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Convoy electrons associated with neutral ejectiles
NOMlB
Nuclear Instruments and Methods in Physics Research B79 (1993) 30-32 North-Holland
Beam Interactions with Materials&Atoms
Convoy electrons associated with neutral ejectiles G.M. Sigaud a, K. Kroneberger b, P. Focke ’ and K-0. Groeneveld nPontifcia Universidade Cat6lica do Rio de Janeiro, Cx. Postal 38071, 22543 RJ, Brazil
b
b Institut jiir Kernphysik, J. W Goethe-Universitiit Frankfurt, D-6000 Frankfurt am Main, Germany ’ Centro Atbmico Bariloche, 8400 S. C. de Bariloche, Argentina
Corwoy electrons are important in the understanding of the interaction processes of swift, heavy ions in matter, providing information about the influence of the projectile on the production and transport of electrons through solids. One crucial point of discussion is whether these convoy electrons are created in the bulk or only upon exit of the projectile from the solid. The observation of convoy electrons associated with neutral ejectiles can provide a stringent test to distinguish between bulk and surface models. Forward electron energy spectra in coincidence with the emergent charge states of H+ and He+ projectiles (100-400 keV/u) impinging upon thin carbon foils have been measured. Convoy electrons were observed associated with neutral as well as charged ejectiles. These results favour bulk models, although the observed yields for neutrals are up to a factor 18 smaller than for charged particles, which indicates that the formation of convoy electrons might occur in the last layers of the solid.
1. Introduction One of the main reasons to study electron production and emission in the interaction of fast heavy ions with solids is the possibility of understanding the transport mechanisms of electrons through the solid and the interactions between electrons and the projectile during this transport. The observation of the velocity spectra of electrons emitted in the forward direction show a sharp cusp-shaped peak when the electron velocity, u,_, matches that of the projectile, up. These so-called “convoy electrons” (CE), which are electrons transferred to low-lying continuum states of the projectile [1,2], can be a special tool for the understanding of the above-mentioned transport mechanisms. CE can be used as a measure of the distance A, over which the projectile and the electron, travelling together through the solid, keep their correlation, providing information on the origin of the CE, i.e. whether they are created in the bulk or upon exit of the projectile from the solid. Another point of interest is to study the influence on this CE-projectile correlation of the projectile charge at the moment of the CE creation and of the subsequent charge-changing processes that may occur eventually. Our aim in this paper is to discuss some of these points, mainly those concerning the place where the CE are created, the attenuation length A, and the dependence of their yield with the charge states of the ejectiles (i.e., projectiles emerging from the solid), that is, whether CE only accompany charged ejectiles, with 0168-583X/93/$06.00
an unscreened Coulomb potential, or also neutral ones. For this purpose, we will analyze some recent experimental results involving light collision systems, with a restricted number of exit channels, in order to get clearer answers.
2. Theoretical models In ion-atom collisions under single-collision conditions, the appearence of the cusp is explained in terms of two basic mechanisms: target electron capture to the continuum of the projectile (ECC) and projectile electron loss to low-lying continuum states (ELC). In the case of CE production from solid targets the answer is not so simple. First, the projectile charge is screened by the electron gas inside the solid and can furthermore change at each collision the projectile suffers. The second question is: are the CE transferred to the projectile continuum states inside the solid or only upon exit of the projectile from it? The models which have been proposed to explain such features can be divided in two branches, namely bulk and surface models. In the bulk models [3-61 the CE production is explained in terms of the so-called four-step picture [6]: a) the preparation of the electronic charge and excitation states of the projectile; b) the production of the CE, via either ECC or ELC, c) its transport through the solid; and d) its transmission through the exit surface. Step b) is strongly dependent on step a), i.e.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
G.M. Sigaud et al. / Convoyelectrons
the projectile state prior to CE formation: with bare projectiles the only possible mechanism is ECC, while, if the projectile has electrons, ELC is dominant, since loss cross sections are larger than the capture ones, for up > 1 a.u. In these models, it is important to compare the attenuation (or transport) length A, with the charge-changing length of the projectile inside the solid, A,,. If A,, > A,,, then the CE yield should be independent of the charge state of the ejectile. In the surface models, free electrons are produced via projectile and/or target ionization. For sufficiently high energy transfer, some of these electrons can follow the projectile as a cloud of entrained electrons [7] or can be captured into wake riding states [8]. Upon exit from the solid, one of these electrons can be captured into a projectile continuum state (free-electron transfer to continuum - FETC), due to the sudden change of the projectile potential from a screened to an unscreened Coulomb potential [8]. Surface models are more likely when the ejectiles are ions; thus, mesurements of CE associated to neutral ejectiles can provide a stringent test on these different interpretations.
