Structure analysis of solid surfaces by multiple scattering of low energy ions

Nuclear Instruments and Methods 203 (1982) 515-522 North-tfolland Publishing Company

515

STRUCTURE ANALYSIS OF SOLID SURFACES BV MULTIPLE SCATTERING OF LOW ENERGY IONS A.J . ALGRA, S.B . LUITJENS *, E.P.Th .M . SUURMEIJER and A.L. BOERS . . epnrnnv, of Applied Plrrsicr, Odeerairr of Grouingen, Mjeaborgh 18. 9747 AG Groningen, 'The Netherlands Received 21 December 1981 and in revised form 27 April 1982 Multiple scattering effects in lowenergy ion scattering (LEIS) can be related to the structure of tlr¢ bombarded solid surf ;-. SGenc examples of the use of multiple scattering phenomena for structure analysis of copper (100) +md (4111) surfaces an: ti--d

1. Introduction Lowenergy ion scattering (LEIS), also known as ion scattering spectroscopy (ISS), is a technique to probe the surface of solids. Usually the target is bombarded by noblegas ions with an energy in thekeV range(1-20 keV), and the energy and yield of scattered ions are measured . It is a generally accepted conviction that the interaction of the projectiles with the target is described correctly by elastic binary collisions, or by asequence of binary collisions . The energy spectra of the scattered particles contain information about the elemental composition [1-3] as well' as the structure of the target surface [4,5,6]. Numerous studies have been devoted to these subjects, and have been reviewed, for instance, by Suurmeijer et al . [7], Buck [81, Heiland et al . [9], Boers [10], and Armour et al . [11]. Due to all theefforts spent, LEIS at the moment has progressed to a useful semi-quantitative technique . Nevertheless, further improvements and refinements are still needed, to increase the'field of its quantitative applicability. In particular, more knowledge has to be gained about the exactshape of the interaction potential - and henceof the cross section -, theionization and neutralization processes and' the inelastic energy losses suffered by the particles. This, however, is known to be highly difficult because in most cases the quantities which can be measured depend on more than one parameter. A well-known example is' the ion yield of scattered noble gas particles which depends on-the scattering cross section as well as the neutralization probability . Due to this entanglement it hasnot been possible up to now to deduce the influence of the factors separately, and we have to content ourselves with approximations . It is " Present address : Philips Research Labs, Eindhoven, The Netherlands. 0167-5087/82/0000-0000/$02.75 0 1982 North-Holland

therefore necessary to arrange a measuring program su that unknown factors other than the one to he inccstigated are excluded as much as possible . For example, to study the interaction potential the scattering cross section should be measured without a disturbing influence of ionization and neutralization processes and multiple scattering effects. This can he achieved by measuring the total yield of particles (neutrals as well as ions) as a function of the experimental parameters using a time-of-flight (TOF) spectrometer 112-14] or a calibrated gas filled stripping cell 1151. A% scattering centers surface atoms should he used which are isolated as much as possible (for instance the edge atoms on stepped surfaces), in order to avoid screening effects. An even more rigorous method to achieve this isolated situation is to scatter on target atoms which are brought into the gas phase (e .g . by evaporation) . Information on charge exchange and neutralization processes can be obtained by measuring tlw ion fraction of scattered particles and the inelastic energy losses. Despite the indicated problems and their precis, influence being unknown . LEIS does lead to-quantitative results under circumstances where appiovitnations of the influencing factors are good enough . This is sometimes the case when relative changes are measured, or when a problem exhibits a certain symmetry. Also in some cases it is possible to calibrate themeasured data, for instance by using reference targct [16,171. In this paper we deal with surface structure analysis try LEIS, using measuring procedures which can be arranged under the just-mentioned category. We will first glance at some interesting features of the energy spectra obtained with LEIS. Next a fewexperiments are described on copper (100) and (410) surfaces, where multiple scattering effects are used for structure analysis . (e.g. lateral or perpendicular spacing of atoms, density and width of atomic steps, position of step edge

516

A.J . Algra et . '. ; Strueture agah'we ul

atoms). Finally, some experimental requirements and features of three modes in which the LEIS technique can be employed are discussed .

