Analysis of the Auger neutralization of He+ at Cu surfaces in low energy ion scattering

Nuclear Instruments and Methods in Physics Research B 267 (2009) 575–577

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Analysis of the Auger neutralization of He+ at Cu surfaces in low energy ion scattering D. Primetzhofer a, S.N. Markin a, I. Juaristi b, J.E. Valdes c, P. Bauer a,* a

Institut für Experimentalphysik, Johannes Kepler Universität, Altenbergstr. 69, A-4040 Linz, Austria Departamento de Fisica de Materiales, Facultad de Ciencias Quimicas, Apartado 1072, E-20080 San Sebastian, Spain c Departamento de Física, Universidád Técnica Federico Santa Maria, Valparaíso, Casilla 110-V, Chile b

a r t i c l e

i n f o

Article history: Received 30 September 2008 Received in revised form 17 October 2008 Available online 29 October 2008 PACS: 34.50.Dy 68.47.De 68.49.Sf 79.20.Rf

a b s t r a c t Recently, strong crystal effects in P+ were observed for He+ and Cu in the Auger neutralization regime, with differences in the ion fraction by up to a factor of three non-equivalent Cu surfaces. In this contribution, it is shown that these findings can quantitatively be described within the jellium model assuming perpendicular velocity scaling of P+. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Ion fraction Low-energy ion scattering Single crystals Auger neutralization Jellium model

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1. Introduction In low energy ion scattering (LEIS) with He+ ions as projectiles, charge exchange is known to occur exclusively via Auger neutralization (AN) as long as the ion energy is below the reionization threshold, Eth. The efficiency of AN is the basis for the well known surface sensitivity of LEIS [1]. For a He+ ion scattered from a surface atom the probability P+ to escape AN on the ways in and out can be written as

 Z  Pþi ¼ exp  CA dt ¼ exp ½hCA iDt   expðv c =v z;i Þ;

v f ¼ k  v 0 , with the kinematic factor k and the incident velocity v0, when electronic losses are neglected. Very recently, marked differences in the deduced ion fraction P+ for different single crystalline and polycrystalline surfaces of a pure element, i.e. Cu, were presented. These crystal effects were analyzed in terms of a free electron gas model, mimicking the crystal structure by a jellium extending into vacuum up to the jellium edge [2,3]. Here, we present a more detailed theoretical analysis of these results, aiming at a better understanding of the role of the Cu 3d-electrons in AN.

ð1Þ 2. Experiment

where i stands for in and out, hCA i is a mean Auger rate along the R trajectory, v c ¼ CA (z) dz is the so called characteristic velocity and vz,i is the perpendicular component of the velocity on the way in and out, i.e. v0  cos a and vf  cos a, respectively. The angle of incidence, a and the exit angle, a are both measured with respect to the surface normal. For a single binary collision with a surface atom, the final velocity of the projectile, vf, is given as

* Corresponding author. E-mail address: [email protected] (P. Bauer). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.10.067

Since the experiments are described in detail in [4], only the main features are summarized here. The measurements were performed using the Time-of-Flight- (TOF-) LEIS setup ACOLISSA [5] with a scattering angle h of 129°. Polar and azimuth scans are possible with a precision of ±0.1° and ±0.2°, respectively. Ions can be separated from neutrals by a post acceleration voltage applied along part of the flight path. In the range of incident angles a < 65° it can be ensured that the whole irradiated spot is visible for the detector, which is a requirement for quantitative yield analysis.

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D. Primetzhofer et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 575–577

Polished single crystals were purchased, polycrystalline films were produced by ex situ evaporation. All surfaces were prepared by cycles of 3 keV Ar+-sputtering and annealing to P400 °C. Auger electron spectroscopy did not show any surface impurities after cleaning; for the single crystals the structure was checked by low-energy electron diffraction (LEED). For neutralization studies, the use of single crystals for neutralization studies is beneficial for various reasons. First, the well defined surface structure leads to a well defined electronic structure. Second, single crystals offer the possibility to limit the information depth to the outermost surface by choosing double alignment conditions. In double alignment geometries, both, the incident beam and the detector are aligned with a low index crystal direction. Thus, backscattering from atoms in deeper layers is much less likely due to channeling and contributions from multiple scattering are strongly reduced due to blocking [6]. Consequently, a TOF-spectrum recorded for a single crystal under these conditions permits to determine the ion fraction P+ from direct comparison of the ion yield A+ and the surface peak of neutrals A0, since they are formed by particles backscattered in a single collision from surface atoms [7,8]. For incidence of the projectiles outside double alignment geometry, an ‘‘apparent ion fraction” P+(a) can be deduced by comparison of A+(0°) and A+(a) considering the change in available scattering centers which assures an equivalent fluence of primary projectiles. Evaluation of the thus obtained ion yield results in an ‘‘apparent ion fraction” since the number of contributing layers is not well known and may considerably exceed one monolayer, depending on AN efficiency and on the possibility of collision induced processes. For He+ and Cu in the AN regime, the backscattered yield is mainly due to interaction with the visible surface atoms. This procedure also permits to determine P+ for polycrystalline targets independent from the method proposed in [9]. It has been shown that for He+ and Cu, both procedures result in concordant P+ values [10]. To evaluate P+(a) via A+(a)/A+(0) is beneficial for both, single crystals in random orientation and polycrystalline targets, since this procedure does not make use of the spectrum of the neutrals. When P+ is deduced [9] from the ratio of the ion yield to the height of the neutral spectrum, A+(a)/H0(kE0), the result may easily be influenced by systematic errors due to multiple scattering events (see Fig. 1) or to lacking knowledge of the inelastic stopping cross section. A further advantage of deducing P+ from A+(a)/A+(0) is that uncertainties of the differential scattering cross section do not contribute since they affect both scattered neutrals and ions in the same way.

