Slow electrons produced by slow multicharged ions on a clean metal surface

KWI B

Nuclear Instruments and Methods in Physics Research B 87 (1994) 130-137 North-Holland

Slow electrons produced by slow multicharged on a clean metal surface

Beam Interactions with Materials&Atoms

ions

F. Aumayr, H. Kurz and HP. Winter * Institut fiir Allgemeine Physik, TU wien, Wiedner Hauptstrasse 8 -IO, A-1040 Wen, Austria

In this progress report we discuss total electron yields for impact of slow (up I 2X lo5 m s-l) multiply charged stripped Ne”+ or Ar’*+) on clean polycrystalline gold, which have recently been determined from the related emission statistics. Within the classical over-barrier model for ion-surface interactions, these data are utilized to complete scenario of the processes initiated by impact of slow multicharged ions on a clean metal surface, on which of slow and fast electrons and X-ray photons as different messengers for the involved electronic transitions will be

1. Introduction

Multiply charged ions (MCI) which approach a metal surface start, within a distance related to both their

initial charge state and the surface work function [l], to capture electrons from the latter into highly excited states, in this way becoming transiently converted into so-called “hollow atoms”. The herewith initiated interplay between electronic states of the projectile and the surface involves, among other processes, rapid resonant electron capture from the surface into the projectile and fast autoionisation and resonant ionisation of the latter until its close contact with the solid. These fast electronic transitions have been modelled [2], based on a recently developed classical overbarrier approach [l], and the resulting slow (E, I 60 eV, cf. section 2) electron emission yields and statistics have been compared with measured data for various slow (up I 2 X lo5 m s-l) highly charged ion species, which were delivered from i) a recoil source pumped by the UNILAC at GSI Darmstadt (e.g. Nel’+, Ar16+, IB+ [2]), and ii) the EBIT facility at Lawrence Livermore National Laboratory (e.g. Arl’+, Xe51+, Thso+ [3]). Atomically clean polycrystalline gold has been used as the target surface within an UHV apparatus at a background pressure of some lo-’ Pa. The resulting total slow electron yields were determined from the

* Corresponding author, tel. +43 1 58801 5710, fax +43 1 564203, e-mail winterQeapv38.tuwien.ac.at.

ions (e.g. fully slow electron unfold a fairly basis the roles discussed.

related slow electron emission statistics, viz. the probabilities for given numbers of slow electrons ejected due to impacts of individual MCI [4]. The such determined slow electron yields increase more or less significantly both with the primary ion charge state and with decreasing projectile impact velocity. As for the most extreme case, for impact of Ne-like Th8’+ ions the emission of about 300 electrons could be observed [3]. However, if the impact velocity is lowered toward its nominal zero-value, the total electron yields apparently level off, which clearly demonstrates the self-acceleration of projectiles toward the surface as the result of their mirror charge [3]. Up to the highest ion charge states so far applied (i.e. q I SO), no saturation of this projectile self-acceleration has been found. Our results provide initial conditions for the further progress of projectiles inside the solid or their possible reflection back into vacuum. It is mainly during these later phases, that the recombination of projectile inner shell vacancies can produce fast Auger electrons and/or soft X-ray photons, which have recently been investigated by a number of research groups.

2. Experimental

methods

The multicharged ions utilized in the present study have first been produced with a recoil ion source

pumped by the UNILAC accelerator at GSI Darmstadt/Germany [2], where ions up to Nelof, Ar16+ and Izsf could be obtained. In a second experimental campaign, we used the EBIT facility at Lawrence Liv-

