- Email: [email protected]
Convoy electrons emitted by 2-MeV He+ ions at grazing incidence on KCl(0 0 1)
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 248 (2006) 21–24 www.elsevier.com/locate/nimb
Convoy electrons emitted by 2-MeV He+ ions at grazing incidence on KCl(0 0 1) K. Nakajima, A. Nakamoto, M. Suzuki, K. Kimura
*
Department of Micro Engineering, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan Received 9 January 2006; received in revised form 3 March 2006 Available online 5 June 2006
Abstract Convoy electrons produced during grazing angle scattering of 2-MeV He+ ions at a clean (0 0 1) surface of KCl are measured to see the effect of the surface track potential. The measurement is performed at 230 C with a beam current far below 1 pA to avoid macroscopic charging. The observed convoy electron energy coincides with the energy of the electron isotachic to the incident ion. This suggests that the effect of the surface track potential is accidentally cancelled out by the surface wake potential. 2006 Elsevier B.V. All rights reserved. PACS: 34.50.Dy; 61.82.Ms; 79.20.Rf Keywords: Convoy electron; Grazing angle scattering; Surface track potential; Surface wake potential; KCl(0 0 1)
1. Introduction When an ion is incident on an atomically flat surface at a grazing angle the ion is subject to a series of correlated small angle scatterings [1]. As a result, the ion is reflected at a specular angle without penetration into the solid. This phenomenon, called specular reflection of fast ion, is very suitable to study ion–surface interactions [2]. In our previous study, we showed that more than three hundred secondary electrons (SEs) are ejected during the specular reflection of 2-MeV He+ at KCl(0 0 1) surfaces [3]. As SE leaves a hole on the surface, emission of such a large number of SEs induces a strong positive surface track potential behind the projectile ion. The induced surface track potential may affect the motion of SEs, if the relaxation time of the created hole is longer than the ion–surface interaction time. Actually, in foil transmission experiments, the influence of the track potential on Auger electrons and convoy elec-
*
Corresponding author. Tel./fax: +81 75 753 5253. E-mail address: [email protected] (K. Kimura).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.186
trons was observed [4,5]. Regarding the surface track potential, however, there have been a limited number of studies so far. A signature of the recapturing of SEs by the surface track potential was observed by measuring the projectile Z-number dependence of the SE yield for 0.5 MeV/amu H+, He2+ and Li2+ reflected from KCl(0 0 1) [3]. More direct evidence of the surface track potential was claimed by Go´metz et al. They measured energy distributions of SEs emitted at forward angles during grazing angle scattering of 60–100 keV H+ at LiF(0 0 1) [6]. In the observed spectrum there was a broad peak at the energy slightly lower than the energy of the electron isotachic to the incident ion. They concluded that the observed broad peak corresponds to the convoy electrons which were decelerated by the surface track potential. Recent careful measurements with a smooth LiF(0 0 1) surface, however, showed that convoy electrons emitted by 100 keV H+ are accelerated by up to 5 eV, which was attributed to the surface wake potential [7]. The observed maximum energy gain (5 eV) is about one third of the energy gain observed with Al(1 1 1) [7]. This is consistent with the dielectric constant of LiF (e = 1.96), which reduces the surface wake potential by (e 1)/(e + 1) 0.3
K. Nakajima et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 21–24
compared with metal surfaces. This indicates that the surface track potential is negligibly small even if the surface track potential exists. In the present paper, we extend their measurement to high energy He+ ions to confirm the existence of the surface track potential. We observe convoy electrons emitted during grazing angle scattering of 2-MeV He+ at KCl(0 0 1). The number of SEs emitted by 2-MeV He+ at KCl(0 0 1) was reported to be 300 [3] while that for 100 keV H+ at LiF(0 0 1) was 10 [6]. This suggests that a strong surface track potential is induced in the present system and a larger deceleration of convoy electrons due to the surface track potential is expected if the surface track potential really exists.