3. Experimental
results and discussion
In figs. 1 and 2 we present forward-electron energy spectra in coincidence with the emergent charge states, qr, of incident H+ (150 keV) and He+ (100, 200 and !OO keV/u), respectively, impinging upon thin (loo-250 A) C foils. All spectra are normalized to the same number of ejectiles with a given charge state. The experiments were performed at the 2.5 MeV Van de Graaff accelerator at the Institut ftir Kemphysik of the University of Frankfurt am Main, Germany. The experimental setup is described in detail elsewhere [9]. It should be mentioned, however, that extreme care was taken to prevent the superposition of the different emergent charge states in the ejectiles detector, in order to guarantee the coincidence between the CE
ELECTRON Fig.
2. Same
as
31
25
50
ELECTRON
75
100
ENERGY
125 (eV)
Fig. 1. Electron energy spectra for 150 keV Hf incident upon a C foil, in coincidence with the emergent charge states qr = 0, 1 and normalized to the same number of ejectiles.
and a specific qf, without contamination from other ones. It can be seen from these figures that convoy electrons can indeed accompany neutral ejectiles. Fig. 2 also shows that the dependence of the CE yields with qr decreases with increasing incident velocity. In fact, the CE yields for He++, He+ and He0 ejectiles, respectively, have ratios 1:0.48:0.055 for 100 keV/u, 1:0.58:0.13 for 200 keV/u, and 1:0.72:0.20 for 400 keV/u projectiles. These two facts indicate that CE are produced inside the solid and their attenuation length increases with the incident velocity. One possible reason for the latter is the so-called Coulomb focussing effect [4], that is the decreasing screening of the projectile nuclear charge inside the solid with increasing velocity. A question which still persists concerns the mechanisms of CE production for neutral ejectiles, since both ELC and ECC are possible. If ELC occurs, the projectile may be neutralized via electron capture into a bound state, either still inside the bulk or upon exit from the solid [lo]. But there can also occur ECC with neutral ejectiles, as has been recently reported by Sarkadi et al. [ll] and Trabold et al. [12] for low-veloc-
ENERGY
(eV)
in fig. 1 for He+ projectiles, for emergent charge states qf = 0, 1, 2. (a) 100 keV/u; (b) 200 keV/u; and (c) 400 keV/u. I. ATOMIC/MOLECULAR
PHYSICS
32
GM Sigaud et al. / Convoy electrons
ity collision systems under single-collision conditions. Unfortunately, the data presented here do not allow one to distinguish between these two mechanisms. Possible experiments to achieve this could involve, for instance, target-thickness dependent measurements for fixed charge states, including thin targets which do not allow the projectiles to reach charge equilibrium within them.
Acknowledgements
This work had been funded by BMFT, Bonn, Germany, under contract no. 060FllO Ti. 439. One of the authors (G.M.S.) acknowledges the hospitality in IKF and partial support from CAPES, Brazil.
[l] G.B. Crooks and M.E. Rudd, Phys. Rev. Lett. 25 (1970) 1599, KG. Harrison and M.W. Lucas, Phys. Lett. A33 (1970) 142. [2] Proc. Symp. Forward Electron Ejection in Ion-Atom Collisions, eds K.O. Groeneveld, W. Meckbach and I.A.
