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Electronic excitation effects on secondary ion emission in highly charged ion–solid interaction
Nuclear Instruments and Methods in Physics Research B 182 (2001) 121±126
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Electronic excitation eects on secondary ion emission in highly charged ion±solid interaction T. Sekioka a, M. Terasawa a,*, T. Mitamura a, M.P. St ockli b, U. Lehnert b, C. Fehrenbach b a
Department of Physics, Himeji Institute of Technology, Himeji, Hyogo 671-2201, Japan b J.R. Macdonald Laboratory, Kansas State University, Manhattan, KS 66506, USA
Abstract In order to investigate the secondary ion emission from the surface of conductive materials bombarded by highly charged heavy ions, we have done two types of experiments. First, we have measured the yield of the sputtered ions from the surface of solid targets of conductive materials (Al, Si, Ni, Cu) bombarded by Xeq (q 15±44) at 300 keV vp 0:30 a:u and at 1.0 MeV vp 0:54 a:u. In view of the secondary ion yields as a function of the potential energy of the projectile, the increase rates below q 35, where the potential energy amounts to 25.5 keV, were rather moderate and showed a prominent increase above q 35. These phenomena were rather strong in the case of the metal targets. Second, we have measured the energy dependence of the yield of the sputtered ions from the surface of solid targets of conductive materials (C, Al) bombarded by Xeq (q 30; 36; 44) between 76 keV vp 0:15 a:u and 6.0 MeV vp 1:3 a:u. A broad enhancement of the secondary ion yield has been found for Al target bombarded by Xe44 . From these experimental results, the electronic excitation eects in conductive materials for impact of slow highly charged heavy ions bearing high potential energy is discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 79.20.Rf; 34.50.Fa; 82.80.Ms Keywords: Electronic excitation eect; Highly charged ion; Secondary ion; Time of ¯ight
1. Introduction The interaction between low energy highly charged ion (HCI) and solid has been an active
* Corresponding author. Tel.: +81-792-67-4924; fax: +81792-66-8868. E-mail address: [email protected] (M. Terasawa).
research area in recent years. One of the many characteristic features of HCI±solid interaction is the very fast relaxation of the ion in the solid [1±5]. Therefore the charge neutralization of HCI impinging on the material proceeds very rapidly and the electronic potential energy of HCI is released very near the surface. Charge state dependent energy loss enhancements have been observed for slow (vp 0:3 a.u.) Xeq and Auq ions at q P 40 transmitted through thin carbon
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 6 6 4 - 4
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T. Sekioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 182 (2001) 121±126
foils, and de-excitation times <10 fs were indicated by pre-equilibrium energy loss enhancements for slow HCI with q > 40 [4]. Mean charge equilibration times of slow HCI like Xe44 and Th75 in thin carbon foils have been found to be only 7 fs [5]. These results show that very highly charged ions are needed to observe strong preequilibrium contributions to the energy loss of slow HCI in solids. A widely used model to study the relaxation of HCI approaching a metal surface is the classical over-the-barrier (COB) model that gives a scenario as follows [6]. The HCI captures electrons from the conducting solid into highly excited Rydberg states at a certain critical distance from the surface via a resonant charge transfer giving rise to the formation of a hollow atom, i.e., an atom whose inner shells remain unoccupied. The hollow atom undergoes de-excitation via Auger or radiative decay that populates lower states. The de-excitation rates are, however, generally too small to allow a full relaxation of the ion before it hits the surface. Recently much eort has been devoted to studies of processes taking place inside the bulk of a metallic target. The formation of a hollow atom in the solid, sometimes called a hollow atom of the second generation, involves a speci®c feature of the metal electrons that are fast enough to develop a dynamic screening cloud. Hollow atom formation and decay in and above metallic and insulating targets has been investigated in great detail by measurements of secondary electron emission, Auger electron and X-ray spectroscopy [7,8]. In this paper, we report the results of our measurements of secondary ion yield from conductive materials (Al, Ni, Cu, Si) bombarded by Xeq (q 15±44) ions beam at 300 keV, and projectile energy dependence of the secondary ion yield from conductive materials (Al, C) bombarded by Xeq (q 30; 36; 44) between 76 keV vp 0:15 a:u and 6.0 MeV vp 1:3 a:u. Together with our earlier data of the secondary ion yield from conductive materials (Al, Ni, Cu, Ag, Si) bombarded by Xeq (q 15±44) ions beam at 1.0 MeV [9], we will discuss the electronic excitation eect in the conductive materials bombarded by slow HCI.