600
2-MeV He INTENSITY (arb. units)
22
400
3. Results and discussion Fig. 1 shows examples of the electron spectra multiplied by electron energy observed at grazing angle scattering of 2-MeV He+ ions for various beam currents. There is a broad convoy electron peak around 250 eV. A broad hump at 600–1000 eV corresponds to the binary electrons. The observed binary peak is extremely broad compared with that observed in ion-foil experiments. Although the broadening of the binary peak is beyond the scope of the present work, we note that similar broadening was reproduced by a classical trajectory Monte Carlo simulation [10]. The arrow shown in Fig. 1 indicates the energy of electrons isotachic to the incident He+ ion. The observed peak energy of the convoy electrons is slightly lower than this energy. The
KCl(001), θ i = 3 mrad 6 pA 25 pA 60 pA
200
0
0
500
1000
ELECTRON ENERGY (eV)
2. Experimental
Fig. 1. Electron spectra multiplied by electron energy observed at grazing angle scattering of 2-MeV He+ ions from KCl(0 0 1). The results for various beam currents are shown: 6 pA (dotted curve), 25 pA (dashed curve) and 60 pA (solid curve). The arrow shows the energy of electron isotachic to the incident He ion.
2-MeV He
PEAK ENERGY (eV)
A single crystal of KCl was cleaved in air and mounted on a high-precision five-axis goniometer in an ultra-high vacuum scattering chamber. After baking the scattering chamber, the KCl(0 0 1) crystal was heated at 250 C for several hours to prepare a clean surface [8]. The surface thus prepared was atomically flat and the mean step density observed by atomic force microscope was about 5 · 103 nm1 [9]. During the measurement, the surface was kept at 230 C to avoid macroscopic charging. A beam of 2-MeV He+ ions from the 4 MV van de Graaff accelerator of Kyoto University was collimated by a series of apertures to less than 0.1 · 0.1 mm2 and a divergence angle less than 0.3 mrad. The beam current was monitored by a vibrating beam chopper installed just before the chamber. The beam was incident on the KCl(0 0 1) at a grazing angle hi = 3 mrad. The energy spectrum of secondary electrons emitted at he = 100 mrad was measured with a 30 parallel-plate electrostatic analyzer (DE/E = 0.04, acceptance angle = 10 msr). The analyzer had small holes in its electrodes, through which reflected ions can pass. The reflected ions passing through the analyzer were detected by a ceramic secondary electron multiplier (ceratron) for monitoring the beam intensity when the beam current was less than 1 pA.
+
+
KCl(001), θ i = 3 mrad
250
200
150
0
20
40
60
BEAM CURRENT (pA) Fig. 2. Observed peak energy of convoy electrons as a function of the beam current.
convoy peak was fitted by a Gaussian function to estimate the peak energy. Fig. 2 shows the observed peak energy as a function of the beam current. The error bars shown in Fig. 2 were estimated from the variation of the peak energies estimated with different fitting regions. The convoy electron energy decreases with increasing beam current, indicating that the effect of the macroscopic charging could not be avoided even though the surface was heated at 230 C. To avoid the macroscopic charging, the beam current was reduced far below 1 pA. Because the beam current was too low to be measured by the beam chopper, the beam intensity was monitored by the count rate of the reflected ions which were measured by the ceratron. Fig. 3 shows the electron spectra multiplied by electron
K. Nakajima et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 21–24 300
INTENSITY (arb. units)
2-MeV He
+
KCl(001), θ i = 3 mrad 1500 cps 4000 cps SnTe(001)
200
100
0
0
500
1000
ELECTRON ENERGY (eV) Fig. 3. Electron spectra multiplied by electron energy observed at grazing angle scattering of 2-MeV He+ ions from KCl(0 0 1). The results for two different ion count rates: 1500 cps (dashed curve) and 4000 cps (solid curve) are shown. The arrow shows the energy of electron isotachic to the incident He ion. The result for SnTe(0 0 1) is shown by dotted curve for comparison.
