- Email: [email protected]
Image charge interaction of multicharged ions in grazing collisions with an Al(111)-surface
NIUMI B
Nuclear Instruments and Methods in Physics Research B 90 (1994) 216-221 North-Holland
Image charge interaction with an Al( 111)-surface
Beam Interactions with Materials&Atoms
of multicharged
ions in grazing collisions
H. Winter and C. Auth * Institut j?ir Kernphysik der Universitiit Miinster, Wilhelm-Klemm-Strasse 9, D-48149 Miinster, Germany
We study the angular distributions of multicharged noble gas ions after under a grazing angle of incidence. The data show pronounced effects of which allow us to deduce the energy gained by the projectile during the discussion of our experimental method and compare the results with an multicharged ions in front of a metal surface.
1. Introduction
The interaction of multiply/ highly charged ions with metal surfaces is a subject of active research in recent years. The field has profited from developments in ion-source technology to provide slow ions in high charge states, i.e. atomic particles having high potential energies. The liberation of this energy in collisions with a solid is connected with a complex multi-electron process, which is an interesting scientific problem and also has the potential for future technological applications. In a number of recent studies - performed primarily by means of electron spectroscopy - essential features of the neutralization mechanisms of multicharged ions above and below metal surfaces have been revealed [l-4]. In the context of this paper, the interactions of these ions in front of the surface, the “above surface are of particular interest. Essential neutralization”, features of this interaction seem to be well described by a classical “over-barrier model” by Burgdijrfer et al. [5], where the neutralization of the ions is initiated at rather large distances from the surface by resonant transitions of conduction band electrons into (multiply excited) Rydberg levels of the projectiles. Via this mechanism, the projectile is assumed to be neutralized completely in front of the topmost layer of surface atoms. However, since the inner shell vacancies of the projectiles will have survived to a major extent, most of the high potential energy stored in the initial projectile ions is still available in close proximity to the surface.
* Corresponding 034962.
author, tel. +49 251 834969, fax +49 251
scattering from a clean and a cesiated Al(lll)-surface the image charge attraction on the incident trajectory approach towards the surface. We present a detailed “over-barrier model” describing the neutralization of
Both at and below the surface plane, close encounters with target atoms and conduction electrons of high densities will rapidly fill the inner shell vacancies to complete the neutralization sequence. Recently we have introduced a method that allows us to deduce image interaction energies in ion-surface scattering experiments [6]. Since these energies are closely related to the effective charge state of the projectile on the incident trajectory, we expect to deduce from corresponding data important information on the neutralization dynamics of multicharged ions in front of the surface of metals.
2 . “Over-barrier model” The classical “over-barrier model” by Burgdiirfer et al. [S] can be considered as a first consequent approach to describe gross features of the extremely complex charge exchange mechanisms between solids and multicharged atoms in a scattering experiment. Arguments for a number of approximations made in this model in particular a justification for the description of the electron tunneling by an “over-barrier” concept - has been given in ref [5] and in a recent review by BurgdGrfer [7]. Since the image charge interaction energies reflect the neutralization sequence in front of the surface, we will apply the model to this part of the trajectory. Furthermore we will use the model in its most simple version: a stepwise neutralization of the incident ions. For an illustration of the description of electron transfer in this model, in Figs. la and lb we plot the effective potentials of the active electron for an ion with core charge Z = 6 at distances of R = 60 a.u.
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E0658-4
(atomic unit of length 0.0529 nm) and Iz = 15 au., respectively, in front of a conducting surface. The potential for the electron results from the Coulomb interaction with the ion core, the image charge interaction due to the presence of the ion core, and the self-image interaction potential (surface potential). At large distances, I? (Fig. la), the atomic and surface potent~aIs are well separated by a large potential barrier, which prevents any transitions of electrons between atom and surface. For smaller distances (Fig. lb), the potential barrier between atom and surface is reduced, and the effective electronic potential is characterized by a saddle point structure, which provides in a classical picture a pathway of resonant transitions for electrons with binding energies smaller than the saddle point poient~al energy. For a description of the image charge potentials by the static and asymptotic limit q/4y (q = charge of particle, y = distance from the surface/image plane), which can be justified for the conditions of our experiments, one finds for the saddle point energy, V,, the simple relation [5,7,8]
sequence, where the effective charge state of the projectile is reduced sequentially by capture of electrans at distances R,(Z) (“staircase model” 191).As a consequence, an ion with initial charge state 4 will gain energy via image charge attraction until the neutralization sequence ceases the acceleration. The gain in energy for the normal motion of the projectile is
The ~~~~oximat~ons made in the derivation of Eq. (3) appear as extremely crude in view of the complex charge exchange mechanisms. However, since the image charge attraction is given by the effective charge state of the projectile, we expect that the effects of ~re)ionizing and consecutive capture events will cancel to a large extent and that the model will reproduce the basic features of the data. Based on Eq. (3), we deduce for PP. 1) a 4 3Y/2-dependence, 2) proportionality with respect to the workfunction u”, 3) no dependence on the type of ion forming the ion core, We will show beIow that these predictions by the simple “staircase-over-barrier modedd” are in fair agreement with our experimental findings.
