- Email: [email protected]
Charge exchange of He+-ions with aluminium surfaces
Nuclear Instruments and Methods in Physics Research B 269 (2011) 1171–1174
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Charge exchange of He+-ions with aluminium surfaces S. Rund, D. Primetzhofer, S.N. Markin, D. Goebl, P. Bauer ⇑ Institut fuer Experimentalphysik, Abt. Atom – und Oberflaechenphysik, Johannes Kepler Universitaet, Altenbergerstr 69, A-4040 Linz, Austria
a r t i c l e
i n f o
Article history: Received 21 September 2010 Received in revised form 12 November 2010 Available online 23 November 2010 Keywords: Low-energy ion scattering Single crystal Neutralization Al(1 1 1) Ion fraction
a b s t r a c t Charge exchange of 4He+ and 3He+ ions with surfaces of polycrystalline aluminium and Al(1 1 1) was investigated in the low-energy ion scattering (LEIS) regime. Ion spectra were recorded for primary energies ranging from 280 to 4000 eV by using an electrostatic analyzer (ESA). A very low threshold energy Eth for collision induced charge exchange (CI) was deduced from the shape of experimental spectra. Ion fractions P+ were evaluated. No systematic difference in P+ was observed for both, the two surfaces investigated and the two different projectiles. Ó 2010 Elsevier B.V. All rights reserved.
R1
vc v j
1. Introduction
Pþj ffi e
LEIS is a commonly used analytical tool for analysis of composition or structure of the topmost atomic layers of solid surfaces [1– 3]. Typically, noble gas ions with energies in the range 0.4 to 10 keV are used as projectiles. The extremely high surface sensitivity is based on the large scattering cross section and the very efficient neutralization of primary ions [4]. Therefore, quantification of experimental LEIS data urgently requires a fundamental understanding of charge exchange. In LEIS, different charge exchange mechanisms are active depending on the primary energy and the atomic species of the projectile [5]. Non-local Auger neutralization (AN) is possible at all primary energies. Collision induced neutralization (CIN) and reionization (CIR) can only contribute for sufficiently close interaction distance between projectile and target atom, which promotes the He 1s level to binding energies resonant with the conduction band, below or above the Fermi level, respectively. This minimum distance rmin corresponds, for given scattering geometry, to an energy threshold Eth. For AN, the neutralization rate dP+ /dt depends on the Auger transition rate C(s) [6] via
where j stands for in or out, respectively, zmin is the minimum distance with respect to the surface plane of atoms, vc denotes the characteristic velocity which is a measure of neutralization efficiency and v\ for the perpendicular velocity of the projectile, i.e. v\in = v0 for perpendicular incidence and v\out = voutcos(ph), with the scattering angle h. Finally, the ion fraction results as þ Pþ ¼ P þ in P out . At E > Eth the ion fraction for single scattering from the first layer is given by
þ
þ
dP dP dt 1 dz ¼ Pþ CðzÞ $ þ ¼ CðzÞ dz ¼ C dz dt v? P
ð1Þ
From Eq. (1), the survival probabilities for incoming and outgoing trajectories are obtained by integration along the distance to the surface: ⇑ Corresponding author. Tel.: +43 732 2468 8516. E-mail address: [email protected] (P. Bauer). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.11.049
v 1 ?j
zmin
CðzÞdz
ffie
?j
;
Pþ ¼ Pþin ð1 PCIN Þ Pþout þ ð1 Pþin Þ P CIR Pþout
ð2Þ
ð3Þ
where the first term includes ions that remained charged along the entire trajectory and the second one describes those who are neutralized on the way in, reionized in a close collision and survived AN on the way out of the sample. In recent studies surface dependent neutralization efficiencies have been observed for He+ ions scattered from different noble metal surfaces [7,8]. A similar behaviour has been observed for experiments performed for He+ ions scattered from an Al single crystalline surface in grazing incidence and successfully explained by theory [9,10]. This study presents complementary information obtained in large angle backscattering experiments using an ESA-LEIS set-up. He+ ions were scattered from polycrystalline Al and Al(1 1 1). The investigation of different surfaces allows to study the influence of the surface orientation and of the information depth on the ion fraction. Furthermore, the presented experiments aimed at a comparison of P+ data obtained from measurements using 3He+ and 4He+ as projectiles. As stated above, AN scales with v\. In contrast,
1172
S. Rund et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 1171–1174
resonant charge exchange strongly depends on the interaction distance and thus, on energy. From these experiments information on the relative importance of different charge exchange mechanisms can be obtained. Furthermore, information on the onset of collision induced charge exchange and thus the corresponding minimum interaction distance to enable resonant charge transfer is sought.