energy observed at two different beam intensities, 1500 (dashed curve) and 4000 cps (solid curve). The spectrum does not depend on the beam intensity, indicating that the macroscopic charging was negligible in this low beam current region. This is consistent with the previous measurement of SE yield emitted by 2-MeV He+ scattered from KCl(0 0 1) at elevated temperatures [11]. The observed convoy electron peak is sharper than that observed at high beam currents (see Fig. 1) and the peak energy agrees with the energy of electrons isotachic to the incident ion (shown by the arrow in Fig. 3), suggesting that the surface track potential does not affect the convoy electrons. This unexpected result might be explained in terms of the surface wake potential. When a projectile ion travels in front of the surface, an image potential is induced. If the velocity of the ion is faster than the velocity of the electrons in the solid, the image potential is delayed behind the projectile by pv/2xs, where v is the ion velocity and xs is the frequency of surface plasmon. This dynamical image potential, called ‘‘surface wake potential’’, accelerates convoy electrons [12,13]. An example of the observed energy spectrum of secondary electrons emitted by 2-MeV He ions at a semiconductor SnTe(0 0 1) surface is shown by a dotted curve in Fig. 3. It is seen that the convoy electrons emitted from SnTe(0 0 1) are accelerated by 30 eV due to the surface wake potential. Even in the insulator surfaces the dynamical image potential is induced although it is reduced by (e 1)/ (e + 1). Actually, image acceleration of multiply charged ions was observed at LiF surfaces [14]. Thus, both the surface track potential and the surface wake potential are induced at insulator surfaces. The present result indicates that the deceleration of convoy electrons due the surface track potential is accidentally cancelled out by the acceleration due the surface wake potential in the present case.
23
The dynamical image force acting on the convoy electron can be estimated by [13]: 2 ðe 1Þ 2xs ðq 1Þe2 ; ð1Þ F ðe þ 1Þ pv when the distance from the surface is much smaller than ˚ in the present case), where q is the projectile pv/2xs (9 A charge number and the effect of the self-image of the convoy electron is included. Substituting e = 4.85 (static dielectric constant of KCl), q = 2 and hxs = 11 eV, the image ˚ . The present result indiforce is calculated to be 0.12 eV/A cates that the dynamical image force is cancelled out by the surface track potential. Thus, the force of the surface track potential acting on the convoy electron produced by 2˚. MeV He+ at KCl(0 0 1) is estimated to be 0.12 eV/A Except for the convoy electron peak, there is another difference between the SE spectra for KCl and SnTe. The yield of low energy electrons for KCl is much smaller than that for SnTe. The low energy SEs emitted at the forward direction are deflected by the surface track potential. As a result, the low energy SE yield is reduced at the forward direction. Although the surface wake potential also affects the motion of low energy SEs, the effect of the surface wake potential is relatively small. This is because the surface wake potential travels with the projectile ion while the low energy SE stays near the place of birth. 4. Conclusion Convoy electrons produced during grazing angle scattering of 2-MeV He+ ions at a clean (0 0 1) surface of KCl are measured. The observed convoy electron peak appears at the energy of the normal convoy electron, indicating that the effect of the surface track potential is accidentally cancelled by the dynamical image potential. The force of the surface track potential acting on the convoy electron is esti˚ . Large reduction of low energy SE mated to be 0.12 eV/A yield emitted at the forward direction was observed, which is attributed to the surface track potential. Acknowledgements We are grateful to the members of the Quantum Science and Engineering Center at Kyoto University for the use of the Van de Graaff accelerator. This work was supported in part by Center of Excellence for Research and Education on Complex Functional Mechanical Systems (COE program) of the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] C. Varelas, R. Sizmann, Surf. Sci. 71 (1978) 129. [2] K. Kimura, M. Hasegawa, Y. Fujii, M. Suzuki, U.Y. Susuki, M. Mannami, Nucl. Instr. and Meth. B 33 (1988) 358. [3] K. Kimura, S. Usui, K. Maeda, K. Nakajima, Nucl. Instr. and Meth. B 193 (2002) 661.
