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Discrete energy losses of fast protons in grazing surface scattering from Al(111)
PhysicsLettersA 168 (1992) 409—415 North-Holland
PHYSICS LETTERS A
Discrete energy losses of fast protons in grazing surface scattering from Al (111) H. Winter and M. Sommer Inst itutfür Kernphysik der Universität Munster, Wilhelm-Klemm-Strasse 9, W-4400 Münster, Germany Received 21 April 1992; revised manuscript received 22 June 1992; accepted for publication 6 July 1992 Communicated by B. Fricke
The energy loss offast protons scattered from a flat and clean A1( 111 )-surface under a grazing angle ofincidence is investigated with high resolution with respect to angle of scattering and energy. We find two different kinds ofmultiple discrete energy losses in the spectra, which weattribute to hopping kind oftrajectories at the surface (“skipping motion”) and to subsurface channeling, respectively. By a variation of the charge state ofthe projectileswe show that chargeexchange affects the image potential close to the surface in a decisive way and plays an important role for the understanding ofthe observed effects.
In recent years Ohtsuki and coworkers have investigated theoretically the scattering of fast protons (v~v0 = Bohr velocity = 1 a.u.) from metal surfaces under grazing angles of incidence. Based on cornputer simulations these authors conclude that due to the dynamical image potential a fraction of scattered projectiles undergoes hopping type of trajectories, the so-called “skipping motion” [1—4].This motion is characterized by multiple approaches of the projectiles to the surface plane, which is expected to result in an increased angular broadening [5] and in a number of discrete energy losses [1—6]for the scattered beam. It turns out that the feature of discrete energy losses is the more direct indication of “skipping motion” type of trajectories, so that expertmental verifications of this effect have been concentrated on the analysis of energy spectra for fast projectiles scattered under grazing angles of incidence. However, discrete energy losses are not an unequivocal signature of such a type of bound motion close to the surface plane. This is obvious from early observations by Lutz et al. [7] in transmission channeling experiments through very thin Au single crystals, which are interpreted in terms of a channeling of projectiles between atomic planes in the bulk of the crystal [8,9]. In ion—surface scattering multiple energy losses have been reported by Hou et al. [10]
and recently by Snowdon et a!. [11,121 for projectiles with energies ofsome keY, and by Kimura et al. [13] for projectiles in the MeV-domain. In all these studies, however,the energy ofmotion normal to the surface plane E~is clearly too high for an interpretation of the data in terms of a “skipping motion” caused by the dynamical image potential [4]. As a consequence, most of the data can be explained by “subsurface channeling” trajectories [6,13], an oscillatory kind of motion between atomic planes below the topmost layer of surface atoms. This motion is closely related to the effects observed and was interpreted already in refs. [7—9].The projectiles enter those subsurface channelsvia e.g. steps at the surface plane and thermal vibrations oflattice atoms at the surface. Subsurface channeling is claimed to be ruled out for the interpretation of the data reported in refs. [11,12], where the authors describe their experiments in terms of a “transient adsorption” mechanism. Recently Stölzle and Pfandzelter have reported on discrete energy losses of 15—75 keV protons in grazing collisions with a smooth polycrystalline graphite surface [14,15]. In part of their data the energy of normal motion is sufficiently small to allow for a “skipping motion” at the surface plane. In addition, the authors found also spectra with multiple discrete energy losses for larger energies of normal motion,
0375-9601/92/S 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
409
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PHYSICS LETTERS A
where a “skipping motion” induced by the image potential can be excluded. All these data are claimed to be reproduced in computer simulations by Sakai et a!. [61, which indicate for small grazing angles “skipping motion” and for larger angles “subsurface channeling”. In this paper we report on investigations of discrete energy losses in grazing collisions of fast protons with an Al( 111)-surface. Depending on the energy ofmotion normal to the surface, we observe two clearly different multiple discrete energy loss spectra, which we attribute also to “skipping motion” and “subsurface channeling”, respectively. In using in addition neutral hydrogen atoms as projectiles, we find further evidence that our spectra for low normal energies E~can be interpretedby the effect of the dynamical image potential on the trajectories above the topmost layer of surface atoms. This interpretation is supported by the study of polar angular distributions in dependence on the charge state of the projectiles. The setup used in our studies is sketched in fig. 1. Protons or neutral hydrogen atoms (produced in a gas-target) of20—100 keY energy are well collimated by sets of vertical and horizontal slits to a divergence of about 0.02°and directed onto the (111)-face of a monocrystalline aluminum sample under grazing angles of incidence ø~,~ 0.2°—1.5°.The target is polished with great care by keeping the deviation between polishing and (111)-plane as small as possible and is treated thereafter in situ by cycles of grazing sputtering with 100—200 keY Ark-ions at 1 ~,,,
gastarget
electric field plates I plates II
~
and heating up to 550°C.Inspection in the final state of the preparation of the target by a “spot profile analysis” LEED [16] yields a rather flat surface with an average terrace width formed by topmost surface atoms larger than about 70 nm. In order to avoid effects of an axial surface channeling on the trajectories, the direction of the projectile beam coincides with a high index crystallographic direction in the surface plane (“random” orientation). This can be adjusted on line by recording the intensity of emitted electrons [171 or simply the target current [18] during an azimuthal rotation of the crystal. This current is found to be enhanced for scattering along low index axial directions [18]. The target is mounted on a precision manipulator and kept under UHY-conditions at a base pressure of about 5 x 10—” mbar. Differential pumping stages at both ends of the UHY-chamber allows to maintain the vacuum against a pressure of about 10—v mbar in the beamline of the accelerator and in the analyzer section. Electric field plates in both pumping stages and a small gas target in the beam-line allow to perform the experiment with neutral and charged projectiles and to separate the charge states of scattered projectiles. Scattered particles are analyzed with help of detector I to obtain (polar) angular distributions or with detector II for energy spectra. Both detectors consist of a channeltron detector behind a circular aperture of about 0.2 mm diameter. The energy of ions is analyzed with a cylindrical 90°electrostatic analyzer of 0.5 m radius with an instrumental res-
°
A
detector I
s~thr Fig. 1. Sketch of the experimental setup.