a
2. Experiment Experiments were performed using the ESA-LEIS set-up MINIMOBIS [11] in the institute of experimental physics at Johannes Kepler University Linz. A beam of He ions with primary energies in the range of 0.2–4 keV is directed towards the sample surface in perpendicular incidence. Primary beam currents range from 0.05 to 2 nA. Ions that are backscattered by h = 136° pass a cylindrical mirror analyzer (CMA) with an azimuth acceptance of 2p and are detected by microchannel plates (MCP). A polished Al(1 1 1) single crystal with a roughness below 0.03 lm and a precision of the orientation of ±0.1° was used. Polycrystalline films are produced by ex situ evaporation. All surfaces were prepared by cycles of Ar+-sputtering and annealing (up to 400 °C). For the single crystal, surface quality was checked by low-energy electron diffraction (LEED). The surfaces where regularly investigated for surface contamination (mainly O and C) by LEIS spectra.
b
3. Results and discussion In LEIS, the ion yield J þ i backscattered from surface atoms ‘‘i’’ of mass mi, is a measure for the atomic surface concentration Ni via
J þi ¼ I0
dri Ni Pþi nþ ; dX
ð4Þ
where I0 denotes the primary ion current, n+ the transmission function of the spectrometer including the detection efficiency, and dr/dX the differential scattering cross section. For the ESA used in this experimental study it was possible to obtain n+ from reference measurements on polycrystalline Cu, for which P+ is known from TOF-LEIS measurements [12]. This permits a quantitative evaluation of the equivalent information deduced from ESA-LEIS experiments [13]. For this calibration, Cu is an ideal candidate since it features high Eth (2 keV). Thus, no reionization processes take place and the ion signal originates from the outermost surface exclusively. A typical spectrum obtained for 1 keV He+ ions scattered from Cu is shown in Fig. 1a. The peak shape can be almost perfectly reproduced by a Gaussian. This should be anticipated since the ion signal originates from single scattered ions in the first layer exclusively and only insignificant inelastic loss straggling are expected. From the width of the ion peaks (FWHM) measured at different energies, it is possible to deduce the system resolution DE. In Fig. 2, DE/E can be obtained from the slope of the Cu data (black triangles), which is found to be almost constant, with a value of 1.5%. In Fig. 1b, a typical spectrum obtained for 1 keV He+ scattered from Al is depicted. The shape of this spectrum is significantly different than for Cu (see Fig. 1a), and a Gaussian fit cannot reproduce the low energy tail of the spectrum. However, since the threshold velocity Eth for Al is claimed to be very small (<500 eV) [14,15], additional charge exchange processes take place and ions scattered in deeper layers may influence the ion yield [7]. When a particle is reionized, its kinetic energy is reduced by DECIR due to the electron promotion process that takes place. In the preceding neutralization, an electron is captured into a state, which is below the Fermi level Ef, typically by several eV. Consecutively, it is shifted above Ef (at distances smaller than rmin), where the electron is transferred to
Fig. 1. (a) Experimental spectrum of 1 keV 4He+ scattered from Cu. A Gaussian perfectly matches the ion distribution. (b) Experimental spectrum of 1 keV 4He+ scattered from Al. The ion peak can be modelled by two Gaussian distributions, survivals and reionized projectiles shifted in energy by DECIR, respectively.