2. Experimental setup Pulsed beams of Xeq (15 6 q 6 44) with initial energies between 76 keV (v 0:15 a.u.) and 6.0 MeV (v 1:3 a.u.) were emitted by the Kansas State University cryogenic electron beam ion source (CRYEBIS) [10] in micro-bunching mode [11], and analyzed with a double-focusing 90° dipole magnet. We used solid metals (Al, Ni, Cu, Ag) and semiconductor (Si) targets. The Si target was N-type (1 0 0) crystal. The incident highly charged Xe ion beam was cast upon the target at 45°. The mass spectra of the secondary ions were measured using a time-of-¯ight (TOF) spectrometer 45 cm in length. A positive voltage of 300 V was applied to the target to extract the secondary ions into the TOF. The FWHM of the time structure of the pulsed beam was typically 0.1 ls. A typical pressure of the target region was 5 10 10 Torr. As it is dicult to obtain start signals by secondary electrons from solid targets, our experiment requires electronic signals indicating the arrival of the incoming ions to measure the ¯ight time of secondary particles. Therefore, a pulsed HCI beam is indispensable for our experiment. The CRYEBIS produced a pulsed, highly charged xenon ion beam in the micro-bunching expulsion mode, in which the ion bunch is divided into several or many micro-bunches. The surface of the target was cleaned by using a 3 kV Ar ion beam shortly before the secondary ion measurement. As the secondary ion yield has signi®cant dependence on the contamination on the surface of the targets, the cleaning procedure is crucial. The demonstration of the cleaning eect is given in Fig. 1 which shows secondary ion mass spectra from an Al target bombarded by (a) Xe30 at 2.5 MeV where the cleaning is insucient, and (b) Xe40 at 1.7 MeV where the cleaning is sucient. The peak assignments are given in the ®gures. In Fig. 1(a), besides the secondary ions of the target materials (Al , Al2 , Al3 ), a big H peak and other peaks due to contamination such as hydrocarbons and oxygen on the target surface are found. We cleaned the targets by 3 kV Ar ion beam irradiation until no contamination peak was seen except hydrogen peaks like Fig. 1(b).
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Fig. 2 shows a typical secondary ion mass spectrum from a C target bombarded by Xe44 at 400 keV. In the spectrum one can see several carbon clusters peaks indicated in the ®gure. The secondary ion yield was obtained after subtracting the background and de-convolution, if necessary. The secondary ion yield was normalized by the integrated beam current, and divided by the estimated detection eciency of the TOF that we obtained by the ion trajectory calculation. In the estimation, we assumed a 100% eciency of the ion detection of the MCP in the TOF. (a)
3. Results and discussion 3.1. Charge dependence of the secondary ion yield
(b) Fig. 1. Secondary ion mass spectra from an Al target bombarded by (a) Xe30 at 2.5 MeV where the cleaning is insucient, and (b) Xe40 at 1.7 MeV where the cleaning is sucient.
Fig. 2. A typical secondary ion mass spectrum from a C target bombarded by Xe44 at 400 keV.