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olution 8Ejnstr/E~5 x l0~.A tilt of the analyzer with respect to A and B (see figure) allows us to obtain energy spectra in dependence on the polar angle of scattering with an angular resolution 8~ana~ 0.02°. The effect ofthe dynamical image potential on the trajectories of the projectiles can be studied directly by comparing angular distributions for neutral and charged projectiles. These distributions are recorded by a scan with detector I in the polar plane of the scattered particles at a distance of 1 in behind the target. The contributions of neutral projectiles before and/or after the scattering event are separated here by electric fields of about 1 kY/cm. From the difference of the signals with and without electric fields we obtain furthermore the angular distributions due to protons. Details on the concept of this “difference method” to measure angular distributions for primary and scattered projectiles resolved with respect to the neutral and positive charge states are described elsewhere [19]. In fig. 2 we display angular distributions of projectiles leaving the surface in the neutral charge state (voltage on field plates II) for incident neutral hydrogen atoms (voltage on plates I; black dots) and for protons (from the difference of the signals with
x 1/120
H
H~ //
/
I
I
I
0
1
2
ANGLE OF SCATTERING
(°)
Fig. 2. Distribution of scattered neutral hydrogen atoms after the interaction of 25 keY hydrogen atoms (black dots) and protons (open circles) with an Al (111)-surface. The solid lines represent best fits ofthe central part of the distributions to a Gaussian lineshape. The single dot on the left side is due to a residual fraction of the (neutral) direct beam, which has passed on top of the target without interaction and which thus represents the direction ofthe projectile beam. The plot ofthis strong signal is suppressed by a factor of 120.
7 September 1992
and without voltage on field plates I; open circles). The data represent well defined distributions of scattered projectiles, where the maximum of the distribution obtained for protons is shifted by ~= 0.13°±0.03°towards larger angles in comparison to the data obtained with neutral projectiles. By means of a He—Ne laser beam directed along the beam axis through all slits, which define the direction and the divergence ofthe projectile beam, we verify that the maximum of the distribution for the neutral projectiles coincides with the direction of specular reflection; i.e. ‘Z = 1.120 and ~ = = 0.56°.The intense narrow peak on the left side (suppressed in fig. 2 by a factor of 120) stems from a residual fraction of the incident neutral beam, which has passed on top of the target without interaction and which thus serves as a reference for the direction of the projectile beam. The angular shift between the two distributions is consistent with the presence of the dynamical image potential. This potential is effective only for charged particles and results on the incoming part of the trajectory in an acceleration ofprotons towards the surface plane. Close to the surface plane the image potential is affected decisively by charge exchange; i.e. here capture and loss of electrons with respect to the hydrogen ls term. Due to the small energies for the motion normal to the surface (E~~eV),charge transfer in grazing fast ion—surface collisions can be described by adiabatic concepts. The effect ofthe high parallel velocity on charge exchange can be incorporated here by a Galilei transformation [20], giving rise to a “Doppler—Fermi—Dirac distribution” [21,22] for the occupation of electronic states in the metal. The transfer of electrons proceeds predominantly via one-electron tunneling and Auger processes and is characterized by consecutive capture and loss events, where hydrogen atoms in the ls ground-state cannot survive from the surface interaction for distances smaller than the so-called survial or “freezing distance” y~[23]. Then only for distances larger than about y, the charge state of the projectiles and an image interaction (for protons) is defined. As a consequence the size of the image potential .
.
.
is affected here in a charactenstic way by the mechanisms of charge exchange. The contributions of electron exchange processes relevant for the image 411
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PHYSICS LETTERS A
potential and the formation of final atomic terms are limited to an interval of distances y well above the surface plane and are represented by a distribution function F(y) peaking at y~[23]. F(y) is determined by the total transition rates and the normal velocity component v~.Zimny et al. [24] have analyzed theoretically the neutralization of protons via one-electron tunneling [25,26] and Auger transfer [27] and deduced a maximum of F(y) at y,~7a.u. from the topmost layer of surface atoms. The image potential energy gained (lost) on the incident (outgoing) trajectory of an ion is equivalent to the energy shift of a level in the corresponding atom [28], and we estimate here from ref. [24] a mean image energy Vim(Ys) = 1.5 ±0.2eY. From the data shown in fig. 2 we obtain from the positions of the maxima (best fit by Gaussian distribution) a mean energy of normal motion for the neutral atoms E~= 25 keY sin2 (0.560 ) = 2.39 eY and for protons E~= 25 keY sin2 (0.69°) = 3.63 eV. The mean image energy gained by the protons on the incident trajectory is then LiE=E~—E~= 1.24 eY. In view~ of the experimental and conceptual uncertainties these data support the assumptions on the mechanisms of charge exchange. A similar, however, more pronounced effect has been observed recently for the scattering of multicharged ions from the same surface [29]. On the outgoing path of the trajectory the result of the image potential is reversed. An energy of about LiE is effective to decelerate and eventually trap ions to the surface plane. Detailed studies on image charge effects for emerging particles confirm such a feature [19J. In summarizing this section, we state that angular distributions recorded for neutral as well as for charged projectiles give direct evidence for the effect of the image potential on the trajectory. The mean image potential energies of relevance here are measured to be of the order of about 1 eY. With help of the electrostatic energy analyzer and detector II (see fig. 1) we record energy spectra of projectiles scattered from the surface. In this work we focus our attention to the appearance ofmultiple energy losses. The energy spectra are measured with high angular resolution by translating the entrance slit of 0.1 mm width and positioned 1.5 m behind the target (B in fig. 1) in the polar plane of scattering. Due to the use of an electrostatic analyzer, we 412
