Sputtering of water ice by keV electrons at 60 K

Surface Science 691 (2020) 121509

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Sputtering of water ice by keV electrons at 60 K a

Robyn M. Meier , Mark J. Loeffler a b c

⁎,b,c

T

Department of Applied Physics and Materials Science, Northern Arizona University, Flagstaff, AZ, 86011, United States Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ, 86011, United States Center for Materials Interfaces in Research and Applications, Northern Arizona University, Flagstaff, AZ 86011, United States

A R T I C LE I N FO

A B S T R A C T

Keywords: Electron bombardment Sputtering Water ice

We have measured the sputtering yield of H2O–ice for 0.5–10 keV electrons at 60 K using microbalance gravimetry. We find that over the energy range studied Y (H2O) changes by a factor of ∼16, indicating that there is a clear relation between Y (H2O) and Se. The laboratory data is qualitatively consistent with previous laboratory studies involving ion irradiation of H2O–ice and the trend with energy is reasonably fit by the most recent model for electron induced sputtering of H2O–ice, although the model overestimates the yield by at least a factor of three. In addition, we show that the irradiation history of our H2O–ice samples can strongly influence the measured Y, as temporary enhancements in Y as high as a factor of six were observed, which we speculate is a result of the release of radiolytically-produced O2 that is trapped below the surface of our H2O–ice samples.

1. Introduction Icy bodies in our Solar System are constantly subjected to different forms of energetic processing, such as ion and electron bombardment and UV photolysis. Over time, these forms of processing can alter the surface significantly by driving solid-phase chemical, physical, and structural changes and possibly even contributing to the exospheres that may form around certain objects. One alteration caused by these forms of processing is the erosion of the surface through a process known as sputtering [1,2]. The erosion of surfaces via sputtering by energetic ions has been studied for decades [2]. One of the important terms associated with sputtering is the sputtering yield, Y, which is defined as the average number of ejected atoms, molecules, and ions per incident particle. Most early measurements investigating sputtering of metals found that Y was roughly linear with the stopping power of the incident projectile, dE/dx. In these cases, the sputtering from the solid was primarily through elastic billiard-ball like collisions (nuclear stopping). Later experiments focusing on condensed gases, such as H2O–ice, showed that in cases where inelastic collisions involving molecular ionization and excitation (electronic stopping) were important, Y could be grossly underestimated if only the nuclear component was considered [3]. Multiple detailed experiments followed to characterize how Y varied with stopping power and irradiation temperature in H2O–ice [4–9], helping to refine our understanding of how sputtering occurs in

H2O–ice. A review on this topic has recently been published [10]. Samples like H2O–ice are not of interest only from a fundamental perspective but also of significant importance to the astronomical community [11,12], as H2O is the primary component found on many cold bodies in our Solar System. For H2O–ice, most experiments have shown that Y ∝ (Se)2 for fast ions [4], where the electronic stopping 1 dE cross section, Se, is equal to N dx , where N is the number density of the material. In addition, it has been shown that sputtered flux from H2O–ice depends strongly on temperature; it is essentially constant below ∼80 K [4,6,13,14] and increases with increasing temperature. This dependence has been attributed to the increased efficiency of formation of O2 at higher temperatures [13,14] and has implications for many of the warmer icy satellites in our Solar System. Measurements of Y induced by energetic electron irradiation of condensed gases have also been reported but to a lesser extent than energetic ions [1,15,16]. However, the majority of those measurements are for condensed gases besides H2O–ice. Most experiments involving H2O–ice have focused on determining mechanisms at play in the chemical evolution of the removed material (e.g., [17–20]). The two groups that have reported Y for H2O–ice have performed experiments over two different energy regimes, and the results suggest there may be a different relation between Y and Se depending on the incident electron energy [21–23]. The earliest studies on electron sputtering of H2O–ice [22,23] showed that Y for 100 keV and 200 keV electrons was ∼1000 times lower than what was observed via protons of similar

⁎ Corresponding author at: Department of Astronomy and Planetary Science, Northern Arizona University, 527 S Beaver St. Bldg. 19, Rm. 209, Flagstaff, AZ 86011, United States. E-mail address: mark.loeffl[email protected] (M.J. Loeffler).

https://doi.org/10.1016/j.susc.2019.121509 Received 27 June 2019; Received in revised form 9 August 2019; Accepted 26 September 2019 Available online 27 September 2019 0039-6028/ © 2019 Elsevier B.V. All rights reserved.

