- Email: [email protected]
Electron yields from solids: a probe for the stopping power of swift charged particles?
HdH B
Nuclear Instruments and Methods in Physics Research B69 (1992) 154-157 North-Holland
Seam Interactions with Materials&Atoms
Electron yields from solids : a probe for the stopping power of swift charged particles? Hermann Rothard
a.t,
Jfergen Schou ', Peter Koschar
a
and Karl-Ontjes Groeneveld
a
lnstinufür Kerephysik der Johann- Wolfgang -Goethe-Untretsttiii, August-Euler-Straße 6, D-6000 Frankfurt am Main 90, Germany Risa National Laboratory, Physics Departnnent, DK-4000 Roskilde, Denmark
Received 21 August 1991 and in revised form 20 November 1991
The kinetic emission of electrons from solid surfaces under swift charged particle bombardment is related to the electronic stopping power S,. of the projectiles. We briefly discuss the question whether the yield y of electrons induced by protons, electrons, and, in particular, heavy ions, molecular ions and clusters is proportional to the stopping power, y - S,. .
l . Protons and electrons The interaction of swift charged particles with projectile velocities r exceeding 1 keV/u with solids leads to the so-called "kinetic emission of electrons" [I-4]. We (restrict the following discussion to the case of swift charged particles, where the kinetic electron emission dominates, and potential emission can be neglected . Also, we will not consider effects correlated with the potential energy of slow multiply charged ions. The basic phenomenon of kinetic electron emission is related to the electronic energy loss of the particles. Consequently, most of the theoretical approaches consider the yield y of electrons ejected per incident projectile to be proportional to the electronic stopping power, y - S~ [5-6], or to the amount of energy deposited near the surface, D(x = 0) = ßS, [6]. Such a proportionality, y = .1*S, has been confirmed for the case of proton bombardment of (metallic) solids in a wide energy range (5 keV <_ E,, :5 24 McV) for a variety of target materials (within, say, ± 15%) [1-4] . The factor A* = A*(Z.,) depends on the target material only and therefore may be called "material parameter" [I-4] . In the case of electron induced secondary electron yields S, no clear proportionality to the stopping power can be stated [4,6]. A comparison of electron yield data obtained with aluminium targets with yields S calculated from the transport theory shows a rough proporNow at : Institut de Physique Nucléaire de Lyon . Groupe des Collisions Atomiques. Université Claude Bernard, 43 Bd. du Il Novembre 1918, F-69622 Villeurbanne, Cedex, France.
tionality of S and stopping power in the energy interval 1 < E,, _< 20 keV [6] . In the energy range below 1 keV, i .e . around the stopping power maximum (0.05 :5 Et, _< 1 keV), "it is difficult to indicate any correlation between the stopping power and the secondary electron yield" [6] . The same statement is valid for electron induced yields S from gold targets (0 .2 _< Et, < 2 .5 keV) [4]. A more detailed discussion can be found in refs . [4,6] and a recent review on electron-induced electron emission in ref. [7] . It is important to note that both from a theoretical as well as from an experimental point of view, H + impact represents the simplest case for the investigation of particle induced electron yields and can be used to test basic theoretical assumptions [1-6]. Because of the multitude of processes involved in electron cm .ssion it is extremely difficult to treat the phenomenon theoretically. Such processes as excitation and ionization of target electrons both from inner shells and the conduction band, collective excitation of and screening of the projectile charge by the nearly free target conduction electrons, elastic and inelastic scattering of the electrons and transmission through the surface have to be taken into account. The situation gets even more complicated if the projectiles carry bound electrons (atoms, heavy ions, molecules, clusters). In these cases, also excitation/ ionization of and screening by projectile electrons have to be considered . The simplest example of a projectile carrying an electron is H". At high projectile velocities r,, >> r,: (t',. being the Fermi velocity of the target electrons), the electron yield y(H") from H" impact on carbon is larger than the proton-induced yield y(H + ) . This can be explained by the additional secondary
0168-583X/92/$05 .0 CO 1992 - Elsevier Science Publishers B.V . All rights reserved
H. Rothard et al. / Electron yields from solids
electron yield induced by the projectile electron, y(e - ), i .e. y(H'~) = y(H + ) + y(e") [1-3,8-9]. At low projectile velocities r,, < r F , the yield y(H ° ) from gold was found to be smaller than y(H + ) due to the screening of the projectile charge which leads to a reduction of the effective charge of the projectile and to a reduced stopping power [3,10]. 2 . Electron yields from heavy ions In the case of swift heavy ions, the proportionality between electron yields and the stopping power only holds if the pre-equilibrium evolution of the charge state of the ions is taken into account. The novel concept of the pre-equilibrium near-surface stopping power S*(x) can be used to describe the electron yield dependence on the projectile atomic number Z p [2,1113]. If the charge equilibration depth A is larger than the escape depth A sp of low energy electrons, A, > A s[., the electron yields are proportional to the effective (near-surface) stopping power y - S* . This quantity is proportional to the square of the penetrationdepth dependent effective ion charge q*(x) in the charge pre-equilibrium, S*(x)- q*(x)` . Thus, S* deviates from tabulated "bulk" stopping power values [14], which implicitly incorporate a proportionality to the square, q,2,(X » A '.), of the effective charge of the ions in the charge equilibrium, S,, - qce .