24
K. Nakajima et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 21–24
[4] G. Schiwietz, P.L. Grande, B. Skogvall, J.P. Biersack, R. Ko¨hrbru¨ck, K. Sommer, A. Schmoldt, P. Goppelt, I. Ka´da´r, S. Ricz, U. Stettner, Phys. Rev. Lett. 69 (1992) 628. [5] G. Xiao, G. Schiwietz, P.L. Grande, N. Stolterfoht, A. Schmoldt, M. Grether, R. Ko¨hrbru¨ck, A. Spieler, U. Stettner, Phys. Rev. Lett. 79 (1997) 1821. [6] G.R. Go´mez, O. Grizzi, E.A. Sa´nchez, V.H. Ponce, Phys. Rev. B 58 (1998) 7403. [7] V.H. Ponce, L.F. de Ferrariis, O. Grizzi, M.L. Martiarena, E.A. Sa´nchez, Nucl. Instr. and Meth. B 232 (2005) 37.
[8] M. Prutton, Surface Physics, Clarendon, Oxford, 1983, p. 9. [9] K. Narumi, Y. Fujii, K. Kimura, M. Mannami, H. Hara, Surf. Sci. 303 (1994) 187. [10] C.O. Reinhold, J. Burgdo¨rfer, K. Kimura, M. Mannami, Phys. Rev. Lett. 73 (1994) 2508. [11] K. Kimura, G. Andou, K. Nakajima, Phys. Rev. Lett. 81 (1998) 5438. [12] J. Burgdo¨rfer, Nucl. Instr. and Meth. B 24–25 (1987) 139. [13] M. Hasegawa, K. Kimura, M. Mannami, J. Phys. Soc. Jpn. 57 (1988) 1834. [14] C. Auth, T. Hecht, T. Igel, H. Winter, Phys. Rev. Lett. 74 (1995) 74.
Nuclear Instruments and Methods in Physics Research B 248 (2006) 21–24 www.elsevier.com/locate/nimb
Convoy electrons emitted by 2-MeV He+ ions at grazing incidence on KCl(0 0 1) K. Nakajima, A. Nakamoto, M. Suzuki, K. Kimura
*
Department of Micro Engineering, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan Received 9 January 2006; received in revised form 3 March 2006 Available online 5 June 2006
Abstract Convoy electrons produced during grazing angle scattering of 2-MeV He+ ions at a clean (0 0 1) surface of KCl are measured to see the effect of the surface track potential. The measurement is performed at 230 C with a beam current far below 1 pA to avoid macroscopic charging. The observed convoy electron energy coincides with the energy of the electron isotachic to the incident ion. This suggests that the effect of the surface track potential is accidentally cancelled out by the surface wake potential. 2006 Elsevier B.V. All rights reserved. PACS: 34.50.Dy; 61.82.Ms; 79.20.Rf Keywords: Convoy electron; Grazing angle scattering; Surface track potential; Surface wake potential; KCl(0 0 1)
1. Introduction When an ion is incident on an atomically flat surface at a grazing angle the ion is subject to a series of correlated small angle scatterings [1]. As a result, the ion is reflected at a specular angle without penetration into the solid. This phenomenon, called specular reflection of fast ion, is very suitable to study ion–surface interactions [2]. In our previous study, we showed that more than three hundred secondary electrons (SEs) are ejected during the specular reflection of 2-MeV He+ at KCl(0 0 1) surfaces [3]. As SE leaves a hole on the surface, emission of such a large number of SEs induces a strong positive surface track potential behind the projectile ion. The induced surface track potential may affect the motion of SEs, if the relaxation time of the created hole is longer than the ion–surface interaction time. Actually, in foil transmission experiments, the influence of the track potential on Auger electrons and convoy elec-
*
Corresponding author. Tel./fax: +81 75 753 5253. E-mail address: [email protected] (K. Kimura).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.186
trons was observed [4,5]. Regarding the surface track potential, however, there have been a limited number of studies so far. A signature of the recapturing of SEs by the surface track potential was observed by measuring the projectile Z-number dependence of the SE yield for 0.5 MeV/amu H+, He2+ and Li2+ reflected from KCl(0 0 1) [3]. More direct evidence of the surface track potential was claimed by Go´metz et al. They measured energy distributions of SEs emitted at forward angles during grazing angle scattering of 60–100 keV H+ at LiF(0 0 1) [6]. In the observed spectrum there was a broad peak at the energy slightly