Then resonant capture of electrons from the surface can proceed for distances R, where the potentiai barrier is reduced to energies of occupied metal states, i.e. V, 2 W (W= work function of metal). Then the transfer of conduction electrons to an ion with core charge Z is possible for distances fl- “X1/2
3. Concept of the experimental method
Since the work function of metals is typically 4 to 5 eV, these “over-barrier” transitions result for muiticharged projectiles in the population of Rydberg levels, whereas the inner shell vacancies cannot be filled at this early stage of neutralizatjon by direct transitions. In simplifying matters, we consider an electron capture
We have recently proposed methods to deduce image jnteraction energies on the in~rn~ng and nutgoing trajectories in ion-surface collisions [6,10], We make use of the specific property of the grazing incidence collision geometry, that the motion of a projectile with energy E0 (velocity us) normal to the surface plane (y-axis) proceeds with energy EY = E, sin’%+,, and the parallel motion with E,, = E, COS~@~~=j E,; @, =
Fig. 1. Electronic potential for an ion of core charge q = 6 at a distance R = 60 a.u. (a) and R = 15 a.u. (b), in front of a metal surface.
218
H. Winter, C. Auth / Nucl. Instr. and Meth. in Phys. Res. B 90 (1994) 216-221
arctan(u,/u,) = u,/u ,,= (Ey/Eo)1/2 is the grazing angle of incidence. The scattering from the surface can be described on the basis of a collective planar potential (“surface channeling”) [ll], which is generally deduced from screened Coulomb interatomic potentials 1121and depends only on the distance from the surface, i.e. on the normal coordinate y. Then the motions normal and parallel with respect to the surface plane are independent, and the projectiles are reflected specularly at a potential plane positioned at ymin = 1 to 2 au. in front of the topmost layer of surface atoms. An ionized projectile with charge q gains (on the incident trajectory via its image charge interaction) the energy AEy4, before this interaction is ceased by the complete screening of the ion core by captured electrons. Then the angle of incidence, Gin, and the normal energy E, defined by the macroscopic collision geometry is modified according to @$ = ( E$q/E,,)‘/z with Ep = E, -k AE,4. Assumption of specular reflection at the surface plane and small energy losses of the projectile gives Q&t = @$,. For the completely neutralized projectile, no image interaction is effective on the outgoing trajectory so that E$“‘
(4)
Then the analysis of the angular distributions for neutral (Gs = 2Qi,) and ionized projectiles allows us to obtain A Ey4.
4. Experiments and results In grazing collisions with surfaces, the projectiles interact with a relatively large lateral extension of the target surface. Therefore, the surface plane has to be maintained at a high level of cleanness and flatness, in order to minimize a broadening of the scattered beam due to binary collisions with defect structures and adsorbed atoms (“dechanneling”). Our experiments are performed under UHV-conditions at a base pressure mbar, and the (Ill&face of an Al of some lo-” monocrystalline sample is prepared by frequent cycles of grazing sputtering (Gin = 2”) with 25 keV Ar+-ions and annealing at temperatures of about 500°C for a couple of minutes. Cleanness of the target surface is monitored via Auger-spectroscopy. From studies with a SPALEED-system [13] we find widths of terraces, formed by atoms at the topmost surface layers, larger than 50 nm with a clear dominance of monoatomic step heights. Such a high quality metal surface is crucial for the studies reported here, since the broadening of the scattered beams due to surface imperfections
Fig. 2. Sketch of the experimental procedure.
reduces the precision in the analysis of data. We will demonstrate this feature by data obtained for the controlled adsorption of Cs-atoms with the help of a Cs-dispenser (SAES-Getters). A sketch of the experimental procedure for recording angular distributions of projectiles scattered from the surface is displayed in Fig. 2. Noble gas ions with energies in the keV-domain are extracted from a Penning ion-source, collimated by sets of vertical and horizontal slits in the differential pumping stages to a sub-mrad divergence, and directed under a grazing angle of incidence @, = OS”-3” onto the Al(lll)surface. Effects of axial surface channeling are avoided by sending the projectiles along high index axial directions in the surface plane (“random orientation”). A careful grounding of all relevant components and a compensation/shielding of magnetic fields to some mG is expected to reduce effects of residual fields on the trajectories of the multicharged ions to a negligible level. The scattered projectiles leave the surface predominantly neutralized (fractions of singly charged ions are typically < 1%) and are detected by a channeltron detector covered with a 0.5 mm diaphragm. This detector is positioned on a precision manipulator, 700 mm behind the target and is translated in the scattering plane via a step-motor drive. In Fig. 3 we show as a representative example of our studies of angular distributions, obtained with 25 keV Are and Ar’+ projectiles, respectively, for the same collimation of the beam and settings of the target. We observe a clear angular shift towards larger angles of scattering for the incident ion beam, relative to the neutral beam. This is the effect expected from the presence of an attractive force on the incident part of the trajectory. The intense peak on the left side of the data is due to a residual fraction of the direct beam that has passed above the target (see Fig. 2) and thus serves as a reference for the direction of the projectile beam. For the analysis of the data in Fig. 3, we fit a Gaussian lineshape to the angular distributions and obtain for the distribution of the Are-projectiles (no image interaction assumed) a scattering angle @s= (1.19 + 0.04)“.
H. Winter, C. Auth /Nucl. Instr. and Meth. in Phys. Res. B 90 (1994) 216-221
219
l 0 n
3 scattering
angle
(“)
3 charge
Fig. 3. Angular distributions for the scattering of 25 keV Are (full circles) and Ar5+ projectiles (open circles) from a clean Al(lll)-surface. The peak on the left side is due to a fraction
4
/
5
J
6
7
state
Fig. 5. Energy gain of Arqf (open circles), Kr4+ (full squares), and Xeqf ions (full circles) in the scattering from an AlUll)-surface as a function of the initial projectile charge q.
of the direct beam that has passed on top of the target (see Fig. 2).