7 September 1992
are sensitive here only to contributions of charged particles (protons). In fig. 3a we show an energy spectrum after the interaction of about 50 keY protons with the surface under a grazing angle of incidence cZ,,, = 0.2°. The spectrum is obtained for the specularly reflected fraction of the beam, i.e. cI~= ~Out• This setting corresponds to an energy of normal motion ~ Ey~=E 5jfl2~jn + Vim = 0.6 eY+ Vim. As a reference we display also the spectrum for the primary projectile beam, which is obtained by a corresponding position of the entrance slit of the energy analyzer. The energy of the direct beam is represented by the few triangles on the right side of the spectrum in fig. 3a, which indicates the good energy resolution of accelerator and analyzer in our measurements. This feature of our setup avoids the need for deconvolution procedures in the analysis of data. The spectrum for the scattered beam (linear and logarithmic representations) shows a pronounced peak due to a regular scattering ofprojectiles from the surface. The energy interval between the peaks of the signals for the direct and scattered beams defines a most probable energy loss ~.E,.A striking feature with respect to the subject ofthis paper is the existence of at least two further peaks at lower energies, which can be identified easily in the linear and in particular in the logarithmic representation of the data. For these peaks we find energy losses LiE 2 2LiE, and ~ Figure 3b shows a spectrum for cP,,. = 0.4°, i.e. ~ = 2.4 eY + Vm (see footnote 1), at otherwise unchanged experimental conditions. It is evident from the figure that aside from a slight hump in the tail of the prominent peak the multiple peak structure has disappeared. In fig. 3c we display a spectrum, which is obtained after a further increase of the angle of incidence to ~ = 0.5° and E~= 3.8 eY+ Vim (see footnote 1). Under these conditions a multiple discrete energy loss structure shows up again, however, the energy losses scale here according to LiE2 2.6LiE1, LiE3 3.8LiE,, —
—
Note that the energies for the transverse motionE~result from the macroscopic settings of projectile beam and target. For protons the effect of the image potential has to be included, i.e. ~ Ey~+ Vim.
Volume 168, number 5,6
PHYSICS LETTERS A
a
and LiE4 5LiE1. The additional peaks have lower intensities and are more shallow than for the data shown in fig. 3a. At even larger angles of incidence cPa,. and in particular larger angles of emergence 0 these structures become more prominent. Detailed
-
(I)
7 September 1992
.
results in this respect will be given in a forthcoming
-
tering of fast protons from a graphite surface [14,15]
-
I
I
35
40
ENERGY
and interpreted by computer simulations [6]. In agreement with those studies we attnbute our results for small energies of normal motion E~to “skipping motion” (fig. 3a) and for larger E~to “subsurface
50
45 (keV)
b
/
.
channeling” (fig. 3c). The experimental observation that the multiple peak structure with equal spacings of energy intervals at E~= 0.6 eV + Vim (an indication of a “skipping motion”) disappears almost for 2.4 eY+ Vim, is consistent with our estimate on effective image potential energies of the order of 1 eY. This gives evidence that the structure in fig. 3a is due
~.
-
-
_____________________________________ 35 40 45 50 I
I
ENERGY
(k~v)
c -
o
~
-
35
40 ENERGY
45
paper [30]. As we have said, similiar observations as described here have been reported recently for the scat-
~o
(keV)
Fig. 3. Energy spectrum observed after the interaction of 50.9 keY protons with an Al( 111)-surface under a grazing angle of incidence ~,,, = 0.2°. The spectra are recorded for specularly reflected projectiles (d,,, ~ The open circles represent a unear plot of the data, the black dots a semiogarithmic plot. The triangles on the right side show the energy distribution obtained for a positioning of the analyzer into the direct proton beam. (b) Same as (a) but ~ (c) Same as (a) but
to the effect of the image potential and indicates that charge exchange plays an essential role for the understanding of “skipping motion”. Support for this statement can be deduced from data obtained with neutral projectiles, where an effect of the image potential on the incoming path of the trajectory is missing (see fig. 2 and discussion above). The neutral hydrogen atoms are formed in a gas-target in the beam-line via proton—”air” collisions, where a mean energy loss of some 10 eY does not affect the energy resolution in our measurements. In fig. 4 we compare data obtained at = 0.2° for about 50 keY protons as projectiles (open circles) and neutral hydrogens atoms (black dots), respectively. The spectra are recorded with a slight sub-specular stetting of the analyzer, where the “skipping motion” structureis found to be increased in comparison to a specular direction [30]. We have normalized both plots of data to the maxima of the prominent peaks and find an enhancement ofthe intensity of the multiple energy loss structure by a factor of about 2. In similar runs at larger 0k,, the mul.
.
tiple peak structure as displayed in fig. 3c is not affected by the charge state of the projectile. These results show the relevance of charge state and charge exchange with respect to the effects ofthe dynamical image potential on the trajectory. On the 413
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H° H
+
50.9 keV,
Al
(111)
0
.
tions from our experimental ratios are, however, not surprising in view of trajectories inside the bulk, where important contributions to the energy loss due
.,
toFinally the excitation of surface are here missing. we summarize thatplasmons we deduce mean effective image potential energies of about 1 eY, which are determined to a wide extent by the mechanisms of charge exchange. Incorporation of these
.
32
36
4044 ENERGY
48
52
(keV)
Fig. 4. Energy spectra for protons after the interaction of 50.9 keY neutral hydrogen atoms (black dots) and protons (open cirdes) with an Al( 111)-surface under a grazing angle ofincidence ~=0.2 and ~ = 0.12 .
incoming trajectory the normal energy of protons is increased from the macroscopic settings E~=E~(effective for neutral atoms) to ~ = E~° + Vim (Y1~~ eY). This energy is about conserved during the reflection from the solid, and the probability for a trapping during the escape from the surface is smaller than for incident neutral atoms with lower E~.