104 1T =ATS" AT= (0.31±0.14)ÄIeV /~
,-10 3 0
w
10 2
r z CD 10 1 w W 100
ZT=6 0 Zp< 6 0 Zp> .6
10-1
qi --qf 100 101 10 2 103 10 4 ELECTRONIC STOPPING POWER Se [eV/íX]
total electron yield YT from ion penetration through carbon foils as a function of the stopping power S,, of light ions (Zp < 6) and heavy ions (Zp z 6) according to ref . I2] . Fig. 1 . Tha
155
This concept is strongly supported by the successful interpretation of the dependence of electron yields on the charge state q, of the incoming ions [10,13,15] and recent calculations of the charge-state dependence of the energy loss [16]. A thorough discussion can be found in ref. [13]. The important assumption of an overall proportionality between cl^ctron yields and the stopping power both for light ions (Z p < 6) and heavy ions (Zp >_ 6) is demonstrated impressively in fig . 1 . It shows the total electron yield YT from carbon foils as a function of the electronic stopping power Se . The yield includes c1cctron emission from the foil in forward as well as in backward direction. A rough proportionality y T = A ~S~ (within a factor of 2) in a wide range of projectile velocities vp (15 kcV/u _< EplMp < 46 McV/u) and projectile nuclear charge Zp (1 < Zt, _< 92) over four decades of electron yield and stopping power values can be stated . The mean value of the proportionality factor is A*T = (0 .31 ± 0.14) A/cV . It is important to note that the data were taken with incident charge states q, close to the mean charge state qt of the emerging ions. This ensures that S* = S, at both surfaces of the foil (compare refs . [2,13]). Recent results obtained at the heavy ion synchrotron of GSI (Darmstadt) indicate that electron yields scale with the stopping power even for relativistic heavy ions (Ar + up to 1 .65 GeV/u) [17] . Finally, it should be noted that strong forward effects concerning the dependence of electron yields on the charge state q, of the ions can only be expected if the thickness d of the foil is smaller than (or comparable to) the charge equilibration depth A., and the range of fast electrons from binary collisions or from projectile ionization (compare refs. [8,15]). 3 . Molecules and clusters At this point, it is interesting to note that a similar argument is valid for molecular ions. Due to Coulomb explosion and multiple scattering, the constituents of a molecule (or a cluster) will have lost their correlation after at most, say, a few 100 A mainly depending on the projectile velocity v p . Thus, strong molecular effects can only be expected to take place near the entrance surface of the target . They should be visible in backward direction (i.e . at the beam entrance side of the target) or with thin foils also in forward direction (see, e .g ., refs. [2,3,8,9,18]) . A detailed discussion of molecular effects in forward electron emission can be found in refs . [2,8,18]. Molecular effects can be observed in the ratio between a physical quantity (such as the stopping power S . or electron yields y) measured with a molecular projectile (or a cluster) and the sum of the values
156
H. Rothard et al. / Electron yields from solids
measured with its atomic constituents . Consequently, it is convenient [1,8,9,16] to define the ratios R(y)
-/(Molecule) Ey(atomicconstituents)
and R(S~)=
SJmolecule) ES,(atomic constituents)
as a measure of the "molecular effect" (R 0 1) . The velocity dependence of the energy loss ratio R(r p ) of H ; in carbon targets has been studied both experimentally and theoretically (see, e .g., refs . [1923]). In general, R is found to increase with r,, . Below a "critical velocity" r_ a reduction, RU t, < r,,) < 1, is observed, whereas at high velocities r,, > v_ an enhancement, R(r p > r~,) > 1, can be seen [19-23] . The critical velocity r ., is found to be on the order of the Fermi velocity r F of the solid [19,20] . Backward electron yields yu from H, molecular ion impact on a variety of target materials mirror the velocity dependence of the stopping power ratio R(S,): Reductions of the electron yield ratio (R < 1) at low velocities (below = 100 keV/u) and enhancements (R> 1) at higher velocities have been reported [13,8,9,18] . This may indicate that the proportionality between energy loss and electron yields is valid also for molecular projectiles. However, "the magnitude and the energy dependence of R is a function of the target material" [3], and the values of the critical velocities t " cr(ZT, Z p ) arc not accurately known. As in the case of H° impact, the explanation for these findings may be the additional secondary electron yield induced by the projectile electrons at high velocities and the screening of the projectile charge at lower velocities [3,10,18] . Similar experimental results have been reported for heavy molecular ions such as O,, N_ CO', Se, and Tc, [2,3,18] . Little is known about electron emission by cluster ion impact at high velocities (i .e . comparable to or higher as c °F) [3,18]. However, studies with small hydrogen clusters I I,, (n = 3, 5, 7, - - - , 15) indicate a similar behaviour as in the case of molecular ions, i.e. R < 1 below = 100 keV/w and R > 1 at velocities exceeding = 120 keV/u [24]. Recent theoretical investigations by Mikkelsen et al. on the energy loss of hydrogen clusters in carbon [20] show an increase of R(S.) with r p starting at R = 0.8 at = 20 keV/u and rising up to R = 1 .1-1 .5 at = 140 keV/u with a critical velocity around rc,(n)=50 keV/u . The magnitude of the "cluster effect" (i .e. R 0 1) depends on the number n of protons in the cluster and has been found to be most pronounced for n >_ 9 [20]. These calculations arc in good agreement with recent experimental results of Ray et al . [19,20]. Also, theoretical investigations by
Abril et al. [23] show an enhanced cluster energy loss at high velocities 2"F :5 r t, _< 101V' R(n) increases with the cluster size up to a value as large as R = 10 for n = 100 . It should be interesting to compare these results on the energy loss of clusters with clcctron yield data from hydrogen cluster impact [18]. 4. Conclusion
In this short survey, we briefly discussed the question whether the yield y of electrons induced by swift electrons, protons, heavy ions, molecular ions and clusters is proportional to the stopping power S. . This proportionality seems to be fulfilled for the particular case of proton impact and possibly in the case of heavy ions. In this latter case, the pre-equilibrium of the charge state evolution and of the stopping power has to be taken into account. With sufficiently thin targets or at sufficiently high ion velocities (which can be achieved with heavy ion accelerators such as GANIL or GSI-SIS [17]) we are generally dealing with pre-equilibrium conditions where stopping power values can differ significantly from equilibrium data. For the case of molecular ions, comparison of theoretical calculations and experimental data suggest a proportionality of electron yields and stopping power ; however, there is clearly a need for further theoretical and experimental investigations on the energy loss of molecular ions. Ion induced electron emission can be a vàjuable test for theories [1,5,6,20-23], which can also be gpplicd to such fields as scanning electron microscopy and all kinds of surface analysis with Auger-, photo- or secondary electrons [25]. New insights into the correlation between electron emission and energy loss can be obtained by simultaneous measurements of electron yields y and stopping power S,, [26] . A wide new field is the study of both forward and backward electron yields from cluster ion interaction with thin foils [18-20,24]. Such electron yield data could be compared to recent calculations and measurements of the energy loss of hydrogen clusters [20-23]. In particular, the target thickness dependence of forward electron yields allows a detailed comparison to theory [2,6] and may yield important information about the influence of "wake effects" [9,18] and electron diffusion on electron emission from solids . Also, results on electron emission in relation to the energy loss of clusters may help to understand such interesting phenomena as collective effects in ion desorption [27] and cluster impact fusion [28]. References [1)
G. 116hler, E.A . Niekisch and J. Treusch (eds .), Particle Induced Electron Emission 11, Springer Tracts in Mod-