1 0.8

+

Cu

He

ion fraction P +

0.6 0.4 v

Cu(110):

0.1 0.08 0.06 0.04 0.0

(11

0)

c

0.2 Cu(100): Cu-poly:

-6

4.0x10

-6

8.0x10

=1

.18

2.0 keV 1.5 keV 2.0 keV 1.9 keV 1.5 keV Markin et. al. (2007) Draxler et. al. (2002)

v

v

=

1.

=1

92

x1

0

.62

5

m

/s

1.2x10

-5

The system of Cu and He+ is of special interest since the threshold energy of reionization is rather high (2100 eV). This permits to study AN in a wide energy range with our setup. Polar scans were performed in the range 15° 6 a 6 65° for Cu(1 0 0) along the [0 0 1] azimuth, for Cu(1 1 0) along the [1–12] azimuth, and for polycrystalline Cu. Note that for a fcc metal like Cu it is reasonable to assume that a polycrystalline surface consists of (1 1 1) facets and therefore exhibits a similar neutralization behavior as Cu(1 1 1). The obtained ion fractions P+ are presented in Fig. 1. It is remarkable that the observed v\-scaling of P+ is in contrast to what is expected from the electron densities of Cu, as calculated in absence of the projectile [11]. These calculations yield a spherically symmetric d-electron density which exceeds the sp-densities everywhere, with very high d-electron density around the Cu ion cores. If the unperturbed electron density was relevant, the neutralization process should be spherically symmetric, too, i.e. independent of the angle of incidence, and P+ should scale with 1/v instead of 1/v\ [1]. Obviously, the observed v\-scaling is a consequence of screening, since a slow ion represents a strong perturbation for the conduction electrons of the sample and, therefore, locally induces large density changes in the conduction electrons of a metal [12]. Comparing P+ obtained for non-equivalent surfaces, face dependent P+ values were observed which differ by up to a factor of three, with P+(110) > P+(100) > P+poly. The most realistic theoretical description of AN would require the knowledge of dynamic electron densities obtained for the sample surface in the presence of the projectile, from (time-dependent) DFT calculations. Since this information is not available, only a qualitative discussion of the observed experimental findings is possible, e.g. by mimicking the sample electrons in the presence of the projectile by a jellium of effective density [13]. This jellium is assumed to extend into vacuum up to the jellium edge at half the interlayer distance dhkl/2 (0.904 Å and 1.044 Å for Cu(1 0 0) and Cu(1 1 1), respectively) [13]. It is obvious that there are more refined ways to model the electron density in front of the surface, but since we are interested in integral quantities only, the simple rectangular density profile is sufficient for our purpose. From this and by making use of the time spent by the projectile in the jellium obtained from Molecular Dynamics calculations (Kalypso [14]) one can deduce effective Auger transition rates CA,j for different surfaces j. Starting from Pþ 1 ¼ exp½hCA i  Dt j  (see Eq. (1)) one obtains concordant values for Cu(1 0 0), Cu(1 1 1) and Cu(1 1 0), i.e. 2.17  1015/s, 2.16  1015/s and 2.19  1015/s, respectively. Within a free electron gas approximation, these Auger rates correspond to a radius per electron in the conduction band rs = 1.94 a.u. [15], i.e. to 2.6 effective free electrons per atom. These results are summarized in Table 1. Due to the non-negligible role played by the 3d electrons, Cu is not really a free electron metal. However, it has been shown that diverse aspects of the interaction of ions with transition metals

x10 5 m/s

(10 0)

c

p c oly

3. Results and discussion

x1

05 m/

1.6x10

s

-5

1/v (s/m) Fig. 1. Ion fraction P+ for He+ and Cu(1 0 0) as a function of 1/v\ = 1/v0\ + 1/vout\ in the AN regime. Also shown are experimental data from [9,10].

Table 1 P+ at 1/v\ = 1.3  105 s/m and mean AN rates hCi for different Cu surfaces deduced from single scattering trajectories and scattering from atoms in the first layer. Also shown are the electron density parameter rs and the effective number of free electrons per Cu atom, Ne,eff. Surface

Cu(1 0 0)

Cu(1 1 0)

Cu(1 1 1)/Cupoly

Dt1 (fs) P+ hCA i (1/fs) rs (a.u.) Ne,eff per atom

0.69 12.1% 2.17 1.948 2.57

0.48 21.5% 2.19 1.949 2.57

0.79 8.3% 2.16 1.944 2.59

D. Primetzhofer et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 575–577

can be properly accounted for within the free electron gas approximation. In this case, it is customary to define an effective free electron gas density from the maximum of the experimental electron energy loss peaks. This kind of approach has been used extensively in studies of the energy loss of slow ions [16,17], of the ion induced kinetic electron emission [18,19] and of the Auger neutralization of highly charged ions [20]. By this procedure the density parameter obtained for Cu is rs = 1.83 a.u. [21], which is very close to the value that we obtain. This shows the importance of the 3d electrons in the Auger neutralization process. Note that if only one free 4s electron per Cu atom were considered one would obtain rs = 2.67 a.u. which would imply an Auger rate that is lower by about an order of magnitude [15]. To conclude, the large differences in neutralization observed for He+ and different Cu surfaces in the LEIS regime, can be even quantitatively explained in terms of a free electron gas model. Whether this is just a fortuitous finding or generally true cannot be decided yet, but requires a more detailed study including metallic samples with a larger variety of electronic properties.

Acknowledgements Support by the Austrian Science Fund FWF (Projects P16173N08 and P16469-N08) is gratefully acknowledged. Daniel Primetzhofer acknowledges a DOC-fellowship of the Austrian Academy of Science.

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