Dynamics of excited state production in the scattering of inert gas atoms and ions from Mg and Al surfaces

surface science ELSEVIER

Surface Science 365 (1996) 353-373

Dynamics of excited state production in the scattering of inert gas atoms and ions from Mg and A1 surfaces L. G u i l l e m o t a, S. L a c o m b e a, V.N. T u a n a, V.A. E s a u l o v a.., E. S a n c h e z b, Y.A. B a n d u r i n c, A.I. D a s h c h e n k o c, V.G. D r o b n i c h c " Laboratoire des Collisions Atomiques et Mol~culaires, (Unit~ associ~e au CNRS), Universitd Paris-Sud, Bat. 351, 91405 Orsay, France b Centro Atomieo Bariloche, Bariloche, Argentina c Department of Physics, Uzhgorod State University, 294000 Uzhgorod, Ukraine

Received 7 November 1995; accepted for publication 2 April 1996

Abstract We present results of a detailed study of the production of electronically excited states in the scattering of ionic or neutral inert gas projectiles in the keV energy range at AI and Mg surfaces. The complementary observation of scattered particles (neutrals or ions), secondary electrons and photons leads to a rather complete description of the successive stages of inelastic scattering events. Efficient neutralisation of the incident ions occurs, when they approach the surface. This is clearly demonstrated by the strong similarities between results obtained for incoming ions and incoming ground-state neutrals. The characteristics of the scattered particle distributions, the observation of scattered ions, and also of some excited states by electron and photon spectroscopy, delineates the decisive importance of short-distance binary collisions with atoms of the surface, in the production of these species. A detailed comparison with the "inverse" collisional systems in the gas phase shows that the same kind of primary excitations as described in the quasi-molecular orbital promotion model are operative and allows, for example, predictions about which of projectile and/or target atoms can be excited. Some very strong differences to gas-phase collisions are also demonstrated. They stress the importance of surface-specific effects, such as the role of resonant or Auger electron transfers between the metal surface and the receding particle in defining the final state population. In particular, an interesting surface-induced core rearrangement effect is emphasised, and different rearrangement mechanisms are presented and discussed. Keywords: PACS: 79.80+W; 79.20Rf; 79.20Nc; 34.50Fa

1. Introduction

We present an experimental investigation of the interactions of inert gas atoms and ions with magnesium and aluminium surfaces in the low keV projectile energy range, focussing on the production of ions and excited states. These collisional systems have been the subject of several experimen* Corresponding author.

tal studies, some dealing with charge transfer processes using time-of-flight (TOF) techniques [ 1-3 ], and others with electron spectroscopy [4-8]. Our goal was to extend the previous measurements via a more complete study including scattered ion and neutral angular distributions, angle-resolved electron spectroscopy and VUV photon spectros c o p y m e a s u r e m e n t s . W e also w i s h e d to p e r f o r m a comparative analysis of the main features observed in t h e l i g h t o f t h e k n o w n p r o p e r t i e s of single atom-atom i n t e r a c t i o n s in g a s - p h a s e collisions,

0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PH S0039-6028 (96) 00716-9

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L. Guillemot et al./Surface Science 365 (1996) 353-373

which has led us to emphasise the role of some very specific surface effects. We concentrated on rare gas ions and Mg and A1 surface systems for two main reasons: firstly the "inverse" systems Mg ÷ and A1 ÷ on rare gas targets have been studied in some detail in the gas phase and are quite well understood, and secondly, Mg and A1 are typical metals for which simple free-electron model descriptions are best suited. They therefore appeared to be ideal objects to permit some complementary theoretical investigations. Some preliminary aspects of this work, involving ionic projectiles, have been addressed in previous short reports [9-11]. In this paper we present new results, including experiments with neutral projectiles, and present a general analysis of all the results obtained, which allows us to arrive at a fairly complete description of the inelastic scattering of ions at metal surfaces. Our experimental work was complemented by numerical simulations using a Marlowe-based code to model the ion-surface interaction. Some enlightening points of these simulations will be also presented.

2. Experimental details One may find full details about the Orsay apparatus in Ref. [ 12]. Briefly, an ion beam, produced in a discharge source, is mass analysed by a Wien filter and after a 90 ° deflection by a quadrupole, is directed onto polycrystalline Mg or A1 samples situated at the centre of a UHV chamber. Polycrystals were used as in most previous experiments because in these experiments, damage to the crystal can occur, and this renders crystal azimuthal dependent inelastic scattering studies, extremely difficult. The conclusions of this paper are not dependent on this point (see Section 6). The set-up is equipped with a sputtering gun located in a separate differentially pumped chamber, and an electron gun for AES diagnostics. The samples were hand polished down to 0.05 #m to give a mirror-like appearance. In-situ preparation consisted of repeated cycles of Ar ÷ sputtering and annealing to 200°C for Mg, or 400°C for A1. As mentioned in Section 1, the electronic structure of Mg and A1 is usually successfully described in

terms of a jellium model and no attempt at a more detailed characterisation is made here. Two tandem parallel plate electrostatic analysers, with a 45 ° entrance angle are set on two separate turntables, allowing for angle-resolved energy measurements of the charged particles. One is for low energy electron spectroscopy (0-100 eV) with a 100 meV resolution for a 10 eV pass energy. The electrons are accelerated or decelerated to this energy using computer-driven zoom optics, which allows the optimisation of the transmission of electrons at every energy. The transmission function of the analyser as a function of the electron energy is not known, and the spectra are therefore not corrected. It should be noted, however, that (except for the lowest energies, typically below ! eV) for the analysis mode adopted, the use of the zoom optics leads to a smoothly decreasing transmission with increasing electron energy. The second analyser, for either ions or high-energy electrons (0-5 keV) has a relative resolution of 0.35%. In addition, a channeltron detector, which allows counting of neutral and/or ionic scattered particles, is located at the back of this analyser, along the entrance axis. The VUV photon spectrometry measurements were made on the Uzhgorod set-up described in Ref. [ 13 ]. The targets were pre-prepared in Orsay from the same polycrystalline rod and similarly polished. In-situ cleaning was achieved by ion bombardment. Ions with final energies up to 30 keV are produced in a discharge source, mass selected, and impinged on the target at a 10° incidence. VUV photons are analysed using a Seya-Namioka diffraction monochromator using a 600 lines per mm grating.