0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDZ 0168-583X(93)E0731-U

F. Aumayr et al. /Nucl. I&r.

and Meth. in Phys. Rex B 87 (1994) 130-137

ermore National Laboratory, Livermore, USA [3], which could deliver as the highest charge states, e.g. A? Xesl” and Th’a+, respectively. Afier passing a differentially pumped stage, the MCI could be further accelerated or decelerated by a four-cylinder lens assembly before hitting the target surface under normal incidence. A sufficiently small spread in initial kinetic ion energy gave access to ‘“nominal” (see below) final impact energies as low as E,, 2 (2 f l)q eV (4 being the projectile charge). Typical ion fluxes at the target surface were, e.g. 104 AI? or 10’ Arr6+ per second. All electrons ejected from the target with energies E, I 60 eV into the full 2a solid angle are deflected by a highly transparent (96%) conical electrode and then, ‘after extraction from the target region, accelerated and focussed onto a surface barrier detector (Canberra PD 100-12-300 AM) connected to +26 kV with respect to the target [4]. Fig. 1 shows a sketch of the last section of the decelerationtarget-detector unit, indicating some typical primary ion and ejected electron trajectories. For the particular case indicated in Fig. 1, the potentials of the various elements have been set in order to decelerate ions Zq+ from an initial kinetic energy of 4OOq eV down to Uq eV, and to extract at the same time the emitted electrons toward the solid state detector. The polycrystalline gold target was regularly sputter-cleaned by means of a built-in 2 keV Ar ’ ion gun. The complete deceleration-target-detector assembly has been kept in UHV at a base pressure below 3 x lo-’ Pa during all measurements by means of a turbomolecular pump and a Ti-sublimation pump with liquid nitrogen-cooled baffle. Any electron emission event induced by an impinging MCI wiil be finished typically within a time interval of less than 101rl s, which is much shorter than the resolution time of the applied detection electronics (2 lop6 s>. Thus, the IZ electrons emitted due to impact of a particular MCI will be registered like one

deceleration electrode (+400 v _ U)

cage (+340 V - U)

0

10

5

131

15

20

25

30

em&&on rnui~ipiici~ n Fig. 2. Measured electron emission statistics (symbols) for impact of Ar”+ ions with three different kinetic energies on clean polycrystalline gold. The dashed lines are fitted Gaussian distributions. The 500 eV data have been compared with a Poissonian (solid line) of the same mean value y, to show its non-applicability in the present case 121.

electron of n X 26 keV rather than n individual 26 keV electrons. Consequently, the area below the rz-th peak of the resulting “electron energy spectra” (henceforth to be abbreviated as ES) will be directly related to the probability W, for emission of p1 electrons. Eq. (1) gives the relation between these individual probabilitics lY, and the total clcctron emission yield y (i.c. the mean number of electrons emitted due to impact of one projectile) y=

i nw,; n=l

2 w,=*. n=O

(1)

The probabilities W, that no electron will bc emitted can generally not be directly determined [4], but become practically negligible for total electron yields y > 3. Electrons reflected from the detector surface (= 15%) will deposit only a part of their kinetic energy

extraction electrode (+1.9 kV - IJ) %?zzmzaF

focussing electrode (+I00 v - U)

electron detector (+26.4 kV - U)

ezzzzzmfinal ion energy E=qUeV

Fig. 1. Settrp for measuring statistics charged ions on a clean gold surface.

of ion-induced slow electron emission (ES) for impact of slow (nominal Ekin 2 2q eV) highly Indicated potentials refer to deceleration of a beam of Zs+ ions from 4009 eV to a nominal kinetic energy of Uq eV on the target surface. IV. INTERACTIONS;

CHANNELING

F. Aumayr et al. /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 130-137

132

3:

20,,,..rp

Neq’ -+ Au 15

3c

10

25

Arq++Au

e 2 s '5.

i? 0 Y

5

20

*at 2

0 : 0

5

10

~(10~

15

20

= al '5.

25

m/s)

Fig. 3. Measured total electron yields vs. nominal impact velocity up for NeqC ions (9 < 10) on a clean gold surface (data from ref. [2]). in the detector and thus add a characteristic background to the ES spectra, which must be quantitatively taken into account in the least-squares fitting of linear combinations of Gaussian-shaped peaks to the measured pulse height spectra, by which means the emission probability distributions W, (cf. Fig. 2) are being derived [4]. From the latter, the total electron yields can be directly calculated via Eqs. (1).

3. Presentation

of selected experimental

results

As described above, by measuring the ES for various projectile species we could determine the corresponding total slow electron yields. The nominal (see below) kinetic energy of these projectiles was varied between typically 2q eV and 400q eV. As typical examples, total slow electron yields y have been plotted vs. projectile velocity up in Fig. 3 for Ne4+ (q 5 10) and in Fig. 4 for Ar@ (q I 16), respectively. In both figures the solid lines represent reasonably good fits to the experimental data [2] according to Eq. (2) which, however, should be taken for guidance only, with C and y, serving as fitting constants, Y&J

=g+,.