Sellin, Lecture Notes in Physics, vol. 213 (Springer, Berlin,
.1984). 131 M.G. Menendez, M.M. Duncan, SD. Berry, LA. Sellin, W. Meckbach, P. Focke and I.B.E. Nemirovsky, Phys. Rev. A33 (1986) 2160. [4] J. BurgdBrfer, in: Lecture Notes in Physics, vol. 294 (Springer, Berlin, 1988) p. 344. [5] J. Kemmler, S. Lencinas, P. Koschar, 0. Heil, H. Rothard, K. Kroneberger, Gy. Szabo and K.O. Groeneveld, Nucl. Instr. and Meth. B33 (1988) 317. [6] P. Koschar, A. Clouvas, 0. Heil, M. Burkhard, J. Kemmler and K.O. Groeneveld, Nucl. Instr. and Meth. B24/25 (19871 153. 171Y. Yamazaki and N. Oda, Phys. Rev. Lett. 52 (1984) 29. [8] N.V. Neelavathi, R.H. Ritchie and W. Brandt, Phys. Rev. Lett. 33 (1974) 302. [9] K. Kroneberger, G.M. Sigaud, H. Rothard, 0. Heil, A. Albert, R. Maier, D. Schliisser, M. Schosnig, H. Trabold and K.O. Groeneveld, Nucl. Instr. and Meth. B67 (1992) 109. [lo] V.P. Zaikov, E.A. Kralkina, V.S. Nikolaev, Yu.A. Fainberg and N.F. Vorobiev, Nucl. Instr. and Meth. B17 (1986) 97. [ll] L. Sarkadi, J. Pal&&, A. Kdver, D. Berenyi and T. Vajnai, Phys. Rev. Lett. 62 (1989) 527. [12] H. Trabold, G.M. Sigaud, D.H. Jakubassa-Amundsen, M. Kuzel, 0. Heil and K.O. Groeneveld, Phys. Rev. A46 (1992) 1270.
Nuclear Instruments and Methods in Physics Research B79 (1993) 30-32 North-Holland
Beam Interactions with Materials&Atoms
Convoy electrons associated with neutral ejectiles G.M. Sigaud a, K. Kroneberger b, P. Focke ’ and K-0. Groeneveld nPontifcia Universidade Cat6lica do Rio de Janeiro, Cx. Postal 38071, 22543 RJ, Brazil
b
b Institut jiir Kernphysik, J. W Goethe-Universitiit Frankfurt, D-6000 Frankfurt am Main, Germany ’ Centro Atbmico Bariloche, 8400 S. C. de Bariloche, Argentina
Corwoy electrons are important in the understanding of the interaction processes of swift, heavy ions in matter, providing information about the influence of the projectile on the production and transport of electrons through solids. One crucial point of discussion is whether these convoy electrons are created in the bulk or only upon exit of the projectile from the solid. The observation of convoy electrons associated with neutral ejectiles can provide a stringent test to distinguish between bulk and surface models. Forward electron energy spectra in coincidence with the emergent charge states of H+ and He+ projectiles (100-400 keV/u) impinging upon thin carbon foils have been measured. Convoy electrons were observed associated with neutral as well as charged ejectiles. These results favour bulk models, although the observed yields for neutrals are up to a factor 18 smaller than for charged particles, which indicates that the formation of convoy electrons might occur in the last layers of the solid.