Fig. 3 shows the incident charge state dependence of the secondary ion yield as a function of the potential energy Epot of the projectile Xeq for (a) Al, Si, Ni, Cu bombarded by 300 keV Xeq (q 15±44), and for (b) Al, Si, Ni, Cu, Ag bombarded by 1.0 MeV Xeq (q 15±44) [9]. The Epot , the sum of the binding energy of electrons removed to form the ions was calculated by a multicon®guration Dirac±Fock code [12]. The charge q of the projectile Xeq is shown in each ®gure. In both spectra, the secondary ion yields increase, as the charge of the incident Xe ion increases for every target, and the yields increase signi®cantly and linearly with Epot at q P 35 (Epot P 25:5 keV), though the increase rate in the region q 6 35 is rather moderate. At the bombarding energy of 300 keV (Fig. 3(a)), one can see a clear dierence in the secondary ion yield from metal (Al, Ni, Cu) targets and a semiconductor (Si) target in the region of q 6 35. The linear increase of the secondary ion yield from Si starts already at q 15 (Epot 2:2 keV). On the contrary, the yield of the secondary ion yield from metal targets remain almost constant at q 6 35. This dierence can be attributed to the rapid neutralization of the electronic excitation in metals. The signi®cant linear increase of the secondary ion yield begins at q P 35 for each metal target. After the eects of incident ion charge in electronic sputtering from LiF and Si at impact
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(a)
(b) Fig. 3. The incident charge state dependence of the secondary ion yield as a function of the potential energy of the projectile Xeq for (a) Al, Si, Ni, Cu bombarded by 300 keV Xeq (q 15±44), and for (b) Al, Si, Ni, Cu, Ag bombarded by 1.0 MeV Xeq (q 15±44). The charge q of the projectile Xeq is shown in parentheses in each ®gure.
of slow Arq HCI q 6 9 were reported [13,14], electronic energy loss process have been found to be important for sputtering of insulators and poor conductors. Defect mediated eect has been proposed as a mechanism to explain the total sputter yields induced by Arq (q P 14) and Xeq q P 27 for LiF and SiO2 [15]. Coulomb explosion model [16] is another model for electronic sputtering of non metals by slow HCIs, in which charge state dependent electronic sputtering was related to electrostatic repulsion of target ions in a charge depleted region at the surface. There are a few reports on the observations of electronic sputtering eects in the interaction of slow HCI with conductors by the measurements of secondary ion emission [9,17,18], energy loss in thin carbon foil [4,19] and pre-equilibrium
eect in the charge state distribution after penetrating the thin carbon foil [5]. All these observation of the enhancement of the electronic excitation eect in conductors have been made by using HCI projectiles with very high potential energy. The strong onset of secondary ion yield from metals bombarded by Xeq (q P 35) found in the present measurement is one of clear evidences of electronic excitation eect in metals when imposed with very high potential energy. The dierences of the secondary ion yields between target materials are correlated with the mass of the target atom. As a general trend, the ion yields are smaller for heavier target atoms. In the COB model [6], de-excitation of hollow atoms in solids progresses via Auger cascades and radiative transitions. It is more dicult for lighter target electrons to ®ll the inner shell vacancies of heavy projectile Zprojectile 54 directly, so that the mean de-excitation time is larger for lighter target. This results in the increased eective charge of hollow atoms and high electronic excitation in lighter target [1,19], which can explain qualitatively the target mass dependence of the secondary ion yield. The secondary ion yields from C target bombarded by Xe44 at bombarding energies 300 keV and 1.0 MeV, which will be shown in Section 3.2, were lower than the ones from Al, but larger than the ones from Si. At present, we cannot give the particular reason for the large secondary ion yield from Al. 3.2. Projectile energy dependence of the secondary ion yield Fig. 4 shows the bombarding energy dependence of the secondary ion yield from the surface of (a) C and (b) Al, bombarded by Xeq (q 30; 36; 44) between 76 keV vp 0:15 a:u and 6.0 MeV vp 1:3 a:u. For C target (Fig. 4(a)), ion yields vary weakly with bombarding energy at q 30; 36 and 44. For Al target (Fig. 4(b)), the ion yields vary weakly with bombarding energies at q 30 and 36, but it has broad enhancement at around 500 keV at q 44. According to TRIM calculation, the maximum of the nuclear stopping power Sn of Xe ion in C
T. Sekioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 182 (2001) 121±126
(a)
(b) Fig. 4. The bombarding energy dependence of the secondary ion yield from the surface of (a) C and (b) Al, bombarded by Xeq (q 30; 36; 44) between 76 keV vp 0:15 a:u and 6.0 MeV vp 1:3 a:u.