3. Ion scattering spectroscopy and charge fraction measurements 3.1. Characteristics of angular distributions We performed measurements of angular distributions of scattered neutrals (N(0)) and ions (I(0)) for grazing incidence angles (~b=3-30°). Some important features of this study concerning Ne + scattering on Mg have been addressed in a prelimi-

L. Guillemot et al./Surface Science 365 (1996) 353-373

nary report [9], which includes a discussion of surface roughness effects. The geometry of the experiment as well as the angle labelling is shown schematically in Fig. 1. Fig. 2a shows typical scattered neutral and ion angular profiles for 2 keV Ne + incident on Mg at a small angle (6°). Scattered neutrals are dominant, and their distribution has a maximum at 0ma, = 11 °, slightly smaller than the specular angle 0sp= 12°. For larger ~b this 0ma, value moves up to larger scattering angles, but shifts further down from the specular value. For example, in the case of q~= 10°, one finds the maximum of the neutral distribution at 15°, 5 ° from the specular value. Results for A1, which are similar to those for Mg, are shown in Fig. 2b. The maximum of the scattered ion distribution is situated at larger angles than for neutrals. Thus for ~b= 6 ° experiments, we find that its maximum lies at 0 = 20 °. This angle increases with increasing ~b. At smaller impact energies, the ion distribution broadens and shifts to significantly larger scattering angles. Simulation of grazing ion-surface scattering have been performed for Ne + on Mg or A1, with a Marlowe-based code [14,15]. This code calculates multiple projectile trajectories in which the ion-surface interaction is modelled as a sequence of binary collisions between the projectile and atoms of the solid. For the ground-state N e - M g or Ne-A1 systems, a Born-Mayer type of diatomic

Electron spectrometer k~//"/ / /

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Fig. 1. G e o m e t r y o f t h e e x p e r i m e n t . ~b is t h e i n c i d e n t a n g l e m e a s u r e d w i t h r e s p e c t to the surface, 0 is the i o n s c a t t e r i n g a n g l e a n d ~ the e l e c t r o n o b s e r v a t i o n angle, b o t h m e a s u r e d w i t h respect t o the i n c i d e n t ion b e a m axis.

355

molecular interaction potential is used. The Mg or A1 polycrystals are simulated by randomly changing the orientation of the crystal cell for each trajectory. Such calculations show that at grazing incidence, the projectiles are scattered very close to the specular angle. The mean value of the scattering profile is exactly specular for q~= 4 ° at 2 keV Ne on Mg (Fig. 3a). When the incidence angle increases, the scattering profile tends to shift closer to the surface than the specular value. This is illustrated by the 10° incidence results, which show a total scattering profile close to an exit angle of 6 ° clearly smaller than the specular value and in good agreement with our experimental findings. For very small incidence angles, scattering becomes superspecular.

3.2. Chargefractions Charge

fractions

defined as Y + = be calculated from the previous neutral and ion distributions. Our results for Mg are in reasonable agreement with previous measurements by Rabalais et al. [1] obtained at 0 = 22 and 45 °. An important feature to be noted is that for a given incident energy, charge fractions are found to depend mainly on exit angle (0-q~) and almost not on the incident angle. We pointed out in Ref. [9] that the angular profile of the Ne/Mg charge fractions can be reasonably well accounted for using a simple exponential dependence on 1/V out±, the inverse perpendicular velocity of the scattered particle in the outgoing trajectory. These results hinted that the presence of scattered ions should result from the following collision scheme: all incoming ions are first neutralised in the incoming path, violent "short distance" binary collisions then occur, leading to excitation/ionisation and the ions we observe are those which survive the neutralisation or de-excitation in the outgoing part of the trajectory. To confirm this "three stage" model of ion production, we performed similar measurements for incoming neutral Ne °. The Ne ° beam was produced by resonant neutralisation of an Ne + beam going through an Ne gas cell. After deflection of the residual ions, the 0.15 ° angular definition of the neutral beam ensures the population of ground-

Nions/(Nions+Nneutrals),

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L. Guillemot et al./Surface Science 365 (1996) 353-373

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state Ne atoms [ 16]. Typical measured ion/neutral angular distributions for the scattering of Ne ° on Mg and A1 are shown in Fig. 2. Here again one finds the main features already mentioned for ionic projectiles. The most interesting feature, shown in Fig. 4, is the striking similarity of the charge fractions obtained with neutral and ion projectiles. These findings are strong evidence that ions in the incoming path are very efficiently neutralised and that the observed scattered ions arise from

"short distance" binary collisions. These produce ionised/excited atoms which survive reneutralisation/de-excitation when receding from the surface. Another notable feature, consistent with this scheme of ion production, can be pointed out on the basis of ion/neutral angular distributions. Let us introduce 0i,el=0io.,-0,eut (the difference in the position of the ion and neutral angular distributions), an angle which can be thought of as revealing the small impact parameter

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contribution in the overall deflection leading to ion production. Measurements for Ne + ions incident at 6 ° on Mg, for Einc = 1, 2 and 3 keV, give 0ine] values of 20, 10 and 7 °, respectively. The striking feature here is the conservation of the product E~.c x 0i~e~, which amounts to roughly 20 keV-deg. This reduced angle E x 0 conservation is a well-known feature in gas-phase a t o m - a t o m collisions. Furthermore, the 20 keV.deg value compares well with the position of the m a x i m u m of the differential cross-section associated with twoelectron excitation of Ne in the gas phase M g + / N e system [17,18]. Of course, a straightforward comparison with a t o m - a t o m interactions should not be pushed too far. One must not forget that ion scattering distributions should be

357

affected in the outgoing way by interactions with the surface, such as reneutralisation or deexcitation. However, it seems reasonable that the lower the incident energy, the higher the deflection of the inelastically scattered particles will be. Another noteworthy feature is that generally, under the same experimental conditions, ion fractions obtained with an Mg target are higher than with an AI target. Thus, the ion fraction integrated over scattering angles, for 2 k e V Ne + projectiles at 6 ° incidence are 15_+1% and 10.5+1% for Mg and A1, respectively. This idea that ion production is triggered by electronic excitations during inelastic binary collisions also stems from energy-loss measurements made by Souda and Aono [19] with He + or Grizzi et al. [3] with Ne +. They reported energy losses which were larger than expected from single elastic scattering with surface atoms. Inelastic energy losses of the order of 20 eV for He + and even He ° scattering give evidence that single excitation occurs in this case. A 40 eV inelastic energy loss reported for Ne + scattering on Mg led Grizzi et al. [3] to propose that double excitation of autoionising states of Ne could be responsible for the important fraction of ions observed. The case of Ar + scattering is somewhat different. In this case, in the energy range 3-10 keV, Rabalais et al. [ 2 ] reported much smaller Ar + fractions. Thus, for a 22 ° scattering angle, the Y ÷ value was of the order of 5% at 6 keV. In this case we also find [9] that the ion fraction follows a an exponential dependence on 1/V°Ut I suggesting that ions are due to violent collisions. We will return to the question of their size later.