In conclusion, our measurements of angular and energy distributions after the grazing scatteringoffast hydrogen atoms and protons from a clean and flat Al (111)-surface give evidence for effects of the dynamical image potential on the trajectories of the protons. For small transverse energies E~we observe multiple energy losses with integral ratios in relation to the loss for a regular scattering LiE,. In our studies we find practically no dependence of LiE, on the angles of scattering, incidence, or emergence. Since the distance of closest approach of a projectile to the 2 a.u. and the topmost of surface layer is here Ymin~ trapping ions by image forces is effectivefor distances larger than about Ys~7 a.u., we except well separated and defmed multiple scatteringevents with the surface. This results in defined multiple energy losses showing integral ratios with respect to LiE 1 as observed here. For larger E the experimental findings are interpreted by “subsurface channeling where one deduces, instead of integral, odd ratios for the multiple energy losses [6,13]. The slight devia414
7 September 1992
mechanisms in the description of projectile trajectories leads to a new and essential aspect in the understanding of “skipping motion”. Whereas in previous work basically [1—6] the of transverse momenta due broadening to the deviation of the col—
lective atomic potentials from an idealized planar continuum potential is crucial to find bound trajectories, this broadening is important but not a prerequisite in our model. Since the relevant part of charge transfer proceeds on the incident and out—
going trajectory in finite intervals of distances around y~,a fraction ofprojectiles can lose simply transverse energy (momentum) by a first capture of electrons at larger distances on the incident path than the survival distances on the exit. Elimination of the image force on the incoming trajectory by using neutral projectiles will increase then the probability for emerging ions to undergo “skipping motion”. This feature is in agreement with our experimental findings. A detailed discussion on the aspects to multiple energy losses in grazing ion—surface scattering and on our model for “skipping motion” will be outlined elsewhere [30]. The assistance of H.W. Ortjohann and M. Bergomaz in the preparation and running of the experiments and helpful discussions with R. Zimny, Professor Y.H. Ohtsuki, and Dr. R. Pfandzelter are gratefullyacknowledged. We thank W. Hassenmeier for his help inby processing the manuscript. This work is supported the Sonderforschungsbereich 216 (Bielefeld/Münster).
References
!!] Y.H. Ohtsuki, K. Koyama and Y. Yamamura, Phys. Rev. B
20 (1979) 5O44~ Y.H. Ohtsuki, Charged beam interaction with solids (Taylor and Francis, London, 1983).
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[2]Y.H. Ohtsuki, R. Kawai and K. Tange, Nuci. Instrum. Methods B 13(1986)193. [3] M. Kato, T. litaka and Y.H. Ohtsuki, Nucl. Instrum. Methods B 33 (1988) 432. (41 Y. Ohtsuki and T. litaka, in: Ion beam interactions with solids, Reports ofthe Special Project Research, Ministry of Education, Science and Culture ofJapan (1988) p. 113. 5) T. Miyamoto, T. litaka and Y.H. Ohtsuki, NucI. Instrum. Methods B 48 (1990) 330. [6] H. Sakai, T. litaka and Y.H. Ohtsuki, Phys. Lett. A 161 (1992) 467. [7]H.O. Lutz, S. Datz, C.D. Moak and T.S. Noggle, Phys. Rev. Lett. 17 (1966) 285. [8]M.T. Robinson, Phys. Rev. 179 (1969) 327. [9] F.H. Eisen and M.T. Robinson, Phys. Rev. B 4 (1971)1457. [101 M. Hou, W. Eckstein and H. Yerbeek, Radiat. Eff. 39 (1978) 107. [11] KJ. Snowdon, D.J. O’Connor and R.J. MacDonald, Phys. Rev. Lett. 61 (1988) 1760. (12] K.J. Snowdon, D.J. O’Connor and RJ. MacDonald, Surf. Sci. 221 (1989) 465. (13] K. Kimura, M. Hasegawa and M. Mannami, Phys. Rev. B 36 (1987) 7. [14)F. Stölzle and R. Pfandzelter, Phys. Lett. A 150 (1990) 315. (15) F. Stölzle and R. Pfandzelter, Surf. Sd. 251/252 (1991) 383. (16] U. Scheithauer, G. Meyer and M. Henzler, Surf. Sci. 178 (1986) 441.
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(17] H.H.W. Feijen, L.K. Verhey, A.L. Boers and F.P.Th.M. Suurmeijer,J. Phys. E6 (1973) 1474. [18) HJ. Ajidra, H. Winter, R. Frohling, N. Kirchner, HJ. Plöhn, W. Wittmann, W. Graser and C. Varelas, Nucl. Instrum. Methods B 170 (1980) 527; H. Winter, to be published. [19] H. Winter, Phys. Rev. A (1992), to be published. H. Winter and J. Leuker, Nuci. Instrum. Methods B (1992), to be published. [20] J.N.M. van Wunnik, R. Brako, K. Makoshi and D.M. Newns, Surf. Sci. 261 (1983) 618. [21]D.M.Newns, Comm. Cond. Mat. Phys. 14 (1989) 295. [22]H. Winter, Comm. At. Mol. Phys. 26 (1991) 287. [23]E.G. Overbosch, B. Rasser, A.D. Tenner and J. Los, Surf. Sci. 92 (1980) 310; .~.th~and J.J.C. Geerlings, Phys. Rep. 190 (1990) 133. [24] R. Zimny, Z. Miskovic, N.N. Nedeljkovic and L.D. Nedeljkovic, Surf~Sci. 255 (1991)135. [25]P. Nordlander and J.C. Tully, Phys. Rev. Lett. 61(1988) 990; Surf~Sci. 211/212 (1989) 207. [26]H. Nienhaus, R. Zimny and H. Winter, Radiat. Eli. Def. Solids 109 (1989) 1. (27] K.J. Snowdon, R. Hentschke, A. NSrmann and W. Heiland, Surf. Sd. 92 (1980) 310. [28]H. Winter, Phys. Rep., submitted. [29]H. Winter, Europhys. Lett. 18 (1992) 207. [30]H. Winter and M. Sommer, to be published.