We check for the specular reflection of the neutral projectile beam using a He-Ne laser beam collimated in the same manner as the particle beam. The direct and the reflected laser beams are analyzed also with our detector by coupling a portion of the stray laser light on the entrance of the channeltron via an optical fiber to a photomultiplier, installed outside the vacuum chamber. The result of a detailed study on the comparison of neutral atom and laser scattering from the target surface is displayed in Fig. 4, where we show for a variation of the tilt of the target the resulting scatter-
loser
scattering
angle
(“)
Fig. 4. Relation of the angles of scattering determined from the reflection of a laser beam and a 25 keV Xe”-beam for the same settings of the target. The solid line indicates the specular reflection condition based on the reflection of the laser beam.
ing angles as measured with the laser and a 25 keV Xe”-beam. The solid line in the figure represents the condition of specular reflection based on the data obtained for the laser beam. The scattering angles deduced from the Xe”-data indicate that the conditions of specular reflection are met in the collision with sufficient accuracy to allow for an analysis of data as outlined above. From the reflection of the Are-atoms and also of the laser beam, we obtain ai,, = Goout= @J2 = 0.60”, so that neutral projectiles approach and leave the surface with the normal energy E, = (2.7 f 0.2) eV. From the distribution for the Ar’+-projectiles we get from @J:,“’ = (1.97 + 0.04)“: @fn+= @,“‘- ai,, = (1.35 + 0.06)0 and Ei”,‘+= (14.2 f 1.3) eV. Thus Ar’+-ions gain an energy AyE: + = (11.5 ? 1.3) eV via image charge interaction on the incident trajectory. In the same manner we perform experiments with 25 keV Arq+, KrQ+, and Xeq+ ions in charge states up to q = 6 and obtain the energies AE,4 as displayed in Fig. 5. The data show a monotonic increase of energy with the projectile charge and are reproduced fairly well by a q312-dependence predicted by the “staircase-over-barrier model”. The solid line in Fig. 5 represents A E;, calculated from Eq. (3) with the work function W= 4.27 eV of the Al(lll)surface as measured in-situ via photoemission with an accuracy of 0.03 eV. We find within the uncertainties of our data the same results for all three noble gas ions. This is expected for the population of Rydberglevels as assumed by the “over-barrier model”. The dependence of the image interaction energies on the work function of the target is probed by the adsorption of about one monolayer of Cs-atoms on the Al(lll)-surface by means of resistive heating of a dispenser. The presence of alkali atoms at the surface III. SURFACE PHENOMENA
H. Winter, C. Auth /Nucl. Instr. and Meth. in Phys. Res. B 90 (1994) 216-221
220
leads to a reduction of the “macroscopic” work function of the target to W = 2.44 eV, as determined by photoemission. The adsorption of the &layer is, however, accompanied by a reduction of the flatness of the surface. This can be seen directly from the angular distributions shown in Fig. 6, obtained for the cesiated target at otherwise unchanged conditions as for the measurements displayed in Fig. 3. We observe a pronounced broadening of both distributions; for the neutral beam, the halfwidth of the peak increases from about 0.25 to 1” by a factor of 4. As we have said, the quality of the target surface is crucial for the precision of the analysis, so that the data shown in Fig. 6 may serve only to demonstrate the expected effects. The dashed vertical line in the figure indicates the peak position for the Ar ‘+-data obtained with the clean target (see Fig. 3). Despite the broadening of the angular distributions, it is evident that the distribution for the Ar5+-ions is shifted towards smaller angles of scattering for the cesiated target. This is equivalent to a reduced energy gain on the incident trajectory as predicted by the “over-barrier model”. For a quantitative analysis, we locate the maximum of the angular distribution for the Ar’+-projectiles at @:,“= (1.62 f O.lS>o and find that the image interaction is reduced from 11.5 eV for the clean target to AE:+= (5.7 _t 2.5) eV for W = 2.44 eV. From the linear dependence with respect to W in Eq. 3, we expect from the model AE:+= (6.6 _t 0.8) eV for the cesiated target from the ratio of the measured workfunctions.
0
1
scattering
2 angle
3 (“)
Fig. 6. Angular distributions of 25 keV Are (open circles) and Arsc projectiles (full circles) for scattering from a cesiated Al(lll)-surface with workfunction W= 2.44 eV. The target settings are the same as for the data shown in Fig. 3. The dashed vertical line indicates the position of the peak for the Ar’+-data observed for the clean target. The intense sharp peak on the left side is due to a fraction of the non-scattered Ar’+-projectile beam and is plotted with a suppression by a factor of 70.
2.0
1.5 e I
1.0
I
:
0.5
i U
1
2
3
scattering
4
5
6
angle
(“)
7
8
Fig. 7. Dependence of the halfwidth of the angular distribution for 2.5 keV Xe”-projectiles on the angle of scattering from an AK11 l&surface.
