Cí L.

Radia/ion Effec/s and Defec/s in Solids, 1989. Vol. 109, pp. 75-80Reprints aV'ailabledirectly from the publisherPhotocopying permitted by lieense only

<!;)1989 Gordon and Breach, Science Publishers, Ine.Printed in Great Britain

INELASTIC ENERGY LOSS OF LOW ENERGY HeSCATTERED FROM Ni(110)

.. b bR. MONREALa, F.FLOREsa, A. NARMANN , W.HEILAND ,S.SCHUBERTb and P.M. ECHENIQUEC -

a Universidad Autonóma de Madrid, Dept. de Materia Condensada, Madrid, Spain;bUniversitiit Osnabrück, FB Physik, Osnabrück, Germany, cEuskal Herriko

Unibertsitatea, Quimicas, Donostia, Euzkadi, The Basque Country

(ReceivedSeptember 19, 1988; in final form October 15, 1988)I

\._ The energy loss of neutralized He in the energy range below 5 keV increases approximately linear with theprimary energy. The losses are of the order of 100 eY. The magnitude of the loss depends on thecrystallographic orientation. These experimental findings are found to be in qualitative agreement withtheoretical estimates based on the non linear stopping power theory.

Key words:energy loss, low energies, energy dependence, crystals.

(-..

. .~ ~ '"\

,,/

1 INTRODUCTION

Inelastic energy losses of slow ions or neutralized ions are an intriguing problem oflow energy ion scattering (ISS) experiments.1 The losses are also of quantitativeimportance for the total reflection of light ions, the range and the implantation oflight ions at low energies.2 Since sputtering at all primary energies is dominated bylow energy collisions the modelling of the inelastic part of the collisions is ofimportance in that field of particle solid interaction as weIl.3,4In low energy ionscattering we can distinguish discrete 10sses1,5-7and "unspecific" 10sses.5,8-10Thediscrete losses are mostly understood in terms of electron promotion models basedon the Fano-Lichten theory.11 The unspecific losses, observed as shifts of theelastic peak position, are often connected with low energy tailS.12-16These peakshapes are difficult to understand, since in most experiments they are affected byneutralization effects and, furthermore, multiple scattering effects may contributeto the intensity in the low energy tail. Very interesting effects may arise fromtrapping into the dynamical image potential, i.e. the skipping motion which causesdiscrete energy loss peaks.17,18

In this paper we will describe experimental results from the scattering of He fromNi(llO). Using a time-of-flight (TOF) system we can measure the energy spectra ofthe neutralized ions, i.e. neutral He, as well as the ion spectra. We wiIl give firstestimates based on a recently developed theory for the nonlinear stopping and thecharge state of partic1es moving in solids.19,20At the base of the theory is thedielectric response function of the solid including Auger processes and resonantcharge exchange processes. These details as well as the calculational accuracy (thedielectric function is calculated e.g. in full Random-Phase-Approximation (RPA))distinguish the results from earlier attempts21,22to describe the stopping of slow(v ~ VF)partic1es in matter. Purely atomic models on the other hand2,23-25have notresulted in satisfactory agreement with experiments.5,8-10On the other hand, weused the same theoretical model recently to estimate the charge state fraction of thescattered He particles and found satisfactory agreement.26,27The energy loss dataprovide a further test of the theory.

75

76 R. MONREAL el al.

2 EXPERIMENT

The experimental setup is an ultra-high-vacuum (UHV) system,Z8equipped with amagnetically analyzed ion beam, target manipulator, ion scattering energyspectrometer at 90° and a TOF system at a fixed scattering angle of 10°. For theexperiments a c1eanNi( 110) surface was used which was ion-polished by prolongedion bombardment (Ne+) at grazing incidence. The ion spectra, the neutral spectraand the energy losses show c1early crystallographic dependences. In a differentvacuum system we found good low energy electron diffraction (LEED) from thecrystal surface. The angle of incidence of the He beam was 5° from the surface(glancing angle). The angular aperture of the detector is 1.2° (full cone). The timeresolution at 1 keV is 5 ns. The calibration of the primary beam velocity, the beamspread on the target, and the scatter in the TOF data add up to errors of ::!:10 eVin the 1- 5 keV energy range covered by the present experiments.