415
PHYSICS LETTERS A
Discrete energy losses of fast protons in grazing surface scattering from Al (111) H. Winter and M. Sommer Inst itutfür Kernphysik der Universität Munster, Wilhelm-Klemm-Strasse 9, W-4400 Münster, Germany Received 21 April 1992; revised manuscript received 22 June 1992; accepted for publication 6 July 1992 Communicated by B. Fricke
The energy loss offast protons scattered from a flat and clean A1( 111 )-surface under a grazing angle ofincidence is investigated with high resolution with respect to angle of scattering and energy. We find two different kinds ofmultiple discrete energy losses in the spectra, which weattribute to hopping kind oftrajectories at the surface (“skipping motion”) and to subsurface channeling, respectively. By a variation of the charge state ofthe projectileswe show that chargeexchange affects the image potential close to the surface in a decisive way and plays an important role for the understanding ofthe observed effects.
In recent years Ohtsuki and coworkers have investigated theoretically the scattering of fast protons (v~v0 = Bohr velocity = 1 a.u.) from metal surfaces under grazing angles of incidence. Based on cornputer simulations these authors conclude that due to the dynamical image potential a fraction of scattered projectiles undergoes hopping type of trajectories, the so-called “skipping motion” [1—4].This motion is characterized by multiple approaches of the projectiles to the surface plane, which is expected to result in an increased angular broadening [5] and in a number of discrete energy losses [1—6]for the scattered beam. It turns out that the feature of discrete energy losses is the more direct indication of “skipping motion” type of trajectories, so that expertmental verifications of this effect have been concentrated on the analysis of energy spectra for fast projectiles scattered under grazing angles of incidence. However, discrete energy losses are not an unequivocal signature of such a type of bound motion close to the surface plane. This is obvious from early observations by Lutz et al. [7] in transmission channeling experiments through very thin Au single crystals, which are interpreted in terms of a channeling of projectiles between atomic planes in the bulk of the crystal [8,9]. In ion—surface scattering multiple energy losses have been reported by Hou et al. [10]
and recently by Snowdon et a!. [11,121 for projectiles with energies ofsome keY, and by Kimura et al. [13] for projectiles in the MeV-domain. In all these studies, however,the energy ofmotion normal to the surface plane E~is clearly too high for an interpretation of the data in terms of a “skipping motion” caused by the dynamical image potential [4]. As a consequence, most of the data can be explained by “subsurface channeling” trajectories [6,13], an oscillatory kind of motion between atomic planes below the topmost layer of surface atoms. This motion is closely related to the effects observed and was interpreted already in refs. [7—9].The projectiles enter those subsurface channelsvia e.g. steps at the surface plane and thermal vibrations oflattice atoms at the surface. Subsurface channeling is claimed to be ruled out for the interpretation of the data reported in refs. [11,12], where the authors describe their experiments in terms of a “transient adsorption” mechanism. Recently Stölzle and Pfandzelter have reported on discrete energy losses of 15—75 keV protons in grazing collisions with a smooth polycrystalline graphite surface [14,15]. In part of their data the energy of normal motion is sufficiently small to allow for a “skipping motion” at the surface plane. In addition, the authors found also spectra with multiple discrete energy losses for larger energies of normal motion,
0375-9601/92/S 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
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Volume 168, number 5,6
PHYSICS LETTERS A
where a “skipping motion” induced by the image potential can be excluded. All these data are claimed to be reproduced in computer simulations by Sakai et a!. [61, which indicate for small grazing angles “skipping motion” and for larger angles “subsurface channeling”. In this paper we report on investigations of discrete energy losses in grazing collisions of fast protons with an Al( 111)-surface. Depending on the energy ofmotion normal to the surface, we observe two clearly different multiple discrete energy loss spectra, which we attribute also to “skipping motion” and “subsurface channeling”, respectively. In using in addition neutral hydrogen atoms as projectiles, we find further evidence that our spectra for low normal energies E~can be interpretedby the effect of the dynamical image potential on the trajectories above the topmost layer of surface atoms. This interpretation is supported by the study of polar angular distributions in dependence on the charge state of the projectiles. The setup used in our studies is sketched in fig. 1. Protons or neutral hydrogen atoms (produced in a gas-target) of20—100 keY energy are well collimated by sets of vertical and horizontal slits to a divergence of about 0.02°and directed onto the (111)-face of a monocrystalline aluminum sample under grazing angles of incidence ø~,~ 0.2°—1.5°.The target is polished with great care by keeping the deviation between polishing and (111)-plane as small as possible and is treated thereafter in situ by cycles of grazing sputtering with 100—200 keY Ark-ions at 1 ~,,,
gastarget
electric field plates I plates II
~
and heating up to 550°C.Inspection in the final state of the preparation of the target by a “spot profile analysis” LEED [16] yields a rather flat surface with an average terrace width formed by topmost surface atoms larger than about 70 nm. In order to avoid effects of an axial surface channeling on the trajectories, the direction of the projectile beam coincides with a high index crystallographic direction in the surface plane (“random” orientation). This can be adjusted on line by recording the intensity of emitted electrons [171 or simply the target current [18] during an azimuthal rotation of the crystal. This current is found to be enhanced for scattering along low index axial directions [18]. The target is mounted on a precision manipulator and kept under UHY-conditions at a base pressure of about 5 x 10—” mbar. Differential pumping stages at both ends of the UHY-chamber allows to maintain the vacuum against a pressure of about 10—v mbar in the beamline of the accelerator and in the analyzer section. Electric field plates in both pumping stages and a small gas target in the beam-line allow to perform the experiment with neutral and charged projectiles and to separate the charge states of scattered projectiles. Scattered particles are analyzed with help of detector I to obtain (polar) angular distributions or with detector II for energy spectra. Both detectors consist of a channeltron detector behind a circular aperture of about 0.2 mm diameter. The energy of ions is analyzed with a cylindrical 90°electrostatic analyzer of 0.5 m radius with an instrumental res-
°
A
detector I
s~thr Fig. 1. Sketch of the experimental setup.