H. Rothard et al. / Electron yields from solids ern Physics 123 (Springer, Heidelberg, 1991) with contributions by D. Hasselkamp, H . Rothard, K.O. Groeneveld, J . Kemmler, P. Varga and H . Winter. [2] H . Rothard, K.O. Groeneveld and J. Kemmler, ibid ., Kinetic electron emission from ion penetration of thin foils in relation to the pre-equilibrium of charge distributions, p . 97. [3] D. Hasselkamp, ibid ., Kinetic electron emission from solid surfaces under ion bombardment, p. 1 . [4] D . Hasselkamp, Habilitationsschrift, Justus-Liebig-Universitiit, Giessen, Germany (1985); see also : Comments Atom . Mol . Phys. 21 (1988) 241, and D. Hasselkamp, S. Hippler, A . Scharmann and T. Schmehl, Ann . Phys. (Leipzig) 47 (1990) 7. [5] J . Devooght, J .C. Dehaes, A. Dubus, M. Cailler, J .P. Ganachaud, M. R6sler and W . Brauer, in : Particle Induced Electron Emission 1, eds . G. Hbhler, E .A . Niekisch and J . Treusch, Springer Tracts in Modern Physics 122 (Springer, Heidelberg, 1991). [6] J . Schou, Scanning Microsc. 2 (1988) 607; see also : Phys. Rev . B22 (1980) 2141 . [7] R. Bindi, H . Lanteri and P . Rostaing, Scanning Microsc. 1 (1987) 1475. [8] K. Kroneberger, A. Clouvas, G . Schlüssler, P. Koschar, J . Kemmler, H. Rothard, C . Biedermann, O . Heil, M . Burkhard and K.O. Groeneveld, Nucl . Instr. and Meth . B29 (1988) 621 . [9] K . Kroneberger et al., J . Phys . (Paris) 50 (1989) C2-99 . [10] G. Lakits, A . Arnau and H. Winter, Phys. Rev. B42 (1990) 15 . [I1] H. Rothard, K . Kroneberger, A. Clouvas, E . Veje, P. Lorenzen, N . Keller, J . Kemmler, W. Meckbach and K.O. Groeneveld, Phys. Rev . A41 (1990) 2521 . [12] A . Clouvas, A . Katsanos, B. Farizon-Mazuy, M. Farizon and M.J. Gaillard, Phys . Rev . B43 (1991) 2498 . [13] H . Rothard, J . Schou and K.O . Groeneveld, Phys . Rev . A45 (1992) in press . [14] J.F. Ziegler, J .P . Biersack and U . Littmark, The Stopping and Range of Ions in Matter (Pergamon, New York, 1985) . [15] P. Koschar et al ., Phys . Rev . A40 (1989) 3632. [16] A. Arnau, M . Pefialba, P .M. Echenique, F. Flores and R.H. Ritchie, Phys . Rev. Lett . 65 (1990) 1024. [17] T . Brohm, H.-G . Clere, B . Voss, C. Ziegler, H. Geissel,
157
M. Kildir, K.H . Schmidt, K. Sümmerer and D .-J. Viera, Verhandl . DPG (VI) 26 (1991) 414; A . Albert, K . Kroneberger, O. Heil, H. Geissel and K.O. Groeneveld, ibid ., p . 707. [18] H . Rothard et al., Phys. Rev . B41 (1990) 3959 . [19] M . Ray, Thesis, Univ. Claude Bernard, Lyon, France (1991). [20] H .H . Mikkelsen, E. Ray, B. Farizon-Mazuy, M .J. Gaillard, J.C. Poizat and J . Remillieux, presented at workshop STOP 91, Middelfart, Denmark, 1991 ; E. Ray, R. Kirsch, H.H. Mikkelsen, J .C. Poizat and J . Remillieux, this issue (STOP 91), Nucl . Instr . and Meth . B69 (1992) 133 . [21] W. Brandt and R.H. Ritchie, Nucl . Instr . and Meth . 132 (1976) 43 . [22] N.R . Arista and A. Gras-Marti, in: Interaction of Charged Particles with Solids and Surfaces, NATO Advanced Study Institute, Alicante, Spain, 1990, eds. F. Flores, H .M . Urbassek, N. Arista and A. Gras-Marti (Plenum, New York, 1991) . [23] I . Abril, M. Vicanek, A . Gras-Marti and N .R . Arista, Proc . 14th . Int. Conf. o n Atomic Collisions in Solids, Salford, UK, 1991, Nucl . Instr . and Meth. B67 (1992) 56, M . Vicanek, presented at workshop STOP 91, Middelfart, Denmark, 1991 . [24] Y. Chanut, J . Martin, R. Salin and H .O. Moser, Surf . Sci . 106 (1981) 563. [25] M . Schosnig, H. Rothard, K. Kroneberger, D. Schlbsser and K.O. Groeneveld, Proc . 2nd Europ. Conf. on Accelerators in Applied Research and Technology, Frankfurt am Main, Germany, 1991, Nucl . Instr. and Meth . B68 (1992) 394 . [26] P. Bauer, O. Benka and E . Steinhauer, Annual Report 1990 of the Institut für Experimentalphysik, Abteilung für Atom- and Kernphysik, Johannes-Kepler-UnN., Linz, Austria, p. 11 . [27] J .P . Thomas, A . Oladipo and M. Fallavier, J . Phys . (Paris) 50 (1989) C2-195 . [28] R .J . Beuhler, G. Friedlander and L. Friedman, Phys . Rev. Lett. 63 (1989) 1292 ; M . Fallavier, J. Kemmler, R. Kirsch, J .C. Poizat, J . Remillieux and J .P. Thomas, Phys. Rev. Lett. 65 (1990) 621 .