4. Electron spectroscopy In this section we present a detailed investigation of electron production during rare-gas ion (He--, Ne ÷ and Ar ÷) scattering on Mg and A1 surfaces. This study involves angle-resolved electron spectroscopy performed in the 0-60 eV energy range and an observation angle range of ct = 0-135 ° observation angle range for a set of incident energies from 400 eV to 5 keV.

L, Guillemot et aL/Surface Science 365 (1996) 353-373

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4.I. General aspect of the spectra Looking at such electron spectra (Fig. 5) one usually sees a smooth continuous distribution, increasing from 0 eV up to a m a x i m u m at 7 or 8 eV and then going down smoothly at higher energies. This broad continuous "background" is commonly attributed to kinetic and potential electron emission. The most interesting features

are the sharp peaks, standing out on the background. They can be ascribed to de-excitation of autoionising states of either scattered projectiles or sputtered target atoms, occurring far enough from the surface to give almost "free-atom like" lineshapes. We have here direct evidence that ion production at Mg or AI surfaces can be triggered by electronic excitations. Fig. 6 a - d show electron spectra produced by

359

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Electron Energy (eV) Fig. 5. Electron energy spectrum obtained by scattering 2 keV Ne + ions on Mg, at 6° incidence. General aspect in the 0-60 eV energy range. Ne + and He + scattering at grazing incidence on Mg and A1. For the Ne + case, the dominant peaks have been identified by Zampieri and Baragiola [20] as being due to Ne** 2pn(3p)3s 2, Ne** 2p4( 1D) 3s 2 (peaks ( 1 ) and (3), respectively). For He ÷ projectiles we observed the production of He** 2s 2 states, as reported previously [ 10,11 ]. As was noted earlier [4], for the N e - M g collisions, similar spectra are obtained using incident ions and neutrals. This was also found by us for the Ne-A1 system Fig. 6e. An important point can be readily seen in Fig. 6e: the major Ne autoionising channel in all cases is Ne** 2p4(3P)3s 2. It can also be seen that projectile autoionising lines become less visible against the background if, for a given incident energy, one goes to large incident angles, and also if, for a given incident angle, the incoming energy increases. Two effects can be thought to lead to such general trends. When increasing the incident angle (or the incoming energy), firstly one reduces the reflection coefficient of the surface, more particles penetrate into the solid and the intensities of the autoionising lines are reduced. Secondly, one increases the background level coming from kinetic electron emission, consequently reducing the autoionising line contrast.

We emphasise here that in both Ne + and He + cases, one witnesses production of excited states of the projectiles. This is not the case if we turn to incoming Ar ÷ ions. In this case, only electron emission from target atoms is observed (Fig. 7). We shall come back to this point later, when excitation mechanisms will be discussed. Recently, production of autoionising states of Ar** was reported in the case of Ar ÷ collisions on A1 and Si [21,22] in contrast with our findings. We have demonstrated elsewhere that what was observed by these authors was due to the spurious presence of Ar 2+ in their incoming ion beam [23]. In the latter case, Ar** production is rendered possible by a different mechanism of resonant neutralisation in the incoming path of the trajectory, as discussed in Ref. [24]. 4.2. Identification of the observed excitations

Let us come back to the particular case of Ne ÷ ions. Beside the main peaks (1) and (3) already mentioned, we observed a set of smaller structures. The Ne autoionising lines we identified for scattering on Mg and A1 are summarised in Table 1. In the case of the A1 surface, the identification

360

L. Guillemot et al./Surface Science 365 (1996) 353-373

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Mg, (c) 2 keV Ne ÷ ions on AI, (d) 2 keV He + on A1. (e) Ne** autoionising lines from collisions of 2 keV Ne ÷ ions, Ne° projectiles with an AI surface. of these structures were discussed by Gallon and Nixon [6] and Xu et al. [7], on the basis of results in gas-phase collision studies of Olsen et al. [25] and Fayeton et al. [17,18]. We have a general good agreement with the findings of those authors. Nevertheless peak (2) which was been reported in Refs. [6,7] at energies close to ours (22.3 eV), is assigned in Ref. [6] to a [2p4(3P)3s3p(3P)]aP/ 3D--*2pS(2P) transition which, according to gasphase experiments [25], is situated at 22.9 eV. Thus we do not think that such an identification can be trusted. The rough energy matching with the 22.1 eV of the r2s2p63s2s--,2pS(2P)] transition is no more appealing since, as stated in Ref. [7],

the requirement of an excitation of a 2s electron of Ne (30 eV lower than a 2p) is very unlikely to occur. It has been proposed by Esaulov [26] that this structure could be part of the natural lineshape of peak (1). Numerical simulations showed that autoionisation of excited particles leaving a surface should produce a characteristic high-energy structure in addition to the almost "free-atom" like lineshape produced by emission very far from the surface. This additional structure, found to be about 2.2 eV above the main peak, would be due to autoionisation occurring very close to the surface, where atomic energy levels are shifted up by the presence of the surface, and resonant ionisation

361

L. Guillemot et al./Surface Science 365 (1996) 353-373 2500

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Electron energy

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(eV)

Fig. 7. Electron energy spectra in the 10-50 eV energy range for 2 keV Ar ÷ ions on Mg. Table 1 Energies and identifications of the various autoionising lines measured in the scattering of Ne ÷ on Mg and AI in the keV incident energy range Peak

Measured energy (eV)

Ne** or Ne ÷** initial state

Ne ÷ or Ne ÷ ÷ final state

Predicted energiesa (eV)

Ionisation energy (eV)

(1) (2)

20.3 22.3

2pS(2P)

20.35

7.1

(3) (4) (5) (6)

