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Elsevier Editorial System(tm) for Nuclear Inst. and Methods in Physics Research, B Manuscript Draft Manuscript Number: IISC18-19R2 Title: Characterization of the Ne-Al scattering potential using low energy ion scattering maps Article Type: Proceedings: IISC-18 Keywords: scattering; channeling; ion-surface interactions; LEIS; aluminum Corresponding Author: Dr. Robert David Kolasinski, Corresponding Author's Institution: Sandia National Laboratories First Author: Robert David Kolasinski Order of Authors: Robert David Kolasinski; Josh A Whaley; Richard A Karnesky; Christopher W San Marchi; Robert Bastasz Manuscript Region of Origin:

18th International Workshop on Inelastic Ion-Surface Collisions (IISC-18)

September 26 – October 1, 2010, Gatlinburg, Tennessee, USA

Nuclear Instruments and Methods in Physics Research, Section B.

MANUSCRIPT COVER PAGE

Title of Paper :Characterization of the Ne-Al scattering potential using low energy

ion scattering maps

Corresponding Author :Robert D. Kolasinski

E-mail :[email protected]

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Cover Letter

α

θ

φ

Fig. 1

Figure 1

Fig. 2

0.01

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10

100

sign

al in

tens

ity (c

ount

s/nC

) ARIES ion energy spectrum3 keV Ne+Al(111)α=67.0° / θ=45° / ϕ=0°

Al(s++)

Al(ss++) Al(s) Al(ss)

(a)

0.01

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10

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relative energy (E/E0)

ARIES ion energy spectrum3 keV Ne+Al(111)α=67.0° / θ=30° / ϕ=0°

(b)

H(r)

Al(s++)

Al(ss++)

Al(s)Al(r)

Figure 2

0.08

0.17

0.25

0.33

0.42

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0.58

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signal intensity(normalized)

Es=0.85

θ=30°

(r,φ)r = radius (0-8 Å)φ = azimuth

MARLOWE

(b)

signal intensity(normalized)

Es=0.82

θ=30°

(r,φ)r = radius (0-8 Å)φ = azimuth

ARIES

(a)

Fig. 3

0.08

0.17

0.25

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Figure 3

EXPERIMENT

SIMULATIONS

Fig. 4

0.50.40.30.20.10.0

1.00.80.60.40.20.0

f=1.2(b)

1.00.80.60.40.20.0

8765432

radial distance (Å)

0.50.40.30.20.10.0

(d) f=0.5scat

terin

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orm

aliz

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0.50.40.30.20.10.0

error (yi -m

ai ) 2

1.00.80.60.40.20.0

f=0.85BEST FIT

(c)

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8765432radial distance (Å)

(a) ARIES polar scan3 keV Ne+Al(111)θ=30° / ϕ=0°

scat

terin

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(n

orm

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Figure 4

0.4

0.3

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0.1

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R-fa

ctor

1.21.00.80.60.4

screening length multiplier (f)

3 keV Ne+Al(111)

Moliére potential fit

Fig. 5

Figure 5

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18th International Workshop on Inelastic Ion-Surface Collisions (IISC-18) / R. D. Kolasinski 1

Characterization of the Ne-Al scattering potential using low energy ion scattering maps

R. D. Kolasinski1, J. A. Whaley, R. A. Karnesky, C. San Marchi, and R. Bastasz

Sandia National Laboratories, Hydrogen and Metallurgical Science Department, Livermore, CA 94551, USA

Abstract – In this study, we examine the scattering of inert-gas ions from Al(111) using low

energy ion scattering (LEIS). These techniques, because of their high surface specificity, provide

structural and compositional information from the first atomic layer of the surface and can be used

to determine the configuration of low-Z adsorbates. Extracting structural information embedded in

LEIS data presents many challenges, given the complex collision processes which ultimately

contribute to the detected scattering and recoil signals. To aid in the interpretation of these data,

we map scattered Ne+ ion signals over a wide range of crystal orientations with respect to the

incident beam in order to investigate a variety of scattering geometries. The signals are also

simulated using a modified version of the MARLOWE binary collision code and related to the

surface structure. We make quantitative comparisons between the simulated results and the

experimental data using reliability factors, and are able to show how the interatomic potential can

be calibrated.