is 160 keV, and that in Al is 180 keV, which are rather near to the bombarding energies of the present measurement. In [19], the average energy loss DEave of Ar18 , 44 Xe and Au69 in thin carbon foil is given as a function of projectile velocity, where strong increase of DEave for Xe44 and Au69 is reported. According to their result, pre-equilibrium eect should exist in C target bombarded by Xe44 , but no prominent enhancement have been found in our secondary ion measurement as a function of bombarding energy. No observation of the enhancement may be explained by the reason that, as the velocity becomes higher, the HCI releases its potential energy in a deeper position in the target, so that the secondary ion emission may be hindered. In [20], synergy of electronic excitations and elastic collision spikes in sputtering from uranium
125
oxide has been reported. They measured secondary ion yields as a function of kinetic energy for Xe27 , Xe44 , Au63 and Au69 . Their results are that, ion yields for Xe projectiles of impact energy between 20 and 500 keV vary only weakly, and the one for Au63 has some structure and the one for Au69 shows a pronounced peak at around 220 keV. A broad enhancement of the secondary ion yield from Al bombarded by Xe44 is found in the present experiment. The target (Al) of the present experiment is much lighter than that (uranium oxide) of [20]. This dierence of mass ratio of the projectile and target may be the reason for the observation of synergy eect in our experiment. We have measured the energy dependence of secondary ion yield up to 6.0 MeV where the electronic stopping power (Se ) dominates over Sn , but we cannot observe the enhancement of the secondary ion emission which may be expected by the increased Se at higher bombarding energy. 4. Conclusions In order to investigate the electronic excitation eect in conductive materials induced by highly charged heavy ions, we have measured mass spectra of secondary ion, ®rst, as a function of incident charge, and second, as a function of projectile energy. In the ®rst experiment, the secondary ion yields showed a signi®cant increase with increasing projectile charge state q, demonstrating the electronic sputtering in the interaction of slow HCIs with conductive materials. In view of the secondary ion yields as a function of the potential energy of the projectile, the increase rate below q 35 (Epot 25:5 keV) were rather moderate and showed a prominent increase above q 35. These phenomena were rather strong in the case of the metal targets which give a clear evidence of electronic excitation eect in metals. In the second experiment, the energy dependence of the secondary ion yields show almost no dependence on projectile energy, except for the case of Al target bombarded by Xe44 where a broad peak around 500 keV of projectile energy
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was found which can be interpreted as a synergy eect of electronic excitation eect and elastic collision spike. Acknowledgements The authors would like to thank Professor T. Mukoyama for his help in the multicon®guration Dirac±Fock calculation. This work was supported by the Grant-in-Aid for Scienti®c Research from the Ministry of Education, Science, Sports and Culture, Japan, and also, by the Division of Chemical Sciences, oce of Basic Energy Sciences, US Department of Energy. References [1] R. Herrmann, C.L. Cocke, J. Ullrich, S. Hagmann, M. Stoeckli, H. Schmidt-Boecking, Phys. Rev. A 50 (1994) 1435. [2] S. Winecki, C.L. Cocke, D. Fry, M.P. St ockli, Phys. Rev. A 53 (1996) 4228. [3] L. Folkerts, S. Schippers, D.M. Zehner, F.W. Meyer, Phys. Rev. Lett. 74 (1995) 2204. [4] T. Schenkel, M.A. Briere, A.V. Barnes, A.V. Hamza, K. Bethge, H. Schmidt-B ocking, D.H. Schneider, Phys. Rev. Lett. 79 (1997) 2030. [5] M. Hattass, T. Schenkel, A.V. Barnes, M.W. Newman, J.W. McDonald, T.R. Niedermayr, G.A. Machicoane, D.H. Schneider, Phys. Rev. Lett. 82 (1999) 4795.