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shield it is estimated to be 10‒100 times lower. Solid H2O samples were vapor-deposited at near-normal incidence at 100 K with a flux of ∼1 × 1015 molecules cm‒2 s‒1 onto an optically flat, gold-mirror electrode of an Inficon IC6 quartz-crystal microbalance (QCM) to column densities between 4 × 1016 and 9 × 1018 molecules cm−2. The stability of the crystal allowed us to distinguish changes of less than 5 × 1013 H2O cm−2 (∼0.1 Hz). During growth, the sample thickness was also monitored using a diode laser (670-nm) that was reflected from the mirror at near normal incidence. After growth, samples were cooled down to 60 K and irradiated with electrons of a specified energy (0.5–10 keV) using an EGG-3103C Kimball Physics electron gun aimed at an incident angle of 12.5°. The electron beam was scanned uniformly over an area slightly larger than the crystal to ensure the entire sample was processed. The beam current was measured before and after the irradiation by moving our Faraday Cup in front of our sample. To prevent loss of secondary electrons from the Faraday Cup, we placed a + 9 V battery in series with the electrometer. During irradiation, the stability of the electron beam was monitored by a thin biased wire collector placed in the electron beam path and biased at –9 V. During the experiments, the beam current typically varied by less than 5%. The ejected flux during irradiation was monitored with a Stanford Research Systems (SRS 100) Residual Gas Analyzer aimed at an incident angle of 12.5°. However, at the low sputtering levels observed in these experiments, we could not confidently distinguish the ejected species from changes in the residual background. Future studies will employ isotopic measurements and possibly involve modification of this portion of our setup to improve its level of sensitivity.

energy [7]. The reported difference is not surprising, as Se for a 100 keV H+ is about 200 times higher than that of a 100 keV e− in H2O–ice [24,25]. Extrapolation of these two data points suggest that Y appears to scale linearly with Se, yet more data points are needed to confidently determine a trend. A more recent study has shown a different, and surprising trend reporting that Y is essentially independent of Se over 0.2 and 10 keV [21], even though Se changes by more than a factor of 10 [26]. This result is interesting in itself, as all previous studies on sputtering of H2O–ice induced by keV ions show that Y depends on Se [4]. Determining the relation between Y and Se is not only of interest from a fundamental perspective but is also critically important to evaluate the role that electrons play in sputtering icy extraterrestrial surfaces. For instance, recent models investigating the sputter-produced atmosphere of Europa, an icy satellite of Jupiter, have relied heavily on recent electron irradiation studies [21] to compare effects caused by ions and electrons [27]. Thus, given the importance of energetic electrons on a variety of icy planetary surfaces [28–30] and the few number of laboratory studies that have measured Y induced by energetic electron irradiation in H2O–ices, we decided to investigate this system in more detail. Specifically, we irradiated thin films of H2O–ice with energetic electrons (0.5–10 keV) at 60 K, while monitoring the mass loss of our ice sample with microbalance gravimetry. These new measurements allowed us to quantitatively determine the relation between Y and Se over a range where Se changes by about a factor of 15, as well as also determine the potential importance of other experimental parameters, such as incident electron flux, sample thickness and irradiation history of the H2O–ice.

2.2. Stopping power and range estimates

2. Experimental details

The stopping power of the electrons in the energy range chosen for this work is overwhelming a result of inelastic energy transfer (Se). In the following, we focus on determining a relation between Y and Se, keeping in mind that the energy adsorbed at the surface of our ice sample can actually be enhanced by a factor proportional to (NSe) due to scattering of the electrons near the surface [15]. To estimate the values of Se, we averaged values from two publications [26,31] and those calculated from ESTAR, which provides values down to 1 keV [24]. The values derived from each reference over the energy range used in our study are within ∼10% or better of the calculated average. To estimate the range of our electrons, we considered two relations pointed out by Johnson [32], which give the range in nm assuming a density of 1 g cm−3:

2.1. Experimental setup All experiments were performed inside a stainless-steel ultra-high vacuum chamber on a radiation-shielded cryostat (Fig. 1). The base pressure of the chamber was ∼3 × 10−9 Torr, and inside the radiation

R (E ) = Rp E α

(1)

where Rp is 45.7 nm and α is 1.75 [33,34] and

R (E ) = 40E (1 + 0.5E )

(2)

where E is in keV [35]. Between 1.5 and 10 keV, these two approximations yield range estimates within ∼10% of one another but deviate significantly at lower energies. For instance, at the lowest energy used in our studies (0.5 keV), the range estimate deviates by ∼60%. As the power law fit given in (1) is based on experimental measurements between 5 and 54 keV [33], unless otherwise noted, we used this extrapolation to estimate projected electron ranges. 2.3. Calculation of the sputtering yield The output frequency of the QCM is related to the areal mass (g cm−2) of the sample through the relation

ΔQ =

−k (f1 − f0 ) f1 f0

(3) −2

where Q is the areal mass (g cm ), f0 and f1 are the initial and final frequencies of the crystal (in Hz), and k is a constant, equal to

Fig. 1. Experimental setup. 2

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R.M. Meier and M.J. Loeffler

Fig. 2. QCM (left axis) and sample temperature (right axis) during irradiation of a 9 × 1018 H2O cm−2 sample with 1 keV electrons at 60 K.