.-hd sur/
K' Wu(410~ (ar6 u)

2. General features of theenew spectra ht figs . 1--3 energy spectra are she ( o'
Fig. 2. Energy spectra of potassium ions scattered through 8=60° from a copper (410) surface, for several E values. Primary beam current density onto the target: 10 y A/cm' : time needed for the recording of one spectrum : I-2 min.

Fig. 1 . Energy spectra of argon ions scattered through 8=40° from a copper (410) surface, for several primary energies E. Primary beam current density onto the target: 10 -7 A/cm2-, time needed for the recording of onespectrum: 2-3 min.

are discussed in ref. 18. For the present study the orientation of the(410)surface waschosen such that the step edges on the surface were parallel to the scattering plane (4-_ [0011) . Consequently, the influence of the steps can be neglected, and it may be expected that the results will be similar to those obtained for a [0011 direction on a (100) surface. First of all we note that, except at low energies in fig .3, the spectra show two more or less well-resolved peaks, the low energy peak of the single collision and thehigh energy peak of thedouble collision. The calculated positions of the pure single and double scattering energies are indicated by E, and Ed, respectively . The experimentally found peak positions appear to depend on Eo and shift towards higher relative energies E/E for decreasing Eo. The reason for this behaviour is the

LJ. ,4l-

c7

,f "did

al. ' Struc'rurc .. ....4"" 1

51 7

05kéV K *~ Cu(100)

1 tp=15° ,tp~[0011,Te300K (arb u )

5

4

3

2

0

_

26

-

28

30 30

32

34

Fig . 4. Scattered ion yields its a function of 8 for 0.5 k,%' potassium ions ecauered from a copper (IM) surface, l'or y - 15° and 0 'ICUI].

Fig . 3. Energy spectra of potassium ions scattered thrcntgh 0 = 30° from acopper (100) surface, for primary energies below 5 keV. Primary beam current density onto the target: 10 A/em 2 ; time needed for the recording of one spcetrum : 1-2 min.

increasing screening influence of neighbour surface atoms for lower Eu. Thereby the main collisions are preceded and followed by a sequence of small defleelions dependent on the angle of incidence y and the angle of emergence ß(see the insert in fig. 1). The pure single and double collisions change into "quasi-single" and"quasi-double" collisions. This "quasi" character of the single and double co?Nions has been discussed extensively by Poelsenia et al. 119,20] . The screening effect indeed takes drastic forms if the energy is lowered below 5 keV while the scattering angle is of the order of 30° or less. This is shown in fig. 3 for scattered potassium ions . For lower Ea the single and double scattering peaks disappear and eventually merge into one multiple scattering peak. The long-range influence of the interaction potential for th°se circumstances is also clearly reflected in the angular distribution. While for higher energies a wide range of scatter-

ing angles is possible 110,21], the angular distribution for0.5 keV potassium and a fixed V of 15° is liniite:i to a narrow region around specular reflection (0= 2t; 1 as shown in fig .4 . Apparently at these low energies the interaction of a projectile with the target should be regarded as a deflection by a rather smooth potential surface resulting in specular reflection. The importance of a small scattering angle in this respect is clearly demonstrated if the 1 and 2 keV spectra measured fnr B -- '(I' (fig. 3) are compared with those obtained for 0--60° (fig. 2) . In the latter case, W and /3 are rather large (30°) and therefore the screening influence of the neighbour atoms is reduced. A second striking feature of the ion spectra is the fact that the relative yield of the double scattering peak (compared with the single scattering yield) increases with E for argon but decreases for potassium. In the latter case the relative yield is determined solely by the cross sections for double and single collisions 1221 (see also section 4) ; the ratio of these cross sections decreases with increasing Et , . For argon, however, the scattering yields are also influenced by ionization and neutralization processes during the interaction. The authors have demonstrated this by measuring the ion fraction of the scattered argon particles 1231, and it appears that the ion fraction for the double collision increases faster with En than the ion fraction for the single collision. Apparently this latter effect overrules the influence of the cross section . Figs. 1 and 2 show that acertain yield remains at the low energy side of the single seratering peak . This low energy tailing is well known in the literature and usually