Keywords: scattering, channeling, ion-surface interactions, LEIS, aluminum

PACS numbers: 34.50.-s, 61.05.Np

1 Mailing address: Sandia National Laboratories, P.O. Box 969, MS 9161, Livermore, CA 94551

Phone: (925) 294-2872 / Electronic Mail: [email protected]

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I. Introduction

Low energy ion scattering (LEIS) and direct recoil spectroscopy (DRS) enable the structure and

composition of the outermost atomic layer of surfaces to be determined. The compositional

information provided by these techniques is reasonably straightforward to interpret, since the

scattered and recoiled particle energies can be directly related to the mass of the collision partner

on the surface using basic kinematic relationships [1]. On the other hand, accurate surface

crystallography using LEIS can be more elusive. The established procedure for determining surface

structure involves calculating shadow cone shapes for the ion-target pairs of interest and

determining the angles of incidence (α) and azimuths (φ) where these shadow cones intersect

neighboring atoms. (See Fig. 1 for angle definitions.) However, this approach can lead to

considerable errors in interpreting scattering data, since the conditions where shadow cone

intersections occur do not necessarily correlate exactly with maxima in scattering intensity. For a

more comprehensive overview of the challenges associated with LEIS surface crystallography, we

refer the reader to Ref. [2].

To circumvent these difficulties, we previously developed a technique where different surface

structures are simulated using binary collision models and are then compared to experimental data

[3]. This involved using reliability factors (R-factors) to make an unbiased, quantitative evaluation

of the best fit between the experiment and model. To make such comparisons, it is useful to

consider which data contain the most information about the surface structure. For this purpose, we

used a mapping technique previously developed by Agostino et al. [4] to acquire scattering signals

over wide ranges of α and φ. A special feature of these maps is that they can be conveniently

rendered in real-space, enabling one to identify important scattering mechanisms and making the

local atomic structure easily recognizable upon inspection.

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In this study, we focus on an alternate application of the aforementioned scattering maps to

cases where the surface structure is already known. One can then apply R-factor analysis in a

similar manner to calibrate the interatomic potential for a surface, a particularly valuable exercise if

the configuration of adsorbed atoms is of interest. This is especially true where the adatoms are

much lighter (e.g. lower Z) than the substrate and have only a minor role in determining how

incident ions are focused along the surface. In this case, adjusting the potentials for the ion-

substrate interaction would clearly enhance the accuracy of adsorbate structural measurements as

well.

We apply the mapping techniques and R-factor analysis described above to the model system 3

keV Ne+→Al(111). This surface has been widely studied, particularly because its interaction with

hydrogen leads to the formation of hydride species [5,6]. In this article, we begin with a description

of our experimental instrumentation, as well as a discussion of our LEIS measurements. We

present ion energy spectra for Ne+→Al(111), and briefly characterize the inelastic losses. A

description of ion scattering maps for the Al(111) surface follows, and a discussion of the R-factor

analysis, which is used to refine the scattering potential, concludes the paper.

II. Experimental and modeling approach

For the measurements described herein, we used an angle-resolved ion energy spectrometer

(ARIES) which has been optimized for detailed LEIS and DRS characterization of low-Z adsorbates

(particularly hydrogen.) A Colutron source produces ions by electron bombardment of a source gas,

and electrostatically accelerates them to a specified energy between 0.5-5 keV. The beam passes

through an E×B filter to remove undesired impurities and is then deflected through a mechanical

bend to remove neutral particles. The beam finally enters an analysis chamber, which is

maintained at a base pressure of 5×10-10 torr. (During operation, this increases to 10-7 torr due to

source gas leakage through the ion gun.) We configured the beam to raster over a 2 mm × 2 mm

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area on the target, adjusting the deflection voltages to maintain an analysis region that is

independent of incidence angle.

An electrostatic analyzer (ESA) collects scattered and recoiled particles over a 2 mm dia.

aperture. The entire detector is mounted within the chamber on a rotatable platform, enabling the

observation angle to be varied between (see Fig. 1 for angle definitions). Our system

is equipped with a 5-axis sample manipulator, which allows for precise alignment of the polar and

azimuthal angles with respect to the incident beam. The polished surface of the Al crystal (MaTecK

GmbH) was aligned to within 0.1° of the (111) plane. We cleaned and ordered the surface by

following previously established procedures [7], which included repeated cycles of sputtering with

Ne+ and annealing to 600 °C.