In view of the relatively large uncertainties of the data, we find quantitative agreement with the predictions based on the “over-barrier model”. However, further efforts in providing alkaline-covered surfaces of higher quality are needed to perform more stringent tests of the model in this respect. We note that our previous experiments with a clean Fe(llO)-surface (work function W = 5.00 eV) [lo] result in the expected larger image interaction energies than reported here for ANlll). A comparison of the widths of the angular distributions reveals a gradual increase with the charge state of the projectile (see Fig. 3 and Figs. in ref. [6] and ref. [lo]). Based on the “over-barrier model”, charge exchange for an ion of charge q is initiated at R, = (2q)‘/‘/W, where the ion has gained the normal energy AE, = Wq3/‘/(4 X 2l”). This is already 75% of the energy deduced from the “staircase-model” in Eq. (31, so that charge exchange can only cause a small additional effect on the spread of normal energies. We test experimentally for this aspect by a study of the angular broadening of the scattered beam as a function of the scattering angle. The experiments are performed with neutral projectiles (25 keV Xe-atoms) in order to exclude image charge effects on the incident part of the trajectory. The data in Fig. 7 show a pronounced increase of the angular halfwidth with angle, where the range of scattering angles from 1.2 to 6.5” corresponds to energies for the normal motion from 2.7 to 80 eV (E, = E, sin’(&/2)). From these data we conclude that the angular broadening, observed for multicharged ions, is basically due to the enhanced normal energies. For larger normal energies, the distances of closest approach for the projectiles to the surface are reduced, and the enhanced corrugation of the collective inter-
H. Winter, C. Auth /Nucl.
Instr. and Meth. in Phys. Res. B 90 (1994) 216-221
atomic potentials leads to an increase of the angular spread of the scattered beam. Comparison of the data in Fig. 7 with data for multicharged ions show that the broadening effects are indeed dominated by the scattering process at the surface plane. Finally we mention that we have also studied the effects of the energy gain on the angle of incidence and the projectile energy. We find for multicharged Xe-ions having angles of incidence from 0.3 to 1.5” and for projectile energies from 20 to 140 keV poor dependences on both parameters. This result is important for the comparison of our data with features predicted by the “over-barrier model”, since in this model the distances for the onset of resonant neutralization do not depend on the dynamics of the scattering process. Details of this aspect will be published elsewhere.
5. Conclusion We have presented studies on the angular distributions after the scattering of multicharged noble gas ions from a metal surface under grazing angles of incidence. The analysis of angular shifts of these distributions allows us to deduce a simple method to determine the energy gained by a charged projectile due to image charge interaction during the approach towards the surface plane. This energy is related closely to the neutralization sequence of multicharged ions in front of the surface plane, so that the image interaction energies yield important information on the charge exchange. Our data provide a basis for rather detailed tests of features predicted from the “over-barrier model” by Burgdijrfer et al [5,7,9]. This model reproduces our experimental results reasonably well. The image interaction energies are of relevance for the scattering experiment itself, since this energy gain has to be taken into account for a correct specification of the collision energy of the projectile with the surface. In this respect, the image charge interaction sets a lower bound for the effective interaction energy with the surface. In recent experiments with Xe-ions, having charges up to q = 33, we observed energies of 1.50 eV [14], which clearly shows that this aspect has to be
taken into account in the scattering charged ions from surfaces.
221
of slow highly
Acknowledgements
We thank H.W. Ortjohann and G. Dierkes for assistance in the preparation and running of the experiments and W. Hassenmeier for his help in processing the manuscript. This work is supported by the Sonderforschungsbereich 216 (Bielefeld/Miinster). References
PI F.W. Meyer, S.H. Overbury, C.C. Havener, P.A. Zeijlmans van Emmichoven and D.M. Zehner, Phys. Rev. Lett. 67 (1991) 723; F.W. Meyer, S.H. Overbury, C.C. Havener, P.A. Zeijlmans van Emmichoven, J. Burgdiirfer and D.M. Zehner, Phys. Rev. A 44 (1991) 7214. 121J. Das, L. Folkerts and R. Morgenstern, Phys. Rev. A 45 (19921 4669; J. Das and R. Morgenstern, Phys. Rev. A 47 (1993) R755. 131H. Kurz, K. Toglhofer, H. Winter, F. Aumayr and R. Mann, Phys. Rev. Lett. 69 (1991) 1140. 141J. Bardsley and B. Penetrante, Comments Atom. Mol. Phys. 27 (1991) 43. 151J. Burgdorfer, P. Lerner and F. Meyer, Phys. Rev. A 44 (1991) 5674. 161H. Winter, Europhys. Lett. 18 (1992) 207. 171J. Burgdorfer, in: Progress in Atomic and Molecular Physics, ed. CD. Lin (World Scientific, Singapore, 1993). B1 H. Winter and R. Zimny, in: Coherence in Atomic Collision Physics, eds. H.J. Beyer et al. (Plenum, New York, 1988) p. 283. 191J. Burgdijrfer and F. Meyer, Phys. Rev. A 47 (1993) R20. [lOI H. Winter, Proc. 6th Int. Conf. on the Physics of HighlyCharged Ions, Manhattan, Kansas, 1992, eds. P. Richard and M. Stockli, (American Inst. of Physics, New York, 1993) p. 583. 1111D.S. Gemmell, Rev. Mod. Phys. 46 (19741 129. [12] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, vol. 1 (Pergamon, New York, 1985). [13] U. Scheithauer, G. Meyer and M. Henzler, Surf. Sci. 178 (1986) 441. [14] H. Winter, C. Auth, R. Schuch and E. Beebe, Phys. Rev. Lett. 71 (1993) 1939.