lower than the energy of the electron isotachic to the incident ion. They concluded that the observed broad peak corresponds to the convoy electrons which were decelerated by the surface track potential. Recent careful measurements with a smooth LiF(0 0 1) surface, however, showed that convoy electrons emitted by 100 keV H+ are accelerated by up to 5 eV, which was attributed to the surface wake potential [7]. The observed maximum energy gain (5 eV) is about one third of the energy gain observed with Al(1 1 1) [7]. This is consistent with the dielectric constant of LiF (e = 1.96), which reduces the surface wake potential by (e 1)/(e + 1) 0.3
K. Nakajima et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 21–24
compared with metal surfaces. This indicates that the surface track potential is negligibly small even if the surface track potential exists. In the present paper, we extend their measurement to high energy He+ ions to confirm the existence of the surface track potential. We observe convoy electrons emitted during grazing angle scattering of 2-MeV He+ at KCl(0 0 1). The number of SEs emitted by 2-MeV He+ at KCl(0 0 1) was reported to be 300 [3] while that for 100 keV H+ at LiF(0 0 1) was 10 [6]. This suggests that a strong surface track potential is induced in the present system and a larger deceleration of convoy electrons due to the surface track potential is expected if the surface track potential really exists.
600
2-MeV He INTENSITY (arb. units)
22
400
3. Results and discussion Fig. 1 shows examples of the electron spectra multiplied by electron energy observed at grazing angle scattering of 2-MeV He+ ions for various beam currents. There is a broad convoy electron peak around 250 eV. A broad hump at 600–1000 eV corresponds to the binary electrons. The observed binary peak is extremely broad compared with that observed in ion-foil experiments. Although the broadening of the binary peak is beyond the scope of the present work, we note that similar broadening was reproduced by a classical trajectory Monte Carlo simulation [10]. The arrow shown in Fig. 1 indicates the energy of electrons isotachic to the incident He+ ion. The observed peak energy of the convoy electrons is slightly lower than this energy. The
KCl(001), θ i = 3 mrad 6 pA 25 pA 60 pA
200
0
0
500
1000
ELECTRON ENERGY (eV)
2. Experimental
Fig. 1. Electron spectra multiplied by electron energy observed at grazing angle scattering of 2-MeV He+ ions from KCl(0 0 1). The results for various beam currents are shown: 6 pA (dotted curve), 25 pA (dashed curve) and 60 pA (solid curve). The arrow shows the energy of electron isotachic to the incident He ion.
2-MeV He
PEAK ENERGY (eV)
A single crystal of KCl was cleaved in air and mounted on a high-precision five-axis goniometer in an ultra-high vacuum scattering chamber. After baking the scattering chamber, the KCl(0 0 1) crystal was heated at 250 C for several hours to prepare a clean surface [8]. The surface thus prepared was atomically flat and the mean step density observed by atomic force microscope was about 5 · 103 nm1 [9]. During the measurement, the surface was kept at 230 C to avoid macroscopic charging. A beam of 2-MeV He+ ions from the 4 MV van de Graaff accelerator of Kyoto University was collimated by a series of apertures to less than 0.1 · 0.1 mm2 and a divergence angle less than 0.3 mrad. The beam current was monitored by a vibrating beam chopper installed just before the chamber. The beam was incident on the KCl(0 0 1) at a grazing angle hi = 3 mrad. The energy spectrum of secondary electrons emitted at he = 100 mrad was measured with a 30 parallel-plate electrostatic analyzer (DE/E = 0.04, acceptance angle = 10 msr). The analyzer had small holes in its electrodes, through which reflected ions can pass. The reflected ions passing through the analyzer were detected by a ceramic secondary electron multiplier (ceratron) for monitoring the beam intensity when the beam current was less than 1 pA.