4.417 × 105 Hz g cm−2 [36]. During irradiation of our samples, we typically see an initial spike in frequency in the QCM, followed by a prolonged steady increase in the frequency (an example is shown in Fig. 2). The former is attributed to an electrical response of the QCM and its height depends on both the electron energy and flux, while the latter is due to actual mass loss of the sample, which we can use to calculate the sputtering yield. To calculate the sputtering yield, we first differentiate (3), obtaining

Fig. 3. Y (H2O) vs. incident electron flux for a 8.15 × 1017 H2O cm−2 sample irradiated with 2 keV electrons at 60 K. The sample was irradiated at an incident angle of 12.5°. Error bars are an estimate of the reproducibility of these data points.

( ) df

(k ) dt1 dQ = dt (f1 )2

(4)

where df1/dt is the slope of the frequency curve shown in Fig. 2. As the incident electron flux, Φ, is constant, we can obtain the sputtering yield in terms of the initial sample (H2O), Y, via the relation:

Y=

dQ dt

N ⎛ A⎞ Φ ⎝ 18 ⎠

(5)

where NA is Avagadro's number and 18 is the mass of one mole of H2O (18 g). We note that recent studies suggest radiolytically produced H2 and O2 likely contribute and perhaps dominate the sputtered flux of electron-irradiated H2O-ice [21,37], which would require an adjustment to these values. However, for simplicity and ease of comparison with previous studies [4,6,9], unless otherwise noted all values of Y are reported in terms of the starting material (H2O). Fig. 4. Y (H2O) vs. areal mass of the deposited sample during irradiation with 2 keV electrons at 60 K. In all cases, the sample was irradiated at an incident angle of 12.5° with an incident flux of 3.3 × 1013 electrons cm−2 s−1. The thickness given in the top axis, assumes the density of the sample is 0.85 g cm−3. Error bars are an estimate of the reproducibility of these data points.

3. Results and discussion 3.1. Flux and thickness dependence on Y One of the underlying goals of this study was to determine the role that electrons play in sputtering surfaces of extraterrestrial icy bodies. Thus, before determining the sputtering yield as function of electron energy, we wanted to ensure that our fluxes were sufficiently low and our samples were sufficiently thick that our derived values were independent of these two parameters. To determine whether the ranges of electron fluxes in which we were operating affected Y (H2O), we irradiated a H2O–ice film with 2 keV electrons at 60 K varying our incident electron flux by more than a factor of ten. As is seen in Fig. 3, this range of fluxes produced no measurable difference in Y (H2O), indicating that possible effects induced by heating of the sample or from coincident electrons producing overlapping tracks are negligible, which is consistent with previous studies on condensed N2 and O2 [15]. To determine how Y (H2O) depended on sample thickness, we irradiated a series of fresh H2O–ice films with 2 keV electrons at 60 K

(Fig. 4). While Y (H2O) begins to increase when the sample thickness is much less than the penetration depth of the electrons, indicating that thin films may also be affected by interactions between the incident electrons and the substrate, as long as the sample thickness was half the range of the electrons, the sputtering yield was independent of the sample thickness. 3.2. Stopping power dependence on Y After we determined the best parameters to mimic extraterrestrial icy bodies, we proceeded to measure Y (H2O) as a function of Se. We used identical experimental conditions in these experiments (temperature, flux, thickness). Fig. 5 shows Y (H2O) as a function of Se and 3

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Fig. 6. Left: Y (H2O) vs. fluence (electrons cm-2) during irradiation of a 9 × 1018 H2O cm−2 ice with 2 keV electrons at 60 K after the sample has been preprocessed with 1 × 1017 electrons cm−2 at the following energies: a) 2.175 keV, b) 2.3 keV, and c) 2.5 keV. The horizontal line shows Y (H2O) for a fresh film irradiated with 2 keV electrons. Right: Summary of studies shown on the left, where Yp is the maximum observed Y–value when irradiating the processed ice with 2 keV electrons, Y0 is the Y–value for a fresh film irradiated with 2 keV electrons, and E0 is the initial irradiation energy. In all cases, the sample was irradiated at an incident angle of 12.5° with an electron flux of 3.3 × 1013 electrons cm−2 s−1. Error bars are derived from reproducibility in the experiments.

Fig. 5. Y (H2O) vs. Se for a 9 × 1018 H2O cm−2 sample irradiated with electrons at 60 K at an incident angle of 12.5° and with an incident flux of 3.3 × 1013 electrons cm−2 s−1. The solid line is a fit to the experimental data: y = a (Se)n with a = 3.6 × 10−4 and n = 1.7. Error bars are derived from reproducibility in the experiments.

energy (inset) after Y (H2O) has reached a constant value. Unlike the recent studies, which showed Y is virtually independent of Se over this energy range [21], we find that Y depends on a power of Se and is highest for the lowest energies studied (highest Se). In fact, over the range of energies we studied, Y (H2O) varies between about 0.016 and 0.26, or by a factor of ∼16.