III. SURFACE PHENOMENA
Nuclear Instruments and Methods in Physics Research B 90 (1994) 216-221 North-Holland
Image charge interaction with an Al( 111)-surface
Beam Interactions with Materials&Atoms
of multicharged
ions in grazing collisions
H. Winter and C. Auth * Institut j?ir Kernphysik der Universitiit Miinster, Wilhelm-Klemm-Strasse 9, D-48149 Miinster, Germany
We study the angular distributions of multicharged noble gas ions after under a grazing angle of incidence. The data show pronounced effects of which allow us to deduce the energy gained by the projectile during the discussion of our experimental method and compare the results with an multicharged ions in front of a metal surface.
1. Introduction
The interaction of multiply/ highly charged ions with metal surfaces is a subject of active research in recent years. The field has profited from developments in ion-source technology to provide slow ions in high charge states, i.e. atomic particles having high potential energies. The liberation of this energy in collisions with a solid is connected with a complex multi-electron process, which is an interesting scientific problem and also has the potential for future technological applications. In a number of recent studies - performed primarily by means of electron spectroscopy - essential features of the neutralization mechanisms of multicharged ions above and below metal surfaces have been revealed [l-4]. In the context of this paper, the interactions of these ions in front of the surface, the “above surface are of particular interest. Essential neutralization”, features of this interaction seem to be well described by a classical “over-barrier model” by Burgdijrfer et al. [5], where the neutralization of the ions is initiated at rather large distances from the surface by resonant transitions of conduction band electrons into (multiply excited) Rydberg levels of the projectiles. Via this mechanism, the projectile is assumed to be neutralized completely in front of the topmost layer of surface atoms. However, since the inner shell vacancies of the projectiles will have survived to a major extent, most of the high potential energy stored in the initial projectile ions is still available in close proximity to the surface.
* Corresponding 034962.
author, tel. +49 251 834969, fax +49 251
scattering from a clean and a cesiated Al(lll)-surface the image charge attraction on the incident trajectory approach towards the surface. We present a detailed “over-barrier model” describing the neutralization of
Both at and below the surface plane, close encounters with target atoms and conduction electrons of high densities will rapidly fill the inner shell vacancies to complete the neutralization sequence. Recently we have introduced a method that allows us to deduce image interaction energies in ion-surface scattering experiments [6]. Since these energies are closely related to the effective charge state of the projectile on the incident trajectory, we expect to deduce from corresponding data important information on the neutralization dynamics of multicharged ions in front of the surface of metals.
2 . “Over-barrier model” The classical “over-barrier model” by Burgdiirfer et al. [S] can be considered as a first consequent approach to describe gross features of the extremely complex charge exchange mechanisms between solids and multicharged atoms in a scattering experiment. Arguments for a number of approximations made in this model in particular a justification for the description of the electron tunneling by an “over-barrier” concept - has been given in ref [5] and in a recent review by BurgdGrfer [7]. Since the image charge interaction energies reflect the neutralization sequence in front of the surface, we will apply the model to this part of the trajectory. Furthermore we will use the model in its most simple version: a stepwise neutralization of the incident ions. For an illustration of the description of electron transfer in this model, in Figs. la and lb we plot the effective potentials of the active electron for an ion with core charge Z = 6 at distances of R = 60 a.u.
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E0658-4
(atomic unit of length 0.0529 nm) and Iz = 15 au., respectively, in front of a conducting surface. The potential for the electron results from the Coulomb interaction with the ion core, the image charge interaction due to the presence of the ion core, and the self-image interaction potential (surface potential). At large distances, I? (Fig. la), the atomic and surface potent~aIs are well separated by a large potential barrier, which prevents any transitions of electrons between atom and surface. For smaller distances (Fig. lb), the potential barrier between atom and surface is reduced, and the effective electronic potential is characterized by a saddle point structure, which provides in a classical picture a pathway of resonant transitions for electrons with binding energies smaller than the saddle point poient~al energy. For a description of the image charge potentials by the static and asymptotic limit q/4y (q = charge of particle, y = distance from the surface/image plane), which can be justified for the conditions of our experiments, one finds for the saddle point energy, V,, the simple relation [5,7,8]
sequence, where the effective charge state of the projectile is reduced sequentially by capture of electrans at distances R,(Z) (“staircase model” 191).As a consequence, an ion with initial charge state 4 will gain energy via image charge attraction until the neutralization sequence ceases the acceleration. The gain in energy for the normal motion of the projectile is
The ~~~~oximat~ons made in the derivation of Eq. (3) appear as extremely crude in view of the complex charge exchange mechanisms. However, since the image charge attraction is given by the effective charge state of the projectile, we expect that the effects of ~re)ionizing and consecutive capture events will cancel to a large extent and that the model will reproduce the basic features of the data. Based on Eq. (3), we deduce for PP. 1) a 4 3Y/2-dependence, 2) proportionality with respect to the workfunction u”, 3) no dependence on the type of ion forming the ion core, We will show beIow that these predictions by the simple “staircase-over-barrier modedd” are in fair agreement with our experimental findings.