[6] J. Burgd orfer, P. Lerner, F.W. Meyer, Phys. Rev. A 44 (1991) 5674. [7] A. Arnau, F. Aumayr, P.M. Echenique, M. Grether, W. Heiland, J. Limburg, R. Morgenstern, P. Roncin, S. Schipers, R. Schuch, N. Stolterfoht, P. Varga, T.J.M. Zouros, H. Winter, Surf. Sci. Rep. 27 (1997) 113. [8] M. Grether, D. Niemann, A. Spieler, N. Stolterfoht, Phys. Rev. A 56 (1997) 3794. [9] T. Sekioka, M. Terasawa, T. Mitamura, M.P. St ockli, U. Lehnert, C.L. Cocke, Nucl. Instr. Meth. B 146 (1998) 172. [10] M.P. Stockli, M. Abdallah, C.Y. Chen, C.L. Cocke, B.D. De Paola, D. Fry, P.E. Gibson, P. Richard, T.N. Tipping, B. Walch, S. Winecki, B. Eastman, Th. Gebel, E. Langer, U. Lehnert, H. Preusse, F. Ullmann, A. Gorges, M. Ramassamy, Rev. Sci. Instrum. 69 (1998) 665, and references therein. [11] M.P. Stockli, Rev. Sci. Instrum. 69 (1998) 649. [12] J.P. Desclaux, Compt. Phys. Commun. 9 (1975) 31. [13] T. Neidhart, F. Pichler, F. Aumayr, HP. Winter, M. Schmidt, P. Varga, Phys. Rev. Lett. 74 (1995) 5280. [14] E.S. Parilis, Z. Phys. D 21 (1991) S127. [15] M. Sporn, G. Libiseller, T. Neidhart, M. Schmidt, F. Aumayr, H.P. Winter, P. Varga, Phys. Rev. Lett. 79 (1997) 945. [16] I.S. Bitenskii, M.N. Murakhmetov, E.S. Parilis, Sov. Phys. ± Tech. Phys. 24 (5) (1979) 618. [17] M. Terasawa, T. Sekioka, T. Mitamura, S. Winecki, M.P. St ockli, C.L. Cocke, Phys. Scr. T 73 (1997) 326, presented at VIII HCI'96, 23±26 September 1996. [18] T. Schenkel, M.A. Briere, H. Schmidt-B ocking, K. Bethge, D.H. Schneider, Phys. Rev. Lett. 78 (1997) 2481. [19] T. Schenkel, A.V. Hamza, A.V. Barnes, D.H. Schneider, Phys. Rev. A 56 (1997) R1701. [20] T. Schenkel, A.V. Barnes, A.V. Hamza, D.H. Schneider, Phys. Rev. Lett. 80 (1998) 4325.
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Electronic excitation eects on secondary ion emission in highly charged ion±solid interaction T. Sekioka a, M. Terasawa a,*, T. Mitamura a, M.P. St ockli b, U. Lehnert b, C. Fehrenbach b a
Department of Physics, Himeji Institute of Technology, Himeji, Hyogo 671-2201, Japan b J.R. Macdonald Laboratory, Kansas State University, Manhattan, KS 66506, USA
Abstract In order to investigate the secondary ion emission from the surface of conductive materials bombarded by highly charged heavy ions, we have done two types of experiments. First, we have measured the yield of the sputtered ions from the surface of solid targets of conductive materials (Al, Si, Ni, Cu) bombarded by Xeq (q 15±44) at 300 keV vp 0:30 a:u and at 1.0 MeV vp 0:54 a:u. In view of the secondary ion yields as a function of the potential energy of the projectile, the increase rates below q 35, where the potential energy amounts to 25.5 keV, were rather moderate and showed a prominent increase above q 35. These phenomena were rather strong in the case of the metal targets. Second, we have measured the energy dependence of the yield of the sputtered ions from the surface of solid targets of conductive materials (C, Al) bombarded by Xeq (q 30; 36; 44) between 76 keV vp 0:15 a:u and 6.0 MeV vp 1:3 a:u. A broad enhancement of the secondary ion yield has been found for Al target bombarded by Xe44 . From these experimental results, the electronic excitation eects in conductive materials for impact of slow highly charged heavy ions bearing high potential energy is discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 79.20.Rf; 34.50.Fa; 82.80.Ms Keywords: Electronic excitation eect; Highly charged ion; Secondary ion; Time of ¯ight
1. Introduction The interaction between low energy highly charged ion (HCI) and solid has been an active
* Corresponding author. Tel.: +81-792-67-4924; fax: +81792-66-8868. E-mail address: [email protected] (M. Terasawa).