Aluminum quickly forms an oxide layer when exposed to small amounts of impurities. While

LEIS and DRS enable identification of surface-adsorbed species, Ne+ does not provide a strong

scattering signal from lighter elements such as C and O. To supplement these measurements, we

used Auger electron spectroscopy (AES), which offers high sensitivity to both species. By

monitoring the amplitudes of the O, C, and Al KLL transition peaks in the derivative AES spectrum,

we verified that surface contaminants were reduced to negligible levels during the preparation of

the surface.

For situations where high computational speed is required (i.e. to simulate complete scattering

maps), the binary collision code MARLOWE [8] is particularly well suited. Our modeling approach

was designed to enable analyses of scattered ion trajectories and collision partners; further details

may be found in Ref. [3]. For Al(111), we incorporated a surface binding energy of 3.36 eV as well

as random, non-correlated thermal displacements using a Debye temperature of 428 K [9]. At least

2×106 particle trajectories (initialized uniformly over a single unit cell on the surface) were needed

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for adequate scattering statistics. The interatomic potentials are used in our simulations are

discussed later in Sections III and IV.

III. Ion energy spectra and real-space scattering maps

Ion energy spectra for 3 keV Ne+→Al(111) are presented in Fig. 2, where the energies of the

scattered particles (E) have been normalized to the incident beam energy, E0. We calibrated the Ne+

beam by passing it directly into our hemispherical analyzer and found the FWHM energy spread to

be <1 eV at E0=3 keV. The largest peak in the spectrum depicted in Fig. 2(a) is indicated with the

notation “Al(s)”, and arises from Ne+ undergoing single elastic collisions with the Al surface atoms.

For a scattering angle corresponding to θ=45°, one would expect the Al(s) peak to occur at

E/E0=0.62; however, inelastic losses shift this to a lower energy. The high-energy peak occurring at

E/E0≈0.75 arises from Ne+ ions undergoing multiple in-plane scattering (hence losing less energy

than a single in-plane collision.) Note that we apply the notation (ss) to contributions arising from

multiple in-plane scattering. An ESA-type detector readily detects multiply-charged particles, as is

evident by the Al(s++) and Al(ss++) signals present in Fig. 2(a). Since double ions pass through an

ESA at half the voltage as their single ion counterparts, Ne2+ scattering from the surface appear as

distinct peaks in the ion energy spectrum.

The prominence of signals arising from Ne2+ is rather striking, as the survival probability for

noble-gas ions scattering from surfaces is typically quite small. However, a small subset of

materials (Mg, Al, and Si) appears to be an exception [10-12]. Also of note in the ion energy

spectrum is the presence of a small broad feature at E/E0=0.49. Both recoiling Al and O can be

detected at this energy. However, based on the AES data discussed previously, we can eliminate

surface-adsorbed O as a possibility.

The remaining measurements discussed in this study were acquired at an observation angle of

θ=30°; an ion energy spectrum for this angle is shown in Fig. 2(b). The main effect of using a

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smaller scattering angle is a shift of the main scattering peaks to higher energies, better revealing

light adsorbates which recoil at low energies. For example, a signal from recoiling hydrogen atoms,

labeled as H(r), is clearly evident in Fig. 2(b). The presence of this hydrogen signal is surprising,

especially given the strong barrier to dissociative chemisorption of H2(g) on Al. However, upon

more careful consideration we found this signal to be from residual H2 which had been fragmented

by the various filaments in our vacuum system. A disadvantage of using a small scattering angle is

that the Al(ss) is now incorporated into the high energy shoulder of the Al(s) peak and is no longer

easily distinguishable. However, the superposition of these peaks is taken into account in our

simulations, and should therefore not affect the quantitative comparisons discussed later in this

paper.

As previously discussed, real-space ion scattering maps reveal the structure of single crystal

surfaces, and can be used to identify important scattering processes. The approach described

herein was initially developed by Agostino et al. [4], and has many similarities to the scattering and

recoiling imaging spectrometry (SARIS) maps pioneered by Rabalais and co-workers. (For further

details on SARIS, refer to Ref. [13].) One key difference is the SARIS maps involve collecting

forward-scattered particles over a wide range of observation angles (θ), making “blocking” effects

easily observable. Agostino’s approach, on the other hand, involves varying the incidence angle (α),

allowing ion focusing along atom rows to be easily visualized. Since each mapping style emphasizes

different scattering mechanisms, one could envision choosing whichever better reveals the

structural information of interest.

