a perfectly smooth surface (as simulated by SDTrimSP, blue) and for the surface of an enstatite (MgSiO₃) pellet used in this study in orange. The latter are obtained using a home-made code based on geometric considerations [8].

 

References

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[3] A. Mutzke et al.: SDTrimSP Version 6.00. Max-Planck-Institut für Plasmaphysik, 2019
[4] P.S. Szabo et al.: Astrophys. J., 891, 100, 2020
[5] G. Hayderer et al.: Rev. Sci. Instrum., 70, 3696, 1999
[6] H. Biber et al.: EPSC2021, online, EPSC2021-526, 2021
[7] N. Jäggi et al.: Icarus, 365, 114492, 2021
[8] C. Cupak et al.: Appl. Surf. Sci., 570, 151204, 2021