Interaction of swift heavy particles with solids: electron emission

NOMB

Nuclear Instruments and Methods in Physics Research B 87 (1994) 149-155 North-Holland

Interaction

Beam interactions with Materials&Atoms

of swift heavy particles with solids: electron emission

Hermann Rothard Centre Interdisciplinaire de Recherches avec les Ions Lourds, Laboratoire M&e CEA-CNRS, GIRL - BP 5133, F-14040 Caen Cedex, France

In this report we focus on electron emission from solids by composite projectiles (such as molecules, clusters or heavy ions) which consist of a multitude of components such as electrons, protons or bare atomic nuclei. We present a selection of recent results concerning electron emission by swift hydrogen clusters (up > 1 a.u.) and fast ( > 2 MeV/u) heavy ions. Topics discussed include electron yields, energy and angular distributions and channeling phenomena. We also discuss electron acceleration by multiple collision sequences in slow collisions.

1. Introduction Swift charged particles interact with matter mainly by electronic processes such as excitation and ionization of the target atoms. Positively charged projectiles may also capture target electrons (to the ground or an excited state). If the projectiles carry electrons, electron loss or projectile excitation may take place. The deexcitation of either excited target atoms or the projectile leads to photon emission and, in particular, to X-ray emission. All of these processes contribute to the loss of kinetic energy of the projectile and the deposition of energy in matter, but (roughly) two-thirds of the projectile energy loss per unit path length leads to ionization and subsequent electron emission from the solid surface. This process, so-called kinetic electron emission, was discovered nearly a hundred years ago [I]. Thus, studying electron emission contributes to a better understanding of particle-matter interaction also involving X-ray and inner shell processes. Usually, electron emission is regarded as a three-step process: production of electrons by primary ionization, transport of the electrons through the solid including secondary electron production by fast electrons due to cascade multiplication and, finally, transmission through the surface and ejection into the vacuum. Electron emission yields are expected to be (roughly) proportional to the electronic energy loss per unit path length [Z-4]. About 90% of all emitted electrons are low energy “true secondary” electrons [3]; electron yield measurements thus provide us with information about these low energy electrons. In this report we focus on electron emission from solids by heavy composite projectiles (such as molecules, clusters or heavy ions of different charge 0168-583X/94/$07.00

states) which consist of a multitude of components, such as electrons, protons or bare atomic nuclei. We present a “pot-pourri” of recent results on electron emission by swift (up 2 1 a.u.) hydrogen clusters (section 2), fast heavy ions (> MeV/u, section 4) including channeling phenomena (section 5). We briefly discuss the interesting mechanism of electron acceleration by multiple collision sequences in slow collisions which may be observable in electron energy distributions from both keV/u heavy ion and cluster interaction with matter (section 3). This choice of topics is, of course, a very personal one, and many interesting topics have to be omitted because of limited space. Extensive reviews on electron emission from solids can be found in refs. [2-41.

2. Swift hydrogen clusters To parameterize the behaviour of composite projectiles during interaction with matter one must refer to deviations from the behaviour of the single components. It has become common practice to introduce the ratio R(n)

(1)

to study the so-called “molecular” or “cluster effect” (i.e., R f 1) and compare, e.g., electron yields y of Hi clusters to yields of independent protons [3]. In analogy to Eq. Cl), one can also define an energy loss (dE/dx) ratio by just replacing y by dE/dx. Quite a number of studies have been performed with fast molecular hydrogen ions and small hydrogen clusters with not more than three atomic components, and a few studies have been devoted to the impact of

0 1994 - Elsevier Science B.V. All rights reserved

SSDI 0168-583X(93)E0734-X

= y(H:)/ny(H+)

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H. Rothard/ Nucl. Instr. and Meth. in Phys.Res. B 87 (1994) 149-155