The elastic energy loss for a binary collision between He and Ni is ca1culatedfrom

'-E/Eo= (M/(Ml + Mz))z (cos 0+ ((M/M1f - sin oZ)1/z)z

to be

E/Eo(o= 10°)=0.9979, (1)

where ois the laboratory scattering angle, MI is the mass of the projectile and Mzthe mass of the target. In our energy range Eo < 5 keV the elastic energy lossAEet< 10 eV for single collisions. Furthermore, at a glancing angle 1/J= 5° the Hepartic1es undergo multiple scattering events such that AEe ~ 10 eV results.Therefore the elastic loss is negligible under the experimental conditions chosen. Ifthe formula (1) for ElEo we set cos 0=1.0 and sinz o=Owe obtain ElEo =1.0.Theinelastic loss Q is for the same approximation reduced from

Q= -(1 + M¡!Mz)EI+(1- M¡!Mz)Eo+2M/Mz ~E1IEO.cos oEo (2)

to Q= Eo- El> a value we measure with an accuracy of ::!:10 eV as stated above. ,W',\.,>

3 RESULTS

The Ni( 110) surface was prepared as described. The TOF spectra were measuredfor energies Eo from 2 keV to 5 keV. Below 2 keV the ion yield from the c1eansurface is so small that the neutral He can be measured only. Above 5 keV ourpost-acceleration voltageZ8 becomes insufficient to separate the ions from theneutials. Neutral spectra for 3 keV and 5 keV are shown in Figures 1 and 2. Thecrystallographic dependence of the ion spectra is within the experimental erroridentical with that of the neutral spectra. They show more scatter since the ionyieldHe +/(HeO+ He +)is of the order of 3%.Z6

The results are surnmarized in Table 1for a random direction.In Table n we list the results obtained when the beam is directed along the [110]

and [001] surface channels. We observe an increase of Q and also a broadening ofthe energy spectra.

He SCATfERING FROM Ni(1tO) 77

lOOHe - Ni (110)

Eo = 2.996 keV~ = 5~-3= 10°

..............Eo

'.

2.40 2.60 2.80

Kinetic Energy (keV)

3.00

FIGURE 1 Energy spectra oí HeOscattered from Ni(110) in a random direction: 1/1is the glancingangle oí incidence, t} is the laboratory scattering angle and Enis the primary kinetic energy oí He +. Thedotted line is the experiment, the solid lines are theoretical resuIts (see text íor details).

.......

0.20

I

O' ................3.80 4.00 4.20 4.40 4.60

Kinetic Energy (keV)

4.80

Eo--r-5.00

FIGURE 2 Same as Figure 1 íor a higher primary ion energy Eo.

VI 0.80-e

Qj<- 0.60>--VIe(!) 0.40-e

/- -,.! 0.20-_./

O

lOOr He - Ni (110)

E = 4.89 keVo

0.80 = 5 -3=10.VI-e

É'U Qj 0.60

>--'iii 0.40e(!)-e

I ..

- - ---

78 R. MONREAL el al.

TABLE 1Inelastic energy losses Q for the scattered Heofrom Ni(11O)at grazing incidence (1jJ= SO,f)= 10°)

Eo(keV)Q(eV)

2.06970

2.996129

4.890276

TABLE IIInelastk energy loss Q for the scattered Heo at 3 keV along different crystallographic direction of

Ni(110). L (A) is a mean estimated trajectory length26(Eo= 3 keV)

r

The trajectory lengths L are estimated using the computer code MARLOWE.29These lengths are defined by the intersection with aplane 1.3 A above theoutermost plane of the Ni cores. The value 1.3 A is chosen from an estimate of thedecay length of the interaction of the He ions with the s-electrons of the Ni( 11O)surface. Even though this definition of L is somewhat arbitrary it gives a goodrelative estimate. L (5 keV, random)=43 A which results in Q/L=6.4 eV/A, Le.there is no simple scaling of the energy loss. The random values of Q(E) fallapproximately on a straight line in agreement with previous results that Q(E) =Eand not proportional to EI12.5.8-1O

4 DISCUSSION

The experimental results can be surnmarized as follows: (i) the inelastic lossincreases approximately proportional to the primary energy, (ii) the losses increase(nonlinear) with trajectory length, (iii) the losses are of the order of 100 eV in theenergy range used and under the conditions used and (iv) the losses for ions andneutrals are about equal.

The last point (iv) is probably due to the fact that the neutralization is almostcomplete for He + under the conditions used, i.e. there are essentially no survivingions. The ions observed are probably due to re-ionization by resonant chargeexchange between excited states of He and the Ni s-electrons.27 We cannot excludethat some ions originate from a violent collision with surface imperfections (steps).