23.5 26.0 26.7 27.7 30.4

(8)

32.0

2pS(ZP) 2pS(ZP) 2pS(2P) 2pS(2P) 2pS(2P) 2p4(3P) 2p4(3p) 2p4(3P) 2p4(aP)3p2F or 2p4(3p)3p2D or 2p4(3P)3p4D

22.9 23.55 25.95 26.7 27.7 27.7 30.56 30.66 31,7 32.0 32.3

4.55 7.0 4.6 3.85 7.0

(7)

[2p4(3P) 3s213P [2p4(3p)3s213p [2p4(3P)3s3p]3P [2p4(ID)3sZJ1D [2p4(1D) 3s3p(3P)]3 p/3D [ 2p4(3P) 3p2(aP)] 1D/1P [2p4(1S)3s211S or [2pa(2D)3s212D [2pa(ZD)3s3p(2p)]4F 2p3(2P)3s2 zp [2pa(2D) 3s23p(2p)]4F [2p3(2D)3s23p(~p)]4F [2p3(ZD)3s23p(2p)]4F

or Auger de-excitation are still active deexcitation channels. A similar structure related to the Ne**lD3s 2 peak can exist at around 25.7 eV. We attribute peaks (4) and (5) to production of excited states with the 2p43s3p and 2p43p 2 configurations. In this 25-28 eV energy range, the findings of the three groups regarding the number or the positions of the peaks are not consistent. The systematic appearance of structures (4) and (5),

precisely at the given positions, together with the very good matching that both agree with the values given by Olsen et al., give us good confidence in the above-mentioned assignment. It is noteworthy that for scattering on an A1 surface, and in contrast with Refs. [6,7], we did not observe any distinguishable structures in this part of the spectrum. In agreement with Xu et al. [7], we assign peaks (6), (7) and (8) to the de-excitation of doubly

362

L. Guillemot et al./Surface Science 365 (1996) 353-373

excited Ne ÷ or triply excited Ne. Indeed, although energy matching with more excited Ne** configurations such as 2p43s3d or 2p43s4s could be met, their ionisation energies are smaller than the Mg or A1 work-function, and render their observation very unlikely because of very efficient reionisation to the solid. For Mg we also clearly observe three structures in the 35-45 eV energy domain. These are signatures of autoionising sputtered target (Mg) atoms and ions. They have been identified by Baragiola [27] to be (i) Mg*2p~3s23p~Mg÷2p63s at 44 eV, (ii) Mg÷*2p53s3p~Mg2+2p 6 at 39.5 eV, and (iii) Mg ÷*2ps3s2~Mg2+2p6 at 35 eV. We notice that target excitations occurred with He ÷, Ne ÷ and Ar ÷ projectiles, and the dominant emission comes from excited neutral Mg. Previously a strong decrease in the intensity of the 44 eV peak for the case of a strongly oxygen-exposed (1000 L of 02) Mg surface was noted [27]. Assuming that oxidation leads to a decrease of resonant neutralisation, the 44 eV line was assigned to Mg**. Further insight can be obtained from the observation of the evolution of these peaks when the Mg surface is exposed to small doses of oxygen (<2 L) [28]. We reported an overall decrease of the three Mg peaks as a function of oxygen exposure. A very interesting feature is the different rate of decrease measured for the three lines. An initial very sharp attenuation of the 44eV peak was observed. However, this 44eV structure, after a sharp decrease, does not vanish completely for high oxygen exposure. This led us to propose that in this structure there must be a small contribution from Mg + *2p53s4s~Mg 2+ 2p 6 at 44.4 eV. In addition, even though both of them decrease slower than the main Mg line, the decrease of the 39.5 eV peak with oxygen exposure is somewhat faster than that of the 35 eV line. This indicates that part of the 39.5eV structure could be due to Mg*2p 53s23p--, Mg ÷ 2p63p transition (40.1 eV). A detailed discussion of target excitation for Ar ÷ collisions with AI has been given by Xu et al. [29].

4.3. Incident energy dependence of the autoionising lines The intensities of the main peaks have been measured for incident energies from 400 eV to

4 keV and incident angles of 6, 15 and 30° (Fig. 8). These intensities are evaluated from peak integrals after a smooth background subtraction and a normalisation to the incident current of ions. Measurements have been performed at a 45 ° collection angle, where the peaks are fairly narrow and symmetric (see discussion on kinematic effects). This renders the integration of the peak after background subtraction more reliable. The main features in Fig. 8 are the following:(i) the production of autoionising states increases with increasing energy, even though a small decrease between 3 and 4 keV at 6° incidence is measured both for the main Ne** and Mg** structures. (ii) The autoionising lines get weaker as the incident angle increases, and (iii) the ratio R(aP1D) of the Ne** (3p)3s2 over Ne** (1D)3s 2 intensities decreases when Einc increases. On the other hand, R(3p/1D) increases with increasing incident angle. Finally, excitation of autoionising states of Ne is found to be more efficient in collision with an Mg than with an A1 target, a trend in keeping with the ion fractions discussed above. The trend in the changes in intensities of the main peaks as a function of incident energies and incident angles can be well accounted for by numerical simulations using the Marlowe code. In order to render the code capable of predictions about the production of projectile excited states, it was modified to include a simple two-state (NeMg and Ne**Mg) quasi-molecular model of excitation processes [15]. The arguments in favour of such a description will be given in Section 6. The excited Ne**Mg state corresponds to Ne** production with an inelasticity of 45 eV. This state was described in the same way as the ground state by a Born-Mayer type potential, whose parameters were adjusted [15] to reproduce the angular differential cross-sections for double excitation of Ne measured in gas-phase collisions [18]. The calculation is performed as follows: for each successive binary collision, a distance of closest approach Pmin is determined, and we introduced a threshold distance Po corresponding to the crossing distance between the Ne + Mg ground-state potential curve and the Ne** 3s2+ Mg excited curve. We assume that collisions with Pmin> PO leave the Ne projectile in its ground state, while collisions with P,~i,
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/'

/ /"

i' f

...... 0 ..... '

/ ......