15

10

5

0

0

5

10 v

20

15

25

(1 O4 m/s)

Fig. 4. Measured total electron yields vs. nominal impact velocity up for Ar4+ ions (q 5 16) on a clean gold surface (data from ref. [2]).

here presented data, they also level off toward the lowest impact energies because of the projectile selfacceleration toward the target due to its image charge, until a complete neutralization is being achieved [3]. For projectile velocities of = 2 X 10’ m/s, corresponding kinetic emission yields are still below 0.5 electrons/ion [5,61 and thus cannot be hold responsible for the flattening of the yield characteristics at high impact velocity. Fig. 5 shows total slow electron yields (full symbols) at up = 5 X lo4 m/s vs. the “total poten-

(2)

Eq. (2) indicates that there are at least two different contributions to the total slow electron yield, of which one depends on the impact-velocity (first term) and the other one is more or less independent of up. As we shall see later on (cf. section 4) these two contributions are probably related to principally different emission processes. Within their charge state manifolds, for both projectiIe species the yields y increase with increasing q and decrease with increasing up, but level off at high impact velocity towards apparently constant values 3~ (cf. Eq. (2)). Although it is not clearly apparent from the

Arq+ 4

Au

v = 4.9xlO’mis P

Fig. 5. Measured total slow electron yields vs. total potential energy (full symbols) and “available” potential energy (open symbols, cf. text), respectively, for impact of slow Arqf (q 5 18) (nominal impact energy 500 eV, up = 4.9X104 m/s) on clean gold (data from refs. [2,3]).

F. Aumayr et al. /Nucl.

133

Instr. and Meth. in Phys. Res. B 87 (1994) 130-137

tial energies” Wq,pot carried by diffently charged Arq+ projectiles, which are defined as

overall uncertainty of our total slow electron amounts to about _t4%.

yields

4 Wq,pot = c

K-l-tz,

(3)

1=1

stands for the ionization potential of where y-i_‘ &-r)+, i.e. the energy necessary to remove one electron from the latter. These ionization potentials have been calculated from Slater’s rules [7] and agree within 5% to the sum of ionization potentials of the corresponding ground state ions according to Kelly and Palumbo [8]. It has been noted by different authors [9-111 that, e.g. for Arq+ ions at 4 5 8, the corresponding total electron yields ~(4) are approximately linearly proportional to the related values Wq,pot.In Fig. 5, the dashed straight line fitted to the y values for q I 8 corresponds to an average energy of about 75 eV, which is needed for emission of one slow electron. For higher q, however, the “cost” for the emission of each further slow electron is increased up to typically 250 eV. Ar ions in charge states q I 8 have completely filled Kand L-shells, whereas for q > 8 a number of (q - 8) L-shell vacancies will be present, and for q = 17 and 18 also one or two K-shell vacancies are developed. These vacancies are not only responsible for the higher costs for emission of slow electrons, but also for ejection of fast electrons and/or X-ray photons during the later stages of the projectile-surface interaction. Further inspection of the Arqf results reveals the surprising fact, that for 8
4. Towards

a complete electron emission

description

of MCI-induced

We have already mentioned that during the neutralization sequence of an MCI in front of the surface, its inner shell vacancies will not become fully recombined until close contact with the surface and will thus be carried into the target bulk, where they will eventually decay under emission of correspondingly fast Auger electrons [12-151 or soft X-ray photons [16-191. Some of the projectile inner shell vacancies might be transferred into the inner shells of target atoms, thus giving rise to fast Auger electron emission from the latter [20]. In some earlier investigations, these projectileand target inner shell vacancy formations have caused characteristic variations of the charge states of scattered projectiles [21] and sputtered target atoms [22] induced by impact of primary MCI in different charge states. So far, reviews of this rapidly emerging field usually adhered to classical concepts [18,23,24,1], but some of the most relevant electronic transitions have also been treated quantum mechanically, as e.g. resonant neutralization (RN [25]> and autoionization (AI [26]). Auger electron transitions in the transiently formed, multiply-excited “hollow atoms” are the more probable the smaller the change in principal quantum numbers for the respective “down-electrons” [10,24,1]. Consequently, most of the “up-electrons” emitted into vacuum will feature rather low kinetic energies. The much faster “above the surface” (henceforth to be shortened as a.s.) electrons produced due to Auger electron transitions into inner shell vacancies before surface impact are comparably rare. With reference to Fig. 6, which indicates the various electronic transitions of interest, we see that during further approach of the particle toward the surface the electrons which have already been emitted via AI can be rapidly replaced by further RN processes. However, these RN transitions have now to compete with other projectile ionization processes, as electron promotion into vacuum due to state shielding (SS)- and image charge-related state shifting (IS), which both will also produce a.s. slow electrons 121.In addition, the neutralized projectiles can become resonantly ionized (RI) by loosing some of their transiently captured electrons into empty surface states. However, as soon as a projectile has reached the surface, all of its electrons still bound in highly excited states will be “peeled off’ [1,2], which provides once more a contribution to the total slow electron yield. If the then transiently re-ionized projectiles enter the target bulk, they will become rapidly re-neutralized IV. INTERACTIONS; CHANNELING

