- Email: [email protected]
Electron capture and loss in the scattering of oxygen atoms and ions on Mg, Al and Ag surfaces
c2-23 __
Nuclear Instruments and Methods in Physics Research B 125 (1997) 283-287 WkJil
__
B
Beam Interactions with Materials&Atoms
!!F! ELSEWIER
Electron capture and loss in the scattering of oxygen atoms and ions on Mg, Al and Ag surfaces M. Maazouz
‘, L. Guillemot
a, T. Schlatholter
b, S. Ustaze a, V.A. Esaulov a.*
a Luhoruroire ties Coffisions Aromiyues et M&cculaires ~Unite’associ& au CNRS), Uniuersiti Paris Sud, b&. 35 1, 9 1405 Ur.wy. France b Fachbereich Physik, Uniuersifiif Osnubriick, Osnabriick, Germany
Abstract We present the results of a study of collisions of 1 to 4 keV oxygen ions and atoms with Mg, Al and Ag surfaces. Formation of O- is in particular investigated. This is an interesting multichannel problem, since the ground state electronic configuration of oxygen 2p4 corresponds to three states and electron capture processes involve three atom c3P, ‘D and IS)-metal continua. We report scattered ion fractions, measured in an angular range extending from 2” to 40” with respect to the surface plane. This allowed us to investigate the characteristics of the resonant charge transfer process for a large range of collision velocities normal to the surface, thus probing the charge transfer process in different atom-surface distance ranges. The ion fractions are found to increase with increasing angle and increasing energy. Similar fractions are obtained for AI and Ag, but significantly higher ones for Mg. Ionisation processes in hard collisions with surface atoms are observed. An electron spectroscopy study was performed and did not reveal any signs of autoionising state (0 * * 2~~3s’) production.
1. Introduction This paper presents the results of an experimental study of electron capture and loss processes in oxygen ion scattering on Mg, Al and Ag surfaces. It extends our recent study of H- formation [ 1,2] on these high work function (4) metal surfaces, with different valence band characteristics. On metal surfaces anion formation involves a transition of an electron from occupied levels of the valence band to the anion level, the latter being downward shifted due to image potential effects as is schematically illustrated in Fig. la. For many common clean metals, the large value of the workfunction (4) requires a large shift and so anion formation can only occur at small atom-surface distances. Electron capture in these cases is favoured by the rapid movement of the atom parallel to the surface, due to a kinematic matching of energies of electrons in the solid [l-6] and in the anion when viewed from the reference frame of the moving atom. Previously [ 1.21, we reported experimental and theoretical results for H- formation in 1 to 4 keV H’ collisions on Mg, Al and Ag surfaces over a large domain of scattering conditions, for which the final charge state distribution is determined at very different atom-surface separations. It was shown that the non perturbative Cou-
* Corresponding author. Email: esaulov@veofl .lcam.u-psud.fr.
pled Angular Mode (CAM) method [7], in conjunction with a semiclassical rate equation approach [3,4] gave a good description of this charge transfer process over a large range of scattering conditions. In this work we studied O- formation in oxygen ion/atom scattering. Oxygen is one of the most common adsorbates and the study of its interaction with metal surfaces is of much practical interest. From a heuristic point of view, the oxygen case is a very interesting one since we deal with a multichannel problem. Indeed the ground state electronic configuration of oxygen 2p4 corresponds to three states: ‘P, ‘D and ‘S. In considering electron capture we then have to consider capture and loss processes corresponding to three atom t3P, ‘D and IS)metal continua as sketched in Fig. la. There exist several theoretical studies of oxygen adsorption on metals and positions and widths of the relevant negative ion affinity level have been calculated [g-12]. Negative ion formation in singly and more recently multiply charged oxygen ion scattering on low workfunction cesiated tungsten 1131 and on a much higher workfunction Au surface [14] have been investigated previously. In the latter work the parallel velocity dependence was in particular explored. Here we present the results for some of the simplest free electron metals like Al, for which some of the theoretical investigations were performed. We also investigated positive ion production. Fig. lb presents a schematic diagram of the characteristics of the metal-oxygen atom systems considered, as well as the
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOl68-583X(96)00807-5
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and Meth. in Phys. Res. B I25 (1997) 283-287
0
0(3P)+e)
E(eV
Mg
Al
Ag
0 ’
0
0 +f3s)
I
*+
I
i
7
I
Io**(3s 2)
I I , I
-I
o+*
I 15
I!(b)
‘9
Fig. I. (a) Schematic diagram of the energy levels of the multichannel metal-oxygen atom/ion system, relevant to O- formation. (b) Schematic diagram of the energy levels of the various metal-oxygen ion systems considered here. The work functions are C$= 3.64 eV (Mg), C#J = 4.4 eV (Al) and 4 = 4.3 eV (Ag) while the Fermi energies arc: cF = 7.08 eV (Mg), Ed = 11.65 eV (Al) and Ed = 5.49 eV (Ag). The electron affinity of O- with respect to the ground ‘P oxygen state is 1.45 eV [8].