Fig. 2. Ion peak width (FWHM) detected at different final energies kE0. For 4He+ scattered from Cu, a linear fit matches the data and yields the energy resolution of the detection system DE/E < 0.015. For 4He+ scattered from Al, ion peaks are found wider due to additional contribution from reionized projectiles. Extrapolation of the data for Al to lower energies permits to determine the reionization threshold Eth.
S. Rund et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 1171–1174
Fig. 3. Experimentally obtained ion fractions for 3He+ and 4He+ projectiles scattered from polycrystalline Al and Al(1 1 1). Also shown are fits to the data according to Eq. (2) and experimental data from [19].
unoccupied states of the conduction band of the target. If the projectile was initially neutralized by Auger neutralization DECIR can be estimated from the position of the unperturbed 1s level and the work function u of the solid to be at most 20 eV. In Fig. 1b, the spectrum can be reasonably well reproduced by a sum of two Gaussians shifted in energy by 20 eV with respect to each other, representing survivals and reionized projectiles, respectively (see Eq. (3)). However, at the low energy onset, a residual contribution cannot easily be fitted within this model. This extra intensity can probably be attributed to a small fraction of ions that survive backscattering from second or third layer atoms. The broadening of the ion peak due to reionization can be used to deduce Eth for Al (see Fig. 2). In comparison to Cu, the widths of the ion peaks (FWHM) obtained from the present experiment on Al, are significantly higher in the whole range of investigated energies and exhibit no energy proportionality. At energies larger than 0.5 keV, the difference is almost constant at 15 eV and only slowly decreases at highest energies. At very low energies, however, the difference becomes very small. This is an indication for fewer contributions from reionized particles. A fit through the data for 4He at very low energies, i.e. kE0 < 0.27 intersects with the extrapolated curve from the experiment on Cu at an energy kE0 1 of 11020 30 eV (red line and red dotted lines, respectively) . From this, Eth 190 eV can be calculated, corresponding to a minimum distance rmin < 0.75 a.u. (employing the ZBL-potential [16]), in good agreement with theoretical prediction [17]. For the evaluation of P+, experimental spectra were fitted by a Gaussian optimized to focus on single scattered contributions from the first layer. A linear background was subtracted, and the tail of ions at lower energies was excluded from the integration. This evaluation was applied to 3He+ and 4He+ projectiles scattered from polycrystalline and Al(1 1 1); the results are shown in Fig. 3. As a result, the data obtained from both surfaces almost coincide. This indicates that for both samples only minor sub-surface contributions have entered the evaluation. This is also in good agreement with similar observations in TOF-LEIS for He+ on Ag [18] indicating that the polycrystalline Al surface consists of azimuthally randomly oriented (1 1 1)-facets. Fig. 3 also shows data obtained in a previous experiment in a smaller energy range by [19] in excellent agreement with the present results.