Then resonant capture of electrons from the surface can proceed for distances R, where the potentiai barrier is reduced to energies of occupied metal states, i.e. V, 2 W (W= work function of metal). Then the transfer of conduction electrons to an ion with core charge Z is possible for distances fl- “X1/2
3. Concept of the experimental method
Since the work function of metals is typically 4 to 5 eV, these “over-barrier” transitions result for muiticharged projectiles in the population of Rydberg levels, whereas the inner shell vacancies cannot be filled at this early stage of neutralizatjon by direct transitions. In simplifying matters, we consider an electron capture
We have recently proposed methods to deduce image jnteraction energies on the in~rn~ng and nutgoing trajectories in ion-surface collisions [6,10], We make use of the specific property of the grazing incidence collision geometry, that the motion of a projectile with energy E0 (velocity us) normal to the surface plane (y-axis) proceeds with energy EY = E, sin’%+,, and the parallel motion with E,, = E, COS~@~~=j E,; @, =
Fig. 1. Electronic potential for an ion of core charge q = 6 at a distance R = 60 a.u. (a) and R = 15 a.u. (b), in front of a metal surface.
218
H. Winter, C. Auth / Nucl. Instr. and Meth. in Phys. Res. B 90 (1994) 216-221
arctan(u,/u,) = u,/u ,,= (Ey/Eo)1/2 is the grazing angle of incidence. The scattering from the surface can be described on the basis of a collective planar potential (“surface channeling”) [ll], which is generally deduced from screened Coulomb interatomic potentials 1121and depends only on the distance from the surface, i.e. on the normal coordinate y. Then the motions normal and parallel with respect to the surface plane are independent, and the projectiles are reflected specularly at a potential plane positioned at ymin = 1 to 2 au. in front of the topmost layer of surface atoms. An ionized projectile with charge q gains (on the incident trajectory via its image charge interaction) the energy AEy4, before this interaction is ceased by the complete screening of the ion core by captured electrons. Then the angle of incidence, Gin, and the normal energy E, defined by the macroscopic collision geometry is modified according to @$ = ( E$q/E,,)‘/z with Ep = E, -k AE,4. Assumption of specular reflection at the surface plane and small energy losses of the projectile gives Q&t = @$,. For the completely neutralized projectile, no image interaction is effective on the outgoing trajectory so that E$“‘
(4)
Then the analysis of the angular distributions for neutral (Gs = 2Qi,) and ionized projectiles allows us to obtain A Ey4.
4. Experiments and results In grazing collisions with surfaces, the projectiles interact with a relatively large lateral extension of the target surface. Therefore, the surface plane has to be maintained at a high level of cleanness and flatness, in order to minimize a broadening of the scattered beam due to binary collisions with defect structures and adsorbed atoms (“dechanneling”). Our experiments are performed under UHV-conditions at a base pressure mbar, and the (Ill&face of an Al of some lo-” monocrystalline sample is prepared by frequent cycles of grazing sputtering (Gin = 2”) with 25 keV Ar+-ions and annealing at temperatures of about 500°C for a couple of minutes. Cleanness of the target surface is monitored via Auger-spectroscopy. From studies with a SPALEED-system [13] we find widths of terraces, formed by atoms at the topmost surface layers, larger than 50 nm with a clear dominance of monoatomic step heights. Such a high quality metal surface is crucial for the studies reported here, since the broadening of the scattered beams due to surface imperfections
Fig. 2. Sketch of the experimental procedure.
reduces the precision in the analysis of data. We will demonstrate this feature by data obtained for the controlled adsorption of Cs-atoms with the help of a Cs-dispenser (SAES-Getters). A sketch of the experimental procedure for recording angular distributions of projectiles scattered from the surface is displayed in Fig. 2. Noble gas ions with energies in the keV-domain are extracted from a Penning ion-source, collimated by sets of vertical and horizontal slits in the differential pumping stages to a sub-mrad divergence, and directed under a grazing angle of incidence @, = OS”-3” onto the Al(lll)surface. Effects of axial surface channeling are avoided by sending the projectiles along high index axial directions in the surface plane (“random orientation”). A careful grounding of all relevant components and a compensation/shielding of magnetic fields to some mG is expected to reduce effects of residual fields on the trajectories of the multicharged ions to a negligible level. The scattered projectiles leave the surface predominantly neutralized (fractions of singly charged ions are typically < 1%) and are detected by a channeltron detector covered with a 0.5 mm diaphragm. This detector is positioned on a precision manipulator, 700 mm behind the target and is translated in the scattering plane via a step-motor drive. In Fig. 3 we show as a representative example of our studies of angular distributions, obtained with 25 keV Are and Ar’+ projectiles, respectively, for the same collimation of the beam and settings of the target. We observe a clear angular shift towards larger angles of scattering for the incident ion beam, relative to the neutral beam. This is the effect expected from the presence of an attractive force on the incident part of the trajectory. The intense peak on the left side of the data is due to a residual fraction of the direct beam that has passed above the target (see Fig. 2) and thus serves as a reference for the direction of the projectile beam. For the analysis of the data in Fig. 3, we fit a Gaussian lineshape to the angular distributions and obtain for the distribution of the Are-projectiles (no image interaction assumed) a scattering angle @s= (1.19 + 0.04)“.
H. Winter, C. Auth /Nucl. Instr. and Meth. in Phys. Res. B 90 (1994) 216-221
219
l 0 n
3 scattering
angle
(“)
3 charge
Fig. 3. Angular distributions for the scattering of 25 keV Are (full circles) and Ar5+ projectiles (open circles) from a clean Al(lll)-surface. The peak on the left side is due to a fraction
4
/
5
J
6
7
state
Fig. 5. Energy gain of Arqf (open circles), Kr4+ (full squares), and Xeqf ions (full circles) in the scattering from an AlUll)-surface as a function of the initial projectile charge q.
of the direct beam that has passed on top of the target (see Fig. 2).
