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Vacuum 73 (2004) 59–63

*Correspondin

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0042-207X/$ - see

doi:10.1016/j.vac

Effects of surface track potential on secondary electronemission and surface stopping power

K. Kimura*, S. Usui, T. Tsujioka, S. Tanaka, K. Nakajima, M. Suzuki

Department of Engineering Physics and Mechanics, Kyoto University, Kyoto 606-8501, Japan

Received 28 August 2003; received in revised form 1 December 2003

Abstract

In ion–surface scattering a positive surface track potential is induced on the surface behind the projectile due to

ionizing collisions. The surface track potential is expected to affect secondary electron emission as well as the energy

loss process of the projectile ions. We measure secondary electron yield induced by 0.5MeV/u H+, He2+, Li2+ and B3+

ions during grazing angle scattering at a KCl(0 0 1) surface. The position-dependent secondary electron production rate

was derived from the observed secondary electron yield. The secondary electron production rate is normalized by the

mean square charge of the reflected ions. The normalized rate decreases with Z1 suggesting that the surface track

potential recapture the secondary electrons. We also measure the energy losses of 0.5MeV/u H+, He2+, Li2+, B3+ and

C4+ ions during grazing angle scattering at a KCl(0 0 1) surface. The observed result suggests that the surface stopping

power is reduced by the surface track potential.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Surface track potential; Specular reflection; KCl(0 0 1); Secondary electron emission; Surface stopping power

1. Introduction

When an ion is incident on an atomically flatsurface at a grazing angle the ion is subject to aseries of correlated small angle scatterings. As aresult, the ion is reflected at a specular anglewithout penetration into the solid. This phenom-enon, called specular reflection of fast ions, is verysuitable to study ion–surface interactions [1,2]. Ina previous study, we have demonstrated that morethan 100 secondary electrons are ejected during thespecular reflection of 0.5-MeV H+ at a KCl(0 0 1)surface [3]. Emission of such a large number ofsecondary electrons induces a strong positive

g author. Fax: +81-75-753-5253.

ss: kimura@kues.kyoto-u.ac.jp (K. Kimura).

front matter r 2003 Elsevier Ltd. All rights reserv

uum.2003.12.037

surface track potential behind the projectile ion.The induced surface track potential affects boththe secondary electrons and the projectile ionitself. Some of emitted secondary electrons maybe recaptured by the surface track potential. Onthe other hand, the projectile ion is acceleratedby the surface track potential. These effects shouldbe more pronounced for higher charged ionswhich induce stronger surface track potentials.In the present contribution, secondary electronyields and energy losses of projectile ions aremeasured for specular reflection of 0.5-MeV/u H,He, Li, B, and C ions at a KCl(0 0 1) surface. Theeffects of the surface track potential on thesecondary electron emission and the surfacestopping power are discussed based on theobserved results.

ed.

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Fig. 1. Observed charge state distribution of 0.5-MeV/u C ions

reflected from KCl(0 0 1).

Fig. 2. Observed secondary electron yield for 0.5-MeV/u H+,

He2+ and Li2+ ions reflected from KCl(0 0 1).

K. Kimura et al. / Vacuum 73 (2004) 59–6360

2. Experimental

A single crystal of KCl was cleaved along (0 0 1)in air and was mounted on a goniometer in a UHVscattering chamber. The crystal was heated atB250�C under UHV conditions to prepare a cleansurface [4]. The surface was kept at 150–180�Cduring the measurement to avoid the surfacecharging by ionic conduction [3]. Beams of 0.5-MeV/u H+, He2+, Li2+, B3+, and C4+ ions fromthe 1.7-MV Tandetron accelerator of KyotoUniversity were collimated by a series of aperturesto less than 0.1� 0.1mm2 and to a divergenceangle less than 0.3 mrad. The typical beam currentwas a few femto amperes or less. The beam wasincident on the target crystal at glancing angles yi

of 1–7 mrad. The azimuth angle was carefullychosen to avoid surface axial channelling. Thespecularly reflected ions were selected by a smallaperture (f ¼ 1 mm) placed 425mm downstreamfrom the target. The energy spectrum of thereflected ions was measured by either a magneticspectrometer (H and He ions) or a silicon surfacebarrier detector (Li, B and C ions). A microchannelplate (MCP) was placed in front of the KCl targetto detect secondary electrons emitted by the ions.The entrance of the MCP was biased atB500V tocollect all secondary electrons emitted. The MCPsignal was measured in coincidence with thereflected ion. The pulse height of the MCP signalis proportional to the number of electrons detected.Thus, the number of the secondary electronsemitted by a single ion can be derived from theobserved pulse height [3]. The charge state dis-tribution of the reflected ions was also measured bymeans of a magnetic charge state analyzer.