1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
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The P+ results shown in Fig. 3 can be fitted by a single exponential according to Eq. (2), which is expected to be valid in the AN regime only. Nevertheless, from the shape of the ion peaks and the evaluation shown above one has to conclude that all data presented are recorded in the reionization regime. Furthermore, calculated Auger rates support the assumption that collision induced neutralization is the dominant neutralization process [17,19]. In this context the question arises how to interpret the fact that data for 3He+ and 4He+ coincide within statistical uncertainty, when P+ is plotted as a function of 1/v\. In other words, the length of the time interval where charge exchange can occur appears to be dominant with respect to the slightly different trajectories obtained for the He isotopes at similar perpendicular velocity. This is possible for two different scenarios: One possibility would be that only AN is active, and v\-scaling of the ion fraction has to be expected from [6]. Alternatively, PCIN may be close to unity in the whole range of investigated energies and the probability for collision induced reionization, PCIR, is increasing slowly with primary energy. The 1/v\-dependence of P+, obtained for He+ and Al, is in contrast to what has been observed for Cu, Ag and Au [8,12], where data show a strong deviation from a behaviour according to Eq. (2) when the projectile energy exceeds Eth. However, in the present investigation all P+ data have been recorded in the regime E Eth. Data for Cu together with an evaluation of PCIR [12] for different energies indicate that PCIN PCIR, and the yield of reionized particles strongly increases at E 3 Eth. This should also apply to the He+/Al case, due to the strong promotion of the He 1s-level in a collision with Al at the interaction distances probed in this study [17]. 4. Conclusions The presented experiments probe the neutralization behaviour of He+ at energies, where resonant charge exchange processes are dominant. A strong influence of collision induced charge transfer on the absolute values P+ is observed in the whole range of energies investigated, as deduced from spectra shape. In comparison to previous investigations on noble metal surfaces, in the present study Eth was found to be extraordinarily low in comparison to the projectile energies used. The data obtained point towards a very small influence of AN on the final charge state. Future investigations by Time-Of-Flight LEIS may help to figure out what would be the exact relative contributions of the different charge exchange processes. Acknowledgement Support by the Austrian Science Fund FWF (Project P20831) is gratefully acknowledged. References [1] H.H. Brongersma, P.A.C. Groenen, J.-P. Jacobs, in: J. Nowotny (Ed.), Science of Ceramic Interfaces, vol. II, Elsevier, New York, 1994, pp. 113–182. [2] E. Taglauer, in: John.C. Vickerman (Ed.), Surface Analysis – The Principle Techniques, Wiley, New York, 1997, p. 215. [3] P. Bauer, in: H. Bubert, H. Jenett (Eds.), Surface and Thin Film Analysis, WileyVCH, Weinheim, 2002, p. 150. [4] H.H. Brongersma, M. Draxler, M. de Ridder, P. Bauer, Surf. Sci. Rep. 62 (2007) 63. [5] R. Souda, M. Aono, Nucl. Instrum. Methods Phys. Res., Sect. B 15 (1986) 114. [6] H.D. Hagstrum, Phys. Rev. 96 (1954) 336. [7] D. Primetzhofer, S.N. Markin, E. Taglauer, P. Bauer, Phys. Rev. Lett. 100 (2008) 231201. [8] D. Primetzhofer, M. Spitz, S.N. Markin, E. Taglauer, P. Bauer, Phys. Rev. B 80 (2009) 125425. [9] Yu. Bandurin, M. Spitz, V.A. Esaulov, L. Guillemot, R.C. Monreal, Phys. Rev. Lett. 92 (2004) 017601. [10] S. Wethekam, D. Valdes, R.C. Monreal, H. Winter, Phys. Rev. B 78 (2008) 075423. [11] R.J.M. Elfrink, Master thesis, Eindhoven University of Technology 1994.
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[12] M. Draxler, R. Gruber, H.H. Brongersma, P. Bauer, Phys. Rev. Lett. 89 (2002) 263201. [13] S.N. Markin, D. Primetzhofer, P. Bauer, Nucl. Instrum. Methods Phys. Res., Sect. B 267 (2009) 634. [14] R. Souda, M. Aono, C. Oshima, S. Otani, Y. Ishizawa, Surf. Sci. 179 (1987) 199. [15] T.M. Thomas, H. Neumann, A.W. Czanderna, J.R. Pitts, Surf. Sci. 175 (1986) L737. [16] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stopping and Range of Ions in Solids, Pergamon Press, New York, NY, USA, 1985.