150

separation of the cluster components can be obtained by studying backward and forward electron emission (from entrance and exit surfaces of thin foils, respectively) separately, as a function of the target thickness d k&7,91. The results obtained for the forward yield ratios R, (Fig. 1, right-hand side) show that correlation effects on electron emission surprisingly persist for separation distances between fragments that are much larger than for other collective effects on charge exchange or energy loss previously observed with cluster beams [9]. As can be seen from Fig. 1, electron yield reduction (R < 1) is not only due to backward emission (beam entrance side of the foils, RB), but also to forward emission (beam exit side, R,) for carbon foils of d = 970 A. A recent comprehensive study of the target thickness dependence of R, and R, showed [6,9] that R,(d) is constant for d > 200 A, whereas R,(d) increases with d. Surprisinglyd R < 1 is still observed with targets as thick as 2800 A at 40 keV/p, i.e. there is still a spatial and/or a long-range Coulombic correlation of the fragments of the cluster proton swarm. Such a correlation could be mediated by plasmons. It has been proposed recently [ll] that bulk plasmons can account for correlation effects in heavy ion i$duced electron emission for distances as large as 500 A. Measurements of the energy distribution N(E) of electrons induced by H,f showed a strong suppression (compared to H,f molecules) of low energy electron emission at E < 20 eV, i.e. in the region of the “true secondary electron peak”, from cascade multiplication [3] both in forward and backward direction 191. This finding may be explained by screening of the projectile nuclear charge by projectile electrons, in particular for soft projectile-electron collisions with large impact pa-

heavy molecular ions. A comprehensive tabulation of publications concerning experimental work on ion induced electron emission from solids (and, of course, electron emission induced by molecules and clusters) has been given by Hasselkamp [3]. Also, electron emission by large clusters consisting of heavy components has been studied, but at velocities far below the regime of “swift” particles considered here. However, up to now, only a few experimental studies of electron emission by fast (u, > 1 a.u.), large (n > 3) clusters have been performed [4-91. Strong suppression (i.e. R(n) < 1) of total secondary electron yields y (the mean number of electrons emitted per incoming projectile) for Au, MO, stainless steel [5] and carbon foil targets [6-91 has been observed with Hz (30 keV/p I E/p I 300 keV/p, n = 1, 2, 3-19, odd), see Fig. 1. The term “suppression” means that fewer electrons are emitted with a cluster containing IZ protons than with n independent protons, even if deviations of the cluster energy loss from the proton energy loss (Le., R f 1) are accounted for! In particular, it was found that the “cluster effect ratio” for electron yields [6,7] from carbon foils (Eq. (1)) does not exhibit the same velocity (u,) dependence as the corresponding ratio R for the energy loss in carbon which was found to be R < 1 for E/p I 50 keV/p and R > 1 for E/p > 50 keV/p [lo]. As a function of cluster size 12,saturation of the reduction effect is observed [6-91 for II > 7 (compare Fig. 1). Atomic or molecular clusters suffer Coulomb explosion and multiple scattering while penetrating into a solid. Consequently, their structure (e.g., the distances rr between the components) is modified depending on the penetration depth x. Thus, much information on collective effects and their evolution with increasing

l

R,(n) 1

I

I

I 300

1 Hn+-i

keV/p

RF(n)

C-FOIL

- T -__--__-__-__________

1.0 -

I ,,.

$__/: v

0

m

0.5

~ 0 120ieY,P v 300i&/P m

1

3

5

7

9

11

13

1

3

5

7

9

ll

13

ri

Fig. 1. Comparison of forward and backward electron yields (in and yF) from the entrance and exit side of thin carbon foils (d = 970 A) measured with Hz clusters (40 keV/p 5 E/p _<300 keV/p as indicated) to electron yields measured with protons of the same velocity [6]. Shown are the “cluster effect ratios” (Eq. (1)) R(n) = -y(Hz)/nr(H+) for backward (left) and forward yields (right) as a function of the cluster size n.

151

H. Rothard /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 149-155 rameter [3]. Surprisingly, in the forward direction only, an enhancement of electron emission’at the high energy side of the peak of electrons from violent binary collisions (the maximum momentum transfer leads to electron velocities around u, = 2 up cos tJ in direction 0) with small impact parameter has been observed [9].