general disposition of levels of oxygen positive ions relevant to this work as will be discussed later.
2. Experiment The apparatus used for the present experiments is described elsewhere [ 1.51.It is equipped for measurement of
electron and ion energy spectra using tandem parallel plate energy analysers, as well as time of flight scattering and direct recoil spectroscopy. O- ions are produced in a discharge source running with a N,O and Ar mixture. They are mass selected and deflected through 90” before entering into the main UHV chamber. The pressure in the chamber is less than 5 X lo-I0 Torr. Some measurements were also performed using a neutral 0” atom beam, obtained by passing the O- beam through a cell containing Ne. This procedure yields a 0” beam in the 3P and ‘D states [ 161. As in our previous study of H- production [ 1,2], polycrystalline Mg, Al and Ag samples were used. These are hand polished to 0.05 pm. In situ preparation consisted in repeated cycles of small angle (less than 10“) Ar+ sputtering and annealing. Because surface flatness is an important consideration in these measurements, in the final stage of preparation the samples were subjected to prolonged more grazing incidence (circa 4”) Ar bombardment. Our previous measurements of the angular distributions of scattered H atoms incident at a 3.5” angle on the surface gave a distribution, which had a FWHM of 6” with a tail extending to large angles [l]. This distribution was wider than reported in some other H grazing incidence studies [ 171. However the residual roughness allows us to study scattering over a wide angular range. The surface cleanliness was ascertained by measuring time of flight (TOF) spectra of scattered and recoiled particles under Ar bombardment. The surface was assumed to be clean when statistically significant peaks of recoiled 0 and H were no longer observed. O- ion fraction measurements were made for ion energies in the 1 keV to 4 keV range. The ion fractions (@,‘/-) for a given scattering angle are defined as the ratio of the scattered positive or negative ion ((N+/-(r))) flux over the total scattered flux (N(JI)). Measurements were made for fixed scattering angles (0) of 7” and 38” using position sensitive 30 mm diameter channelplate detectors set at a distance of 2.2 m from the scattering centre at the end of time of flight analysis tubes. These detectors are equipped with three discrete anodes. Deflector plate assemblies set before the channelplates allowed to separate and simultaneously count the incoming positive and negative ions and neutrals. The sample orientation was varied and incidence angies (o) changed in the 2” to 36” range (as measured with respect to the surface plane), allowing us to sample a similar exit angle (I)) range with the 38” detector. Measurements with the 7” detector allowed determinations of @- for both grazing incidence and exit angles. Initially charge fractions were determined by counting scattered ions and neutrals using a continuous beam in order to minimise data acquisition times and eliminate any effects due to oxygen implantation. Time of flight spectra for each charge state were then recorded for various angular settings and the charge fractions were determined from these spectra. In all these measurements
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we assumed an equal detector efficiency for ions and neutrals. This assumption seems reasonable on the basis of our earlier studies of negative ion scattering [16] for keV energies.
3. Results Typical results of ion fraction measurements for the Mg surface are shown in Fig. 2 for 1 and 3 keV incident ion energies as a function of the exit angle I) to the surface. Both negative and positive backscattered ions are observed. The overall trend of these results is that the ion fractions are found to increase as a function of increasing 1,4and ion energy. In order to verify the infIuence of the incoming ion trajectory on @+I- we performed measurements with both O- and 0” beams. Similar results were obtained using the neutral and negative ion beam indicating that the memory of the incident ion charge state is lost. Our
measurements performed for small exit angles for both grazing incidence and large angle incidence were also in good mutual general agreement, once one takes into account differences in final energy. These are large for Mg but small for the case of scattering on Ag considered below. These measurements correspond to a sum over energy losses of scattered particles and could also contain a
A
0
%A
0
A
CwreCWd
%I
I
0
.