134

F. Aumayr et al. /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 130-137 peeling off

SS+IS

*,

a8

t

“;

promotion YaC”“m IeYe,

t

:g5 R @.U.)

+ approaching MCI metal S-DOS Fig. 6. States of a neutralizing multicharged ion approaching a metal surface (schematical). All processes considered in the discussed model calculations are indicated. Electrons having been captured via RN may be emitted via AI, promotion into vacuum due to screening/image shifts (SS + IS) and “peeling off” all those electrons which cannot stay bound inside the solid. In addition, electrons can be recaptured via RI into the solid.

into such tightly bound shells which can readily exist inside the solid [27,1,2]. Because of this dramatic change in the projectile’s population distribution, still remaining inner shell vacancies can now be recombined much faster than above the surface. This can give rise to the major share of the observed fast Auger electrons, which for the sake of clarity shall be called “below-surface” (henceforth to be shortened as b.s.) fast Auger electrons [13,28]). Alternatively, the projectile and target vacancy recombination can be achieved via soft X-ray emission [16191. We note, however, that these X-ray photons can also escape from deeper layers in the target bulk than the b.s. fast Auger electrons, because of which their characteristics also cover a somewhat later stage of the projectile neutralization and de-excitation inside the solid. The fast b.s. Auger electrons can produce secondary electrons inside the solid, which will further contribute to the total slow electron yields. Under favourable backscattering conditions some projectile inner shell vacancies might even survive until the projectile has left again the target bulk, and will then be recombined above the surface, thus contributing to the a.s. fast Auger electron emission [15]. Recently, it has been demonstrated in considerable detail [28] how the above described projectile inner shell vacancies decay above and below the surface, and that the resulting fast Auger electrons can deliver important informations on the involved recombination processes above and below the surface. The fast Auger electron emission is thus of great basical interest, but its absolute contribution to the total electron yield is much smaller than that of the

slow electrons, which is caused by the much more efficient utilization of the projectile potential energy for ejecting slow electrons than fast electrons. The time available for electronic transitions above the surface is principally limited by the image charge acceleration of the projectiles. As an example, for impact of N6+ on clean tungsten, a time of about 80 fs can be deduced from the classical over-barrier model [l]. However, the emission of projectile K-shell electrons above the surface is hampered because of the almost “inverted” electronic population of the transient “hollow” projectile. On the other hand, after having entered the target bulk, a nitrogen K-shell vacancy cannot survive longer than a few fs [20,28], because now the rapidly filled projectile L- and M-shells permit much faster KLL- and KLM Auger transitions than possible above the surface. It should be added that the image-charge acceleration-limited time until surface impact decreases with increasing projectile charge state according to qp1j4. Consequently, the generally small ratio between a.s. and b.s. fast Auger electron yields can be convincingly explained. On the other hand, no quantitative comparisons have been made between slow and fast electron yields induced by MCI on clean metal surfaces, except for impact of N6+ and Arg+ on clean tungsten [29]. At low impact energies (0.9 keV for N6+ and 1.35 keV for Ar9+), the contribution of the nitrogen K-shell electrons to the total electron yield was about 7% and that of the argon L-shell electrons about l%, and the related total electron yields were about 5.5 [14,29] and 10 electrons per MCI Ill], respectively. Further evidence that the contributions by the fast Auger electrons to the total electron yields are comparably small, can be gained from some absolutely measured Auger electron energy distributions for impact of A++ (q 2 9) and of H-like MCI (C’+, N6+, 08+) on clean tungsten [14]. In the latter measurements, the involved lowest impact energies of about 100 eV at 45” angle of incidence correspond to “perpendicular” impact energies of about 50 eV to which, however, the respective energy gains due to image charge acceleration, i.e. about 12, 16 and 20 eV, respectively, must be added. The typical KLL Auger peak intensities are about 4 x 10m3 electrons/(MCI sr eV>. By taking into account the probable origin of the b.s. Auger electrons inside the metal for their transport into vacuum, we may conclude that for each fast electron ejected inside the solid as the result of a projectile K-shell vacancy recombination, typically only 0.3 fast electrons will escape and thus contribute to the total electron yield. This number, if compared with the respective total electron yields, shows that the contributions by fast Auger electrons are indeed relatively small. It has also been demonstrated that the number of fast Auger electrons produced in Ar L-shell vacancy recombina-