3. Fast electrons from slow collisions:

“Fermi-shuttle”

acceleration

Recently, Baragiola et al. [12] found exponentially decreasing high energy tails in electron spectra induced by l-6 keV He and Ar ion bombardment of metals. This low-level tail extended up to several keV and thus corresponds to electrons which are much faster than can be expected from binary collisions only, even if the initial electron momentum distribution, the Compton profile, is taken into account. Sigmund et al. [13] suggested that these tails could be due to electrons which are accelerated in multiple collisions sequences between projectile and target atoms. This Fermi-acceleration scheme has also been discussed in connection with cluster-induced fusion (see, e.g., refs. 114,151). Such an acceleration mechanism should be visible in a more clearcut way by measuring doubly differential electron spectra from e.g., HL cluster penetration of thin foils in forward (beam) direction. The simplest scenario is reflection of a projectile electron by a target atom and subsequent reflection at one of the projectile protons. It should be possible to detect such electrons as enhanced electron emission around and below E, = 9 (m,/m,)E, (corresponding to three times the projectile velocity compared to two times up for single binary encounter collisions) when comparing cluster induced to proton induced forward (0 = 0’) electron spectra [16]. The two primary processes, i.e. loss of projectile electrons and 2u, binary encounter electron emission, are clearly visible in electron spectra from thin foils 13,171.A preliminary search with H: and H’ (500 keV/p) traversing very thin carbon foils (d < 150 A) showed electron emission far beyond the binary peak and also at v, = 3u,, but did not reveal any indications of enhancements of electron emission by H; [16].

4. Fast heavy ions A multitude of experiments have been performed on electron emission induced by slow (u, < 1 a.u.> or medium velocity heavy ions (< 1 MeV/u) [3], but only a few studies with fast heavy ions have been published [18-271. One can distinguish between two different types of experiments: studies of electron emission yields y [22-241, or measurements of doubly differential elec-

tron energy spectra d’y(e>/dE the observation angle 0 [l&21].

da

as a function

of

4.1. Electron spectra and angular distributions Besides investigations of electron emission related to channeling phenomena 125,261 (section 51, to our knowledge the only systematic measurements of energy and angular distributions have been performed at VICKSI in Berlin with Ne ions [18,19,27] and at the Super HILAC in Berkeley with U ions (3.5 and 8.5 MeV) [20,21] with thin carbon foils (5-100 kg/cm’>. The dependence of the electron emission cross section on energy and angle [18,21] and also the evolution with target thickness [20] have been calculated within two different theoretical approaches: Sparrow, Olson and Schneider performed a Monte Carlo simulation of electron transport in solids. The primary ionization of target atoms was determined by the nCTMC (n-body classical trajectory Monte Carlo) method [20]. An interesting result of the simulations is the evolution of the binary electron (2~~) peak structure with target thickness from single collision conditions to multiple collisions including wide angle scattering of electrons. Schiwietz et al. [1X,19,21] utilized semi-empirical atomic ionization cross sections and an analytical method, the SELAS (separation of energy loss and angular scattering) approximation, to account for the transport of electrons to the surface. By comparison of calculation and experiment, it was even possible to determine the population of excited projectile states (n 2 2) and the ground state (n = 11 inside the solid, because the Rydberg state population P,, remains the only free parameter thus allowing a fit of the theoretical curves to the experimental data [19,21]. A remarkable result is the observation [27] of an energy shift of carbon KLL Auger electrons emitted from polypropylene foils with respect to Auger emission from carbon foils, which can be attributed to the influence of the heavy ion nuclear track potential. 4.2. Electron yields Ion induced electron yields from thin foils may analyzed within a semi-empirical model proposed Koschar et al. 1231.The target thickness dependence forward (or) and backward (~a) electron yields can described by simple equations Yn(d) =

rn(m>[l-

exp( -d/A,)],

Yn(d) = ~r(=~)[l -P, -Ps

be by of be

(2)

exp( -d/A,)

exp( -d/A,)],

(3)

where A, and A,(A, > A,) are characteristic transport lengths for low energy electrons (say, E < 100 eV) and IV. INTERACTIONS; CHANNELING

152

H. Rothard / Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 149-155 “““I

q

high energy “S” electrons, respectively; /3a is the socalled “partition factor” describing the fraction of the lost projectile energy dE/dx leading to 6 electron emission from violent binary collisions with a small impact parameter; and p, = (1 - &) is the fraction dissipated in soft collisions with large impact parameters. This means that

YT Ni k+ (15.2 M&/u)

100

?2 F ti g

10

(dE/dx)

I

1

,,,,,,

IO

I

,I,,IbI

1000

100

TARGET THICKNESS [Irgr/cm*l

Fig. 2. Total (T), forward (F) and backward electron yields (B) as a function of target thickness (carbon foils) obtained with fast (up = 23 au., E = 13.6 MeV/u) heavy (Art6+) ions at GANIL in Caen [24]. Also included are ys = or - +ySE(labeled 6) and ~sn = in + yF (labeled SE), see text, as well as or data obtained with Niz6+ (15.2 MeV/u) [29] (labeled Ni). Also shown are the results of least square fits of Eqs. (2) and (3) to the experimental yn and yF values and of the sum of these equations to the sum of the low energy electron yields, YSE = YB + YF.