51 8

A .J. Algru er af. / Srrurture unulr.ci.c , J .coGd .surJhrec

attributed to the scattering from deeper layers [12,24] . In addition, three-dimensional effects at the surface, such as zig-zag collisions, may play a role as well (the ,-,pression "zig-zag" indicates that the projectiles are scattered by two target atoms in the Ist layer which do n..x both lie in the scattering plane :defined by the incoming trajectory and the detector) 12S] . In general, the latter category of collisions will also contribute to the scattering yield in the vicinity of and above the single collision energy [ 19,22,25], and can even give rise to discrete peaks in the spectra [261. For the experimental configuration in figs . 1 and 2, however, the cross section for possible zig-zag events can be calculated to be relatively small, while the final energies of particles after these zig-zag trajectories lie below E and thus only contribute to the tail [221. The low yield in the tail with respect to the yield in the surface peaks indicates that LEIS is a "surface sensitive" technique . We define a normalized surface sensitivity factor S as S - (I.~Ib)i (I.+I6), (1) where /, and / n are the maximum yield in the single scattering peak and the yield in the low energy tail just at the left of the single scattering peak, respectively (see the insert in fig. 5) . When noble gas ions are used as projectiles and only the scattered ions are detected, the large value of S is usually attributed to the phenomenon of preferential neutralization of ions scattered from below the surface. From recent TOF measurements of the yield of both neutrals and iems by Buck et al. 1121 and Luitjens et al . [141, it appears, however, that the large scattering cross section is also important. Due to the large cross section an attenuation of both the primary and the reflected fluxes evolves on their way through the solid. The neutralization and attenuation effects are represented by the surface sensitivity factors S and S., respectively, with S = 9' !nn S = a.-at, and t +rlb a,+at, . The yields I, and I b are proportional to a.n,} and o 6q,; , respectively . The resulting surface sensitivity, S, is now given by (2) S,=(S.+S.)/ (I +S.S.) .

An inspection of figs. 1 and 2 reveal that S(Ar) and S(K) decrease with increasing Eo. As argued above, this can be attributed to the dependence of the cross section a on Eo. This is confirmed if we plot the factor S(K), derived from the potassium spectra in fig. 2, versus the cross section a for single scattering of potassium on copper through 0=60° . The cross sections were calculated with the Thomas-Fermi-Moli6re potential using the theoretical screening length as given in ref. 19 . For potassium scattered from copper we have found that the

Ar-C.u(410), fig 1 0 - 40',41-20',,pa10011 .T :540K K* Cu (410), fig 2 0=6W,~-30', ~O a[M1] , T =540K - Cakuiated

1a

oe

f_ s.

06 04

ü~

02

0o

Eo(KeV)

"

ï,~_ 2

ts' 1 n ts'tb

s sa 1 -S~Sa

5(K1 : Sa (N) = Sa(Ar)

4

5 6,

4

~3r6

1~

2 12

' 14

Fig. 5 . The relation be!ween the surface sensitivity factor S and the cross section a, for single scattering through 0=60°. The thin line connecting the experimental values for S(K) (" ), obtained from fig. 2, serves to guide the eye. The experimental values for S(Ar) (X), obtained from fig. 1 are also given . Note that in this case 0=40° instead of 60° and thus only the E horizontal scale holds. For the definition of S, S and S.. see the text. ion fraction is virtually independent of the final energy 127], i .e . is the same for particles scattered from the surface and from the bulk; thus S(K)=0. Consequently, it follows from eq. (2) that S(K) =S.(K), i - . for potassium the surface sensitivity is totally due to the attenuation effect . As can be seen in fig . 5 . a clear relationship exists between S(K) and a thus demonstrating the increasing transparency of the surface for increasing Eo . From the ion fractions 9+ given in ref. 23 we derive that for argon in the keV region S(Ar) is about 0.670.75 (shaded area in fig . 5). The scattering c-oss sections for argon and potassium are about the same and, hence, S(Ar)-S.(K). Thus, using eq. (2), S(Ar) can be calculated by substituting a mean value for S(At) (=0.71) and S(Ar) = S4(K) . Fig. 5 shows the result. Finally, the values of S(Ar), derived from fig. 1 are also indicated. Note, however, that in this case 0 = 40b and ~ = 20° . As expected, the surface sensitivity with argon ions as projectiles is larger than with potassium ions. However, from fig. 5 it can also be deduced that for argon the surface sensitivity for low energies (below about 5 keV) is mainly due to the attenuation effect (S(Ar) larger than S(Ar)), whereas for higher energies (above about 10 keV) S is the more important factor. These boundaries are valid for argon on copper, and will generally depend on the scattering geometry (defined by 4, 0 and p) and the kind of projectile and target atoms .