F. Aumayr et al. /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 130-137

tions is about proportional to the number of the initially present vacancies. Considering the fact that when increasing the projectile charge state by one, the respective total slow electron yields will rise surely by more than one electron/ion (cf. Figs. 3-51, we conclude that the total yield fractions of the fast Auger electrons should even gradually diminish with increasing projectile charge. Some of these b.s. fast Auger electrons may produce secondary electrons inside the metal (see above). Consideration of the typical yields for such processes under the given geometrical conditions leads to not much more than one slow b.s. secondary electron for each scattered b.s. fast Auger electron. Consequently, the yields of b.s. slow secondary electrons and those of b.s. fast Auger electrons should be of comparable size, and the bs. slow electrons should therefore remain considerably less important than the slow electrons produced before and upon surface impact. It is thus the latter which provide the by far dominant contribution to the total electron yield for slow MCI-metal surface collisions. In ref. [2] we have shown that according to the classical over-barrier (COB) model, the bulk of slow electron emission is provided on the one hand by AI (this contribution depends strongly on the impact velocity up> and on the other hand by SS-, IS- and peeling off (PO) processes, which all depend rather weakly on up. It is thus tempting to ascribe, at least qualitatively, the AI contribution to the first term of Eq. (2), and all the other contributions to its second term. At present, our COB modelling is insensitive to the projectile species, since we have assumed highly excited hydrogenic projectile states into which the electrons are being captured until the surface impact. In this context it is of interest to compare total slow electron yields measured for equally fast, but chemically different projectiles in equal charge states, e.g. the data given in ref. [2] for the rare gas ions Nelof, Ar”+, Kri”+ and Xei’+, respectively. This comparison shows that, as a general tendency, for lighter ions (i.e. lower Z> the impact-velocity dependence of the yield becomes obviously flatter, whereas at any given up its absolute value will gradually increase. This is explained in the following, qualitative way. Under otherwise equal circumstances, the AI transition probabilities should become larger, the heavier the projectile, because of the relatively more complex projectile level structures. This should cause a flatter impact velocity dependence for lighter projectiles, because the influence of RI stays approximately independent of the projectile structure. Inside the solid, however, the energy of fast Auger electrons ejected from the projectile is larger for lower Z, which causes a comparably more efficient production of b.s. secondary electrons, and thus a slighly increased total slow electron yield, as experimentally observed.

135

Finally we ask ourselves, where the large potential energy carried by a highly charged ion into the metal target actually ends up. As we have seen, during the neutralisation in front of the surface, most of the inner shell vacancies remain empty and their corresponding excitation energy will be carried into the solid to be spent later on for emission of fast b.s. Auger electrons which, however, do not contribute significantly to the total electron yield (see above). Given the limited time in front of the surface because of the projectile’s image-charge acceleration (see above), for impact of Arq+ all of the (q - 8) inner shell vacancies will practically survive until surface contact and thus a substantial potential energy remains stored, which is not made available for a.s. slow electron emission via the earlier outlined RN-AI-PO processes. If we subtract such “withheld” excitation energy from the above defined total potential energy Wq,pot, the remaining part fits much better into the linear relation for total slow electron yields for q I 8, as shown in Fig. 5. These so-called “available” potential energies have been calculated by assuming that during the projectile neutralization the captured electrons can really cascade down into the projectile M-shells. The apparent discrepancy between the such corrected total potential energies and their extrapolated values (dashed straight line in Fig. 5) demonstrates that with increasing q the a.s. captured electrons actually cannot make it down into the M-shell until the surface impact. In addition, for the H-like and the fully stripped Ar projectiles, de-excitation can also take place via X-ray emission [17], which fact might be responsible for the comparably larger discrepancies at q = 17 and 18 between the above defined “usable” potential energy and our linear extrapolation made for q I 8.