1. Introduction One of the main reasons to study electron production and emission in the interaction of fast heavy ions with solids is the possibility of understanding the transport mechanisms of electrons through the solid and the interactions between electrons and the projectile during this transport. The observation of the velocity spectra of electrons emitted in the forward direction show a sharp cusp-shaped peak when the electron velocity, u,_, matches that of the projectile, up. These so-called “convoy electrons” (CE), which are electrons transferred to low-lying continuum states of the projectile [1,2], can be a special tool for the understanding of the above-mentioned transport mechanisms. CE can be used as a measure of the distance A, over which the projectile and the electron, travelling together through the solid, keep their correlation, providing information on the origin of the CE, i.e. whether they are created in the bulk or upon exit of the projectile from the solid. Another point of interest is to study the influence on this CE-projectile correlation of the projectile charge at the moment of the CE creation and of the subsequent charge-changing processes that may occur eventually. Our aim in this paper is to discuss some of these points, mainly those concerning the place where the CE are created, the attenuation length A, and the dependence of their yield with the charge states of the ejectiles (i.e., projectiles emerging from the solid), that is, whether CE only accompany charged ejectiles, with 0168-583X/93/$06.00
an unscreened Coulomb potential, or also neutral ones. For this purpose, we will analyze some recent experimental results involving light collision systems, with a restricted number of exit channels, in order to get clearer answers.
2. Theoretical models In ion-atom collisions under single-collision conditions, the appearence of the cusp is explained in terms of two basic mechanisms: target electron capture to the continuum of the projectile (ECC) and projectile electron loss to low-lying continuum states (ELC). In the case of CE production from solid targets the answer is not so simple. First, the projectile charge is screened by the electron gas inside the solid and can furthermore change at each collision the projectile suffers. The second question is: are the CE transferred to the projectile continuum states inside the solid or only upon exit of the projectile from it? The models which have been proposed to explain such features can be divided in two branches, namely bulk and surface models. In the bulk models [3-61 the CE production is explained in terms of the so-called four-step picture [6]: a) the preparation of the electronic charge and excitation states of the projectile; b) the production of the CE, via either ECC or ELC, c) its transport through the solid; and d) its transmission through the exit surface. Step b) is strongly dependent on step a), i.e.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
G.M. Sigaud et al. / Convoyelectrons
the projectile state prior to CE formation: with bare projectiles the only possible mechanism is ECC, while, if the projectile has electrons, ELC is dominant, since loss cross sections are larger than the capture ones, for up > 1 a.u. In these models, it is important to compare the attenuation (or transport) length A, with the charge-changing length of the projectile inside the solid, A,,. If A,, > A,,, then the CE yield should be independent of the charge state of the ejectile. In the surface models, free electrons are produced via projectile and/or target ionization. For sufficiently high energy transfer, some of these electrons can follow the projectile as a cloud of entrained electrons [7] or can be captured into wake riding states [8]. Upon exit from the solid, one of these electrons can be captured into a projectile continuum state (free-electron transfer to continuum - FETC), due to the sudden change of the projectile potential from a screened to an unscreened Coulomb potential [8]. Surface models are more likely when the ejectiles are ions; thus, mesurements of CE associated to neutral ejectiles can provide a stringent test on these different interpretations.
3. Experimental
results and discussion
In figs. 1 and 2 we present forward-electron energy spectra in coincidence with the emergent charge states, qr, of incident H+ (150 keV) and He+ (100, 200 and !OO keV/u), respectively, impinging upon thin (loo-250 A) C foils. All spectra are normalized to the same number of ejectiles with a given charge state. The experiments were performed at the 2.5 MeV Van de Graaff accelerator at the Institut ftir Kemphysik of the University of Frankfurt am Main, Germany. The experimental setup is described in detail elsewhere [9]. It should be mentioned, however, that extreme care was taken to prevent the superposition of the different emergent charge states in the ejectiles detector, in order to guarantee the coincidence between the CE
ELECTRON Fig.