We check for the specular reflection of the neutral projectile beam using a He-Ne laser beam collimated in the same manner as the particle beam. The direct and the reflected laser beams are analyzed also with our detector by coupling a portion of the stray laser light on the entrance of the channeltron via an optical fiber to a photomultiplier, installed outside the vacuum chamber. The result of a detailed study on the comparison of neutral atom and laser scattering from the target surface is displayed in Fig. 4, where we show for a variation of the tilt of the target the resulting scatter-
loser
scattering
angle
(“)
Fig. 4. Relation of the angles of scattering determined from the reflection of a laser beam and a 25 keV Xe”-beam for the same settings of the target. The solid line indicates the specular reflection condition based on the reflection of the laser beam.
ing angles as measured with the laser and a 25 keV Xe”-beam. The solid line in the figure represents the condition of specular reflection based on the data obtained for the laser beam. The scattering angles deduced from the Xe”-data indicate that the conditions of specular reflection are met in the collision with sufficient accuracy to allow for an analysis of data as outlined above. From the reflection of the Are-atoms and also of the laser beam, we obtain ai,, = Goout= @J2 = 0.60”, so that neutral projectiles approach and leave the surface with the normal energy E, = (2.7 f 0.2) eV. From the distribution for the Ar’+-projectiles we get from @J:,“’ = (1.97 + 0.04)“: @fn+= @,“‘- ai,, = (1.35 + 0.06)0 and Ei”,‘+= (14.2 f 1.3) eV. Thus Ar’+-ions gain an energy AyE: + = (11.5 ? 1.3) eV via image charge interaction on the incident trajectory. In the same manner we perform experiments with 25 keV Arq+, KrQ+, and Xeq+ ions in charge states up to q = 6 and obtain the energies AE,4 as displayed in Fig. 5. The data show a monotonic increase of energy with the projectile charge and are reproduced fairly well by a q312-dependence predicted by the “staircase-over-barrier model”. The solid line in Fig. 5 represents A E;, calculated from Eq. (3) with the work function W= 4.27 eV of the Al(lll)surface as measured in-situ via photoemission with an accuracy of 0.03 eV. We find within the uncertainties of our data the same results for all three noble gas ions. This is expected for the population of Rydberglevels as assumed by the “over-barrier model”. The dependence of the image interaction energies on the work function of the target is probed by the adsorption of about one monolayer of Cs-atoms on the Al(lll)-surface by means of resistive heating of a dispenser. The presence of alkali atoms at the surface III. SURFACE PHENOMENA
H. Winter, C. Auth /Nucl. Instr. and Meth. in Phys. Res. B 90 (1994) 216-221
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leads to a reduction of the “macroscopic” work function of the target to W = 2.44 eV, as determined by photoemission. The adsorption of the &layer is, however, accompanied by a reduction of the flatness of the surface. This can be seen directly from the angular distributions shown in Fig. 6, obtained for the cesiated target at otherwise unchanged conditions as for the measurements displayed in Fig. 3. We observe a pronounced broadening of both distributions; for the neutral beam, the halfwidth of the peak increases from about 0.25 to 1” by a factor of 4. As we have said, the quality of the target surface is crucial for the precision of the analysis, so that the data shown in Fig. 6 may serve only to demonstrate the expected effects. The dashed vertical line in the figure indicates the peak position for the Ar ‘+-data obtained with the clean target (see Fig. 3). Despite the broadening of the angular distributions, it is evident that the distribution for the Ar5+-ions is shifted towards smaller angles of scattering for the cesiated target. This is equivalent to a reduced energy gain on the incident trajectory as predicted by the “over-barrier model”. For a quantitative analysis, we locate the maximum of the angular distribution for the Ar’+-projectiles at @:,“= (1.62 f O.lS>o and find that the image interaction is reduced from 11.5 eV for the clean target to AE:+= (5.7 _t 2.5) eV for W = 2.44 eV. From the linear dependence with respect to W in Eq. 3, we expect from the model AE:+= (6.6 _t 0.8) eV for the cesiated target from the ratio of the measured workfunctions.
0
1
scattering
2 angle
3 (“)
Fig. 6. Angular distributions of 25 keV Are (open circles) and Arsc projectiles (full circles) for scattering from a cesiated Al(lll)-surface with workfunction W= 2.44 eV. The target settings are the same as for the data shown in Fig. 3. The dashed vertical line indicates the position of the peak for the Ar’+-data observed for the clean target. The intense sharp peak on the left side is due to a fraction of the non-scattered Ar’+-projectile beam and is plotted with a suppression by a factor of 70.
2.0
1.5 e I
1.0
I
:
0.5
i U
1
2
3
scattering
4
5
6
angle
(“)
7
8
Fig. 7. Dependence of the halfwidth of the angular distribution for 2.5 keV Xe”-projectiles on the angle of scattering from an AK11 l&surface.