5. Conclusions An innovative method for measuring the statistics of particle-induced slow electron emission has been combined with an advanced production of slow highly charged ions, to determine precisely total slow electron yields for impact of slow MCI on an atomically clean, polycrystalline gold surface. With the such obtained experimental data, the dynamics of neutralization, deexcitation and slow electron emission during MCI interaction with the target surface have been analyzed within the classical over-barrier model. Quite realistic estimates for the involved neutralization and de-excitation time scales could be obtained, and the observed relative fractions of slow and fast electrons to the total electron yields could be well explained. Our simulations have shown that the processes of IS + SS promotion and peeling-off deliver slow electrons almost independently of the projectile’s impact velocity, whereas a IV. INTERACTIONS: CHANNELING

136

F. Aumayr et al. /Nucl. Instr. and Meth. in Phys. Rex B 87 (1994) 130-137

strongly impact-velocity dependent part results primarily from the autoionization cascades before and upon the projectile’s impact on the surface. The present study describes in a fairly complete manner the interactions of slow highly charged ions with metal surfaces and the resulting emission of slow and fast electrons and soft X-ray photons.

Acknowledgments The authors thank Dr. R. Mann (GSI Darmstadt, Germany) and Dr. D. Schneider and coworkers (Lawrence Livermore Nat. Lab., USA) for fruitful cooperations. Work has been supported by Fonds zur Fiirderung der Wissenschaftlichen Forschung (Proj. no. P8315TEC) and by Kommission zur Koordination der Kemfusionsforschung at the Austrian Academy of Sciences.

References [l] F. Burgdiirfer, P. Lerner and F.W. Meyer, Phys. Rev. A

44 (1991) 567% F. BurgdEirferand F.W. Meyer, Phys. Rev. A 47 (1993) R20. [Zj H. Kurz, K. Tijglhofer, HP. Winter, F. Aumayr and R. Mann, Phys. Rev. Lett. 69 (1992) 1140; H. Kurz, I?. Aumayr, C. Lemell, K. T(iglhofer and HP. Winter, Phys. Rev. A 48 (1993) 2182. [3] F. Aumayr, H. Kurz, D. Schneider, M.A. Briere, J.W. McDonald, C.E. ~nn~ngh~ and HP. Winter, Proc. XVIII ICPEAC Aarhus, Denmark (1993) p. 767; F. Anmayr, H. Kun, D. Schneider, M.A. Briere, C.E. Cunningham, J.W. McDonald and HP. Winter, 71 (1993) 1943. [4] G. Lakits, F. Aumayr and HP. Winter, Rev. Sci. Instr. 60 (1989) 3151; F. Aumayr, G. Lakits and HP. Winter, Appl. Surf. Sci. 47 (1991) 139. 151 G. Lakits, F. Aumayr, M. Heim and HP. Winter, Phys. Rev. A 42 (1990) 5780. [6] K. Tiiglhofer, F. Aumayr and HP. Winter, Surf. Sci. 281 (1993) 142. [7] J.C. Slater, Phys. Rev. 36 (1930) 57. [S] R.L. Kelly and L.J. Palumbo, Atomic and Ionic Emission Lines below 2000 A NRL (1973). [9] H.D. Hagstrum, Phys. Rev. 104 (1956) 672. [lo] U.A. Arifov, L.M. Kishinevskii, E.S. Mukhamadiev and E.S. Parilis, Sov. Phys. Tech. Phys. 18 (1973) 118. [ll] M. Delaunay, S. Dousson, R. Geller, P. Varga, M. Fehringer and HP. Winter, Proc. 14th ICPEAC, Palo Alto, USA (1985) p. 477; M. Delaunay, M. Fehringer, R. Geller, D. Hitz, P. Varga and HP. Winter, Phys. Rev. I3 35 (198714232.