Xe37+ (27 MeViu)

=P,(dE/dx)

+P,(dE/dx),

(4) with & + pa = 1. The model is described in more detail in refs. [3,23]. An example of the dependence of total (T), forward (F) and backward electron yields (B) on target thickness with obtained fast (v, = 23 a.u., E = 13.6 MeV/u) heavy (Ar16+) ions at GANIL in Caen [28] is shown in Fig. 2. For comparison, yr data obtained with Niz6+ (15.2 MeV/u) at UNILAC ‘in Darmstadt [29] have been included (labeled Ni). The total yields yr can be obtained easily by measuring the ion induced target current; the forward and backward electron yields (yn and yF) have been measured with two cylindrical Faraday cups on the beam entrance and exit side of thin carbon foils, as described in ref. [30]. Electrons with energies exceeding about 100 eV which are emitted in

L! !!A

-> SI 0

n

58

62

66

70

ELECTRON VELOCITY [a u.]

< 110 > CHANNELED

40 @pxit

[deg]

60

ELECTRONVELOCITY[a.u.]

Fig. 3. (a) Dependence of convoy electron yields Y(4)/ Y(random) on the angle of incidence 4 with respect to the (111) plane of a Au single crystal (d = 1200 A) bombarded with protons (E = 0.17-1.9 MeV). (b is measured in units of Lindhards critical angle of channeling +crit = 0.59/G [MeV] (from ref. [32]). (b) Forward (0 = 0”) electron spectra obtained with Xe37i (27 MeV/u, up = 33 a.u.> incident on a silicon crystal (d = 21 pm) in random orientation and (110) axial alignment (as indicated) at GANIL (from ref. [25]). (c) Enlargement of the high-energy side of the binary encounter electron peak (from Fig. 3b), normalized to the same maximum peak height.

H. Rothard /Nucl. Instr. and Meth. in Phys. Res. B 87 (1994) 149-155

the extreme forward (O-15”) or backward (165-180”) direction can escape from the cups. Thus, ys = yr - ysn (labeled 6 in Fig. 2) with ysE = yn + yF (labeled SE) gives qualitative information about high-energy electrons (E > 100 eV>, whereas ySE is a measure of low energy electron emission. Also shown are the results of least square fits of Eqs. (2) and (31 to the experimental ~a and yF values and of the sum of these equations to the sum of the low energy electron yields, ysn = yn + yF. In good agreement with a value of Pa = 0.59 * 0.05 for S (3.9 MeV/u) and pa = 0.54 k 0.05 for C (1 MeV/u) [231, we find & = 0.55 + 0.02 from our measurements with Arq+ (13.6 MeV/u, 4 = 16-18). This means that although they represent only about 15% of the total electron yield, siightly more than 50% of the projectile kinetic energy loss by ionization is transferred to high energy electrons. The values of the characteristic transport lengths are found to be much ltrger in the ca:e of Ar (13.6 MeV/u): As = 180 + ZOA (A, = 15 & 3 A fof S and C) and A, = 9000 + 600 A with hsO= 300 i 26 A for C (1 MeV/u) and ha = 1200 + 50 A for S (3.9 IvfeV/u). This result is easy to understand for A,: the maximum momentum transfer, the mean electron energy and, consequently, the range of the fast electrons increase with projectile velocity. In order to study high energy electron emission (compare ys in Fig. 2) in more detail, we recently measured the evolution of doubly differential electron spectra d2y@)/dE da (E = 100 eV to 40 keV, 0 = o-180”) with carbon target thickness for Araf (13.6 MeV/u, a = 17-18) 1311.