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olution 8Ejnstr/E~5 x l0~.A tilt of the analyzer with respect to A and B (see figure) allows us to obtain energy spectra in dependence on the polar angle of scattering with an angular resolution 8~ana~ 0.02°. The effect ofthe dynamical image potential on the trajectories of the projectiles can be studied directly by comparing angular distributions for neutral and charged projectiles. These distributions are recorded by a scan with detector I in the polar plane of the scattered particles at a distance of 1 in behind the target. The contributions of neutral projectiles before and/or after the scattering event are separated here by electric fields of about 1 kY/cm. From the difference of the signals with and without electric fields we obtain furthermore the angular distributions due to protons. Details on the concept of this “difference method” to measure angular distributions for primary and scattered projectiles resolved with respect to the neutral and positive charge states are described elsewhere [19]. In fig. 2 we display angular distributions of projectiles leaving the surface in the neutral charge state (voltage on field plates II) for incident neutral hydrogen atoms (voltage on plates I; black dots) and for protons (from the difference of the signals with
x 1/120
H
H~ //
/
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0
1
2
ANGLE OF SCATTERING
(°)
Fig. 2. Distribution of scattered neutral hydrogen atoms after the interaction of 25 keY hydrogen atoms (black dots) and protons (open circles) with an Al (111)-surface. The solid lines represent best fits ofthe central part of the distributions to a Gaussian lineshape. The single dot on the left side is due to a residual fraction of the (neutral) direct beam, which has passed on top of the target without interaction and which thus represents the direction ofthe projectile beam. The plot ofthis strong signal is suppressed by a factor of 120.
7 September 1992
and without voltage on field plates I; open circles). The data represent well defined distributions of scattered projectiles, where the maximum of the distribution obtained for protons is shifted by ~= 0.13°±0.03°towards larger angles in comparison to the data obtained with neutral projectiles. By means of a He—Ne laser beam directed along the beam axis through all slits, which define the direction and the divergence ofthe projectile beam, we verify that the maximum of the distribution for the neutral projectiles coincides with the direction of specular reflection; i.e. ‘Z = 1.120 and ~ = = 0.56°.The intense narrow peak on the left side (suppressed in fig. 2 by a factor of 120) stems from a residual fraction of the incident neutral beam, which has passed on top of the target without interaction and which thus serves as a reference for the direction of the projectile beam. The angular shift between the two distributions is consistent with the presence of the dynamical image potential. This potential is effective only for charged particles and results on the incoming part of the trajectory in an acceleration ofprotons towards the surface plane. Close to the surface plane the image potential is affected decisively by charge exchange; i.e. here capture and loss of electrons with respect to the hydrogen ls term. Due to the small energies for the motion normal to the surface (E~~eV),charge transfer in grazing fast ion—surface collisions can be described by adiabatic concepts. The effect ofthe high parallel velocity on charge exchange can be incorporated here by a Galilei transformation [20], giving rise to a “Doppler—Fermi—Dirac distribution” [21,22] for the occupation of electronic states in the metal. The transfer of electrons proceeds predominantly via one-electron tunneling and Auger processes and is characterized by consecutive capture and loss events, where hydrogen atoms in the ls ground-state cannot survive from the surface interaction for distances smaller than the so-called survial or “freezing distance” y~[23]. Then only for distances larger than about y, the charge state of the projectiles and an image interaction (for protons) is defined. As a consequence the size of the image potential .
.
.
is affected here in a charactenstic way by the mechanisms of charge exchange. The contributions of electron exchange processes relevant for the image 411
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potential and the formation of final atomic terms are limited to an interval of distances y well above the surface plane and are represented by a distribution function F(y) peaking at y~[23]. F(y) is determined by the total transition rates and the normal velocity component v~.Zimny et al. [24] have analyzed theoretically the neutralization of protons via one-electron tunneling [25,26] and Auger transfer [27] and deduced a maximum of F(y) at y,~7a.u. from the topmost layer of surface atoms. The image potential energy gained (lost) on the incident (outgoing) trajectory of an ion is equivalent to the energy shift of a level in the corresponding atom [28], and we estimate here from ref. [24] a mean image energy Vim(Ys) = 1.5 ±0.2eY. From the data shown in fig. 2 we obtain from the positions of the maxima (best fit by Gaussian distribution) a mean energy of normal motion for the neutral atoms E~= 25 keY sin2 (0.560 ) = 2.39 eY and for protons E~= 25 keY sin2 (0.69°) = 3.63 eV. The mean image energy gained by the protons on the incident trajectory is then LiE=E~—E~= 1.24 eY. In view~ of the experimental and conceptual uncertainties these data support the assumptions on the mechanisms of charge exchange. A similar, however, more pronounced effect has been observed recently for the scattering of multicharged ions from the same surface [29]. On the outgoing path of the trajectory the result of the image potential is reversed. An energy of about LiE is effective to decelerate and eventually trap ions to the surface plane. Detailed studies on image charge effects for emerging particles confirm such a feature [19J. In summarizing this section, we state that angular distributions recorded for neutral as well as for charged projectiles give direct evidence for the effect of the image potential on the trajectory. The mean image potential energies of relevance here are measured to be of the order of about 1 eY. With help of the electrostatic energy analyzer and detector II (see fig. 1) we record energy spectra of projectiles scattered from the surface. In this work we focus our attention to the appearance ofmultiple energy losses. The energy spectra are measured with high angular resolution by translating the entrance slit of 0.1 mm width and positioned 1.5 m behind the target (B in fig. 1) in the polar plane of scattering. Due to the use of an electrostatic analyzer, we 412