Nuclear Instruments and Methods in Physics Research B69 (1992) 154-157 North-Holland
Seam Interactions with Materials&Atoms
Electron yields from solids : a probe for the stopping power of swift charged particles? Hermann Rothard
a.t,
Jfergen Schou ', Peter Koschar
a
and Karl-Ontjes Groeneveld
a
lnstinufür Kerephysik der Johann- Wolfgang -Goethe-Untretsttiii, August-Euler-Straße 6, D-6000 Frankfurt am Main 90, Germany Risa National Laboratory, Physics Departnnent, DK-4000 Roskilde, Denmark
Received 21 August 1991 and in revised form 20 November 1991
The kinetic emission of electrons from solid surfaces under swift charged particle bombardment is related to the electronic stopping power S,. of the projectiles. We briefly discuss the question whether the yield y of electrons induced by protons, electrons, and, in particular, heavy ions, molecular ions and clusters is proportional to the stopping power, y - S,. .
l . Protons and electrons The interaction of swift charged particles with projectile velocities r exceeding 1 keV/u with solids leads to the so-called "kinetic emission of electrons" [I-4]. We (restrict the following discussion to the case of swift charged particles, where the kinetic electron emission dominates, and potential emission can be neglected . Also, we will not consider effects correlated with the potential energy of slow multiply charged ions. The basic phenomenon of kinetic electron emission is related to the electronic energy loss of the particles. Consequently, most of the theoretical approaches consider the yield y of electrons ejected per incident projectile to be proportional to the electronic stopping power, y - S~ [5-6], or to the amount of energy deposited near the surface, D(x = 0) = ßS, [6]. Such a proportionality, y = .1*S, has been confirmed for the case of proton bombardment of (metallic) solids in a wide energy range (5 keV <_ E,, :5 24 McV) for a variety of target materials (within, say, ± 15%) [1-4] . The factor A* = A*(Z.,) depends on the target material only and therefore may be called "material parameter" [I-4] . In the case of electron induced secondary electron yields S, no clear proportionality to the stopping power can be stated [4,6]. A comparison of electron yield data obtained with aluminium targets with yields S calculated from the transport theory shows a rough proporNow at : Institut de Physique Nucléaire de Lyon . Groupe des Collisions Atomiques. Université Claude Bernard, 43 Bd. du Il Novembre 1918, F-69622 Villeurbanne, Cedex, France.
tionality of S and stopping power in the energy interval 1 < E,, _< 20 keV [6] . In the energy range below 1 keV, i .e . around the stopping power maximum (0.05 :5 Et, _< 1 keV), "it is difficult to indicate any correlation between the stopping power and the secondary electron yield" [6] . The same statement is valid for electron induced yields S from gold targets (0 .2 _< Et, < 2 .5 keV) [4]. A more detailed discussion can be found in refs . [4,6] and a recent review on electron-induced electron emission in ref. [7] . It is important to note that both from a theoretical as well as from an experimental point of view, H + impact represents the simplest case for the investigation of particle induced electron yields and can be used to test basic theoretical assumptions [1-6]. Because of the multitude of processes involved in electron cm .ssion it is extremely difficult to treat the phenomenon theoretically. Such processes as excitation and ionization of target electrons both from inner shells and the conduction band, collective excitation of and screening of the projectile charge by the nearly free target conduction electrons, elastic and inelastic scattering of the electrons and transmission through the surface have to be taken into account. The situation gets even more complicated if the projectiles carry bound electrons (atoms, heavy ions, molecules, clusters). In these cases, also excitation/ ionization of and screening by projectile electrons have to be considered . The simplest example of a projectile carrying an electron is H". At high projectile velocities r,, >> r,: (t',. being the Fermi velocity of the target electrons), the electron yield y(H") from H" impact on carbon is larger than the proton-induced yield y(H + ) . This can be explained by the additional secondary
0168-583X/92/$05 .0 CO 1992 - Elsevier Science Publishers B.V . All rights reserved
H. Rothard et al. / Electron yields from solids
electron yield induced by the projectile electron, y(e - ), i .e. y(H'~) = y(H + ) + y(e") [1-3,8-9]. At low projectile velocities r,, < r F , the yield y(H ° ) from gold was found to be smaller than y(H + ) due to the screening of the projectile charge which leads to a reduction of the effective charge of the projectile and to a reduced stopping power [3,10]. 2 . Electron yields from heavy ions In the case of swift heavy ions, the proportionality between electron yields and the stopping power only holds if the pre-equilibrium evolution of the charge state of the ions is taken into account. The novel concept of the pre-equilibrium near-surface stopping power S*(x) can be used to describe the electron yield dependence on the projectile atomic number Z p [2,1113]. If the charge equilibration depth A is larger than the escape depth A sp of low energy electrons, A, > A s[., the electron yields are proportional to the effective (near-surface) stopping power y - S* . This quantity is proportional to the square of the penetrationdepth dependent effective ion charge q*(x) in the charge pre-equilibrium, S*(x)- q*(x)` . Thus, S* deviates from tabulated "bulk" stopping power values [14], which implicitly incorporate a proportionality to the square, q,2,(X » A '.), of the effective charge of the ions in the charge equilibrium, S,, - qce .