/

/'

t

i i i

1o

=

t

I

0

2000

I

~

4000

I

~

2000

I

=

4000

Projectile Energy

I

,

2000

I

4000

(eV)

Fig. 8. Relative intensities of the Ne**, N e ÷* a n d M g * * peaks as a function of the energy of the incident Ne ÷ ions, for (a) $ = 6 °, (b) = 15 °, (c) ~ = 30 c',

lead to Ne** with unit probability. Results of the simulation are presented in Fig. 9, which shows the number of escaping doubly excited Ne** as well as the total scattering yield as a function of the incidence angle. The total reflectivity of the metal surface decreases monotonically with increasing incidence. This shows that an increasing number of projectiles end up inside the solid when

the incident angle increases. The Ne** yield exhibits a different shape, with a maximum at ~b= 6 °. It is understandable that for very small ~b (< 6°), the projectile cannot get close enough to the surface atoms to give rise to excited Ne. When the incidence angle increases, excitations produced by close collisions are favoured, but the yield is weighted by the fall-off of the total reflectivity of

10000

5000 ~

'

~

~

Ne**

z

0 0

I 2

I 4

L _ 6

I 8

L 10

I 12

I 14

I0 16

incidence angle (fleg) Fig. 9. Results of simulation of the total scattering yield for 2 keV N¢ ÷ collisions with M g ( Q ) and the Ne** yield ( l l ) as a function

of the incidence angle ~b.

364

L. Guillemot et al./Surface Science 365 (1996) 353-373

the surface, as mentioned above. We measured a maximum intensity for 4 = 6 °, which then gets weaker for larger ~ (15 and 30°). In view of the very approximate character of the simulation, which furthermore neglects surface-induced deexcitation processes, we only stress here a qualitative agreement. The decrease of the simulated Ne** yield is much stronger than actually observed with electron emission.

4.4. Angle-resolved spectra

I

I

I

= 6 deg

oo ¢.-. ¢-.

The projectile autoionising lineshapes and positions change with the incident ion energy as well as with the angle of detection. As was first proposed by Zampieri et al. [4] and later confirmed by Pepper and Aron I-5], this is due to electron ejection kinematics determined by the fast movement of the emitting particles leaving the surface. Thanks to our continuously rotating electron spectrometer, we have been able to make a detailed analysis of the variation in position and shapes of the peaks due to the Ne projectiles as well as those due to sputtered target atoms as a function of the observation angle [10]. Here we shall only summarise the main points of this discussion. Fig. 10 shows the two dominant Ne** (3p/zD)3s2 peaks obtained by Ne ÷ scattering on Al, for three detec~tion angles ~. For small ~ (forward detection) the !i~peaks are found at high electron energies and !I exhibit a dominant high-energy contribution with a low energy tail. For large ct (backward detection), the peaks are shifted to lower energies, the maximum now being at low energy with a higherenergy tail. We showed that this behaviour can be accounted for in terms of kinematic effects, considering an angular and energy distribution of the emitting particles given by the experimentally determined ion distributions. This distribution, which sharply rises to reach maximum at 20 °, extends to rather large scattering angles, where the associated elastic energy losses are also large. Most of the particles scattered into small angles are flying towards the analyser for small ct, giving rise to the high-energy maximum, and are flying away from it for large ~, giving rise to the low-energy maximum. On the other hand, the large-angle and low-energy part of the scattering distribution gives

i

60 deg

I "J 18

a

I 20

~

I 22

i

I

,

24

I 26

28

Electron Energy (eV) Fig. 10. Ne** (aP)3s 2 and Ne** (iD)3s2 autoionising lines for three electron observation angles ct, for 2 keV Ne ÷ ions incident on A1.

rise to the low-energy tail of the peaks for small ct, and a high-energy hump for large ~. At medium detection angles, an intermediate situation is found where the peak is more symmetric. We showed that the main features mentioned above can be simulated with a simple kinematics model using an experimentally determined angular profile of the scattered ions where in addition to an inelastic energy loss a single-collision elastic scattering loss is assumed for a given angle. The major point to be noted is that the relative importance of the low/high energy tail of the peaks for small/large ~, is representative of the relative importance of the small scattering angle part of the scattered particle distribution with respect to the larger angle part. In other words, the evolution of the autoionising lineshapes when observation goes from forward to backward position reflects the angular and energetic distribution of the emitted particles. An important and interesting feature is clearly visible in Fig. 10, and is indicated by an arrow.

365

L. Guillemot et al./Surface Science 365 (1996) 353-373

743.7 and 735.9 ~, (2p53s[J = 1/2, 3/2]~2p61S) has been observed indicating that a substantial resonant neutralisation of scattered ions occurs. Some preliminary calculations by Nordlander [31], using a complex scaling method, show that Ne + ions approaching a Mg surface should be very efficiently resonantly neutralised to Ne 3s at distances between 12 and 6 au. At smaller distances, image charge effects shift the Ne 3s energy level above the Fermi level, where strong resonant ionisation occurs. Since our results suggest that efficient neutralisation to ground-state Ne ° takes place in the incoming trajectory this must be due to efficient Auger neutralisation and de-excitation processes, as shown by some preliminary estimates [32]. In the outgoing path, Ne + ions leaving the surface should also suffer efficient resonant neutralisation to Ne 3s at distances larger than 6 au, which must give rise to the resonant emission measured. We observed some additional VUV lines. The lines at about 405 and 445 A correspond to Ne + transitions (2p4(3p) o r ( 1 D ) 3 s ~ 2 p S ( Z P ) ) , and are

The low/high energy tail of the peak in forward/backward collection is much more pronounced for the Ne** (1D)3s2 state, showing a noteworthy difference in the scattering profiles of the atoms in (1D)3s2 o r (3p)3s2 excited states. Using the procedure outlined in Ref. [30] with the assumption of in-plane scattering, we were able to estimate the corresponding scattered atom distributions. They are shown in Fig. 11, and confirm that the angular distribution of scattered Ne** (3p)3s2 is shifted to smaller angles, or closer to the surface, as compared with Ne** (1D)3s2 one.

5. Photon spectroscopy

Some additional information about excited-state production can be obtained by looking at photon emission. These measurements were made for 15 keV Ne + ions, incident at 10°, on Mg surface. They are shown in Fig. 12. In agreement with previous findings of Rabalais et al. [2] on the Mg surface, strong Ne I resonance line emission at 1,2

!