[17] N.P. Wang, Evelina A. García, R. Monreal, F. Flores, E.C. Goldberg, H.H. Brongersma, P. Bauer, Phys. Rev. A 64 (2001) 012901. [18] D. Primetzhofer, S.N. Markin, J.I. Juaristi, E. Taglauer, P. Bauer, Nucl. Instrum. Methods Phys. Res., Sect. B 267 (2009) 624. [19] E.C. Goldberg, R. Monreal, F. Flores, H.H. Brongersma, P. Bauer, Surf. Sci. 440 (1999) L875.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Charge exchange of He+-ions with aluminium surfaces S. Rund, D. Primetzhofer, S.N. Markin, D. Goebl, P. Bauer ⇑ Institut fuer Experimentalphysik, Abt. Atom – und Oberflaechenphysik, Johannes Kepler Universitaet, Altenbergerstr 69, A-4040 Linz, Austria
a r t i c l e
i n f o
Article history: Received 21 September 2010 Received in revised form 12 November 2010 Available online 23 November 2010 Keywords: Low-energy ion scattering Single crystal Neutralization Al(1 1 1) Ion fraction
a b s t r a c t Charge exchange of 4He+ and 3He+ ions with surfaces of polycrystalline aluminium and Al(1 1 1) was investigated in the low-energy ion scattering (LEIS) regime. Ion spectra were recorded for primary energies ranging from 280 to 4000 eV by using an electrostatic analyzer (ESA). A very low threshold energy Eth for collision induced charge exchange (CI) was deduced from the shape of experimental spectra. Ion fractions P+ were evaluated. No systematic difference in P+ was observed for both, the two surfaces investigated and the two different projectiles. Ó 2010 Elsevier B.V. All rights reserved.
R1
vc v j
1. Introduction
Pþj ffi e
LEIS is a commonly used analytical tool for analysis of composition or structure of the topmost atomic layers of solid surfaces [1– 3]. Typically, noble gas ions with energies in the range 0.4 to 10 keV are used as projectiles. The extremely high surface sensitivity is based on the large scattering cross section and the very efficient neutralization of primary ions [4]. Therefore, quantification of experimental LEIS data urgently requires a fundamental understanding of charge exchange. In LEIS, different charge exchange mechanisms are active depending on the primary energy and the atomic species of the projectile [5]. Non-local Auger neutralization (AN) is possible at all primary energies. Collision induced neutralization (CIN) and reionization (CIR) can only contribute for sufficiently close interaction distance between projectile and target atom, which promotes the He 1s level to binding energies resonant with the conduction band, below or above the Fermi level, respectively. This minimum distance rmin corresponds, for given scattering geometry, to an energy threshold Eth. For AN, the neutralization rate dP+ /dt depends on the Auger transition rate C(s) [6] via
where j stands for in or out, respectively, zmin is the minimum distance with respect to the surface plane of atoms, vc denotes the characteristic velocity which is a measure of neutralization efficiency and v\ for the perpendicular velocity of the projectile, i.e. v\in = v0 for perpendicular incidence and v\out = voutcos(ph), with the scattering angle h. Finally, the ion fraction results as þ Pþ ¼ P þ in P out . At E > Eth the ion fraction for single scattering from the first layer is given by
þ
þ
dP dP dt 1 dz ¼ Pþ CðzÞ $ þ ¼ CðzÞ dz ¼ C dz dt v? P
ð1Þ
From Eq. (1), the survival probabilities for incoming and outgoing trajectories are obtained by integration along the distance to the surface: ⇑ Corresponding author. Tel.: +43 732 2468 8516. E-mail address: [email protected] (P. Bauer). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.11.049
v 1 ?j
zmin
CðzÞdz
ffie
?j
;
Pþ ¼ Pþin ð1 PCIN Þ Pþout þ ð1 Pþin Þ P CIR Pþout
ð2Þ
ð3Þ
where the first term includes ions that remained charged along the entire trajectory and the second one describes those who are neutralized on the way in, reionized in a close collision and survived AN on the way out of the sample. In recent studies surface dependent neutralization efficiencies have been observed for He+ ions scattered from different noble metal surfaces [7,8]. A similar behaviour has been observed for experiments performed for He+ ions scattered from an Al single crystalline surface in grazing incidence and successfully explained by theory [9,10]. This study presents complementary information obtained in large angle backscattering experiments using an ESA-LEIS set-up. He+ ions were scattered from polycrystalline Al and Al(1 1 1). The investigation of different surfaces allows to study the influence of the surface orientation and of the information depth on the ion fraction. Furthermore, the presented experiments aimed at a comparison of P+ data obtained from measurements using 3He+ and 4He+ as projectiles. As stated above, AN scales with v\. In contrast,
1172
S. Rund et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 1171–1174
resonant charge exchange strongly depends on the interaction distance and thus, on energy. From these experiments information on the relative importance of different charge exchange mechanisms can be obtained. Furthermore, information on the onset of collision induced charge exchange and thus the corresponding minimum interaction distance to enable resonant charge transfer is sought.