2. Same
as
31
25
50
ELECTRON
75
100
ENERGY
125 (eV)
Fig. 1. Electron energy spectra for 150 keV Hf incident upon a C foil, in coincidence with the emergent charge states qr = 0, 1 and normalized to the same number of ejectiles.
and a specific qf, without contamination from other ones. It can be seen from these figures that convoy electrons can indeed accompany neutral ejectiles. Fig. 2 also shows that the dependence of the CE yields with qr decreases with increasing incident velocity. In fact, the CE yields for He++, He+ and He0 ejectiles, respectively, have ratios 1:0.48:0.055 for 100 keV/u, 1:0.58:0.13 for 200 keV/u, and 1:0.72:0.20 for 400 keV/u projectiles. These two facts indicate that CE are produced inside the solid and their attenuation length increases with the incident velocity. One possible reason for the latter is the so-called Coulomb focussing effect [4], that is the decreasing screening of the projectile nuclear charge inside the solid with increasing velocity. A question which still persists concerns the mechanisms of CE production for neutral ejectiles, since both ELC and ECC are possible. If ELC occurs, the projectile may be neutralized via electron capture into a bound state, either still inside the bulk or upon exit from the solid [lo]. But there can also occur ECC with neutral ejectiles, as has been recently reported by Sarkadi et al. [ll] and Trabold et al. [12] for low-veloc-
ENERGY
(eV)
in fig. 1 for He+ projectiles, for emergent charge states qf = 0, 1, 2. (a) 100 keV/u; (b) 200 keV/u; and (c) 400 keV/u. I. ATOMIC/MOLECULAR
PHYSICS
32
GM Sigaud et al. / Convoy electrons
ity collision systems under single-collision conditions. Unfortunately, the data presented here do not allow one to distinguish between these two mechanisms. Possible experiments to achieve this could involve, for instance, target-thickness dependent measurements for fixed charge states, including thin targets which do not allow the projectiles to reach charge equilibrium within them.
Acknowledgements
This work had been funded by BMFT, Bonn, Germany, under contract no. 060FllO Ti. 439. One of the authors (G.M.S.) acknowledges the hospitality in IKF and partial support from CAPES, Brazil.
[l] G.B. Crooks and M.E. Rudd, Phys. Rev. Lett. 25 (1970) 1599, KG. Harrison and M.W. Lucas, Phys. Lett. A33 (1970) 142. [2] Proc. Symp. Forward Electron Ejection in Ion-Atom Collisions, eds K.O. Groeneveld, W. Meckbach and I.A.
Sellin, Lecture Notes in Physics, vol. 213 (Springer, Berlin,
.1984). 131 M.G. Menendez, M.M. Duncan, SD. Berry, LA. Sellin, W. Meckbach, P. Focke and I.B.E. Nemirovsky, Phys. Rev. A33 (1986) 2160. [4] J. BurgdBrfer, in: Lecture Notes in Physics, vol. 294 (Springer, Berlin, 1988) p. 344. [5] J. Kemmler, S. Lencinas, P. Koschar, 0. Heil, H. Rothard, K. Kroneberger, Gy. Szabo and K.O. Groeneveld, Nucl. Instr. and Meth. B33 (1988) 317. [6] P. Koschar, A. Clouvas, 0. Heil, M. Burkhard, J. Kemmler and K.O. Groeneveld, Nucl. Instr. and Meth. B24/25 (19871 153. 171Y. Yamazaki and N. Oda, Phys. Rev. Lett. 52 (1984) 29. [8] N.V. Neelavathi, R.H. Ritchie and W. Brandt, Phys. Rev. Lett. 33 (1974) 302. [9] K. Kroneberger, G.M. Sigaud, H. Rothard, 0. Heil, A. Albert, R. Maier, D. Schliisser, M. Schosnig, H. Trabold and K.O. Groeneveld, Nucl. Instr. and Meth. B67 (1992) 109. [lo] V.P. Zaikov, E.A. Kralkina, V.S. Nikolaev, Yu.A. Fainberg and N.F. Vorobiev, Nucl. Instr. and Meth. B17 (1986) 97. [ll] L. Sarkadi, J. Pal&&, A. Kdver, D. Berenyi and T. Vajnai, Phys. Rev. Lett. 62 (1989) 527. [12] H. Trabold, G.M. Sigaud, D.H. Jakubassa-Amundsen, M. Kuzel, 0. Heil and K.O. Groeneveld, Phys. Rev. A46 (1992) 1270.