I

I

1,0

0

1D3s 2

•

3p3s2

O• 0,8

•O

O

0,6

O •

O

0,4

O

0,2

0,0~

O

~:) i

0

•

I

20

I

I

40

i

I

60

i

®

®

I

i

80

100

Scattering Angle ( deg ) Fig. 11. Simulation of the state selected Ne** (3P)3s 2 and Ne** (1D)3s 2 scattering profiles, constructed from the measured kinematicsinduced shape of the autoionising lines (see text). 2 keV Ne ÷ ions on A1.

L. Guillemot et al./Surface Science 365 (1996) 353-373

366

(a)

104

1' 1"~t~ • .

..

..

t--t

1o \ / 10~

, ,-~l

300

,,1~,~,1,,,~1

400

500

600

700

800

,,

900

,,

1000

1100

(Angstr6m) '~IC~

T 104

~

t'~ ~t~

~

c~N

Co)

ua

1"

1"

/ 10 3

,A ,i, 300

400

i

500

i

t

600

i

i

700

i

i

t

i

800

i

i

i

900

i

i

I

i

1000

i

i

t

1100

(Angstr6m) Fig. 12. V U V photon spectra obtained from 15 keY Ne + ions scattering at 10 ° incidence on (a) M g surface and (b) A1 surface.

of comparable intensity. This differs from what we have at smaller impact energies, where the 2p4(3p) core state gives the strongest contribution in the autoionising lines, but agrees with the

trend observed in our data for energies in the 1 - 5 k e V range discussed in Section 4.3. Two lines assigned to Ne ÷ and Ne ÷ ÷ with a 2s hole are also measured. At 461~, one sees the

L. Guillemot et al./Surface Science 365 (1996) 353-373

2s2p6(2S)-~2s22pS(2p)Ne +* transition, and at about 490A a Ne 2+ transition 2s2pS(3p)~ 2s22p4(3P). It should be stressed that such ionisation of Ne leading to a vacancy in the 2s shell must be much more important here with 15 keV projectiles than at lower energies (a few keV), as in the electron emission measurements discussed in Section 4.

6. Excitation mechanisms: discussion The experimental results presented in the previous sections reveal that in ion-surface scattering at keV energies, a major role is played by shortdistance binary collisions in the production of either electronically excited states or ions. In this section we will present a detailed comparative analysis of our results with atom-atom interactions of the same species in gas-phase collisions in order to point out the similarities and differences. This will shed light on some important specific effects due to the solid target.

6.1. Comparison of results for gas-phase and surface collisions Gas-phase experiments have been performed with metallic ion projectiles (Mg + or A1+) colliding on neutral Ne or Ar targets. They consisted of ion energy-loss spectroscopy [17,18,33] and secondary electron spectroscopy [25]. The ion energy losses revealed single and double excitation of Ne, preferentially to the 3p orbital, as well as some Mg + * projectile excitations. Electron spectroscopy showed dominant excitation of the Ne** (1D)3s2 and Ne** (1D)3s3p autoionising states. All the peaks corresponding to Ne** (3P)nl,nl')) configurations are found to be much smaller. As far as Mg is concerned, the measured autoionising lines are assigned to Mg +* 2p 5 3s31 (l=s,p,d)~ MgZ+(2p6+e-). In the case of the Ar target, only projectile metal excitation is observed. The general trend of excitation of He and Ne (but not Ar), that we observe for collisions with surfaces agrees with the data for gas-phase collisions. We also compared the efficiency of the production of Ne** states as a function of the

367

collision energy, with the total cross-sections measured in the gas phase. Fig. 13 shows a comparative plot of the summed intensities of the Ne** (3p)3s 2 and Ne** (ID)3s 2 states and also of the Mg +* and Mg* states in Ne + collisions on the Mg surface, and the corresponding total crosssections measured for Mg+/Ne collisions in the gas phase as a function of the incident projectile energy. They are shown to exhibit a comparable dependence on Ei.c. It is very interesting to note that the dynamics of Ne excitations in collisions on an Mg surface follows the general trend observed in the pure atom-atom interaction. These similarities suggest that the same primary excitation mechanisms are operative in gas phase collisions and in collisions involving a solid target. We do observe some strong surface-specific effects which are: (i) the higher excited states, such as Ne** 3s3p or Ne** 3p2 lines, strongly excited in gas-phase collisions, are observed with very weak intensities, (ii) the Ne** (3p)3s2 line, hardly seen in the gas phase, is dominant, and (iii) Ne +** or Ne*** structures, absent in the gas phase, are observed. In the following we shall briefly discuss the primary excitation mechanisms on the basis of the gas-phase description of atomic collisions, and then consider the surface-specific effects.

6.2. The quasi-molecular model Over the past thirty years, in the field of atomic collisions at intermediate energies (keV), a great deal of experimental results have been successfully described in the framework of a quasi-molecular interpretation using the molecular orbital (MO) promotion model proposed by Fano and Lichten [34] and Barat and Lichten [35]. It allows correlation, from separate atoms to united atom, of the quasi-molecular orbitals according to some simple conservation rules. This brings about strong promotions of some orbitals. The collisional excitation and ionisation processes originate via electron transition from these strongly promoted orbitals. Such a correlation diagram is shown in Fig. 14 for the Ne-Mg system. The striking feature is the strong promotion of the 4fa MO which correlates to the 2p atomic orbital (AO) of Ne. The formation

368

L. G u i l l e m o t et a l . / S u r f a c e S c i e n c e 3 6 5 ( 1 9 9 6 ) 3 5 3 - 3 7 3

10000

,

i

,

,

10

.y. 1000

-.t o

•

o o . . . . . . .

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1

o

.--

. . . . . . .

: : ..

100

:

o a -.t X

o

o

o

gas

,

I

0

•

solid{

i

phase {

I

1000

2000

i

Ne**

o

0,1

Mg*

Ne" Mg+*

...... I

3000

~o

o i

I

4000

5000

Projectile Energy (eV) Fig. 13. Comparative plot of the yield of excited Ne** autoionising states produced in collision on an Mg surface and the crosssection for the production of excited Ne in the "inverse" collision, Mg ÷ on Ne in the gas phase.

4p

4~a

• •

. ..- 3p Mg 3s Mg

4s.

3d.

.........3 p . ~ ...3p~ 3s

_.

3d~

3p. 3s

J

..o°...o...

2p Ne 2s Ne 2p Mg 2s Mg

2p.~.............. 2so 2p 2s.