A .J. .11,4rn et ul. / Srrus9ure 3. Demarcation of the multiple scattering regime We define the multiple scattering regime by those ranges of experimental parameters for which multiple scattering effects, in principle, can be used to determine the structure of a solid surface. This implies that multiple scattering effects should be measurable (for example a shift of the single collision peak or the occurrence of the double collision peak) ,nd, also, distinguishable from other effects (i .e. the overlap of different peaks must be small). It appears that both requirements can be related to the cross section for single scattering, a, . From several sets of spectra obtained with different projectiles on copper we have deduced empirical values for the lower and the upper limits of o,. The observability of multiple scattering effects appears to he A', while guaranteed if o, is larger than about 3 x 10 a sufficient energy resolution is ensured for o, values smaller than about 0.5 A= . The range of suitable 0 values depends on E and on the masses of the projectile and the target atoms, and can be estimated using the above limits for (Y, . The results arc indicated in fig. 6 for pot ssium, sodium and lithium scattered from copper. Because it will often be advantageous to have a wide range of 0 values available for the experiments, it can be deduced from fig . 6 that the primary energy should be chosen not too high. A, already noted in the previous section, also the surface ,ensitivity is benefited by a low primary energy. Concerning the choice of e% and A we finally remark that, irrespective of E and 0, small sß and ß values (i .e . in the order of 2-10°) always result in multiple scatter-

sum/vsr.s sJ.vulid .sur(ust"s

ing due to the atomic screening on th '~ incoming and outgoing paths, respectively (101 . 4. The use of multiple zcatterfnp, effects , Multiple scattering effects canhe classified into tltr":e categories : (i) The influence on the sir.gi~ scattering peak . (ii) The occurrence of péaks caused by particles scattered mainly by two defk:ctiom (plain-double collisions and zig-gag collisions). (iii) The occurenceof other peaks duo to nteractions with more than two target atoms. In the following we will discuss some examples of each category. (i) As has been mentioned before, under certain circumstances the screening influence of neighbour surface atotus has ;t noticeable effect on tile single scattering energy . An increase of the energy E,t, of the quasi-single peak can be observed for small E, and y or /3. Since the distance of the neighbour atoms is directly involved, a dependence of E,t, on the azimuthal orientation of the target surface with respect to the scattering plane is to be expected . Fig. 7 shows our measurements with 6 keV argon on copper (100) for y5 - 11113), 10011 and (011 with it decreasing interatomic distance d of

4

1t tarb u)

6 keV Ar' Cu (100) 6 c 30', 4)- 15', T= 5"70K

0.e6 a8e o90 a92 a9e E/Ea Fig. 7. Three energy spectra of 6 keV argon ions, specularly reflected from a copper (100) surface through 0_- 30°, and showing the dependence of E,t, 'on the interatomic distatee d=2.55 . 3 .61, 5 .71 Afor ¢=10111.1WI1 and 1013), respectively. Further experimental conditions are as in fig. 1 .

ae2 -0s4 Fig. 6. Demarcation of the 6- and E -multiple scattering rev ; roes for potassium, sodium and lithium, respectively, scattered from a copper surface . The boundaries of the regions are calculated froom the requirement 0 .03-_o,<0.5 A' using the Thomas- Fermi-MohBre potential .

511)

>2t1

!E

A .J . .4(grn rr al. Bk2V P.r'

;

Slru
.ee(âio)

0 : x', T . 540 K

loot)

-36A

-y :10131

a=57A

Fig. K. The relation between E., 'E and y for a fixed 0 =- alto and for argon scatlemd from a copper (410) surface, for two interatomic distances .