Nevertheless a theoretical discussion of the results (i)-(iii) has to take intoaccount that the particles are for some part of their trajectories charged and neutralrespectively. As shown previouslyl9 the stopping for charged particles at lowvelocities is larger than for neutrals. This can be shown (linear theory) using for thestopping power (energy loss/unit distance) the approximation (in atornic units):

2

J

oo

J

oo dq

¡

1

}0+

S=- wdw - 1m - .p'nv2 o w/vq e(q, w) ,

where po= (2 - 2 Po? (3)

and p+ = (2 - Po?

/"u

Direction Random [110] [001]

Q(eV) 129 195 156L(A) 32 54 61Q/L (eV/A) 4.0 3.6 2.6

\.)

- -- - -

.

He SCATTERING FROM Ni(1tO) 79

C)

Po is in each case the Fourier transform of the appropriate electronic charge, e( q, w)the dielectric function, q the wave vector, w the frequency, v the (constant) particlevelocity. The evaluation of Eq. (3) yields for v(3 keV) = 0.17 a.u. andv(5 keV)=0.22 a.u., stopping power ratio of S(He+)/S(HeO):::::3. With dQ= yv dsfor the differential energy loss, where y is the "surface friction coefficient", ds thedifferential path length, we estimate the values for dQ/ds given in Table ID. Thebulk value 1 = 0.65 a.u. from Ref. 19 is estimated to be smaller at the surface toYs0:::::0.30 to account for the lower electron density at the surface. We also estimatethe factor between S(He+) and S(HeO) to be larger than 3 at the surface, firstestimates give an approximate value of 5. A detailed discussion of these effects willbe given elsewhere.30

The values found are in satisfactory qualitative agreement with the experimentalresults. The proportionality Qoc E follows approximately from Q= YVAA. HereAA= V. íA is the mean ion survivallength, íA the Auger decay time. yand íA can beassumed to vary slowly with energy, so Q-ocv2 foIlows immediately. The quality ofthis approximation wiIl depend on the relative contribution of "ionic" and "neutral"stopping (Table ID).

TABLEIIITheoreticalvaluesfor the differentialenergyloss for ions and neutralsat 3 and 5 keV primaryenergy

For a more detailed comparison with the experiment we make use of the Augerneutralization rate estimated previously27 and the trajectory lengths shown in Tablen. In a first approximation the number of ions N+ decreases as

o dN+ /dt= - N+( vI AA)'

where AA is the mean survivallength, with AA(3keV)=7 A and AA(5keV)=9 A.With dQs°=y{° v ds for the neutrals (the suffix s stands for surface) anddQ= (Ys+ - Ys°}vdsfor the contribution from the ions we obtain

dN/dQ-exp{ -(Q- QO)/(y/ - YsO)V2íJ9(Q- QO), (4)

where e is the step function and íA=AAfv. Evaluation of Eq. (4) gives the solidlines in Figures 1 and 2. The peak position of the calculated peak agrees with theexperimental result, but the calculated peak is too narrow. We note, however, thatthe slope of the low energy part of the peaks agree too. We realize that we neglectedin our present treatment three facts which will lead to a peak broadening: thedecrease of the electron density above the surface, the distribution of the scatteringtrajectories (different elastic 10ssesf6 and the straggling of the inelastic 10SS.31Theimportance of these influences can qualitatively be seen from the change of theenergy loss spectra when going from random to channeling directions. We wiIlinclude especially the straggling in our forthcoming publication.30

I

Energyparticle Ions Neutrals

3keV 12.5 eV/Á 2.5 eV/Á5keV 16.0 eV/Á 3.2 eV/Á

--

80 R MONREAL el al.

5 SUMMARY

We have used the same theoretical approach for an understanding of the inelasticlosses of slow ions/neutrals when interacting with a surface as previously for theunderstanding of charge state fraction experiments. In both cases we find goodqualitative agreement of theory and experimento The theory based on the dielectricresponse function of the solid is leading to a better understanding of the particlesolid interactitm. We obtain a unified point of view compared to the previoussituation where charge exchange processes and energy loss processes have been thesubject of quite different theoretical models.

ACKNOWLEDGEMENT

Financial support by the Deutsche Forschungsgemeinschaft (DFG), the Deutscher AkademischerAustauschdienst (DAAD) as part of the Acciones Integradas Hispano-Alemanas and the ComisiónAsesora de Investigación Científica y Técnica (contract no. 0388-84) is gratefully acknowledged.Thanks are due to K-J. Snowdon for helpful discussions.

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