5. Channeling While the dependence of electron production on the projectile-target electron collision impact parameter is included in the semi-empiri~l theory (by the rough distinction between soft and close collisions, see Eqs. (2)-(4)), channeling studies yield the interesting possibility to study the dependence of target ionization on the projectile-target nucleus collision impact parameter distribution. Under channeling conditions, when the ion trajectory is confined within a channel of a crystal lattice, coilisions with small impact parameter are largely suppressed. This means that the ions encounter mostly outer shell electrons (“nearly free electron target”) when channeled. Thus, by comparing electron emission under random impact and channeling conditions, one cannot only study the dependence of electron production of the electron density encountered by the projectile, but also distinguish ionization of inner and outer shells of the target atoms.

153

This is demonstrated in Fig. 3b which shows forward (B = 0”) electron spectra obtained with Xe37+ (27 MeV/u, up = 33 a.u,) incident on a silicon crystal (d = 21 pm) in random orientation and (110) axial alignment (as indicated) at GANIL (from ref. [25]). The most striking difference is the strong reduction of convoy electron [3,17] emission (cusp-shaped peak at u, = up) for channeled ions: the convoy electron yield is reduced by a factor of 13. This result is related to the suppression of convoy electron production either by capture of target electrons to the continuum (ECC) or loss of projectile electrons to low-lying projectile continuum states (ELC). In particular, electron loss is largely suppressed (incoming charge state, 11,= 37; most probable final charge state, 4; = 50 for random inci- t dence; and qfc = 44 under channeling conditions f251). The binary encounter eiectron yield (around 2 up = 66 a.u.) is reduced by a factor of 3.2, because it is directly proportional to the electron density encountered by the projectile which is strongly reduced if the projectile trajectories remain confined within a crystal channel. A closer inspection of the high-energy side of the binary electron peak shows that the momentum distribution of these electrons is much smaller when channeled (Fig. 3c), because the contribution of SiKshell electrons to the Compton profile is suppressed f25]. In random incidence, the broad initial momentum dis~bution of these strongly bound inner-shell electrons leads to wings at the high- (and low-) energy side of the binary peak. Similar results have been obtained by Kudo et al. [26] who studied backward electron emission from thick Si and GaAs single crystals bombarded with MeV/u ions. They obtained the effective ion charges as a function of the profile atomic number ZP and various channeling conditions. The dependence of electron emission on the minimum distance of approach to the target atoms b,,(4) can be studied by varying the angle of incidence 4 from random incidence to channeling conditions (4 < #&, as the ion trajectories remain confined in a region outside the minimum impact parameter bmin(4). As an example, Fig. 3a (from ref. [32]) shows the dependence of convoy electron yields Y(+)/Y(random) on the angle of incidence 4 with respect to the (111) plane of Au single crystal (d = 1200 A) bombarded with protons (E = 0.17-1.9 MeV). d, is measured in units of Lindhards critical angle of channeling fbcrlt= 0.58,’ 6 [MeVl. One observes a strong dependence of convoy electron yield on the crystal orientation (but not so strong as in the case of axial channeling discussed above; here we are dealing with planar channehng). Y(4ctii,)/(random) decreased with increasing velocity and follows the velocity dependence of the emerging neutrai charge state fraction f;,(H) 1321.This may be a hint that in this

IV. INTERACTIONS; CXIABNELING

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H. Rothard /Nucl. Instr. and Meth. in Phys. Rex B 87 (1994) 149-155

case convoy electrons are produced by indirect electron loss to the continuum (IELC) [3,32], i.e. electron capture from the target with subsequent ELC.

ville), G. Schiwietz (Berlin), J. Schou (Roskilde), and P. Sigmund (Odense). References

6. Conclusions In this report we presented a selection of recent results on electron emission with different types of composite projectiles such as molecular and cluster ions and swift heavy ions. The collective action of the multiple components leads to deviations from single particle behaviour. It is interesting to note that quite similar observations such as strong suppression of low energy electron emission (compared to the case of protons, and always taking into account the different energy loss of different projectiles) have been made with hydrogen clusters [5] and heavy ions [33,34] at velocities below 300 keV/u. In both cases saturation is reached as a function of cluster size n and atomic number 2, for the clusters at n > 7 (compare Fig. 1) and for the heavy ions at Z, > 6 [33,34]. An explanation may be found in screening of the projectile charge by either target or projectile electrons. Surprisingiy, even with fast heavy (MeV/u) ions, electron yields are reduced [22-24,281. It is a challenging question whether all of these results may be understood in a universal picture involving the same physical processes such as, for example, the above-mentioned screening effects, long range plasmon excitation [ll] or wake effects [24]. Further insight into differences and similarities of collective effects observed with clusters and heavy ions will be obtained by measurements with heavier particles and by extending studies to higher energy, i.e. the > 10 MeV/u region for heavy ions [31] and the > 100 keV/u region for clusters. In particular, the recent development of high energy beams of heavy ion clusters (such as C,+ and Au;) at tandem accelerators in Orsay and Erlangen [35] opens a promising new field of investigation.