+
+
KCl(001), θ i = 3 mrad
250
200
150
0
20
40
60
BEAM CURRENT (pA) Fig. 2. Observed peak energy of convoy electrons as a function of the beam current.
convoy peak was fitted by a Gaussian function to estimate the peak energy. Fig. 2 shows the observed peak energy as a function of the beam current. The error bars shown in Fig. 2 were estimated from the variation of the peak energies estimated with different fitting regions. The convoy electron energy decreases with increasing beam current, indicating that the effect of the macroscopic charging could not be avoided even though the surface was heated at 230 C. To avoid the macroscopic charging, the beam current was reduced far below 1 pA. Because the beam current was too low to be measured by the beam chopper, the beam intensity was monitored by the count rate of the reflected ions which were measured by the ceratron. Fig. 3 shows the electron spectra multiplied by electron
K. Nakajima et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 21–24 300
INTENSITY (arb. units)
2-MeV He
+
KCl(001), θ i = 3 mrad 1500 cps 4000 cps SnTe(001)
200
100
0
0
500
1000
ELECTRON ENERGY (eV) Fig. 3. Electron spectra multiplied by electron energy observed at grazing angle scattering of 2-MeV He+ ions from KCl(0 0 1). The results for two different ion count rates: 1500 cps (dashed curve) and 4000 cps (solid curve) are shown. The arrow shows the energy of electron isotachic to the incident He ion. The result for SnTe(0 0 1) is shown by dotted curve for comparison.
energy observed at two different beam intensities, 1500 (dashed curve) and 4000 cps (solid curve). The spectrum does not depend on the beam intensity, indicating that the macroscopic charging was negligible in this low beam current region. This is consistent with the previous measurement of SE yield emitted by 2-MeV He+ scattered from KCl(0 0 1) at elevated temperatures [11]. The observed convoy electron peak is sharper than that observed at high beam currents (see Fig. 1) and the peak energy agrees with the energy of electrons isotachic to the incident ion (shown by the arrow in Fig. 3), suggesting that the surface track potential does not affect the convoy electrons. This unexpected result might be explained in terms of the surface wake potential. When a projectile ion travels in front of the surface, an image potential is induced. If the velocity of the ion is faster than the velocity of the electrons in the solid, the image potential is delayed behind the projectile by pv/2xs, where v is the ion velocity and xs is the frequency of surface plasmon. This dynamical image potential, called ‘‘surface wake potential’’, accelerates convoy electrons [12,13]. An example of the observed energy spectrum of secondary electrons emitted by 2-MeV He ions at a semiconductor SnTe(0 0 1) surface is shown by a dotted curve in Fig. 3. It is seen that the convoy electrons emitted from SnTe(0 0 1) are accelerated by 30 eV due to the surface wake potential. Even in the insulator surfaces the dynamical image potential is induced although it is reduced by (e 1)/ (e + 1). Actually, image acceleration of multiply charged ions was observed at LiF surfaces [14]. Thus, both the surface track potential and the surface wake potential are induced at insulator surfaces. The present result indicates that the deceleration of convoy electrons due the surface track potential is accidentally cancelled out by the acceleration due the surface wake potential in the present case.