Fig. 3(a) portrays an experimental scattering map for 3 keV Ne+→Al(111) acquired using our

ARIES instrument. We collected ion signals over complete 360° rotations in azimuth (φ) in 2° steps,

while incrementing the polar angle between 59.94°≤α≤81.46°. This was sufficient to sweep the

shadow cones over an 8 Å radius within the first layer surface plane in 0.25 Å steps. In total, each

map consists of 4887 individual measurements, with the intensity patterns rendered using a first-

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order interpolation between these points. All scattering signals were collected at a constant

observation angle of θ=30°. Once a complete data set has been acquired, the maps are transformed

from a (α,φ) phase space into real-space coordinates by considering where the shadow cone at a

particular incidence angle intersects the first surface plane of atoms. The map coloration indicates

the intensity of the Al(s) peak (E/E0=0.82), corresponding to single in-plane scattering. Our

experimental data are reproduced satisfactorily by MARLOWE, as illustrated in Fig. 3(b). This set of

calculations incorporates a Molière potential [14] using a Firsov screening length [15] adjusted

using a multiplicative factor of 0.85. (Details regarding the calculation of the screening length

which best fit the experimental data appear in the following section.) Note that we have taken

advantage of the surface symmetry by simulating only 0°≤φ≤120° and assembling the complete

map by reflection. Since the second layer of atoms may contribute to the scattering intensity at

lower angles of incidence, it was necessary to consider a 120° domain, rather than 60°.

The intensity patterns in the scattering maps arise depending on how adjacent atoms are

shadowed. Immediately evident is the six-fold symmetry of the patterns, which conform to the

parabola-shaped lines included in the Fig. 3(a) overlay. Each of these lines represents the angular

coordinates where the shadow cone from a reference site (located at the center of the map)

intersects its nearest neighbors. A more complete explanation of the mechanisms which contribute

to the intensity patterns can be found in Ref. [3]. To determine the intersection conditions, we used

the Oen shadow cone formula discussed in Ref. [16].

One benefit of the maps is that atom positions can be roughly assigned to the apex of each of the

red parabolic curves, as illustrated in the Fig. 3(b) overlay. Accurate atom positions can be

determined through more detailed comparisons between the MARLOWE simulations and ARIES

data. The rendering of the map in real space is mainly a convenience, but is also beneficial in

interpreting other features in the maps. This is especially true at the map periphery, where a

strong increase in scattering intensity is evident where the shadow lines converge.

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While every effort was made to minimize the ion dose during the acquisition of the scattering

map (each data point required 100 nC total dose), the accumulation of damage to the surface over

time remains a concern. Strikingly, we did not observe any degradation of our scattering signals

with ion fluence. There are several plausible explanations as to why this would be the case. First, it

is important to recognize that LEIS is not strongly sensitive to surface damage. Scattering from

atoms in non-crystalline regions should only add a uniform background rather than destroy the

structural variation in intensity generated by scattering from the crystalline portions. Also, the

activation energy for Al adatom migration on Al(111) has been observed to be rather low [17],

suggesting that some reordering of the surface is possible. In addition, it is likely that the

sputtering rate of the material from the incident beam is sufficiently fast so as to prevent damage

accumulation in the material.

To further address this issue, we improved the experimental procedure discussed in Ref. [3] by

monitoring only the scattered ion energies of interest, rather than acquiring a full spectrum at each

(α, φ) position. This lowers the fluence needed to acquire a map by a factor of five. Further

reductions could be realized by using a time of flight system (with a MCP), which would have a

much higher detection efficiency than our present ESA. While there is some variation of the

intensity patterns with azimuth, this is likely due to a slight misalignment of the crystal surface,

rather than damage accumulation.

IV. Interatomic potential calibration using R-factors

Accurately modeling low energy ion-surface collisions requires the selection of an appropriate

interatomic potential. Unless detailed sputtering or recoil calculations are of interest, the repulsive

portion of the potential typically dominates the ion-solid interaction [18]. It is common practice to

model the short-range repulsion with a screened Coulomb potential, and in the absence of prior

knowledge of the surface structure the ZBL empirical fit is generally suitable for this purpose [19].

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However, one may wish to calibrate the interatomic potential for a specific ion-target combination.

This is typically accomplished by adjusting the screening length (a) to control how quickly the

potential decays as a function of distance (r) away from the scattering center.

The approach we pursue here is to calibrate the potential by simulating scattering intensities

along a representative subset of Al(111) azimuths using MARLOWE and adjusting the screening

length by a simple multiplicative factor. We then perform a quantitative comparison between the

experiment and model using R-factors:

In the expression above, yi and ai are individual points within the experimental and simulated

scattering maps, respectively. The simulated data are scaled by a single multiplicative factor

which is determined by a weighted least squares scheme so that the error between the two data

sets is minimized. N refers to the number of points in each data set.