5.71, 3,61 and 2.55 A, respectively. It appears that E, t , increases considerably with decreasing d. The relation between E,t, and d can be used to determine the latter quantity as has been demonstrated for nickel (110) upon oxygen adsorption by Verheij et al. [28] and for copper (410) by thepresent authors 1181 . A particular suitable method to compare the calculated and treasured E,l, values is to use a fixed scattering angle anddecrease theangleof incidence ~P. In fig . 8 the tneasured E,l, energies are presented as a function of y for 8 kcV argon on copper (410) for two azimuthal orientations ¢=[001] (d=3 .61 A) and 0 '[013] (d= 5.71* A). We observe that the larger interatomic distance gives rise to a smaller increase of El, with decreasing ¢, dnd that the :.mall difference of the d values has a very distinct effect . The dependence of F,t, on d can be exploited to determine interatomic distances up to about 5 A by acomparison with computer calculations or with the F,-,y curves measured for known dvalues [ 18]. (ii) The existence of the plain double collision (i .e. the projectiles are deflected by two neighbour surface ;toms both lying in the incidence plane) was first predicted by Parilis et al. [29]. The dependence of the relative yield Id of the double scattering peak with respect to the yield I; of thesingle scattering peak was extensively investigated by Mashkova and Molchanov [30]. We have demonstrated [22] that the ratio R' of I., and I,' can be employed to measure the interatomic distance of nmghbour atoms in low index directions at

nf suhd .-rJn,,

the surface. When the experimental conditions are chosen such that the possible influences of ionization and neutralization processes and zig-zag collisions are eliminated, the following relations are obtained : R, = R=f(0)Eod 2 ,

(3)

where f(B) is a function of 0 which can be calculated front the cross sections involved, and R is the ratio of the total yields (ions + neutrals) of the single and plain double collisions . Ri , Rl and R4' , R, are the R' , R values for directions with interatomic distances d, and d_, respectively . The requirement that ionization and neutralization processes should have no influence excludes the use of noble gas ions when only the scattered ions are detected. To measure correct R values with noble gas ions the total scattering yield (ions+neutrals) should be detected . Alternatively, alkali ions can be used as projectiles because these particles have the same ion fraction after a single or double collision [22,31] and )fence R' =R . The requirement that zig-zag collisions should not contribute in the energy range of interest (E,-E.) re. stricts the application of this simple measuring method, at first glance, to low index directions, because in other directions the influence of zig-zags may be considerable [22,25,26]. Although the interpretation of the energy spectra under these circumstances is more complicated than for the low index directions, a reasonably good description can be given with a simple two-atom model [22]. Front the comparison of measured and calculated spectra information about the relative positions of tie target atoms can be deduced even in such high index directions. (iii) If appropriate experimental conditions are chosen the projectiles can he scattered in a well-defined way by interactions with more than two target atoms . A well-known example of this phenomenon are the peaks observed by Begemann et al. [5] due to particles scattered from a damaged surface . In these experiments very small angles ,y or ß (a few degrees) were used . In that case scattering from the undamaged parts of the surface is not possible (see, for instance, the discussions in the review papers about the chain model), and only scattering from more or less isolated atoms at step edges can contribute to the yield. For small ,0 values there are two possible scattering trajectories denoted with F and F' by Begemann et al, The F peak is due to single scattering from the isolated atoms, while the multiple scattering peak F' is due to particles first scattered from the isolated atoms by about Bt =0- 2ß followed by a specular reflection from the surface through 02. = 2ß (see the insert in fig. 9). An example of the damage peaks is given in fig . 9.