research area in recent years. One of the many characteristic features of HCI±solid interaction is the very fast relaxation of the ion in the solid [1±5]. Therefore the charge neutralization of HCI impinging on the material proceeds very rapidly and the electronic potential energy of HCI is released very near the surface. Charge state dependent energy loss enhancements have been observed for slow (vp 0:3 a.u.) Xeq and Auq ions at q P 40 transmitted through thin carbon
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 6 6 4 - 4
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T. Sekioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 182 (2001) 121±126
foils, and de-excitation times <10 fs were indicated by pre-equilibrium energy loss enhancements for slow HCI with q > 40 [4]. Mean charge equilibration times of slow HCI like Xe44 and Th75 in thin carbon foils have been found to be only 7 fs [5]. These results show that very highly charged ions are needed to observe strong preequilibrium contributions to the energy loss of slow HCI in solids. A widely used model to study the relaxation of HCI approaching a metal surface is the classical over-the-barrier (COB) model that gives a scenario as follows [6]. The HCI captures electrons from the conducting solid into highly excited Rydberg states at a certain critical distance from the surface via a resonant charge transfer giving rise to the formation of a hollow atom, i.e., an atom whose inner shells remain unoccupied. The hollow atom undergoes de-excitation via Auger or radiative decay that populates lower states. The de-excitation rates are, however, generally too small to allow a full relaxation of the ion before it hits the surface. Recently much eort has been devoted to studies of processes taking place inside the bulk of a metallic target. The formation of a hollow atom in the solid, sometimes called a hollow atom of the second generation, involves a speci®c feature of the metal electrons that are fast enough to develop a dynamic screening cloud. Hollow atom formation and decay in and above metallic and insulating targets has been investigated in great detail by measurements of secondary electron emission, Auger electron and X-ray spectroscopy [7,8]. In this paper, we report the results of our measurements of secondary ion yield from conductive materials (Al, Ni, Cu, Si) bombarded by Xeq (q 15±44) ions beam at 300 keV, and projectile energy dependence of the secondary ion yield from conductive materials (Al, C) bombarded by Xeq (q 30; 36; 44) between 76 keV vp 0:15 a:u and 6.0 MeV vp 1:3 a:u. Together with our earlier data of the secondary ion yield from conductive materials (Al, Ni, Cu, Ag, Si) bombarded by Xeq (q 15±44) ions beam at 1.0 MeV [9], we will discuss the electronic excitation eect in the conductive materials bombarded by slow HCI.
2. Experimental setup Pulsed beams of Xeq (15 6 q 6 44) with initial energies between 76 keV (v 0:15 a.u.) and 6.0 MeV (v 1:3 a.u.) were emitted by the Kansas State University cryogenic electron beam ion source (CRYEBIS) [10] in micro-bunching mode [11], and analyzed with a double-focusing 90° dipole magnet. We used solid metals (Al, Ni, Cu, Ag) and semiconductor (Si) targets. The Si target was N-type (1 0 0) crystal. The incident highly charged Xe ion beam was cast upon the target at 45°. The mass spectra of the secondary ions were measured using a time-of-¯ight (TOF) spectrometer 45 cm in length. A positive voltage of 300 V was applied to the target to extract the secondary ions into the TOF. The FWHM of the time structure of the pulsed beam was typically 0.1 ls. A typical pressure of the target region was 5 10 10 Torr. As it is dicult to obtain start signals by secondary electrons from solid targets, our experiment requires electronic signals indicating the arrival of the incoming ions to measure the ¯ight time of secondary particles. Therefore, a pulsed HCI beam is indispensable for our experiment. The CRYEBIS produced a pulsed, highly charged xenon ion beam in the micro-bunching expulsion mode, in which the ion bunch is divided into several or many micro-bunches. The surface of the target was cleaned by using a 3 kV Ar ion beam shortly before the secondary ion measurement. As the secondary ion yield has signi®cant dependence on the contamination on the surface of the targets, the cleaning procedure is crucial. The demonstration of the cleaning eect is given in Fig. 1 which shows secondary ion mass spectra from an Al target bombarded by (a) Xe30 at 2.5 MeV where the cleaning is insucient, and (b) Xe40 at 1.7 MeV where the cleaning is sucient. The peak assignments are given in the ®gures. In Fig. 1(a), besides the secondary ions of the target materials (Al , Al2 , Al3 ), a big H peak and other peaks due to contamination such as hydrocarbons and oxygen on the target surface are found. We cleaned the targets by 3 kV Ar ion beam irradiation until no contamination peak was seen except hydrogen peaks like Fig. 1(b).
T. Sekioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 182 (2001) 121±126
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Fig. 2 shows a typical secondary ion mass spectrum from a C target bombarded by Xe44 at 400 keV. In the spectrum one can see several carbon clusters peaks indicated in the ®gure. The secondary ion yield was obtained after subtracting the background and de-convolution, if necessary. The secondary ion yield was normalized by the integrated beam current, and divided by the estimated detection eciency of the TOF that we obtained by the ion trajectory calculation. In the estimation, we assumed a 100% eciency of the ion detection of the MCP in the TOF. (a)
3. Results and discussion 3.1. Charge dependence of the secondary ion yield
(b) Fig. 1. Secondary ion mass spectra from an Al target bombarded by (a) Xe30 at 2.5 MeV where the cleaning is insucient, and (b) Xe40 at 1.7 MeV where the cleaning is sucient.
Fig. 2. A typical secondary ion mass spectrum from a C target bombarded by Xe44 at 400 keV.
Fig. 3 shows the incident charge state dependence of the secondary ion yield as a function of the potential energy Epot of the projectile Xeq for (a) Al, Si, Ni, Cu bombarded by 300 keV Xeq (q 15±44), and for (b) Al, Si, Ni, Cu, Ag bombarded by 1.0 MeV Xeq (q 15±44) [9]. The Epot , the sum of the binding energy of electrons removed to form the ions was calculated by a multicon®guration Dirac±Fock code [12]. The charge q of the projectile Xeq is shown in each ®gure. In both spectra, the secondary ion yields increase, as the charge of the incident Xe ion increases for every target, and the yields increase signi®cantly and linearly with Epot at q P 35 (Epot P 25:5 keV), though the increase rate in the region q 6 35 is rather moderate. At the bombarding energy of 300 keV (Fig. 3(a)), one can see a clear dierence in the secondary ion yield from metal (Al, Ni, Cu) targets and a semiconductor (Si) target in the region of q 6 35. The linear increase of the secondary ion yield from Si starts already at q 15 (Epot 2:2 keV). On the contrary, the yield of the secondary ion yield from metal targets remain almost constant at q 6 35. This dierence can be attributed to the rapid neutralization of the electronic excitation in metals. The signi®cant linear increase of the secondary ion yield begins at q P 35 for each metal target. After the eects of incident ion charge in electronic sputtering from LiF and Si at impact
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T. Sekioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 182 (2001) 121±126
(a)
(b) Fig. 3. The incident charge state dependence of the secondary ion yield as a function of the potential energy of the projectile Xeq for (a) Al, Si, Ni, Cu bombarded by 300 keV Xeq (q 15±44), and for (b) Al, Si, Ni, Cu, Ag bombarded by 1.0 MeV Xeq (q 15±44). The charge q of the projectile Xeq is shown in parentheses in each ®gure.