104 1T =ATS" AT= (0.31±0.14)ÄIeV /~
,-10 3 0
w
10 2
r z CD 10 1 w W 100
ZT=6 0 Zp< 6 0 Zp> .6
10-1
qi --qf 100 101 10 2 103 10 4 ELECTRONIC STOPPING POWER Se [eV/íX]
total electron yield YT from ion penetration through carbon foils as a function of the stopping power S,, of light ions (Zp < 6) and heavy ions (Zp z 6) according to ref . I2] . Fig. 1 . Tha
155
This concept is strongly supported by the successful interpretation of the dependence of electron yields on the charge state q, of the incoming ions [10,13,15] and recent calculations of the charge-state dependence of the energy loss [16]. A thorough discussion can be found in ref. [13]. The important assumption of an overall proportionality between cl^ctron yields and the stopping power both for light ions (Z p < 6) and heavy ions (Zp >_ 6) is demonstrated impressively in fig . 1 . It shows the total electron yield YT from carbon foils as a function of the electronic stopping power Se . The yield includes c1cctron emission from the foil in forward as well as in backward direction. A rough proportionality y T = A ~S~ (within a factor of 2) in a wide range of projectile velocities vp (15 kcV/u _< EplMp < 46 McV/u) and projectile nuclear charge Zp (1 < Zt, _< 92) over four decades of electron yield and stopping power values can be stated . The mean value of the proportionality factor is A*T = (0 .31 ± 0.14) A/cV . It is important to note that the data were taken with incident charge states q, close to the mean charge state qt of the emerging ions. This ensures that S* = S, at both surfaces of the foil (compare refs . [2,13]). Recent results obtained at the heavy ion synchrotron of GSI (Darmstadt) indicate that electron yields scale with the stopping power even for relativistic heavy ions (Ar + up to 1 .65 GeV/u) [17] . Finally, it should be noted that strong forward effects concerning the dependence of electron yields on the charge state q, of the ions can only be expected if the thickness d of the foil is smaller than (or comparable to) the charge equilibration depth A., and the range of fast electrons from binary collisions or from projectile ionization (compare refs. [8,15]). 3 . Molecules and clusters At this point, it is interesting to note that a similar argument is valid for molecular ions. Due to Coulomb explosion and multiple scattering, the constituents of a molecule (or a cluster) will have lost their correlation after at most, say, a few 100 A mainly depending on the projectile velocity v p . Thus, strong molecular effects can only be expected to take place near the entrance surface of the target . They should be visible in backward direction (i.e . at the beam entrance side of the target) or with thin foils also in forward direction (see, e .g ., refs. [2,3,8,9,18]) . A detailed discussion of molecular effects in forward electron emission can be found in refs . [2,8,18]. Molecular effects can be observed in the ratio between a physical quantity (such as the stopping power S . or electron yields y) measured with a molecular projectile (or a cluster) and the sum of the values
156
H. Rothard et al. / Electron yields from solids
measured with its atomic constituents . Consequently, it is convenient [1,8,9,16] to define the ratios R(y)
-/(Molecule) Ey(atomicconstituents)
and R(S~)=
SJmolecule) ES,(atomic constituents)
as a measure of the "molecular effect" (R 0 1) . The velocity dependence of the energy loss ratio R(r p ) of H ; in carbon targets has been studied both experimentally and theoretically (see, e .g., refs . [1923]). In general, R is found to increase with r,, . Below a "critical velocity" r_ a reduction, RU t, < r,,) < 1, is observed, whereas at high velocities r,, > v_ an enhancement, R(r p > r~,) > 1, can be seen [19-23] . The critical velocity r ., is found to be on the order of the Fermi velocity r F of the solid [19,20] . Backward electron yields yu from H, molecular ion impact on a variety of target materials mirror the velocity dependence of the stopping power ratio R(S,): Reductions of the electron yield ratio (R < 1) at low velocities (below = 100 keV/u) and enhancements (R> 1) at higher velocities have been reported [13,8,9,18] . This may indicate that the proportionality between energy loss and electron yields is valid also for molecular projectiles. However, "the magnitude and the energy dependence of R is a function of the target material" [3], and the values of the critical velocities t " cr(ZT, Z p ) arc not accurately known. As in the case of H° impact, the explanation for these findings may be the additional secondary electron yield induced by the projectile electrons at high velocities and the screening of the projectile charge at lower velocities [3,10,18] . Similar experimental results have been reported for heavy molecular ions such as O,, N_ CO', Se, and Tc, [2,3,18] . Little is known about electron emission by cluster ion impact at high velocities (i .e . comparable to or higher as c °F) [3,18]. However, studies with small hydrogen clusters I I,, (n = 3, 5, 7, - - - , 15) indicate a similar behaviour as in the case of molecular ions, i.e. R < 1 below = 100 keV/w and R > 1 at velocities exceeding = 120 keV/u [24]. Recent theoretical investigations by Mikkelsen et al. on the energy loss of hydrogen clusters in carbon [20] show an increase of R(S.) with r p starting at R = 0.8 at = 20 keV/u and rising up to R = 1 .1-1 .5 at = 140 keV/u with a critical velocity around rc,(n)=50 keV/u . The magnitude of the "cluster effect" (i .e. R 0 1) depends on the number n of protons in the cluster and has been found to be most pronounced for n >_ 9 [20]. These calculations arc in good agreement with recent experimental results of Ray et al . [19,20]. Also, theoretical investigations by
Abril et al. [23] show an enhanced cluster energy loss at high velocities 2"F :5 r t, _< 101V' R(n) increases with the cluster size up to a value as large as R = 10 for n = 100 . It should be interesting to compare these results on the energy loss of clusters with clcctron yield data from hydrogen cluster impact [18]. 4. Conclusion
In this short survey, we briefly discussed the question whether the yield y of electrons induced by swift electrons, protons, heavy ions, molecular ions and clusters is proportional to the stopping power S. . This proportionality seems to be fulfilled for the particular case of proton impact and possibly in the case of heavy ions. In this latter case, the pre-equilibrium of the charge state evolution and of the stopping power has to be taken into account. With sufficiently thin targets or at sufficiently high ion velocities (which can be achieved with heavy ion accelerators such as GANIL or GSI-SIS [17]) we are generally dealing with pre-equilibrium conditions where stopping power values can differ significantly from equilibrium data. For the case of molecular ions, comparison of theoretical calculations and experimental data suggest a proportionality of electron yields and stopping power ; however, there is clearly a need for further theoretical and experimental investigations on the energy loss of molecular ions. Ion induced electron emission can be a vàjuable test for theories [1,5,6,20-23], which can also be gpplicd to such fields as scanning electron microscopy and all kinds of surface analysis with Auger-, photo- or secondary electrons [25]. New insights into the correlation between electron emission and energy loss can be obtained by simultaneous measurements of electron yields y and stopping power S,, [26] . A wide new field is the study of both forward and backward electron yields from cluster ion interaction with thin foils [18-20,24]. Such electron yield data could be compared to recent calculations and measurements of the energy loss of hydrogen clusters [20-23]. In particular, the target thickness dependence of forward electron yields allows a detailed comparison to theory [2,6] and may yield important information about the influence of "wake effects" [9,18] and electron diffusion on electron emission from solids . Also, results on electron emission in relation to the energy loss of clusters may help to understand such interesting phenomena as collective effects in ion desorption [27] and cluster impact fusion [28]. References [1)
G. 116hler, E.A . Niekisch and J. Treusch (eds .), Particle Induced Electron Emission 11, Springer Tracts in Mod-