---------

2pa I s Ne

J ls Ti

J

l s Mg

J

R Fig. 14. Correlation diagram of the diabatic molecular orbital of the Ne-Mg system.

of singly or doubly excited states of Ne thus occurs through promotion of its outermost 2p electrons. For the He/Mg pair, the excitation process arises from transitions from the promoted 3da MO, which correlates to ls AO of He. This is not the case with the Ar/Mg system, in which sharp promotion affects the 2p AO of Mg, the 3p valence orbitals of Ar being only weakly promoted. Similar considerations are valid for inert gas/A1 systems. As we showed above, these predictions of the Fano-Barat-Lichten model also agree with our results for Mg and A1 surfaces. This strongly suggests that in ion-surface interactions we are dealing with the same kind of primary excitation mechanisms as described by the molecular orbital promotion model. Indeed, it should be remembered that promotion effects come from the overlapping of the involved AO (2p for Ne and Mg) when the collision partners are close enough 1-34,35]. In the solid, Mg atoms are located far away from each other (about 6 au) so that their 2p orbitals should not be much affected with respect to the free atom orbitals. Experimental and theoretical studies in gas-phase collisions of Mg and A1 scattering on Ne indicate that the internuclear distances at which the transition from the promoted 4fa MO can

L. Guillemot et al./Surface Science 365 (1996) 353-373

occur are of the order of 1.2 and 0.85 a.u., respectively. From simple geometrical considerations, one can see that the excitation cross-sections will decrease when going from Mg to A1. This also corresponds to the trend we observe. Nevertheless, in the case of a solid target, a major difference should be stressed. The promotion of the molecular orbitals discussed above occurs in the presence of the metal conduction band continuum. It has been proposed by Joyes [36], that this conduction band should be accounted for in the correlation diagram (Fig. 14). In this interpretation, the binary collisions would lead to ionisation of the projectile as soon as the promoted orbitals cross above the Fermi level of the metal. The short-distance interaction would then lead to the production of either Ne + 2p 5 or Ne 2+ 2p 4 ions coming out of the surface. Production of excited states would then be due to electron captures taking place in the outgoing way of the Ne particles. We will now consider in more detail excitedstate production for the N e - M g system and then consider the surface specific effects noted above. In Ne-Mg/A1 collisions, excitation processes originate as a result of one or two 4ftr electron transitions from the initial 3dzc44fa2 state. This leaves the 3dTt4 core unchanged which then leads to the production of the 2pa(1D) or (zS) core states of Ne** when the particles separate. This explains the dominance of the 2p4(XD)nlnT states observed in the electron spectra in gas-phase collisions. Direct excitation of Ne** 2pa(3p)3s 2 requires a hole in the 3drc quasi-molecular orbital during the collision and can only occur in a Ne ÷ collision, since we would then be dealing with the two 3dTz44fa and 3dTr34ftr2 states. At first glance our observations for solid targets do not fit into this scheme. However, in most cases they can be fairly simply accounted for. The first point, the absence of high lying states, can be simply understood in terms of differences induced by resonant electron capture and loss processes close to the surface, and the effects of image-potential related energy level shifts. Indeed, the binding energy of an electron in the Ne** 2p43s 2 state relative to its parent Ne + * 2p43s state is about 7 eV, while that of the higher-lying states

369

is less than 5 eV. These can be resonantly ionised near the surface in particular because of image potential shifts, and clearly the more strongly bound 3s 2 states remain stable for smaller atom-surface distances. If one thinks of the formation of these states in terms of electron capture by receding ions, then the Ne** 2p43s 2 states would be the first to be formed and are dominantly populated [37]. As gas-phase collisions lead to an almost exclusive population of Ne** 2p4(XD)nlnT autoionising states, the very strong appearance of an Ne** 2p4(3p)3s 2 for Ne + and also Ne ° interaction with solid Mg points out to a major core rearrangement effect induced by the presence of the surface. This conclusion is supported by the above-mentioned dominance of the triplet core-state for small angle scattering, as deduced from the angle-resolved observation of the electron spectra. This very interesting effect will be addressed in the Section 6.3. Structures corresponding to deexcitation of Ne ÷* 2p3nlnT and Ne* 2p3nlnTn'T ' states are not observed with gas targets, and are not understandable in the framework of the MO promotion model assuming an Ne ° projectile. In the quasi-molecular description one can understand the double excitation of Ne through the promotion of the 4fa orbital, but an excitation of a third electron is required to end up with 2p 3 core configuration. This excitation processes can be attributed to double collisions with Mg target atoms, where the first encounter would yield secondary Ne ÷ ions. In the following N e + - M g binary collision, the quasi-molecular system can find itself in either the 3d~z34fcr2 or the 3dTrn4ftr configuration. The 3dTt34fir2 states of the system can then lead to a Ne 3+ 2p 3 core state which afterwards, by electron capture, can produce the observed Ne +* 2p3nlnT or Ne* 2p3nlnTn'T ' autoionising lines. Some measurements by Xu et al. [7] further support this double collision scheme. Plotting the intensities of peaks (1), (3) and (7) as a function of the Ne + incident energy, they found an appearance threshold for peaks (1) and (3) of about 200 eV different and lower than the 400eV for peak (7). This finding points out the different nature of the processes leading to peaks (1) and (3) on the one hand, and peak (7) on the other hand. The higher

370

L. Guillemot et al./Surface Science 365 (1996) 353-373

appearance threshold of the latter is consistent with the assumption that it requires a sequence of two "binary" collisions.

away from a metallic surface, are imaginable. These should be effective on a larger atom-surface distance range.

6.3. Core rearrangement processes

6.3.2. Atomic Auger de-excitation A possible process which can be thought of as the asymptotic limit of the previous process (Fig. 15b) is direct atomic Auger de-excitation, where a transition between the (ID) and (3p) core states of Ne, situated 3.2 eV apart in the free-atom limit, is accompanied by electron emission from the valence band in the limit of 3.2 eV under the Fermi level.

In order to explain the dominant production of the 3p3s2 state one has to envisage core rearrangement processes triggered by the presence of the surface. Four different possibilities can be proposed.