Consider Fig. 4, which illustrates the effect of calibrating the interatomic potential. Here we

have simulated the variation in scattering intensity as a function of α for 3 keV Ne+ scattered along

the <100> azimuth on the Al(111) surface. In effect, these conditions correspond to a radial cross

section of the scattering maps shown in Fig. 3 along φ=0°. Case (a) illustrates the experimental data,

whereas cases (b-d) are MARLOWE simulations which have been performed for different potential

screening lengths. For this purpose, we used the Molière theoretical potential used in conjunction

with the following screening length (developed by Firsov [15]):

In the above expression, a0 is the Bohr radius, whereas Z1 and Z2 are the atomic numbers of the

incident particle and target atoms. It is important to note that the Moliere potential is theoretically

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derived, in contrast to the ZBL formulation which is an empirical fit to many pair potentials. A more

thorough discussion of the theoretical underpinnings can be found in Ref. [19].

Fig. 4(a) shows the Ne+ scattering intensity along the <100> direction on the Al(111) surface as

a function of incidence angle. For a better comparison with the scattering maps in Fig. 3, α has been

transformed to radial distance d, based on the shadow cone intersection along the surface. The

lower panels portray scattering signals simulated using a screening factor that has been adjusted by

scalar multipliers of f=1.2 (b), 0.85 (c), and 0.5 (d). For each set of simulations, the dashed curve

plots the error term, . To better clarify the differences between each of the simulated

cases, we have normalized each data set to its maximum value.

Several important points may be extracted from the data presented in Fig. 4. First, the figure

clearly shows the sensitivity of the scattering signals to changes in the screening length. This is not

unexpected, as any modification effectively alters the strength of the potential, and by extension the

shadow cone shape. This affects both the broadness of the various features observed in polar scans,

as well as the incidence angle where these signals occur. In addition, the error curves illustrate

how the R-factor technique outlined here emphasizes the quality of the alignment between the

different features of the experimental and simulated scattering maps. A misalignment is captured

in an especially prominent manner in case (b), which corresponds to f=1.2. In this situation, the

leading edge prior to the maximum in scattering intensity is not reproduced well by the simulation.

This discrepancy is ameliorated by decreasing the screening length multiplier to f=0.85, as in case

(b), whereas further decreases produce unrealistic scattering behavior at grazing incidence,

illustrated by case (c).

While considering R-factor calculations along a single azimuth provides some insight into which

simulations best fit the experimental data, incorporating a larger data set improves the accuracy of

the fitting procedure dramatically [3]. A complete R-factor comparison for a range of screening

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length scaling factors appears in Fig. 5. Here we have compared experimental and simulated

scattering signals along 18 azimuths on the crystal surface for 8 different screening lengths. The

best fit between the experiment and model is indicated by the lowest R-factor value, which in this

case corresponds to f=0.85 and represents a fairly mild correction to the potential. (Further

refinement of this estimate could be obtained by considering a series of additional screening

lengths close to this value.) It is worth comparing this result with the findings of O’Connor and

Biersack [20], who performed a detailed evaluation of the Molière potential. Based on potential

calculations and experimental data for a wide range of ion-target combinations, the authors suggest

the following generalized correction to the screening length for the Molière potential:

where μ=0.045 and β=0.54. For specific case of Ne+→Al, fopt=0.84, which is comparable to the value

obtained through our analysis.

V. Concluding remarks

We have demonstrated how the Ne-Al interatomic potential can be calibrated using low energy

scattering maps. The basic procedure involves first using LEIS to collect ion signals over a wide

range of shadowing geometries. In this specific case we considered the model system 3 keV

Ne+→Al(111). Using MARLOWE allowed us to efficiently simulate scattering for different potential

screening lengths, and R-factor analyses made it possible to accurately compare these results with

the experimental data. For the Molière potential, a mild correction to the screening length provides

the best match with the experimental data.

When confronted with the task of modeling low energy ion scattering from surfaces, several

different options are available. The ZBL fit or Molière potential (coupled with O’Connor and

Biersack’s correction factor) are known to work well for many ion-target combinations, and could

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generally be expected to provide a satisfactory first approximation to the scattering potential.