23
The dynamical image force acting on the convoy electron can be estimated by [13]: 2 ðe 1Þ 2xs ðq 1Þe2 ; ð1Þ F ðe þ 1Þ pv when the distance from the surface is much smaller than ˚ in the present case), where q is the projectile pv/2xs (9 A charge number and the effect of the self-image of the convoy electron is included. Substituting e = 4.85 (static dielectric constant of KCl), q = 2 and hxs = 11 eV, the image ˚ . The present result indiforce is calculated to be 0.12 eV/A cates that the dynamical image force is cancelled out by the surface track potential. Thus, the force of the surface track potential acting on the convoy electron produced by 2˚. MeV He+ at KCl(0 0 1) is estimated to be 0.12 eV/A Except for the convoy electron peak, there is another difference between the SE spectra for KCl and SnTe. The yield of low energy electrons for KCl is much smaller than that for SnTe. The low energy SEs emitted at the forward direction are deflected by the surface track potential. As a result, the low energy SE yield is reduced at the forward direction. Although the surface wake potential also affects the motion of low energy SEs, the effect of the surface wake potential is relatively small. This is because the surface wake potential travels with the projectile ion while the low energy SE stays near the place of birth. 4. Conclusion Convoy electrons produced during grazing angle scattering of 2-MeV He+ ions at a clean (0 0 1) surface of KCl are measured. The observed convoy electron peak appears at the energy of the normal convoy electron, indicating that the effect of the surface track potential is accidentally cancelled by the dynamical image potential. The force of the surface track potential acting on the convoy electron is esti˚ . Large reduction of low energy SE mated to be 0.12 eV/A yield emitted at the forward direction was observed, which is attributed to the surface track potential. Acknowledgements We are grateful to the members of the Quantum Science and Engineering Center at Kyoto University for the use of the Van de Graaff accelerator. This work was supported in part by Center of Excellence for Research and Education on Complex Functional Mechanical Systems (COE program) of the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] C. Varelas, R. Sizmann, Surf. Sci. 71 (1978) 129. [2] K. Kimura, M. Hasegawa, Y. Fujii, M. Suzuki, U.Y. Susuki, M. Mannami, Nucl. Instr. and Meth. B 33 (1988) 358. [3] K. Kimura, S. Usui, K. Maeda, K. Nakajima, Nucl. Instr. and Meth. B 193 (2002) 661.
24
K. Nakajima et al. / Nucl. Instr. and Meth. in Phys. Res. B 248 (2006) 21–24
[4] G. Schiwietz, P.L. Grande, B. Skogvall, J.P. Biersack, R. Ko¨hrbru¨ck, K. Sommer, A. Schmoldt, P. Goppelt, I. Ka´da´r, S. Ricz, U. Stettner, Phys. Rev. Lett. 69 (1992) 628. [5] G. Xiao, G. Schiwietz, P.L. Grande, N. Stolterfoht, A. Schmoldt, M. Grether, R. Ko¨hrbru¨ck, A. Spieler, U. Stettner, Phys. Rev. Lett. 79 (1997) 1821. [6] G.R. Go´mez, O. Grizzi, E.A. Sa´nchez, V.H. Ponce, Phys. Rev. B 58 (1998) 7403. [7] V.H. Ponce, L.F. de Ferrariis, O. Grizzi, M.L. Martiarena, E.A. Sa´nchez, Nucl. Instr. and Meth. B 232 (2005) 37.
[8] M. Prutton, Surface Physics, Clarendon, Oxford, 1983, p. 9. [9] K. Narumi, Y. Fujii, K. Kimura, M. Mannami, H. Hara, Surf. Sci. 303 (1994) 187. [10] C.O. Reinhold, J. Burgdo¨rfer, K. Kimura, M. Mannami, Phys. Rev. Lett. 73 (1994) 2508. [11] K. Kimura, G. Andou, K. Nakajima, Phys. Rev. Lett. 81 (1998) 5438. [12] J. Burgdo¨rfer, Nucl. Instr. and Meth. B 24–25 (1987) 139. [13] M. Hasegawa, K. Kimura, M. Mannami, J. Phys. Soc. Jpn. 57 (1988) 1834. [14] C. Auth, T. Hecht, T. Igel, H. Winter, Phys. Rev. Lett. 74 (1995) 74.