(Yp) for these cases is between ∼3–6 times higher than the equilibrium yield (Y0) measured for the fresh H2O–ice sample. The shape of the curve is virtually identical to what has been reported in cases where radiolytic O2 has been produced in H2O–ice [8,9], suggesting that this short-term increase in Y (H2O) is driven by production of O2 below the ice surface in our samples as well. To evaluate how prevalent this effect is under our experimental conditions, we performed more experiments as in Fig. 6 (left) but expanded the energy range of our first irradiation step. A summary of those results are shown in Fig. 6 (right), where we plot Yp/Y0 vs. E0, where E0 is the initial electron energy. From these results, it appears that if the initial irradiation energy was lower than the subsequent irradiation energy, then the resulting Y measured in the 2nd step would be similar for the case of fresh ice sample. However, if the initial irradiation energy was higher than the subsequent irradiation energy, then the resulting Yp measured in the 2nd step could be up to six times higher than in the case of fresh ice. Other experiments, not shown here, showed that similar trends were seen if our 2nd irradiation step involved electrons at other energies (e.g., 1st irradiation with 8 keV electron and 2nd irradiation with 5 keV electrons). We point out that this enhancement was much less pronounced if the 2nd irradiation energy was ≤1 keV. While the cause for this difference at low energies is not obvious, we suspect that this observation is akin to what was observed by Teolis et al. [8,9]. In those studies, they found that if they irradiated a processed H2O–ice sample with 4.5 keV ions, as compared to 100 keV ions, they needed to remove ∼5 times more material before an enhancement of Y was observed. They hypothesized that the 4.5 keV ions did a better job depth profiling the actual O2 distribution in the processed sample, while the 100 keV ions enabled the trapped O2 to diffuse towards the surface, causing the enhancement in Y to appear at shallower depths. Analogously, our results suggest that electrons with energies ≤1 keV may simply be less efficient in initiating O2 diffusion towards the surface than electrons with energies ≥2 keV. As a consequence, we may not have removed enough material to see an enhancement in Y when we irradiated our processed ice with electrons at energies that were ≤1 keV. Of course, this would also imply that O2 diffusion initiated by energetic electrons depends strongly on the incident electron energy, something we hope to test in

3.3. Influence that irradiation history has on Y During the course of our experiments, it was clear that in our fresh samples, Y (H2O) quickly reached a constant value, which are the values reported in Fig. 5. However, we also noticed that if we changed the irradiation energy of the electrons after Y (H2O) had reached equilibrium, the resulting Y (H2O) of the processed film could be significantly higher than if we had simply irradiated a fresh film. An effect similar to this has been reported previously by Teolis et al. [8,9], yet they observed these variations in Y (H2O) while changing the irradiation temperature of a processed sample. Regardless, they clearly show this transient increase in Y (H2O) in the processed sample is a result of the formation and buildup of radiolytically produced O2. With this in mind, we set out to quantify this effect further in our experiments by performing a set of systematic experiments given below. After deposition at 100 K, we cooled our sample to 60 K and irradiated it with electrons of a prechosen energy to a fluence of 1 × 1017 electrons cm−2. Next, we changed the incident electron energy and monitored Y (H2O). Selected results from our studies are shown in Fig. 6. Fig. 6 (left) compares Y (H2O) vs. electron fluence measured for 2 keV electrons for four different cases: no preprocessing and preprocessing with 2.175, 2.3, and 2.5 keV electrons. In all cases, where the ice sample has been preprocessed, there is a strong fluence dependence on Y (H2O). Generally, the peak and the FWHM of the curve increase with increasing energy. An estimate given for the amount of material removed to reach the peak in Y is 0.8 ± 0.1 nm, 2.1 ± 0.7 nm, and 3.1 ± 1.8 nm for the 2.175, 2.3 and 2.5 keV electron experiments respectively, assuming the sputtered material has a density of 0.85 g cm−3. In all cases it appears that if the sample is irradiated long enough, Y (H2O) will return to the value measured for the unprocessed ice sample, as is expected in sputtering of molecular solids [32]. The data in Fig. 6 (left) shows that the maximum sputtering yield 4