a
2. Experiment Experiments were performed using the ESA-LEIS set-up MINIMOBIS [11] in the institute of experimental physics at Johannes Kepler University Linz. A beam of He ions with primary energies in the range of 0.2–4 keV is directed towards the sample surface in perpendicular incidence. Primary beam currents range from 0.05 to 2 nA. Ions that are backscattered by h = 136° pass a cylindrical mirror analyzer (CMA) with an azimuth acceptance of 2p and are detected by microchannel plates (MCP). A polished Al(1 1 1) single crystal with a roughness below 0.03 lm and a precision of the orientation of ±0.1° was used. Polycrystalline films are produced by ex situ evaporation. All surfaces were prepared by cycles of Ar+-sputtering and annealing (up to 400 °C). For the single crystal, surface quality was checked by low-energy electron diffraction (LEED). The surfaces where regularly investigated for surface contamination (mainly O and C) by LEIS spectra.
b
3. Results and discussion In LEIS, the ion yield J þ i backscattered from surface atoms ‘‘i’’ of mass mi, is a measure for the atomic surface concentration Ni via
J þi ¼ I0
dri Ni Pþi nþ ; dX
ð4Þ
where I0 denotes the primary ion current, n+ the transmission function of the spectrometer including the detection efficiency, and dr/dX the differential scattering cross section. For the ESA used in this experimental study it was possible to obtain n+ from reference measurements on polycrystalline Cu, for which P+ is known from TOF-LEIS measurements [12]. This permits a quantitative evaluation of the equivalent information deduced from ESA-LEIS experiments [13]. For this calibration, Cu is an ideal candidate since it features high Eth (2 keV). Thus, no reionization processes take place and the ion signal originates from the outermost surface exclusively. A typical spectrum obtained for 1 keV He+ ions scattered from Cu is shown in Fig. 1a. The peak shape can be almost perfectly reproduced by a Gaussian. This should be anticipated since the ion signal originates from single scattered ions in the first layer exclusively and only insignificant inelastic loss straggling are expected. From the width of the ion peaks (FWHM) measured at different energies, it is possible to deduce the system resolution DE. In Fig. 2, DE/E can be obtained from the slope of the Cu data (black triangles), which is found to be almost constant, with a value of 1.5%. In Fig. 1b, a typical spectrum obtained for 1 keV He+ scattered from Al is depicted. The shape of this spectrum is significantly different than for Cu (see Fig. 1a), and a Gaussian fit cannot reproduce the low energy tail of the spectrum. However, since the threshold velocity Eth for Al is claimed to be very small (<500 eV) [14,15], additional charge exchange processes take place and ions scattered in deeper layers may influence the ion yield [7]. When a particle is reionized, its kinetic energy is reduced by DECIR due to the electron promotion process that takes place. In the preceding neutralization, an electron is captured into a state, which is below the Fermi level Ef, typically by several eV. Consecutively, it is shifted above Ef (at distances smaller than rmin), where the electron is transferred to
Fig. 1. (a) Experimental spectrum of 1 keV 4He+ scattered from Cu. A Gaussian perfectly matches the ion distribution. (b) Experimental spectrum of 1 keV 4He+ scattered from Al. The ion peak can be modelled by two Gaussian distributions, survivals and reionized projectiles shifted in energy by DECIR, respectively.