Acknowledgements I am indebted to my collaborators in Caen (A. Cassimi, C. Caraby, B. Gervais, J.-P. Grandin, P. Jardin), Lyon (A. Billebaud, M. Chevallier, D. Dauvergne, M. Fallavier, R. Kirsch, J.C. Poizat, J. Remillieux, J.P. Thomas), Frankfurt am Main (K.O. Groeneveld, M. Jung, P. Koschar, J. Kemmler, R. Maier, M. Tobisch) and Thessaloniki (A. Clouvas) who all contributed to parts of the work presented here. Thanks to D. Schneider (Livermore) for sending a preprint. And, finally, I would like to thank all those who inspired this work, including in particular R. Baragiola (Charlottes-

Dl M.P. Villard, J. Phys. Theor. Appl. 8 (1899) 5.

w J.

Devooght, J.C. Dehaes, A. Dubus, M. Cailler, J.P. Ganachaud, M. Rijsler and W. Brauer, Particle Induced Electron Emission I, Springer Tracts in Modern Physics 122, eds. G. Hohler and E.A. Niekisch (Springer, Berlin, 1991). [31 D. Hasselkamp, H. Rothard, K.O. Groeneveld, J. Kemmler, P. Varga and H. Winter, Particle Induced Electron Emission II, Springer Tracts in Modern Physics 123, eds. G. Hiihler and E.A. Niekisch (Springer, Berlin, 1991). 141 R. Baragiola (ed.), Ionization of Solids by Heavy Particles, NATO AS1 Ser. B, vol. 306 (Plenum, New York, 1993). [51 Y. Chanut, J. Martin, R. Salin and H.O. Moser, Surf. Sci. 106 (1981) 563. t61 H. Rothard, D. Dauvergne, M. Fallavier, K.O. Groeneveld, R. Kirsch, J.C. Poizat, J. Remillieux and J.P. Thomas, Radiat. Eff. and Defects in Solids 126 (1992) 373. [71 H. Rothard, J.P. Thomas, J. Remillieux, J.C. Poizat, R. Kirsch, K.O. Groeneveld, M. Fallavier and D. Dauvergne, in ref. [4], p. 275. Bl N.V. de Castro Faria, B. Farizon-Mazuy, M. Farizon, M.J. Gaillard, G. Jalbert, S. Ouaskit, A. Clouvas and A. Katsanos, Phys. Rev. A 46 (1992) R3594. Dl H. Rothard, A. Billebaud, D. Dauvergne, M. Fallavier, K.O. Groeneveld, R. Kirsch, J.C. Poizat, J. Remillieux and J.P. Thomas, Preprint (1994), to be published; Institut fiir Kernphysik Frankfurt am Main, Annual Report IKF-52 (1992) 27; B. Fricke et al. (eds), Arbeitsbericht “Energiereiche atomare St&se” (Kassel/FRG) EAS-14 (1993) p. 157. DO1 M. Ray, R. Kirsch, H.H. Mikkelsen, J.C. Poizat and J. Remillieux, Nucl. Instr. and Meth. B 69 ((1992) 133. 1111Y. Yamazaki, K. Kuroki, T. Azuma, K. Komaki, H. Watanabe, N. Kakutani, T. Hasegawa, M. Sekiguchi and T. Hattori, Phys. Rev. Lett. 70 (1993) 2702. [I21 R.A. Baragiola, E.V. Oliva, A. Bonnano and F. Xu, Phys. Rev. A 45 (1992) 5286. t131 P. Sigmund, in ref. [4]. t141 M. Hautala, Z. Pan and P. Sigmund, Phys. Rev. A 44 (1991) 7428. u51 J. Burgdofer, J. Wang and R.H. Ritchie, Phys. Scripta 44 (1991) 391. t161 M. Jung, M. Schosnig, M. Tobisch, K.O. Groeneveld and H. Rothard, private communication, 1992; Institut fiir Kernphysik Frankfurt am Main, Annual Report IKF-52 (1992) 26. However, Fermi shuttle electron acceleration can possibly also be observed in backward emission; see Suarez et al. (5th Workshop on Fast IonAtom Collisions, Debrecen, 1993) Nucl. Instr. and Meth. B 86 (1994) 197. [17] H. Rothard et al., Nucl. Instr. and Meth. B 48 (1990) 616. [18] G. Schiwietz, J.P. Biersack, D. Schneider, N. Stolterfoht, D. Fink, V.J. Montemajor and B. Skogvall, Phys. Rev. B 41 (1990) 6262.