H. Rothard et al. / Electron yields from solids ern Physics 123 (Springer, Heidelberg, 1991) with contributions by D. Hasselkamp, H . Rothard, K.O. Groeneveld, J . Kemmler, P. Varga and H . Winter. [2] H . Rothard, K.O. Groeneveld and J. Kemmler, ibid ., Kinetic electron emission from ion penetration of thin foils in relation to the pre-equilibrium of charge distributions, p . 97. [3] D. Hasselkamp, ibid ., Kinetic electron emission from solid surfaces under ion bombardment, p. 1 . [4] D . Hasselkamp, Habilitationsschrift, Justus-Liebig-Universitiit, Giessen, Germany (1985); see also : Comments Atom . Mol . Phys. 21 (1988) 241, and D. Hasselkamp, S. Hippler, A . Scharmann and T. Schmehl, Ann . Phys. (Leipzig) 47 (1990) 7. [5] J . Devooght, J .C. Dehaes, A. Dubus, M. Cailler, J .P. Ganachaud, M. R6sler and W . Brauer, in : Particle Induced Electron Emission 1, eds . G. Hbhler, E .A . Niekisch and J . Treusch, Springer Tracts in Modern Physics 122 (Springer, Heidelberg, 1991). [6] J . Schou, Scanning Microsc. 2 (1988) 607; see also : Phys. Rev . B22 (1980) 2141 . [7] R. Bindi, H . Lanteri and P . Rostaing, Scanning Microsc. 1 (1987) 1475. [8] K. Kroneberger, A. Clouvas, G . Schlüssler, P. Koschar, J . Kemmler, H. Rothard, C . Biedermann, O . Heil, M . Burkhard and K.O. Groeneveld, Nucl . Instr. and Meth . B29 (1988) 621 . [9] K . Kroneberger et al., J . Phys . (Paris) 50 (1989) C2-99 . [10] G. Lakits, A . Arnau and H. Winter, Phys. Rev. B42 (1990) 15 . [I1] H. Rothard, K . Kroneberger, A. Clouvas, E . Veje, P. Lorenzen, N . Keller, J . Kemmler, W. Meckbach and K.O. Groeneveld, Phys. Rev . A41 (1990) 2521 . [12] A . Clouvas, A . Katsanos, B. Farizon-Mazuy, M. Farizon and M.J. Gaillard, Phys . Rev . B43 (1991) 2498 . [13] H . Rothard, J . Schou and K.O . Groeneveld, Phys . Rev . A45 (1992) in press . [14] J.F. Ziegler, J .P . Biersack and U . Littmark, The Stopping and Range of Ions in Matter (Pergamon, New York, 1985) . [15] P. Koschar et al ., Phys . Rev . A40 (1989) 3632. [16] A. Arnau, M . Pefialba, P .M. Echenique, F. Flores and R.H. Ritchie, Phys . Rev. Lett . 65 (1990) 1024. [17] T . Brohm, H.-G . Clere, B . Voss, C. Ziegler, H. Geissel,
157
M. Kildir, K.H . Schmidt, K. Sümmerer and D .-J. Viera, Verhandl . DPG (VI) 26 (1991) 414; A . Albert, K . Kroneberger, O. Heil, H. Geissel and K.O. Groeneveld, ibid ., p . 707. [18] H . Rothard et al., Phys. Rev . B41 (1990) 3959 . [19] M . Ray, Thesis, Univ. Claude Bernard, Lyon, France (1991). [20] H .H . Mikkelsen, E. Ray, B. Farizon-Mazuy, M .J. Gaillard, J.C. Poizat and J . Remillieux, presented at workshop STOP 91, Middelfart, Denmark, 1991 ; E. Ray, R. Kirsch, H.H. Mikkelsen, J .C. Poizat and J . Remillieux, this issue (STOP 91), Nucl . Instr . and Meth . B69 (1992) 133 . [21] W. Brandt and R.H. Ritchie, Nucl . Instr . and Meth . 132 (1976) 43 . [22] N.R . Arista and A. Gras-Marti, in: Interaction of Charged Particles with Solids and Surfaces, NATO Advanced Study Institute, Alicante, Spain, 1990, eds. F. Flores, H .M . Urbassek, N. Arista and A. Gras-Marti (Plenum, New York, 1991) . [23] I . Abril, M. Vicanek, A . Gras-Marti and N .R . Arista, Proc . 14th . Int. Conf. o n Atomic Collisions in Solids, Salford, UK, 1991, Nucl . Instr . and Meth. B67 (1992) 56, M . Vicanek, presented at workshop STOP 91, Middelfart, Denmark, 1991 . [24] Y. Chanut, J . Martin, R. Salin and H .O. Moser, Surf . Sci . 106 (1981) 563. [25] M . Schosnig, H. Rothard, K. Kroneberger, D. Schlbsser and K.O. Groeneveld, Proc . 2nd Europ. Conf. on Accelerators in Applied Research and Technology, Frankfurt am Main, Germany, 1991, Nucl . Instr. and Meth . B68 (1992) 394 . [26] P. Bauer, O. Benka and E . Steinhauer, Annual Report 1990 of the Institut für Experimentalphysik, Abteilung für Atom- and Kernphysik, Johannes-Kepler-UnN., Linz, Austria, p. 11 . [27] J .P . Thomas, A . Oladipo and M. Fallavier, J . Phys . (Paris) 50 (1989) C2-195 . [28] R .J . Beuhler, G. Friedlander and L. Friedman, Phys . Rev. Lett. 63 (1989) 1292 ; M . Fallavier, J. Kemmler, R. Kirsch, J .C. Poizat, J . Remillieux and J .P. Thomas, Phys. Rev. Lett. 65 (1990) 621 .