However, if one wishes to calibrate the interatomic potential for a particular surface, the R-factor

analysis we describe here appears to be a reasonable approach. The R-factor analysis avoids many

of the problems associated with calibrating potentials based upon a more rudimentary shadow

cone analysis, where maxima in scattering intensity do not necessarily correlate well with the

conditions where shadow cones intersect neighboring atoms. With this in mind, application of the

analysis techniques described in this paper to more complex systems (including adsorbates)

appears particularly promising.

Acknowledgements

We express our appreciation to Dorian Balch, Rion Causey, Dean Buchenauer, and William

Wampler for helpful discussions regarding this work. Sandia is a multiprogram laboratory

operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of

Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

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Figure Captions

Fig. 1: Schematic of LEIS scattering geometry, illustrating the angle of incidence (α),

observation angle (θ) and azimuth (φ). Note that φ=0° has been aligned with the <100> close-

packed surface directions.

Fig. 2: Ion energy spectra for 3 keV Ne+→Al(111) at scattering angles of θ=45° (a) and θ=30°

(b). Peaks due to in-line single (s) and double (ss) scattering are indicated for both Ne+ and

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65


Ne2+ (++). In panel (b), the location of a peak due to recoiling hydrogen (r) is also noted. Note

that we have used a logarithmic scale to emphasize the location of the H(r) peak in case (b).

Fig. 3: Experimental (a) and simulated (b) ion scattering maps for the Al(111) surface. All

measurements were obtained at an observation angle of θ=30°. Overlay: red lines indicate

shadow line positions, circular markers indicate atom positions. Atoms indicated in light blue

contribute to the scattering intensity; dark blue indicates a given atom is shadowed from the

incident beam.

Fig. 4: Polar scans along the <100> direction on Al(111), where the incidence angle (α) has

been transformed to a real-space distance along the surface (d). Case (a) shows experimental

ARIES data, whereas cases (b-d) are MARLOWE calculations for different screening lengths.

The dashed line indicates the error term at each point.

Fig. 5: Comparison of R-factors for a range of screening length scaling factors. (The curve is

intended only to guide the eye.)

Response to referee comments

R. Kolasinski

12 November 2010

We appreciate the fast response of the referee, along with the insightful comments. To address the

concern about damage to the crystal surface, we considered the surface diffusion mechanism

suggested by the referee. We found published values for activation energies for Al adatom

migration on different crystal surface planes in a review paper by Kellogg [Surf. Sci. Rep. 21 (1994)

1]. Assuming the diffusion exhibits Arrhenius-type behavior, the diffusion coefficient will be:

For most Al surfaces, a pre-factor of D0=10-3 cm2/s and diffusion activation energy of Ed≈0.43 eV are

typical. This value of Ed is based upon field ion microscope measurement and estimated from an

“onset” temperature, where surface migration is first visible. Onset for single atom migration on

Al(110) has been observed at T≈154 K, and the Al(111) surface appears to be a special case where

the migration activation energy is especially low [Barth, Phys. Rev. Lett. 84 (2000) 1732]. In any

case, these data show atom migration (and therefore some level of reordering) at room

temperature is possible. Of course, these values may change depending on the presence of steps or

other surface effects.

Another explanation is that the sputtering rate of the material is sufficiently fast so as to prevent

damage accumulation in the material. The referee mentions that, assuming a sputtering yield of 1,

the incident beam wears away 100 monolayers of surface material. However, for the grazing angles

of incidence considered in this study, most of the surface damage will be concentrated in the first

15 monolayers or so. The shallow depth of damage makes it easier for the incident beam to sputter

away regions of the surface which contain damage.

An additional possibility as to why surface damage effects are not observed is that LEIS is just not

very sensitive to surface damage. Scattering from atoms in non-crystalline regions should only add

a uniform background rather than destroy the structural variation in intensity generated by

scattering from the crystalline portions. If the signal carries structural information in direct

proportion to the number of surface atoms existing in locally ordered areas, then structural

information could be obtained even on a significantly damaged surface.

We do not have enough information to definitively claim which of these mechanisms predominates,

and an in-depth discussion of each is probably beyond the scope of this work. However, we

reorganized the discussion of surface damage at the end of Section 3, bringing the above points to

the reader’s attention.

Finally, we emphasize that the methods applied here could also be implemented in a time-of-flight

detection system (using an MCP detector), which would reduce the needed dose by orders of

magnitude. This would make acquiring the maps essentially “dose-free”, thereby circumventing

some of the limitations of our ESA setup.

*Response to Reviewers

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