6.3.1. The quasi-molecular rearrangement This mechanism has been proposed by Olsen et al. 1-38] to explain the weak excitation of Ne** 2p4(3p)nlnT in collisions of Na + with Ne in gasphase collisions. The strong promotion of the 4fa orbital will lead to a more repulsive character of the 3dr?4fanlnT molecular state, as compared to a 3dzanlnT state. The first one, which can correlate to states with 2p4(3P) Ne 2÷ core, then crosses at some short distance the second one which correlates to states with 2pn(1D)Ne 2÷ core. In the receding phase, a two-electron transition at this crossing point, corresponding to a 3drc/4f~relectron exchange mediated by a Rydberg electron, allows the formation of 2p4(aP) core state of Ne (Fig. 15a). Zampieri et al. [4] first proposed that this mechanism, later discussed in Refs. [7,10,39], could be very efficient in the case of a solid target because of the huge electron "reservoir" available to make the transition. It must be noted that the presence of the metal valence band allows the above-discussed transition to take place not only at the crossing distance but also further away, involving metal electrons down to 3.2 eV (triplet-singlet core separation) under the Fermi level. Two questions remain concerning this quasi-molecular rearrangement process and its efficiency. (i) It has to happen when the projectile moves away from the surface, in a rather short time range beyond the crossing point, but still within the brief lifetime of the quasimolecule, which is about 10 -15 s for a 1 keV Ne. (ii) The molecular states are themselves shifted by the presence of the surface, so that the energetics of the system is not known. Some more "atomic'-like processes, in terms of a delocalised interaction of an excited atom flying

6.3.3. "Resonant" autoionisation of excited projectiles This process, proposed in Ref. [37], is illustrated in Fig. 15c. In the case of an Ne 2+ 1D, moving away from the surface, resonant electron capture can lead to Ne +* ~Dnl for distances greater than 2 au. Beyond, there is a range of distances where the Ne +* 1Dnl state is bound with respect to its own parent Ne 2+ 1D+metal continuum, but autoionising with respect to the Ne 2+ 3p + metal continuum lying 3.2 eV lower. Resonant autoionisation of Ne +* 1Dnl to the Ne 2+ 3p+metal continuum followed by further resonant electron captures can produce the observed Ne** 3pnlnT states. 6.3.4. Auger neutralisation of Ne ÷* 2p3nlnT In the discussion about peak (7), we emphasised that double collisions can result in the production of Ne + * 2p33s 2. The filling by Auger neutralisation of a 2p vacancy of such excited species can lead to the production of a 2p 4 core configuration. A "sudden" transition should produce state populations with statistical weights of 9 for 3p, 5 for 1D and 1 for 1S. It is clear that all the proposed core rearrangement mechanisms may be active during the actual scattering of Ne on Mg or A1 surfaces. They are consistent with the fact that conversion to Ne** 3p is favoured for lower collision energies and scattering trajectories closer to the surface, i.e. for longer interaction times with the surface. They justify the strong population of either Ne** 2p4(3p)3s/ observed by electron spectroscopy, or

371

L. Guillemot et al./Surface Science 365 (1996) 353-373

vcuum!

(c)

(a)

V

~z4nlX

I D nl nT

nTL'

3p nl n'l'

b

R

(d) Ne ++ ID + metal (vacuum)

(b)

R

Ne++ 3p + metal (vacuum) 1D+ metal( E v) 3p+metal(~v)

V

Ne+ , l D nl Ne+ , 3p nl 1D nl nT 3~ nil n'l'~.

3p nl nT

"R Fig. 15. Schematic of the Ne** (3P)3s2--*Ne** (ID)3s 2 core rearrangement mechanisms: (a) quasi-molecular, (b) atomic Auger deexcitation, (c) resonant autoionisation followed by electron capture.

Ne +* 2p4(aP)3s measured by VUV spectrophotometry. At present it is not possible to delineate the respective role of these processes, and we cannot state that any of these mechanisms should be strongly dominant. Because of its local character, the quasi molecular process should be active over shorter distances (2-4 au) during dissociation of the quasi-molecule. This is not the case for the "atomic" rearrangements, which can play an important role as long as the atom-surface separation is not too large. These processes should therefore be very efficient since the interaction time will be long. Thus if we consider Ne** scattering at an angle of 15 ° from the surface plane, a characteristic 6 au vertical atom-surface distance would correspond to a 25 au distance along the surface plane. These conclusions are supported by recent measurements on an AI(111) monocrystalline surface

[40], where the azimuthal dependence of the rearrangement process was investigated experimentally and in computer simulations.

7. Conclusion

We present results of a detailed study of production of electronically excited states, in the scattering of ionic or neutral inert gas projectiles, in the keV energy range, at A1 or Mg surfaces. The complementary observation of scattered particles (neutrals or ions), secondary electrons and photons leads to a rather complete description of the successive stages of the inelastic scattering events. Strong similarities in the results obtained for incoming ions and incoming ground-state neutrals are observed. This shows that efficient neutralisation

372

L. Guillemot et al./Surface Science 365 (1996) 353-373

of the incident ions occurs in the incoming trajectory. The characteristics of the scattered particle distributions, the observation of scattered ions and also of excited Ne**, Ne +* and Ne ++* states by electron and photon spectroscopy, delineates the decisive importance of short-distance binary collisions with surface atoms in the production of these species. A detailed comparison with the "inverse" collisional systems in the gas phase, i.e. collisions of Mg and A1 ions with inert gases, shows that the same kind of primary excitations as described in the quasi-molecular orbital promotion model are operative. We do observe some very strong differences with gas-phase collisions, however. These stress the importance of surface-specific effects: the role of resonant and Auger electron transfers between the metal surface and the receding particle in defining the final-state population. This leads to a dominant population of the energetically lowestlying excited states. Angle-resolved electron spectroscopy measurements clearly indicate the importance of an interesting surface-induced core rearrangement process leading to a transformation o f 1D core states to 3p c o r e states. Different rearrangement mechanisms are presented and discussed. The characteristics of the scattered particle angular distributions and the dependence of the Ne** excited production on incident energy and angle could be modelled using a modified version of the Marlowe code which includes a two-state molecular model of excitation. In this paper we did not address the characteristics of the shapes of the electron energy distributions, in particular the strong continuous background. This background has contributions from a variety of processes including a number of Auger neutralisation and de-excitation channels. Some of these channels have been discussed in detail by, for example, Zeijlmans van Emmichoven et al. [41] and us [24,26]. It should be noted that at present, calculations of the shapes of spectra in neutralisation of, for example, doubly charged ions [41] still rely on a number of fitting parameters, and can only be regarded as giving a qualitative insight into the role of various processes.

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