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future studies. 3.4. Modeling the sputtering yield Irradiation of H2O–ice not only removes material from the surface via sputtering but also changes the chemical composition of the sample. Compositional changes are not only evident in the irradiated ice [38–41] but are also reflected in the sputtered flux, as radiolytically produced H2 and O2 have been identified using mass spectrometry in both electron [37,42] and ion irradiated ices [5,8,14,43,44]. Previous experiments have shown that the composition of the sputtered flux depends strongly on a number of parameters including temperature, incident projectile and radiation history. Isotopic studies using both light and heavy ions have shown that at temperatures below ∼80 K, the majority of the sputtered flux is H2O molecules [43,45], although the estimated relative amounts do vary. Regardless, as the temperature increases, the sputtered contribution from O2 (and presumably H2)1 increases significantly and even becomes the dominant sputtered component. In addition, as noted above detailed depth profiling studies have shown that the concentration of radiolytically produced O2 is not constant with depth [8,9]. The majority of electron irradiation studies have focused on mechanisms involved in O2 production from H2O–ice, rather than determining the composition of the sputtered flux [19,20,42]. However, these studies have shown that, like previous ion irradiation experiments, the sputtered flux of O2 is relatively constant below ∼50 K but increases significantly with temperature [19]. Two more recent studies have estimated the contribution of H2 and O2 to the sputtered flux of H2O–ice and concluded that these radiolytically produced species may be the dominant components, implying that perhaps the erosion of H2O–ice via energetic electrons is driven by a different mechanism than for fast ions [21,37]. Future isotopic studies, such as those using H218O [9], should allow one to determine whether these recent observations at 90–100 K are preserved at lower temperatures in electron irradiated ice, or if the trend at low temperatures is more similar to what has been observed for ions. Recent modeling efforts related to the erosion of H2O–ice via energetic electrons and ions have focused on using existing laboratory data to produce an empirical relationship between measurable parameters, such as E, Se and Y [6,10,11]. In those models, the relation for energetic electrons focuses on the released O2 [10], while the relation for the ions gives expressions for both H2O [6,11] and O2 [10]. The relation for O2 is given by the expression [10]:

YO2 (E , T , β ) =

εgO02 x 0

⎡1 − e− r0cosβ ⎣

r 0cosβ ⎛1 x0 ⎤ ⎜

⎦⎝

+ q0 e

−

Q ⎞ kB T ⎟

⎠

Fig. 7. Y vs. electron energy measured in our experiments (●) and previous ones: ■ [21] and ▲ [22,23], assuming the sputtered flux is entirely O2 (scaling values in Figure 5 and in [21–23] by 18/32) compared with the model given in Eq. (6). In the cases where the incident angle for irradiation was not normal incidence, we scaled the down the data by a factor of cos(θ)1.3, where θ in the incident angle [47]. The curves produced from the model are as follows: using the electron range estimate given in (1) and multiplying the resulting curve produced by (6) by 0.25 (solid line) and using the range estimate given in (2) and multiplying the resulting curve produced by (6) by 0.15 (dashed line).

Teolis et al. [10] to fit the existing laboratory data [46], although at that point it was not clear whether the difference was due to measurement or model error. If the sputtered flux is not predominately O2 at the lower temperatures of our experiments, then the difference between our data and the existing model would be more significant. 3.5. Comparison to previous studies Besides comparing our laboratory data to the most recent model, we can also compare our results with previous experiments with light ions and electrons, keeping in mind that the utility of this exercise may be limited if the composition of the sputtered flux depends strongly on incident projectiles (see above). A straightforward way to compare across different experiments is to plot Y (H2O) as a function of electron stopping cross section (Se) with the assumption that all the mass loss is due to H2O, as in Fig. 5. Our results appear to be generally in-line with previous studies (Fig. 8), except for the recent electron irradiation studies reported in [21], which appear to be significantly higher than our studies and even, in some cases, higher for ions of the same Se. Some of this discrepancy may simply be a consequence of the difference in irradiation temperature (60 vs. 90 K), as O2 production, and hence Y, will increase with temperature. However, the observed independence of Y with Se in [21] suggests that perhaps the use of processed samples to determine Y, which in some cases can cause short-term enhancements in Y due to O2 buildup (see Fig. 6), may have been a contributing factor as well. In an attempt to compare our results with previous ion irradiation experiments more quantitatively, we can fit our data with an expression relating Y with Se. Obtaining a relation in this manner is desirable because it can give insight into what mechanisms are driving sputtering at a fundamental level. It has already been well established that when Se is the dominant form of energy transfer for energetic ions, Y ∝ Se2, which is hypothesized to indicate that pairs of excitations are overlapping at the surface, causing this non-linear behavior. Deviations from this have been observed [48] but are thought to be due to different projectile charge states and contribution from elastic collision processes [4], as the regime in which elastic collisions dominate sputtering is

(6)

where the effective energy, ε, is essentially equal to the incident energy (E) for electrons, T is temperature, β is the incident angle, r0 is the range of electrons at normal incidence, x0 is the depth at which O2 forms below the surface (2.8 ± 0.4 nm) and gO02 is the surface radiolysis yield per unit energy (5 ( ± 0.5) × 10−3 eV−1). To account for the increasing O2 yield with temperature, the last term was derived from an empirical fit to existing data, yielding q0 = 103 ( ± 102) and Q = 0.06 ± 0.01 eV. In our case, T ∼60 K, this temperature dependent term only provides a correction of ∼1%. Fig. 7 compares our data to the model given in (6) with the assumption that the entire sputtered flux is O2 and using the two approximations for r0 given in (1) and (2). The laboratory data is best fit by this model if the approximation for r0 given in (1) is used and the model is scaled down by ∼0.25 (solid line in Fig. 7), although the measured fall-off of Y with energy is steeper than predicted. Coincidentally, the applied scaling factor is similar to the one (0.3) used by 1 Sputtered H2 is typically assumed to be 2 (YO2) when the equilibrium fluence is reached.