Fig. 2. Ion peak width (FWHM) detected at different final energies kE0. For 4He+ scattered from Cu, a linear fit matches the data and yields the energy resolution of the detection system DE/E < 0.015. For 4He+ scattered from Al, ion peaks are found wider due to additional contribution from reionized projectiles. Extrapolation of the data for Al to lower energies permits to determine the reionization threshold Eth.
S. Rund et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 1171–1174
Fig. 3. Experimentally obtained ion fractions for 3He+ and 4He+ projectiles scattered from polycrystalline Al and Al(1 1 1). Also shown are fits to the data according to Eq. (2) and experimental data from [19].
unoccupied states of the conduction band of the target. If the projectile was initially neutralized by Auger neutralization DECIR can be estimated from the position of the unperturbed 1s level and the work function u of the solid to be at most 20 eV. In Fig. 1b, the spectrum can be reasonably well reproduced by a sum of two Gaussians shifted in energy by 20 eV with respect to each other, representing survivals and reionized projectiles, respectively (see Eq. (3)). However, at the low energy onset, a residual contribution cannot easily be fitted within this model. This extra intensity can probably be attributed to a small fraction of ions that survive backscattering from second or third layer atoms. The broadening of the ion peak due to reionization can be used to deduce Eth for Al (see Fig. 2). In comparison to Cu, the widths of the ion peaks (FWHM) obtained from the present experiment on Al, are significantly higher in the whole range of investigated energies and exhibit no energy proportionality. At energies larger than 0.5 keV, the difference is almost constant at 15 eV and only slowly decreases at highest energies. At very low energies, however, the difference becomes very small. This is an indication for fewer contributions from reionized particles. A fit through the data for 4He at very low energies, i.e. kE0 < 0.27 intersects with the extrapolated curve from the experiment on Cu at an energy kE0 1 of 11020 30 eV (red line and red dotted lines, respectively) . From this, Eth 190 eV can be calculated, corresponding to a minimum distance rmin < 0.75 a.u. (employing the ZBL-potential [16]), in good agreement with theoretical prediction [17]. For the evaluation of P+, experimental spectra were fitted by a Gaussian optimized to focus on single scattered contributions from the first layer. A linear background was subtracted, and the tail of ions at lower energies was excluded from the integration. This evaluation was applied to 3He+ and 4He+ projectiles scattered from polycrystalline and Al(1 1 1); the results are shown in Fig. 3. As a result, the data obtained from both surfaces almost coincide. This indicates that for both samples only minor sub-surface contributions have entered the evaluation. This is also in good agreement with similar observations in TOF-LEIS for He+ on Ag [18] indicating that the polycrystalline Al surface consists of azimuthally randomly oriented (1 1 1)-facets. Fig. 3 also shows data obtained in a previous experiment in a smaller energy range by [19] in excellent agreement with the present results.