7 September 1992
are sensitive here only to contributions of charged particles (protons). In fig. 3a we show an energy spectrum after the interaction of about 50 keY protons with the surface under a grazing angle of incidence cZ,,, = 0.2°. The spectrum is obtained for the specularly reflected fraction of the beam, i.e. cI~= ~Out• This setting corresponds to an energy of normal motion ~ Ey~=E 5jfl2~jn + Vim = 0.6 eY+ Vim. As a reference we display also the spectrum for the primary projectile beam, which is obtained by a corresponding position of the entrance slit of the energy analyzer. The energy of the direct beam is represented by the few triangles on the right side of the spectrum in fig. 3a, which indicates the good energy resolution of accelerator and analyzer in our measurements. This feature of our setup avoids the need for deconvolution procedures in the analysis of data. The spectrum for the scattered beam (linear and logarithmic representations) shows a pronounced peak due to a regular scattering ofprojectiles from the surface. The energy interval between the peaks of the signals for the direct and scattered beams defines a most probable energy loss ~.E,.A striking feature with respect to the subject ofthis paper is the existence of at least two further peaks at lower energies, which can be identified easily in the linear and in particular in the logarithmic representation of the data. For these peaks we find energy losses LiE 2 2LiE, and ~ Figure 3b shows a spectrum for cP,,. = 0.4°, i.e. ~ = 2.4 eY + Vm (see footnote 1), at otherwise unchanged experimental conditions. It is evident from the figure that aside from a slight hump in the tail of the prominent peak the multiple peak structure has disappeared. In fig. 3c we display a spectrum, which is obtained after a further increase of the angle of incidence to ~ = 0.5° and E~= 3.8 eY+ Vim (see footnote 1). Under these conditions a multiple discrete energy loss structure shows up again, however, the energy losses scale here according to LiE2 2.6LiE1, LiE3 3.8LiE,, —
—
Note that the energies for the transverse motionE~result from the macroscopic settings of projectile beam and target. For protons the effect of the image potential has to be included, i.e. ~ Ey~+ Vim.
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a
and LiE4 5LiE1. The additional peaks have lower intensities and are more shallow than for the data shown in fig. 3a. At even larger angles of incidence cPa,. and in particular larger angles of emergence 0 these structures become more prominent. Detailed
-
(I)
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.
results in this respect will be given in a forthcoming
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tering of fast protons from a graphite surface [14,15]
-
I
I
35
40
ENERGY
and interpreted by computer simulations [6]. In agreement with those studies we attnbute our results for small energies of normal motion E~to “skipping motion” (fig. 3a) and for larger E~to “subsurface
50
45 (keV)
b
/
.
channeling” (fig. 3c). The experimental observation that the multiple peak structure with equal spacings of energy intervals at E~= 0.6 eV + Vim (an indication of a “skipping motion”) disappears almost for 2.4 eY+ Vim, is consistent with our estimate on effective image potential energies of the order of 1 eY. This gives evidence that the structure in fig. 3a is due
~.
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-
_____________________________________ 35 40 45 50 I
I
ENERGY
(k~v)
c -
o
~
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40 ENERGY
45
paper [30]. As we have said, similiar observations as described here have been reported recently for the scat-
~o
(keV)
Fig. 3. Energy spectrum observed after the interaction of 50.9 keY protons with an Al( 111)-surface under a grazing angle of incidence ~,,, = 0.2°. The spectra are recorded for specularly reflected projectiles (d,,, ~ The open circles represent a unear plot of the data, the black dots a semiogarithmic plot. The triangles on the right side show the energy distribution obtained for a positioning of the analyzer into the direct proton beam. (b) Same as (a) but ~ (c) Same as (a) but
to the effect of the image potential and indicates that charge exchange plays an essential role for the understanding of “skipping motion”. Support for this statement can be deduced from data obtained with neutral projectiles, where an effect of the image potential on the incoming path of the trajectory is missing (see fig. 2 and discussion above). The neutral hydrogen atoms are formed in a gas-target in the beam-line via proton—”air” collisions, where a mean energy loss of some 10 eY does not affect the energy resolution in our measurements. In fig. 4 we compare data obtained at = 0.2° for about 50 keY protons as projectiles (open circles) and neutral hydrogens atoms (black dots), respectively. The spectra are recorded with a slight sub-specular stetting of the analyzer, where the “skipping motion” structureis found to be increased in comparison to a specular direction [30]. We have normalized both plots of data to the maxima of the prominent peaks and find an enhancement ofthe intensity of the multiple energy loss structure by a factor of about 2. In similar runs at larger 0k,, the mul.
.
tiple peak structure as displayed in fig. 3c is not affected by the charge state of the projectile. These results show the relevance of charge state and charge exchange with respect to the effects ofthe dynamical image potential on the trajectory. On the 413
Volume 168, number 5,6
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H° H
+
50.9 keV,
Al
(111)
0
.
tions from our experimental ratios are, however, not surprising in view of trajectories inside the bulk, where important contributions to the energy loss due
.,
toFinally the excitation of surface are here missing. we summarize thatplasmons we deduce mean effective image potential energies of about 1 eY, which are determined to a wide extent by the mechanisms of charge exchange. Incorporation of these
.
32
36
4044 ENERGY
48
52
(keV)
Fig. 4. Energy spectra for protons after the interaction of 50.9 keY neutral hydrogen atoms (black dots) and protons (open cirdes) with an Al( 111)-surface under a grazing angle ofincidence ~=0.2 and ~ = 0.12 .
incoming trajectory the normal energy of protons is increased from the macroscopic settings E~=E~(effective for neutral atoms) to ~ = E~° + Vim (Y1~~ eY). This energy is about conserved during the reflection from the solid, and the probability for a trapping during the escape from the surface is smaller than for incident neutral atoms with lower E~.