IO
0
3 keV
X
I keV
I
.
20
I
30
,
40
Exit angle (deg.) Fig. 2. Ion fractions for oxygen scattering on Mg. The TOF corrected data is explained in the text. The final scattering energies corresponding to scattering through 38” are of 2.2 keV and 0.74 keV.
20
25
30
35
40
Time(p sec.) Fig. 3. Time of flight spectra for 3 keV oxygen scattering on Mg for specular 19” incidence and scattering conditions. The main peak for a flight time of less than 25 ps is due to 0, while the structure at 30 ~LSis due to sputtered Mg.
contribution from recoiled surface atoms. In order to assess these, TOF spectra for scattered ions and neutrals were measured for some scattering configurations. Typical results for 3 keV incident ions for scattering with (Y= 19” for I)= 19” are shown in Fig. 3. The spectrum shows structures due to single and multiple scattering. One can also observe a structure corresponding to recoiled Mg atoms. The spectra of scattered ions had a similar general shape, but Mg- ions are not observed. This is not surprising, since for Mg there only exists a metastable Mg (3~3~~) ‘P excited state bound by 0.32 eV [8]. Using these spectra, ion fractions corresponding to the scattered oxygen alone were evaluated, by integrating the peak areas as indicated by the vertical bars in the figure. The resulting fraction was normalised to the summed fraction reported above. The results are indicated in Fig. 2. where the error bars correspond to statistical errors in the evaluation of peak integrals in the TOF spectra after background subtraction and also the scatter of the original data used in the normalisation. Note that in the following when presenting the charge fractions the value of the final energy for single scattering will be indicated. Since the energy width of the TOF spectrum is not large the ion fraction for various values of the energy in the spectrum do not vary significantly. The positive ion fractions @+ were smaller than @-. They increase with increasing exit angles and grow with increasing incident energy. These O+ must be produced in inelastic binary collisions with surface atoms. Several inelastic processes may lead to the observation of the O+, including both single and double ionisation processes leading to O+, O+* or 02+ production. To identify the ionisation processes leading to O+ formation we measured electron spectra and searched for any evidence of structures that could be attributed to 0 * * autoionising state VI. NEUTRALISATION AND CHARGE EXCHANGE
M. Maazouz et al./Nucl. Instr. and Meth. in Phys. Res. B I25 (1997) 283-287
286
formation. Such structures were observed in our previous study of inelastic scattering of He and Ne ions [I&19]. These may be considered signatures of doubly charged ion production. As discussed previously [l&20] the prcductionofO** may be considered a result of a sequence of electron capture events: O*+ + 0+ * -+ 0 * * The lowest lying relevant excited states are schematised in Fig. lb. Decay by autoionisation of 0’ * could be responsible for
10
0
20
10
30
40
5.0
5.5
Exit angle (deg.) A
. t
0.1 /
x 10
A
A
* 4 1.
.00: 4keV 4keV 1 keV
A oo+: AllAg
i
TOF corrected
1
0,01 0
40
10 Exit angle (deg.) I
’
I
’
1
.
(b)
4.0
4.5
Workfunction (ev)
.
lr .
3.5
.
d
i
.
for 1 keV oxygen scattering on the different surfaces studied. (b) Ion fractions for different metals plotted for grazing incidence conditions as a function of the metal workfunctions.
Fig. 5. (a) Comparison of ion fractions measured
A
0
A *
1
A A
0
*
*
h
.
&A
0+
O-
* A
4keV
0
1keV
A
(0+x 10)
Exit angle (deg.) Fig. 4. Ion fractions for scattering on (a) Al and (b) Ag surfaces. The nature of the TOF corrected data is explained in the text. The final scattered particle energies are of 0.77 keV and 3.08 keV for Al and 0.94 keV and 3.96 keV for Ag.