3. Results and discussion

Fig. 1 shows the observed charge state distribu-tion of reflected C ions when 0.5-MeV/u C4+ ionswere incident on the KCl(0 0 1). The charge statefractions and mean square charge /q2S are shownas a function of yi: The fraction of C

4+ ion is mostdominant (B60%) and the charge state distribu-tion hardly depends on yi: The observed chargestate distributions for other ions show similar

yi -dependence, i.e. the charge state distributionsare almost independent of yi: The observed meansquare charges are 1.0, 3.9, 8.1, 14.9 and 19.2 for0.5-MeV/u H, He, Li, B and C ions, respectively.Fig. 2 shows observed secondary electron yield g

as a function of yi for various ions. In a previouspaper [3], we have shown that the position-dependent secondary electron production ratePðxÞ can be derived from the observed secondaryelectron yield gðyiÞ as

PðxÞ ¼ �1

2pE

dV ðxÞdx

ffiffiffiffiffiffiffiffiffiffiE

V ðxÞ

sgð0Þ

(

þZ p=2

0

dgðyÞdy

����y¼

ffiffiffiffiffiffiffiV ðxÞ

E

qsin u

du

9=;; ð1Þ

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Fig. 3. Position-dependent secondary electron production rate

derived from observed secondary electron yield for 0.5-MeV/u

H, He and Li ions at a KCl(0 0 1) target.

Fig. 4. Normalized position-dependent secondary electron

production rate for 0.5-MeV/u H, He, and Li ions at a

KCl(0 0 1) target.

Fig. 5. Observed energy loss of 0.5-MeV/u H, He, Li, B and C

ions reflected from KCl(0 0 1).

K. Kimura et al. / Vacuum 73 (2004) 59–63 61

where E is the ion energy, V ðxÞ the continuumsurface potential and x the distance from thesurface. We employed the universal potential [5] tocalculate V ðxÞ: Fig. 3 shows the derived position-dependent secondary electron production rate.The production rate decreases with x and x

dependence is almost the same for all projectiles.The normalized secondary electron productionrate, PðxÞ=/q2S; is shown as a function of x inFig. 4. Because the ionization cross-section isroughly proportional to /q2S; the normalizedrate PðxÞ=/q2S is expected to be independentof the projectile atomic number Zp: The normal-ized production rate, however, decreases withZp: This suggests that a part of emitted electronsare recaptured by the surface track potentialbecause heavier ions induce stronger surface trackpotentials.Fig. 5 shows the observed energy loss, DE; of the

reflected ions as a function of yi: The energy lossincreases with Zp and slightly increases with yi:The surface stopping power has two componentsin the present case. One is the normal electronicstopping power and the other is the acceleration by

the surface track potential. While the normalelectronic stopping power is proportional to/q2S; the acceleration by the surface trackpotential has a different q dependence. In thefirst-order approximation, the induced surfacetrack potential, which is created by the secondaryelectron emission, is proportional to q2 and so the

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K. Kimura et al. / Vacuum 73 (2004) 59–6362

acceleration is proportional to q3: Thus theeffective stopping power is given by

SðxÞ ¼ /q2SSpðxÞ �/q3SFpðxÞ; ð2Þ

where Sp and Fp are the normal stopping powerand the acceleration by the surface track potential,respectively, for protons.The position-dependent stopping power SðxÞ

can be derived from the observed energy lossDEðyiÞ using a similar procedure to Eq. (1) [6]:

SðxÞ ¼ �1

2pE

dV ðxÞdx

ffiffiffiffiffiffiffiffiffiffiE

V ðxÞ

sDEð0Þ

(

þZ p=2

0

dDEðyÞdy

����y¼

ffiffiffiffiffiffiffiffiffiffiffiV ðxÞ=E

psinu

du

): ð3Þ

The derived position-dependent stopping powerwas normalized by /q2S and is shown as afunction of x in Fig. 6. The normalized stoppingpower SðxÞ=/q2S decreases with Zp as is sug-gested by Eq. (2). The acceleration by the surfacetrack potential can be estimated from the observedstopping powers, S1ðxÞ and S2ðxÞ; for two different