In conclusion, our measurements of angular and energy distributions after the grazing scatteringoffast hydrogen atoms and protons from a clean and flat Al (111)-surface give evidence for effects of the dynamical image potential on the trajectories of the protons. For small transverse energies E~we observe multiple energy losses with integral ratios in relation to the loss for a regular scattering LiE,. In our studies we find practically no dependence of LiE, on the angles of scattering, incidence, or emergence. Since the distance of closest approach of a projectile to the 2 a.u. and the topmost of surface layer is here Ymin~ trapping ions by image forces is effectivefor distances larger than about Ys~7 a.u., we except well separated and defmed multiple scatteringevents with the surface. This results in defined multiple energy losses showing integral ratios with respect to LiE 1 as observed here. For larger E the experimental findings are interpreted by “subsurface channeling where one deduces, instead of integral, odd ratios for the multiple energy losses [6,13]. The slight devia414
7 September 1992
mechanisms in the description of projectile trajectories leads to a new and essential aspect in the understanding of “skipping motion”. Whereas in previous work basically [1—6] the of transverse momenta due broadening to the deviation of the col—
lective atomic potentials from an idealized planar continuum potential is crucial to find bound trajectories, this broadening is important but not a prerequisite in our model. Since the relevant part of charge transfer proceeds on the incident and out—
going trajectory in finite intervals of distances around y~,a fraction ofprojectiles can lose simply transverse energy (momentum) by a first capture of electrons at larger distances on the incident path than the survival distances on the exit. Elimination of the image force on the incoming trajectory by using neutral projectiles will increase then the probability for emerging ions to undergo “skipping motion”. This feature is in agreement with our experimental findings. A detailed discussion on the aspects to multiple energy losses in grazing ion—surface scattering and on our model for “skipping motion” will be outlined elsewhere [30]. The assistance of H.W. Ortjohann and M. Bergomaz in the preparation and running of the experiments and helpful discussions with R. Zimny, Professor Y.H. Ohtsuki, and Dr. R. Pfandzelter are gratefullyacknowledged. We thank W. Hassenmeier for his help inby processing the manuscript. This work is supported the Sonderforschungsbereich 216 (Bielefeld/Münster).
References
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20 (1979) 5O44~ Y.H. Ohtsuki, Charged beam interaction with solids (Taylor and Francis, London, 1983).
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[2]Y.H. Ohtsuki, R. Kawai and K. Tange, Nuci. Instrum. Methods B 13(1986)193. [3] M. Kato, T. litaka and Y.H. Ohtsuki, Nucl. Instrum. Methods B 33 (1988) 432. (41 Y. Ohtsuki and T. litaka, in: Ion beam interactions with solids, Reports ofthe Special Project Research, Ministry of Education, Science and Culture ofJapan (1988) p. 113. 5) T. Miyamoto, T. litaka and Y.H. Ohtsuki, NucI. Instrum. Methods B 48 (1990) 330. [6] H. Sakai, T. litaka and Y.H. Ohtsuki, Phys. Lett. A 161 (1992) 467. [7]H.O. Lutz, S. Datz, C.D. Moak and T.S. Noggle, Phys. Rev. Lett. 17 (1966) 285. [8]M.T. Robinson, Phys. Rev. 179 (1969) 327. [9] F.H. Eisen and M.T. Robinson, Phys. Rev. B 4 (1971)1457. [101 M. Hou, W. Eckstein and H. Yerbeek, Radiat. Eff. 39 (1978) 107. [11] KJ. Snowdon, D.J. O’Connor and R.J. MacDonald, Phys. Rev. Lett. 61 (1988) 1760. (12] K.J. Snowdon, D.J. O’Connor and RJ. MacDonald, Surf. Sci. 221 (1989) 465. (13] K. Kimura, M. Hasegawa and M. Mannami, Phys. Rev. B 36 (1987) 7. [14)F. Stölzle and R. Pfandzelter, Phys. Lett. A 150 (1990) 315. (15) F. Stölzle and R. Pfandzelter, Surf. Sd. 251/252 (1991) 383. (16] U. Scheithauer, G. Meyer and M. Henzler, Surf. Sci. 178 (1986) 441.
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(17] H.H.W. Feijen, L.K. Verhey, A.L. Boers and F.P.Th.M. Suurmeijer,J. Phys. E6 (1973) 1474. [18) HJ. Ajidra, H. Winter, R. Frohling, N. Kirchner, HJ. Plöhn, W. Wittmann, W. Graser and C. Varelas, Nucl. Instrum. Methods B 170 (1980) 527; H. Winter, to be published. [19] H. Winter, Phys. Rev. A (1992), to be published. H. Winter and J. Leuker, Nuci. Instrum. Methods B (1992), to be published. [20] J.N.M. van Wunnik, R. Brako, K. Makoshi and D.M. Newns, Surf. Sci. 261 (1983) 618. [21]D.M.Newns, Comm. Cond. Mat. Phys. 14 (1989) 295. [22]H. Winter, Comm. At. Mol. Phys. 26 (1991) 287. [23]E.G. Overbosch, B. Rasser, A.D. Tenner and J. Los, Surf. Sci. 92 (1980) 310; .~.th~and J.J.C. Geerlings, Phys. Rep. 190 (1990) 133. [24] R. Zimny, Z. Miskovic, N.N. Nedeljkovic and L.D. Nedeljkovic, Surf~Sci. 255 (1991)135. [25]P. Nordlander and J.C. Tully, Phys. Rev. Lett. 61(1988) 990; Surf~Sci. 211/212 (1989) 207. [26]H. Nienhaus, R. Zimny and H. Winter, Radiat. Eli. Def. Solids 109 (1989) 1. (27] K.J. Snowdon, R. Hentschke, A. NSrmann and W. Heiland, Surf. Sd. 92 (1980) 310. [28]H. Winter, Phys. Rep., submitted. [29]H. Winter, Europhys. Lett. 18 (1992) 207. [30]H. Winter and M. Sommer, to be published.
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