A.J. Algra er al. ."' Srrurrure m.nlr .+i.r oj solid .rnrfi ca,

521

Using a collision type similar to the F' collision the authors have shown that the energy EP of the so-called "plateau peak" [181 depends on the width 1 of the terrace of the stepped surface from which the particles were scattered . The relation between L'r and f can he calculated using a semiempirical theory [181 . This yields a method to determine the width to terraces between 15 and 60 A with an inaccuracy of about 5 A. S, Expcrintental requirements and features

0 1.~.-L~.t."" l 082

E y (Ar) Ee(N) .. . 0-84

.. . ..: : . .:.. a . 1- ._ssL,,lti..tJ 094 086 I " 088 090 092 . E/EO

Fig. 9. Energy spectra of potassium and argon ions scattered from atomic steps on Cu (100); 8=30°, tL==26° . The potassium spectrum shows the F and F' damage peaks, the argon spectrum only the F peak . Both spectra are normalized in height at the F peak . Primary beam current density onto the target : argon : 10 - ' A/cm', potassium : 10 °' A,; cnt 2 ; time needed fot the recording of one spectrum : argon : 2-3 min, potassium : 1-2 minutes .

representing spectra of argon and potassium ions scattered through B =30° front a copper (100) surface for/3 =0-~ =4° . Both spectra arenormalized in height at the F peak . Although the yield of the peaks appears to be a function of the degree of ion-induced damage. one has to be careful in interpreting the argon results as a measure of the density of damage sites. While for potassium, namely, the F' peak is present, this peak is completely missing in the argon spectrum. Again, neutralization processes influence the yield since all argon ions scattered via F' trajectories are neutralized [32,331 .

As was intimated before . LEIS in general is perand formed with noble gas ions the yield of only the scattered ions is monitored. Apart front historical grounds, several rearons can be indicated why the Iccltnique until recently has been employed only in this mode . First, there is the fact that for noble gas suns effective preferential neutralization occtus of those ions which have penetrated the surface Iaycr before Ica,ing the target, thus giving noble gas LEIS a relatively high surface sensitivity. Furthermore, the chwnical itxrtness of noble gas ions is attractive, certainly in view of the relatively large current densities (~ 10 ^ A"'cin2 1 and doses (,~Z 10' ions/ctn`) used in the past. Finally, energy analysis of the scattered ions can easily be carried out using electrostatic devices . In recent years, due to impro ed detection techniques - enabling the use of very low primary betun currents -- and the evidenced importance of theattenuation effect, this situation has changed and two other LEIS modes have been developed, namely LEIS-TOE (noble gas ions as primary particles ; detection of all scattered particles with a time-of-flight (TOE) spectrometer) [14,18,341 and alkali LEIS (alkali ions as printparticles; detection of scattered ions only) 122,351 . As v:,:, discussed, in section 2, the surface sensitivity of LEIS in both these nodes is nrtintaiaed to a lingo extent, thus giving the "in principle" possibility' of quantitative surface analysis . In addition, the re.,luced

Table I Some characteristics of the three LEIS modes; +-good. + -+ very good . -- po.,r Primary ions:

Noble gas ions

Detection of :

Ions

Characteristics: Preferential neutralization Constant ion fraction Relative required ion dose Chemical inertness Simplicity of ion source Simplicity of detection