5

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temperatures, the irradiation history of our H2O–ice sample can strongly influence the measured Y of our samples, causing temporary enhancements as high as a factor of six, which is a result of radiolytically O2 trapped in below the surface of the H2O–ice. Thus, to ensure accurate results, measurements of Y (H2O) should be done using fresh ices. Future studies will focus on determining the relative proportions of the components sputtered from H2O–ice at low temperatures, as well as determine how Y varies with temperature for electrons at this energy range. Acknowledgements This research was supported by start-up funding provided by Northern Arizona University and from NSF Grant # 1821919. References [1] J. Schou, Sputtering of frozen gases, Nuc. Instr. Met. B 27 (1987) 188–200. [2] P. Sigmund, Theory of sputtering. I. Sputtering yield of amorphous and polycrystalline targets, Phys. Rev. 184 (1969) 383–416. [3] W.L. Brown, W.M. Augustyniak, L.J. Lanzerotti, Linear and non-linear processes in the erosion of H2O ice by fast light-ions, Phys. Rev. Lett. 45 (1980) 1632–1635. [4] R.A. Baragiola, R.A. Vidal, W. Svendsen, J. Schou, M. Shi, D.A. Bahr, C.L. Atteberrry, Sputtering of water ice, Nuc. Instr. Met. B 209 (2003) 294–303. [5] A. Bar-Nun, G.b. Herman, M.L. Rappaport, Y. Mekler, Ejection of H2O, O2, H2 and H from water ice by 0.5–6 Kev H+ and Ne+ ion-bombardment, Surf. Sci. 150 (1985) 143–156. [6] M. Fama, J. Shi, R.A. Baragiola, Sputtering of ice by low-energy ions, Surf. Sci. 602 (2008) 156–161. [7] M. Shi, R.A. Baragiola, D.E. Grosjean, R.E. Johnson, S. Jurac, J. Schou, Sputtering of water ice surfaces and the production of extended neutral atmospheres, J. Geophys. Res. 100 (1995) 26387–26395. [8] B.D. Teolis, J. Shi, R.A. Baragiola, Formation, trapping, and ejection of radiolytic O2 from ion-irradiated water ice studied by sputter depth profiling, J. Chem. Phys. 130 (2009) 134704. [9] B.D. Teolis, R.A. Vidal, J. Shi, R.A. Baragiola, Mechanisms of O2 sputtering from water ice by keV ions, Phys. Rev. B 72 (2005) 245422. [10] B.D. Teolis, C. Plainaki, T.A. Cassidy, U. Raut, Water ice radiolytic O, H, and H2O2 yields for any projectile species, energy, or temperature: a model for icy astrophysical bodies, J. Geophys. Res. 122 (2017) 1996–2012. [11] R.E. Johnson, M.H. Burger, T.A. Cassidy, F. Leblanc, M. Marconi, W.H. Smyth, Composition and detection of Europa's sputter-induced atmosphere, in: R.T. Pappalardo, W.D. McKinnon, K. Khurana (Eds.), Europa, Univ. Arizona Press, Tucson, 2009, pp. 507–527. [12] R.E. Johnson, J. Schou, Sputtering of inorganic insulators, in: P. Sigmund (Ed.), Sputtering of inorganic insulators, Fundamental Processes in the Sputtering of Atoms and Molecules (1993) 403–493. [13] J.W. Boring, R.E. Johnson, C.T. Reimann, J.W. Garret, W.L. Brown, K.J. Marcantonio, Ion-induced chemistry in condensed gas solids, Nuc. Instr. Met. Res. 218 (1983) 707–711. [14] C.T. Reimann, J.W. Boring, R.E. Johnson, J.W. Garrett, K.R. Farmer, W.L. Brown, K.J. Marcantonio, W.M. Augustyniak, Ion-induced molecular ejection from D2O ice, Surf. Sci. 147 (1984) 227–240. [15] O. Ellegaard, J. Schou, H. Sorensen, P. Borgesen, Electronic sputtering of solid nitrogen and oxygen by keV electrons, Surf. Sci. 167 (1986) 474–492. [16] J. Schou, O. Ellegaard, P. Bøgesen, H. Sørensen, The erosion of condensed gases by keV electron bombardment, in: W. Brenig, D. Menzel (Eds.) Desorption Induced by Electronic Transitions DIET II, SpringerBerlin, Heidelberg1985, pp. 170–176. [17] T.M. Orlando, M.T. Sieger, The role of electron-stimulated production of O2 from water ice in the radiation processing of outer solar system surfaces, Surf. Sci. 528 (2003) 1–7. [18] X. Pan, A.D. Bass, J.-P. Jay-Gerin, L. Sanche, A mechanism for the production of hydrogen peroxide and the hydroperoxyl radical on icy satellites by low-energy electrons, Icarus 172 (2004) 521–525. [19] N.G. Petrik, A.G. Kavetsky, G.A. Kimmel, Electron-stimulated production of molecular oxygen in amorphous solid water, J. Phys. Chem. B 110 (2006) 2723–2731. [20] M.T. Sieger, W.C. Simpson, T.M. Orlando, Production of O2 on icy satellites by electronic excitation of low-temperature water ice, Nature 394 (1998) 554–556. [21] A. Galli, A. Vorburger, P. Wurz, A. Pommerol, R. Cerubini, B. Jost, O. Poch, M. Tulej, N. Thomas, 0.2 to 10 keV electrons interacting with water ice: radiolysis, sputtering, and sublimation, Plan. Spac. Sci. 155 (2018) 91–98. [22] H.G. Heide, On the irradiation of organic-samples in the vicinity of ice, Ultramicroscopy 7 (1982) 299–300. [23] H.G. Heide, Observations on ice layers, Ultramicroscopy 14 (1984) 271–278. [24] M.J. Berger, J.S. Coursey, M.A. Zucker, J. Chang, Stopping-Power and Range Tables for Electrons, Protons, and Helium Ions, NIST, Physical Measurement Laboratorary, 2005, http://www.nist.gov/pml/data/star/index.cfm. [25] J.F. Ziegler, Stopping and range of ions in matter SRIM2010, , 2010. [26] Z. Francis, S. Incerti, M. Karamitros, H.N. Tran, C. Villagrasa, Stopping power and