O+ production. Preliminary calculations 1211show that the lowest lying autoionising level of 0 2p23s2 should lie at an energy of 17.75 eV above the ground state of 0 2p4 (3P). No such structure was observed. This indicates that though from the point of view of molecular orbital correlation diagrams for the diatomic 0-Mg collisional system these states or their parents could be excited, they are not efficiently populated near the surface. Indeed the binding energy of the electron with respect to the parent 2~~3s state is of 5.25 eV. Near the surface the energy level of this state is shifted and lies above the Fermi level and hence it can suffer resonant ionisation. As the excited atom in the 2p23s state moves away from the surface the 2p23s2 state can be produced by resonant capture at atom-surface
M. Maazouz et al./Nucl.
Instr. and Meth. in Phys. Res. B 125 (1997) 283-287
greater than about Sa,. In the Ne case, states with a similar binding energy (e.g. Ne’ * 2p43s3p) are produced with a significantly lower efficiency (by a factor of 100) than the lowest states Ne * * 2p43s2 bound by about 7eV [l&-20]. Such a low efficiency would render them difficult to observe in this experiment, where the incident O- current was smaller than that of Ne by an order of magnitude. distances
3.1. Results for Al and Ag surfaces and a general comparison A similar study of scattering on Al and Ag surfaces was performed. The ion fractions obtained in the continuous beam mode as for Mg, are shown in Fig. 4a and b for 1 and 4 keV incident ion energies as a function of the exit angle to the surface t,!~.The overall trend of these results is similar to the Mg case. The negative ion fraction is found to increase as a function of I,!Jand attains a value of close to 20% for large angles at 4 keV for Al. An analysis of the TOF spectra was performed and the ion fractions were corrected taking into account the contribution from recoiled surface atoms. These results are indicated in Fig. 4a, b as for the Mg case. As for Mg we observed some positive ion production. A search for any signs of autoionising states of 0” was performed by measuring electron spectra, but again none were observed. Fig. 5a presents a comparison of the ion fractions for the Mg, Al and Ag targets for a 1 keV incident energy. As may be seen the ion fraction is the largest for the Mg case. This may be intuitively related to the smaller value of the work function of Mg, though as was previously demonstrated for H- this is not the only important factor, when parallel velocity effects are important. However at our low
keV energies these should not play a very important role. It is interesting to compare this data with the ion fractions obtained for grazing incidence scattering for a Au target [14], with a workfunction of 5.4 eV. The negative ion fractions for the various targets deduced from our data and that of [14] for a parallel velocity of 0.1 a.u. (4 keV) and an exit angle of 2”, are plotted as a function of the workfunction in Fig. 5b. The ion fractions nicely follow a general trend of a strong increase with a decrease in the workfunction. At present, parameter free, theoretical calculations of O- production for the extended range of scattering conditions presented here for the various metal surfaces are not available. Calculations in good agreement with O- formation in grazing incidence scattering on Au [14] have been recently performed [22]. We hope that our results will
287
stimulate further theoretical investigations tems.
of these sys-
Acknowledgements The authors are grateful to V. Sidis for performing the theoretical estimate of the 0 ‘. state energy. Our thanks for interesting discussions are due to A. Borisov, D. TeilletBilly and J.P. Gauyacq. The participation of T.S. in these experiments was made possible through funding via the European Community HCM network.