H. Rothard /Nucl.

Instr. and Meth. in Phys. Res. B 87 (1994) 149-155

[19] G. Schiwietz, Rad. Eff. and Defects in Solids 112 (1990)

195; ref. [4], p.197. [20] R.A. Sparrow, R.E. Olson and D. Schneider, J. Phys. B 25 (1992) L295. 1211D. Schneider, G. Schiwietz and D. Dewitt, Phys. Rev. A 47 (1993) 3945. [22] A. Koyama, T. Shikata, H. Sakairi and E. Yagi, Jpn. J. Appl. Phys. 21 (1992) 1216. [23] P. Koschar, K. Kroneberger, A. Clouvas, M. Burkhard, W. Meckbach, 0. Heil, J. Kemmler, H. Rothard, K.O. Groeneveld, R. Schramm and H.D. Betz, Phys. Rev. A 40 (1989) 3632. [24] J.E. Borovsky and D.M. Suszcynsky, Phys. Rev. A 43 (1991) 1416 and 1433. [25] S. Andriamonje et al., in: High Energy Ion-Atom Collisions, Springer Lecture Notes in Physics 376, eds. D. Berenyi and G. Hock (Springer, Berlin, 1991) p. 221. [26] H. Kudo, K. Shima, K. Masuda and S. Seki, Phys. Rev. B 43 (1991) 12729 and 12726. 1271 G. Schiwietz, P. Grande, B. Skogvall, J.P. Biersack, R. Kohrbruck, K. Sommer, A. Schmoldt, P. Goppelt, I. Kadar, S. Ritz and U. Stettner, Phys. Rev. Lett. 69 (1992) 628. [28] H. Rothard, A. Cassimi, B. Gervais, J.-P. Grandin, P. Jardin, A. Billebaud, M. Chevallier, J.C. Poizat, J. Remillieux, K.O. Groeneveld, M. Jung and R. Maier, private communication (19931, to be published.

1.55

1291 R. Latz, Thesis, J.W. Goethe Univ., Frankfurt am Main, Germany, 1984. 1301H. Rothard, et al., Phys. Rev. A 41 (1990) 2521. [31] A. Billebaud, C. Caraby, A. Cassimi, M. Chevallier, B. Gervais, J.P. Grandin, M. Jung, P. Jardin and H. Rothard, private communication (1993), to be published. 1321P. Koschar, N. Cue, R. Kirsch, J. Remillieux, J.C. Poizat, J. Kemmler, H. Rothard and K.O. Groeneveld, Institut fiir Kernphysik, Frankfurt am Main, Annual Report IKF47 (1988) 13; P. Koschar et al., Verhandl. DPG (VI), 27 (1992) 1243. [33] H. Rothard, J. Schou and K.O. Groeneveld, Phys. Rev. A 45 (1992) 1701. [34] A. Clouvas et al., Phys. Rev. B 43 (1991) 2498. [35] C. Schoppmann et al., Nucl. Instr. and Meth. B 82 (1993) 156. K. Boussofiane-Baudin, A. Brtmelle~ P. Chaurand, J. Depauw, S. Della-Negra, P. Hakansson and Y. LeBeyec, submitted to Int. J. Mass Spectrom. Ion. Process. (1993). Proc. Conf. on Polyatomic Ion Impact on Solids and Related Phenomena, St. Malo, June 6-10, 1993, Nucl. Instr. and Meth., in press; S. Della-Negra et al., Nucl. Instr. and Meth. B 74 (1993) 453.

IV. INTERACTIONS;

CHANNELING