yes no 1 + + +

Alkali ion, Ions + neutrals

10 '-Ill

Ions no yes Ill

1 -Ill

522

A .J. A~gra er al. ; Srrutvure «a«ft'.ris t'l .cnlid .r«r(<«e".e

bombarding dose required to detect the same number of scattered particles in these two modes is very attractive. We have summarized the important characteristics of the three LEIS modes in table l . The choice for one of the three LEIS modes will depend on the subject of investigation. For the study of surface structures using multiple scattering phenomena a low bombarding dose, u variable B, a small aperture angle .1B and a good energy resolution of the detection system are required. Consequently, in such studies theuseof alkali ions and a rotatable electrostatic energy analyser is agood choice. This work is part of the research program of the "Stichting vo or Fundamenteel Onderzmk der Materie" (FOM) with financial suppert from the "Nederlandse Organisatic voor Zuiver Wetenschappelijk Onderurek" (ZWO). Refetrnars C. Brunei, Z. Phys . 147 (1957) 161 . B.V . Panin, So, Phys. JETP 15 (1962) 215 . lit D.P. Smith, J. Appl. Phys . 38 (t967) 340, 141 H.H. Brongersma, J . Vac. Sci . Technol . 1 1 (1974) 231 . 151 S.H.A . 9egemann and A.L. Boors, Surface Ski. 30 (1972) 134 . 161 W. Heiland. ILG. Schaffler and E. Taglauer, Surface Sci. 35 (1973) 381 . 171 E.P.Th .M. Suurmeijer and A.L . Boers, Surface Sci. 43 (1974) 309. [81 T.M . Buck, Methods of surface analysis, ed ., A.W. Czanderna (Elsevier. NewYork, 1975) p. 75. 191 W, lleiland and E. Taglauer. Surface Sci. 68 (1977) 96. 1101 A.L. &e,,,, Surface Sci. 63 (1977) 475. [111 D.G. Armour, J.A. van den Berg and L.K. Verheij, J . 1 Radk anulytical Chemistry 48 (1979) 359, 121 T.M . Buck, Y.S. Chen, G.H . Wheatley and W.F. van der Wcg, surface Sci. 47 (1975) 244. 1131 S.A. Agamy and J.E. Robinson . Nucl . Instr . an d Meth. 149 (1978) 595 . 1141 S.B. Luitjens, A.J . Algra, E.P.Th.M. Suurmeijer and A.L. Boors, Appl. Phys. 21 (1980) 205. 111 121

[151 W. Eckstein. V .A . Molchanov and H. Verbeek. Nucl. Instr . and Meth . 149 (1978) 599. 1161 H.H. Brongersma and P.M . Mul. Surface Sci. 35 (1973) 393. [ 171 G.C. Nelson, Surface Sci . 59 (1976) 310 . 1181 A.J. Algra, S.B. Luitjens, E.P.Th .M . Suurmeijer and A.L. Boors, Surface Sci . 100 (1980) 329. 1191 B. Poelsema. L.K . Verheij and A.L. Boers, Surface Sci. 55 (1976)445 . 1201 B. Poelsema, L.K. Verheij andA.L. Boers, Surface Sci. 60 (1976) 485 ; 64 (1977) 537. 1211 V.M . Kivilis, E.S. Parilis and N. Yu. Turaev, Sov. Phys . Dokl. 12 (1967) 328. [221 A.J . Algra, S.B. Luitjens, H. Borggreve, E.P.Th .M . Suurmeijer and A.L. Boers, Radiation Effects 62 (1982) 7 . [231 S.B. Luitjens, A.J . Algra, E.P.Th .M. Suurmeijerand A.L . Boers, Surface Sci . 99 (1980) 652. 1241 Sec, for instance. O.B. Firsov, E.S. Mashkova and V.A . Molchanov, Radiat . Eff. 18 (1973) 257; D.J. Ball, T.M . Buck, D. McNair and G.H . Wheatley, Surf. Sci . 30 (1972) 69. 1251 E. Taglauer, W. Englert, W. Heiland and D.P. Jackson, Phys. Rev. Lett . 45 (1980) 740. 126] D.J. O'Connor and R.J. McDonald, Radial. Eff. 45 (1980) 205. 1271 A.J . Algra, E. van Loenen, J. Duyzer, E.P .Th.M . Suurmeijer and A.L. Boers, Radiation Effects 60 (1982) 173. [281 L.K. Verheij, J.A. van den Berg andD.G. Armour, Surface Sci. 84 (1979) 408. 1291 E. Parilis and N.Yu. Turaev, Sov. Phys. Dokl . 10 (1965) 212. [30] E.S. Mashkova and V.A. Molchanov, Sov. Phys. Solid State 8 (1966) 1206. [311 This has been demonstrated by the authors for 2-10 keV K', Na' and Li' on Cu (100) and polycrystalline gold ; see also ref. 22. 1321 S.L . Nizhnaya, E.S. Parilis and N. Yu. Turaev, Radial . Eff. 40 (1979) 23 . [331 S.B. Luitjens, A.J . Algra, E.P.Th.M . Suurmeije r and A.L. Boers, Surface Sci. 100 (1980) 315 . [341 T.M . Buck, G.H . Wheatley, G.L. Miller, D.A.H . Robinson and Y.S . Chen, Nucl. Instr. and Meth . 149 (1978) 592 . [351 1. Terzic, D. Ciric and B. Perovic. Surface Sci . 85 (1979) 149 .