of slow Arq HCI q 6 9 were reported [13,14], electronic energy loss process have been found to be important for sputtering of insulators and poor conductors. Defect mediated eect has been proposed as a mechanism to explain the total sputter yields induced by Arq (q P 14) and Xeq q P 27 for LiF and SiO2 [15]. Coulomb explosion model [16] is another model for electronic sputtering of non metals by slow HCIs, in which charge state dependent electronic sputtering was related to electrostatic repulsion of target ions in a charge depleted region at the surface. There are a few reports on the observations of electronic sputtering eects in the interaction of slow HCI with conductors by the measurements of secondary ion emission [9,17,18], energy loss in thin carbon foil [4,19] and pre-equilibrium
eect in the charge state distribution after penetrating the thin carbon foil [5]. All these observation of the enhancement of the electronic excitation eect in conductors have been made by using HCI projectiles with very high potential energy. The strong onset of secondary ion yield from metals bombarded by Xeq (q P 35) found in the present measurement is one of clear evidences of electronic excitation eect in metals when imposed with very high potential energy. The dierences of the secondary ion yields between target materials are correlated with the mass of the target atom. As a general trend, the ion yields are smaller for heavier target atoms. In the COB model [6], de-excitation of hollow atoms in solids progresses via Auger cascades and radiative transitions. It is more dicult for lighter target electrons to ®ll the inner shell vacancies of heavy projectile Zprojectile 54 directly, so that the mean de-excitation time is larger for lighter target. This results in the increased eective charge of hollow atoms and high electronic excitation in lighter target [1,19], which can explain qualitatively the target mass dependence of the secondary ion yield. The secondary ion yields from C target bombarded by Xe44 at bombarding energies 300 keV and 1.0 MeV, which will be shown in Section 3.2, were lower than the ones from Al, but larger than the ones from Si. At present, we cannot give the particular reason for the large secondary ion yield from Al. 3.2. Projectile energy dependence of the secondary ion yield Fig. 4 shows the bombarding energy dependence of the secondary ion yield from the surface of (a) C and (b) Al, bombarded by Xeq (q 30; 36; 44) between 76 keV vp 0:15 a:u and 6.0 MeV vp 1:3 a:u. For C target (Fig. 4(a)), ion yields vary weakly with bombarding energy at q 30; 36 and 44. For Al target (Fig. 4(b)), the ion yields vary weakly with bombarding energies at q 30 and 36, but it has broad enhancement at around 500 keV at q 44. According to TRIM calculation, the maximum of the nuclear stopping power Sn of Xe ion in C
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(a)
(b) Fig. 4. The bombarding energy dependence of the secondary ion yield from the surface of (a) C and (b) Al, bombarded by Xeq (q 30; 36; 44) between 76 keV vp 0:15 a:u and 6.0 MeV vp 1:3 a:u.
is 160 keV, and that in Al is 180 keV, which are rather near to the bombarding energies of the present measurement. In [19], the average energy loss DEave of Ar18 , 44 Xe and Au69 in thin carbon foil is given as a function of projectile velocity, where strong increase of DEave for Xe44 and Au69 is reported. According to their result, pre-equilibrium eect should exist in C target bombarded by Xe44 , but no prominent enhancement have been found in our secondary ion measurement as a function of bombarding energy. No observation of the enhancement may be explained by the reason that, as the velocity becomes higher, the HCI releases its potential energy in a deeper position in the target, so that the secondary ion emission may be hindered. In [20], synergy of electronic excitations and elastic collision spikes in sputtering from uranium
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oxide has been reported. They measured secondary ion yields as a function of kinetic energy for Xe27 , Xe44 , Au63 and Au69 . Their results are that, ion yields for Xe projectiles of impact energy between 20 and 500 keV vary only weakly, and the one for Au63 has some structure and the one for Au69 shows a pronounced peak at around 220 keV. A broad enhancement of the secondary ion yield from Al bombarded by Xe44 is found in the present experiment. The target (Al) of the present experiment is much lighter than that (uranium oxide) of [20]. This dierence of mass ratio of the projectile and target may be the reason for the observation of synergy eect in our experiment. We have measured the energy dependence of secondary ion yield up to 6.0 MeV where the electronic stopping power (Se ) dominates over Sn , but we cannot observe the enhancement of the secondary ion emission which may be expected by the increased Se at higher bombarding energy. 4. Conclusions In order to investigate the electronic excitation eect in conductive materials induced by highly charged heavy ions, we have measured mass spectra of secondary ion, ®rst, as a function of incident charge, and second, as a function of projectile energy. In the ®rst experiment, the secondary ion yields showed a signi®cant increase with increasing projectile charge state q, demonstrating the electronic sputtering in the interaction of slow HCIs with conductive materials. In view of the secondary ion yields as a function of the potential energy of the projectile, the increase rate below q 35 (Epot 25:5 keV) were rather moderate and showed a prominent increase above q 35. These phenomena were rather strong in the case of the metal targets which give a clear evidence of electronic excitation eect in metals. In the second experiment, the energy dependence of the secondary ion yields show almost no dependence on projectile energy, except for the case of Al target bombarded by Xe44 where a broad peak around 500 keV of projectile energy
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