Fig. 8. Comparison of our measurements of Y (●) with those found in the literature for light ions (H+): ◊ [3], ○ [4], and □ [5], and electrons (e−): ■ [21] and ▲ [22,23]. The constant yield at low Se in [5] is due to the contribution from nuclear collisions (Sn). In the cases where the incident angle for irradiation was not normal incidence, we scaled the down the data by a factor of cos(θ)1.3, where θ in the incident angle [47].

dominated by single collisions and is linear with Sn. The most recent analytical expression developed by Fama et al. [6] fits the sputtering data for H2O–ice induced by ion irradiation quite well but there is not an equivalent counterpart for electron irradiation. Thus, another way to compare our results to previous ion irradiation experiments is to use a simple power law relation:

Y = a (Se )n

(7)

where a is a constant and n is determined empirically from the data. Our data is best fit with Y ∝ Se1.7, which is shown in Fig. 5. This a stronger dependence than has been observed for electron irradiation of condensed O2 and N2 (∼1.2; [15]) and than what is extrapolated from 100 to 200 keV electron irradiation experiments of H2O–ice (n∼1; [22,23]). However, it is slightly lower than the quadratic dependence observed for MeV protons [3,49] but higher to what was determined by compiling results from a number of different keV ion experiments (n∼1.3; [48]). This superlinear relation between Y with Se suggests that sputtering of H2O–ice by keV electrons over this energy range is more similar to fast ions than was previously thought [4]. Whether this non-linear behavior implies overlapping of excitation pairs at the surface are driving the observed sputtering, as it does for fast ions, than single collision events will require more detailed modeling of electron interaction with H2O–ice. 4. Conclusions We have measured the sputtering yield of H2O–ice for 0.5–10 keV electrons at 60 K using microbalance gravimetry, finding that over the energy range studied Y (H2O) for fresh films changes by a factor of ∼16 and is independent of our chosen irradiation flux and sample thickness. Furthermore, we show that our measured trend with energy is a qualitatively reasonable fit to the most current model, although the model overestimates the yield by at least a factor of three. In addition, our measured relation between Y and Se is qualitatively consistent with previous ion irradiation experiments, following a simple power law relation with n ∼1.7. This suggests that sputtering via energetic electrons may also be non-linear with Se, possibly driven by overlapping pairs of excitations at the surface, rather than single collision events, as previously suspected. Finally, we also find that, even at these low 6

Surface Science 691 (2020) 121509

R.M. Meier and M.J. Loeffler

[27] [28]

[29] [30]

[31] [32] [33]

[34] [35] [36] [37]

[38]