D21 D.M. Zehner, S.H. Overbury, CC. Havener, F.W. Meyer and W. Heiland, Surf. Sci. 178 (1986) 359. 1131F.W. Meyer, S.H. Overbury, CC. Havener, P.A. Zeijlmans van Emmichhoven and D.M. Zehner, Phys. Rev. I.&t. 67 (1991) 723; F.W. Meyer, S.H. Gverbury, CC. Havener, P.A. Zeijlmans van Emmichhoven, J. Burgdorfer aud D.M. Zehner, Phys. Rev. A 44 (1991) 7214; P.A. Zeijlmans van Emmichhoven, CC. Havener, I.G. Hughes, D.M. Zehner and F.W. Meyer, Phys. Rev. A 47 (1993) 3998. [I41 S.T. de Zwart, Ph.D. thesis, Rijksuniversiteit Groningen (1987); S.T. de Zwart, A.G. Drentje, A.L. Boers and R. Morgenstern, Surf. Sci. 217 (1989) 298; L. Folkerts, Ph.D. thesis, ~jksunive~iteit Groningen (1992); L. Folkerts and R. Morgepstern, Europhys. Lett. 13 (1990) 377. t151 R. Kohrbriick, K. Sommer, J.P. Biersack, J. Bleck-Neuhaus, S. Schippers, P. Roncin, D. Lecler, F. Fremont and N. Stolterfoht, Phys. Rev. A 45 (1992) 4653; J.W. McDonald, D. Schneider, M.W. Clark and D. Dewitt, Phys. Rev. Lett. 68 (1992) 2297. 1161 E.D. Donets, Physica Scripta T3 (1983) 11; Nucl. Instr. and Meth. B 9 (1985) 522. [I71 J.P. Briand, L. de Billy, P. Charles, S. Essabaa, P. Briand, R. Geller, J.P. Desclaux, S. Bliman and C. Ristori, Phys. Rev. L&t. 6.5 (1990) 1.59;Phys. Rev. A 43 (1991) 565. USI H.J. AndrZ et al., Proc. 17th ECPEAC, Interaction of multiply charged ions at low velocities with surfaces and solids, Brisbane, Australia, 1991, eds. W.R. MacGillivray, I.E. McCarty and M.C. Standages (TOP, 19921 p. 89. [19] M.W. Clark, D. Schneider, D. Dewitt, J.W. McDonald, R. Bruch, U.I. Safronova, I.Y. Tolstikhina and R. Schuch, Phys. Rev. A 47 (1993) 3983; M. Schulz, C.L. ticke, S. Hagm~, M. Stiickli and H. Schmidt-B~c~ng, Phys. Rev. A 44 (1991) 1653. [20] S.T. de Zwart et al., lot. cit. 1141; F.W. Meyer, C.C. Havener, S.H. Gverbury, K.J. Snowdon, D.M. Zehner, W. Heiland and H. Hemme, Nucl. Instr. and Meth. B 23 (1987) 234; K.J. Snowdon, C.C. Havener, F,W. Meyer, S.H. Gverbury, D.M. Zehner and W. Heiland, Phys. Rev. A 38 (1988) 2294; S. Schippers, Thesis, Univ. Osnabriick, Germany, (Shaker, 1992); S. Schippers, S. Hustedt, W. Heiland, R. Kiihrbriick, J. Bleck-Neuhaus, J. Kemmler, D. Lecler and N. Stolterfoht, Phys. Rev. A 46 (1992) 4003. [21] S.T. de Zwart, T. Fried, U. Jellen, A.L. Boers and A.G. Drentje, J. Phys. 3 18 (1985) L623. [22] ST. de Zwart, T. Fried, D.O. Boerma, R. Hoekstra, A.G. Drentje and A.L. Boers, Surf. Sci. 177 (19861 L939. [23] P. Varga, Appl. Phys. A 44 (1987) 31; P. Varga, Comments At. Mol. Phys. 27 (1989) 111. [24] H.J. AndrI, in: Physics of Highly Ionized Atoms, ed. R. Marrus, NATO-AS1 Series B vol. 201 (Plenum, 1989) p. 377; H.J. And& Nuci. Instr. and Meth. B 43 (1989) 306;

F. Aumayr et al. /Nucl. Instr. and Meth. in Phys. Rex B 87 (1994) 130-137

H.J. Andr% et al., Z. Phys. 21 (1991) S135, see also ref.

ml. [25] U. Wille, Phys. Rev. A 45 (1992) 3004. [26] N. Vaeck and J.E. Hansen, J. Phys. B 24 (1991) L469. [27] A. Mazarro, P.M. Echenique and R.H. Ritchie, Phys. Rev. B 27 (1983) 4117.

137

1281J. Das and R. Morgenstern, Phys. Rev. A 47 (1993) R755; J. Das and R. Morgenstern, Comm. At. Mol. Phys. 29 (1993) 205. [29] M. Fehringer, M. Delaunay, R. Geller, P. Varga and HP. Winter, Nucl. In&r. and Meth. B 23 (1987) 245.

TV. INTERACTiONS;

CHANNELING