1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
1173
The P+ results shown in Fig. 3 can be fitted by a single exponential according to Eq. (2), which is expected to be valid in the AN regime only. Nevertheless, from the shape of the ion peaks and the evaluation shown above one has to conclude that all data presented are recorded in the reionization regime. Furthermore, calculated Auger rates support the assumption that collision induced neutralization is the dominant neutralization process [17,19]. In this context the question arises how to interpret the fact that data for 3He+ and 4He+ coincide within statistical uncertainty, when P+ is plotted as a function of 1/v\. In other words, the length of the time interval where charge exchange can occur appears to be dominant with respect to the slightly different trajectories obtained for the He isotopes at similar perpendicular velocity. This is possible for two different scenarios: One possibility would be that only AN is active, and v\-scaling of the ion fraction has to be expected from [6]. Alternatively, PCIN may be close to unity in the whole range of investigated energies and the probability for collision induced reionization, PCIR, is increasing slowly with primary energy. The 1/v\-dependence of P+, obtained for He+ and Al, is in contrast to what has been observed for Cu, Ag and Au [8,12], where data show a strong deviation from a behaviour according to Eq. (2) when the projectile energy exceeds Eth. However, in the present investigation all P+ data have been recorded in the regime E Eth. Data for Cu together with an evaluation of PCIR [12] for different energies indicate that PCIN PCIR, and the yield of reionized particles strongly increases at E 3 Eth. This should also apply to the He+/Al case, due to the strong promotion of the He 1s-level in a collision with Al at the interaction distances probed in this study [17]. 4. Conclusions The presented experiments probe the neutralization behaviour of He+ at energies, where resonant charge exchange processes are dominant. A strong influence of collision induced charge transfer on the absolute values P+ is observed in the whole range of energies investigated, as deduced from spectra shape. In comparison to previous investigations on noble metal surfaces, in the present study Eth was found to be extraordinarily low in comparison to the projectile energies used. The data obtained point towards a very small influence of AN on the final charge state. Future investigations by Time-Of-Flight LEIS may help to figure out what would be the exact relative contributions of the different charge exchange processes. Acknowledgement Support by the Austrian Science Fund FWF (Project P20831) is gratefully acknowledged. References [1] H.H. Brongersma, P.A.C. Groenen, J.-P. Jacobs, in: J. Nowotny (Ed.), Science of Ceramic Interfaces, vol. II, Elsevier, New York, 1994, pp. 113–182. [2] E. Taglauer, in: John.C. Vickerman (Ed.), Surface Analysis – The Principle Techniques, Wiley, New York, 1997, p. 215. [3] P. Bauer, in: H. Bubert, H. Jenett (Eds.), Surface and Thin Film Analysis, WileyVCH, Weinheim, 2002, p. 150. [4] H.H. Brongersma, M. Draxler, M. de Ridder, P. Bauer, Surf. Sci. Rep. 62 (2007) 63. [5] R. Souda, M. Aono, Nucl. Instrum. Methods Phys. Res., Sect. B 15 (1986) 114. [6] H.D. Hagstrum, Phys. Rev. 96 (1954) 336. [7] D. Primetzhofer, S.N. Markin, E. Taglauer, P. Bauer, Phys. Rev. Lett. 100 (2008) 231201. [8] D. Primetzhofer, M. Spitz, S.N. Markin, E. Taglauer, P. Bauer, Phys. Rev. B 80 (2009) 125425. [9] Yu. Bandurin, M. Spitz, V.A. Esaulov, L. Guillemot, R.C. Monreal, Phys. Rev. Lett. 92 (2004) 017601. [10] S. Wethekam, D. Valdes, R.C. Monreal, H. Winter, Phys. Rev. B 78 (2008) 075423. [11] R.J.M. Elfrink, Master thesis, Eindhoven University of Technology 1994.
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[12] M. Draxler, R. Gruber, H.H. Brongersma, P. Bauer, Phys. Rev. Lett. 89 (2002) 263201. [13] S.N. Markin, D. Primetzhofer, P. Bauer, Nucl. Instrum. Methods Phys. Res., Sect. B 267 (2009) 634. [14] R. Souda, M. Aono, C. Oshima, S. Otani, Y. Ishizawa, Surf. Sci. 179 (1987) 199. [15] T.M. Thomas, H. Neumann, A.W. Czanderna, J.R. Pitts, Surf. Sci. 175 (1986) L737. [16] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stopping and Range of Ions in Solids, Pergamon Press, New York, NY, USA, 1985.
[17] N.P. Wang, Evelina A. García, R. Monreal, F. Flores, E.C. Goldberg, H.H. Brongersma, P. Bauer, Phys. Rev. A 64 (2001) 012901. [18] D. Primetzhofer, S.N. Markin, J.I. Juaristi, E. Taglauer, P. Bauer, Nucl. Instrum. Methods Phys. Res., Sect. B 267 (2009) 624. [19] E.C. Goldberg, R. Monreal, F. Flores, H.H. Brongersma, P. Bauer, Surf. Sci. 440 (1999) L875.