Sci. 52 (2004) 371–378. [39] K.P. Hand, R.W. Carlson, H2O2 production by high-energy electrons on icy satellites as a function of surface temperature and electron flux, Icarus 215 (2011) 226–233. [40] M.J. Loeffler, U. Raut, R.A. Vidal, R.A. Baragiola, R.W. Carlson, Synthesis of hydrogen peroxide in water ice by ion irradiation, Icarus 180 (2006) 265–273. [41] M.H. Moore, R.L. Hudson, IR detection of H2O2 at 80 K in ion-irradiated laboratory ices relevant to Europa, Icarus 145 (2000) 282–288. [42] N.G. Petrik, G.A. Kimmel, Electron-stimulated sputtering of thin amorphous solid water films on Pt(111), J. Chem. Phys. (2005) 123. 054702. [43] W.L. Brown, W.M. Augustyniak, K.J. Marcantonio, E.H. Simmons, J.W. Boring, R.E. Johnson, C.T. Reimann, Electronic sputtering of low-temperature molecularsolids, Nucl Instrum Meth B 1 (1984) 307–314. [44] J. Benit, W.L. Brown, Electronic sputtering of oxygen and water molecules from thin films of water ice bombarded by MeV Ne+ ions, Nucl. Instrum. Meth B 46 (1990) 448–451. [45] R.A. Baragiola, C.L. Atteberry, C.A. Dukes, M. Famá, B.D. Teolis, Atomic collisions in solids: astronomical applications, Nucl. Instrum. Meth B 193 (2002) 720–726. [46] B.D. Teolis, G.H. Jones, P.F. Miles, R.L. Tokar, B.A. Magee, J.H. Waite, E. Roussos, D.T. Young, F.J. Crary, A.J. Coates, R.E. Johnson, W.L. Tseng, R.A. Baragiola, Cassini finds an oxygen-carbon dioxide atmosphere at Saturn's icy moon rhea, Science 330 (2010) 1813–1815. [47] R.A. Vidal, B.D. Teolis, R.A. Baragiola, Angular dependence of the sputtering yield of water ice by 100 keV proton bombardment, Surf. Sci. 588 (2005) 1–5. [48] M. Shi, D.E. Grosjean, J. Schou, R.A. Baragiola, Particle-emission induced by ionization tracks in water ice, Nucl. Instrum. Meth B 96 (1995) 524–529. [49] W.L. Brown, W.M. Augustyniak, E. Brody, B. Cooper, L.J. Lanzerotti, A. Ramirez, R. Evatt, R.E. Johnson, Energy-dependence of the erosion of H2O ice films by H and He ions, Nucl. Instrum. Met. Phys. Res. B 170 (1980) 321–325.

ranges of electrons, protons and alpha particles in liquid water using the Geant4DNA package, Nucl. Instrum. Meth B 269 (2011) 2307–2311. A. Vorburger, P. Wurz, Europa's ice-related atmosphere: the sputter contribution, Icarus 311 (2018) 135–145. J.F. Cooper, P.D. Cooper, E.C. Sittler, S.J. Sturner, A.M. Rymer, Old faithful model for radiolytic gas-driven cryovolcanism at Enceladus, Planet. Space Sci. 57 (2009) 1607–1620. J.F. Cooper, R.E. Johnson, B.H. Mauk, H.B. Garrett, N. Gehrels, Energetic ion and electron irradiation of the icy Galilean satellites, Icarus 149 (2001) 133–159. C. Paranicas, D.G. Mitchell, S.M. Krimigis, D.C. Hamilton, E. Roussos, N. Krupp, G.H. Jones, R.E. Johnson, J.F. Cooper, T.P. Armstrong, Sources and losses of energetic protons in Saturn's magnetosphere, Icarus 197 (2008) 519–525. J.C. Ashley, Stopping power of liquid water for low-energy electrons, Rad. Res. 89 (1982) 25–31. R.E. Johnson, Energetic Charged Particle Interactions with Atmospheres and Surfaces, Springer-Verlag, New York, 1990. A.E. Grun, Lumineszenz-photometrische messungen der energieabsorption im strahlungsfeld von elektronenquellen eindimensionaler fall in luft, Z. Natur 12 (1957) 89–95. P.M. Banks, G. Kockarts, Aeronomy, Academic Press, New York, 1973. H.G. Paretzke, Radiation track structure theory, in: G.R. Freeman (Ed.), Kinetics of Non-Homogeneous Processes, Wiley & Sons, New York, 1987, pp. 89–170. C.S. Lu, O. Lewis, Investigation of film-thickness determination of oscillating quartz resonators with large mass load, J. Appl. Phys. 43 (1972) 4385–4390. A.G.M. Abdulgalil, A. Rosu-Finsen, D. Marchione, J.D. Thrower, M.P. Collings, M.R.S. McCoustra, Electron-promoted desorption from water ice surfaces: neutral gas phase products, ACS Earth Space Chem. 1 (2017) 209–215. O. Gomis, M.A. Satorre, G. Strazzulla, G. Leto, Hydrogen peroxide formation by ion implantation in water ice and its relevance to the Galilean satellites, Planet. Space

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