In view of the relatively large uncertainties of the data, we find quantitative agreement with the predictions based on the “over-barrier model”. However, further efforts in providing alkaline-covered surfaces of higher quality are needed to perform more stringent tests of the model in this respect. We note that our previous experiments with a clean Fe(llO)-surface (work function W = 5.00 eV) [lo] result in the expected larger image interaction energies than reported here for ANlll). A comparison of the widths of the angular distributions reveals a gradual increase with the charge state of the projectile (see Fig. 3 and Figs. in ref. [6] and ref. [lo]). Based on the “over-barrier model”, charge exchange for an ion of charge q is initiated at R, = (2q)‘/‘/W, where the ion has gained the normal energy AE, = Wq3/‘/(4 X 2l”). This is already 75% of the energy deduced from the “staircase-model” in Eq. (31, so that charge exchange can only cause a small additional effect on the spread of normal energies. We test experimentally for this aspect by a study of the angular broadening of the scattered beam as a function of the scattering angle. The experiments are performed with neutral projectiles (25 keV Xe-atoms) in order to exclude image charge effects on the incident part of the trajectory. The data in Fig. 7 show a pronounced increase of the angular halfwidth with angle, where the range of scattering angles from 1.2 to 6.5” corresponds to energies for the normal motion from 2.7 to 80 eV (E, = E, sin’(&/2)). From these data we conclude that the angular broadening, observed for multicharged ions, is basically due to the enhanced normal energies. For larger normal energies, the distances of closest approach for the projectiles to the surface are reduced, and the enhanced corrugation of the collective inter-
H. Winter, C. Auth /Nucl.
Instr. and Meth. in Phys. Res. B 90 (1994) 216-221
atomic potentials leads to an increase of the angular spread of the scattered beam. Comparison of the data in Fig. 7 with data for multicharged ions show that the broadening effects are indeed dominated by the scattering process at the surface plane. Finally we mention that we have also studied the effects of the energy gain on the angle of incidence and the projectile energy. We find for multicharged Xe-ions having angles of incidence from 0.3 to 1.5” and for projectile energies from 20 to 140 keV poor dependences on both parameters. This result is important for the comparison of our data with features predicted by the “over-barrier model”, since in this model the distances for the onset of resonant neutralization do not depend on the dynamics of the scattering process. Details of this aspect will be published elsewhere.
5. Conclusion We have presented studies on the angular distributions after the scattering of multicharged noble gas ions from a metal surface under grazing angles of incidence. The analysis of angular shifts of these distributions allows us to deduce a simple method to determine the energy gained by a charged projectile due to image charge interaction during the approach towards the surface plane. This energy is related closely to the neutralization sequence of multicharged ions in front of the surface plane, so that the image interaction energies yield important information on the charge exchange. Our data provide a basis for rather detailed tests of features predicted from the “over-barrier model” by Burgdijrfer et al [5,7,9]. This model reproduces our experimental results reasonably well. The image interaction energies are of relevance for the scattering experiment itself, since this energy gain has to be taken into account for a correct specification of the collision energy of the projectile with the surface. In this respect, the image charge interaction sets a lower bound for the effective interaction energy with the surface. In recent experiments with Xe-ions, having charges up to q = 33, we observed energies of 1.50 eV [14], which clearly shows that this aspect has to be
taken into account in the scattering charged ions from surfaces.
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of slow highly
Acknowledgements
We thank H.W. Ortjohann and G. Dierkes for assistance in the preparation and running of the experiments and W. Hassenmeier for his help in processing the manuscript. This work is supported by the Sonderforschungsbereich 216 (Bielefeld/Miinster). References
PI F.W. Meyer, S.H. Overbury, C.C. Havener, P.A. Zeijlmans van Emmichoven and D.M. Zehner, Phys. Rev. Lett. 67 (1991) 723; F.W. Meyer, S.H. Overbury, C.C. Havener, P.A. Zeijlmans van Emmichoven, J. Burgdiirfer and D.M. Zehner, Phys. Rev. A 44 (1991) 7214. 121J. Das, L. Folkerts and R. Morgenstern, Phys. Rev. A 45 (19921 4669; J. Das and R. Morgenstern, Phys. Rev. A 47 (1993) R755. 131H. Kurz, K. Toglhofer, H. Winter, F. Aumayr and R. Mann, Phys. Rev. Lett. 69 (1991) 1140. 141J. Bardsley and B. Penetrante, Comments Atom. Mol. Phys. 27 (1991) 43. 151J. Burgdorfer, P. Lerner and F. Meyer, Phys. Rev. A 44 (1991) 5674. 161H. Winter, Europhys. Lett. 18 (1992) 207. 171J. Burgdorfer, in: Progress in Atomic and Molecular Physics, ed. CD. Lin (World Scientific, Singapore, 1993). B1 H. Winter and R. Zimny, in: Coherence in Atomic Collision Physics, eds. H.J. Beyer et al. (Plenum, New York, 1988) p. 283. 191J. Burgdijrfer and F. Meyer, Phys. Rev. A 47 (1993) R20. [lOI H. Winter, Proc. 6th Int. Conf. on the Physics of HighlyCharged Ions, Manhattan, Kansas, 1992, eds. P. Richard and M. Stockli, (American Inst. of Physics, New York, 1993) p. 583. 1111D.S. Gemmell, Rev. Mod. Phys. 46 (19741 129. [12] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, vol. 1 (Pergamon, New York, 1985). [13] U. Scheithauer, G. Meyer and M. Henzler, Surf. Sci. 178 (1986) 441. [14] H. Winter, C. Auth, R. Schuch and E. Beebe, Phys. Rev. Lett. 71 (1993) 1939.
III. SURFACE PHENOMENA