Fig. 6. Position-dependent stopping power of KCl(0 0 1)

normalized by the mean square charge /q2S of reflected ions

for 0.5MeV/u H, He, Li, B and C ions.

projectile ions,

FpðxÞ ¼ S1ðxÞ=/q21S� S2ðxÞ=/q22S�

=

/q32S=/q22S�/q31S=/q21S� �

; ð4Þ

where subscripts 1 and 2 denote two differentprojectiles. The acceleration FpðxÞ was estimatedfrom the data for protons and He. The obtainedFpðxÞ is shown in Fig. 7 (a thick solid line). Theabsolute value of the acceleration due to thesurface track potential is about 15–20% ofthe observed proton stopping power.The acceleration due to the surface track

potential can be estimated by using a simplemodel. The secondary electron leaves a hole on thesurface. There are two dominant mechanisms forsecondary electron emission in the present case.One is the direct excitation of target electrons byion–electron collisions and the other is decay ofplasmons excited by the projectile ion. Because thelifetime of a plasmon is larger than the ion–surfaceinteraction time, the holes created by the lattermechanism hardly affect the surface stoppingpower. Therefore we consider only the former

Fig. 7. Acceleration of 0.5-MeV proton at a KCl(0 0 1) surface

due to the surface track potential estimated from the measured

results (solid line). Calculated results with a simple model are

also shown.

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K. Kimura et al. / Vacuum 73 (2004) 59–63 63

mechanism to estimate the effect of the surfacetrack potential on the stopping power.The hole production rate at x; i.e. the number of

electrons directly excited over the vacuum level bythe projectile proton per unit path length can becalculated with a binary encounter model,

PnðxÞ ¼2pe4

mv2

Xi

niðxÞ1

ei

�1

2mv2

� �; ð5Þ

where v is the ion velocity, m the electron mass,niðxÞ the electron density of the ith shell averagedover the plane parallel to the surface and ei is itsbinding energy. We employed Hartree–Fock wavefunctions of isolated K and Cl atoms to calculateniðxÞ [7]. The created holes are able to move with ahopping velocity vH ¼ Wa=ð4 hÞ; where W (B0.1a.u. for Cl 3p band) is the band width, a ¼ 8:4 a.u.is the interatomic distance and h is the Planckconstant. The estimated hopping velocity for Cl 3phole is, however, as low as 0.03 a.u., which is muchsmaller than the projectile velocity (4.5 a.u.). Thus,the mobility of the hole can be neglected in thecalculation. On the other hand, the holes arescreened by the secondary electrons when they arecreated but the screening becomes weaker as thesecondary electrons move away from the surface.A simple model is introduced to describe thescreening, i.e. a screening function F(t, x)=1�exp(VSEt/x) is used, where VSE is the velocityof the secondary electrons and t is the time aftersecondary electron creation. Using this simplemodel, the force acting on the 0.5-MeV proton(the correction of the stopping power) can becalculated. The calculated results at yi ¼ 5 mradfor various VSE are shown in Fig. 7. Theacceleration force in the outgoing path is slightlylarger than that in the incoming path. The resultswith VSE¼ 0:521 a.u. are the same order ofmagnitude as the experimental result, showingthat the present simple model gives a reasonabledescription of the surface track potential.

4. Conclusions

The secondary electron yields emitted by var-ious ions of 0.5MeV/u during grazing anglescattering at a KCl(0 0 1) surface were measured.From the observed result the position-dependentsecondary electron production rate was derived.The obtained production rate, normalized by themean square charge of the scattered ions, wasfound to decrease with the projectile Z number,indicating that the emitted secondary electrons arerecaptured by the surface track potential inducedby the projectile ion. The energy losses of thereflected ions were also measured. The position-dependent stopping power derived from theobserved energy losses suggests that the surfacestopping power is reduced by the surface trackpotential.

Acknowledgements

We are grateful to the members of QuantumScience and Engineering Center of Kyoto Uni-versity for use of the Tandetron accelerator.

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