References
[II
M. Maazouz, R. Baragiola, A. Borisov, D. Teillet-Billy, V.A. Esaulov, S. Lacombe, J.P. Gauyacq and L. Guillemot, Surf. Sci. 364 ( 1996)L568. M M. Maazouz. R. Baragiola, A. Borisov, D. Teillet-Billy, V.A. Esaulov, S. Lacombe, J.P. Gauyacq and L. Guillemot, Phys. Rev. B (19%), submitted. [31 J.N.M. van Wunnick, R. Brako, K. Makoshi and D.M. Newns Surf. Sci. 126 (1983) 618. t41 R. Brako and D.M. Newns, Rep. Prog. Phys. 52 (1989) 655. t51 1. Los and J.J.C. Geerlings, Phys. Rep. 190 (1990) 133. I61 R. Zimny, H. Nienhaus and H. Winter, Radiat. Eff. Def. in Solids 109 (1989) 9. [71 D. Teillet-Billy and J.P. Gauyacq, Surf. Sci. 229 (1990) 343. [81 V. Esaulov, Ann. Phys. Fr. 11 (1986) 493. [91 N.D. Lang, Phys. Rev. B 27 (1983) 2019. [lOI P. Nordlander, Phys Rev. B 46 (1992) 2584. t111 B. Bahrim, D. Teillet-Billy and J.P. Gauyacq, Surf. Sci. 316 (1994) 189. [121 B. Bahrim, D. Teillet-Billy and J.P. Gauyacq, Phys. Rev. B 50 (1994) 7860. (131 H.M. van Pixteren, C.F.A. van OS, R.M.A. Heeren, R. Rodnik, J.J.C. Geerlings and J. Los, Europhys. Let. 10 (1989) 715. [141 L. Folkerts, S. Scippers, D.M. Zehner and F.W. Meyer, Phys Rev. Lett. 74 (1995) 2204. [ISI V. Esaulov, L. Guillemot, 0. Grizzi, M. Huels, S. Lacombe and Vu Ngoc Tuan, Rev. Sci. Ins@. 67 (1996) 135. [I61 V. Esaulov, D. Dhuicq and J.P. Gauyacq, J. Phys. B 11 (1978) 1049. [171 F. Wyputta, R. Zimny and H. Winter, Nucl. Instr. and Meth. B 58 (1991) 379. 1181 V. Esaulov, S. Lacombe, L. Guillemot and Vu Ngoc Tuan, Nucl. Instr. and Meth. B 100 (1995) 232. [I91 L. Guillemot, S. Lacombe, V.N. Tuan, V.A. Esaulov, E. Sanchez, Y.A. Bandurin, A.I. Dashchenko and V.G. Drobnich, Surf. Sci. 365 (1996) 353. 1201V. Esaulov, 1. Phys: Cond. Matter C 6 (1994) L699. [Zll V. Sidis, private communication. t221 A. Borisov, D. Teillet-Billy and J.P. Gauyacq, private communication; to be published.
VI. NEUTRALISATION
AND CHARGE
EXCHANGE
Nuclear Instruments and Methods in Physics Research B 125 (1997) 283-287 WkJil
__
B
Beam Interactions with Materials&Atoms
!!F! ELSEWIER
Electron capture and loss in the scattering of oxygen atoms and ions on Mg, Al and Ag surfaces M. Maazouz
‘, L. Guillemot
a, T. Schlatholter
b, S. Ustaze a, V.A. Esaulov a.*
a Luhoruroire ties Coffisions Aromiyues et M&cculaires ~Unite’associ& au CNRS), Uniuersiti Paris Sud, b&. 35 1, 9 1405 Ur.wy. France b Fachbereich Physik, Uniuersifiif Osnubriick, Osnabriick, Germany
Abstract We present the results of a study of collisions of 1 to 4 keV oxygen ions and atoms with Mg, Al and Ag surfaces. Formation of O- is in particular investigated. This is an interesting multichannel problem, since the ground state electronic configuration of oxygen 2p4 corresponds to three states and electron capture processes involve three atom c3P, ‘D and IS)-metal continua. We report scattered ion fractions, measured in an angular range extending from 2” to 40” with respect to the surface plane. This allowed us to investigate the characteristics of the resonant charge transfer process for a large range of collision velocities normal to the surface, thus probing the charge transfer process in different atom-surface distance ranges. The ion fractions are found to increase with increasing angle and increasing energy. Similar fractions are obtained for AI and Ag, but significantly higher ones for Mg. Ionisation processes in hard collisions with surface atoms are observed. An electron spectroscopy study was performed and did not reveal any signs of autoionising state (0 * * 2~~3s’) production.
1. Introduction This paper presents the results of an experimental study of electron capture and loss processes in oxygen ion scattering on Mg, Al and Ag surfaces. It extends our recent study of H- formation [ 1,2] on these high work function (4) metal surfaces, with different valence band characteristics. On metal surfaces anion formation involves a transition of an electron from occupied levels of the valence band to the anion level, the latter being downward shifted due to image potential effects as is schematically illustrated in Fig. la. For many common clean metals, the large value of the workfunction (4) requires a large shift and so anion formation can only occur at small atom-surface distances. Electron capture in these cases is favoured by the rapid movement of the atom parallel to the surface, due to a kinematic matching of energies of electrons in the solid [l-6] and in the anion when viewed from the reference frame of the moving atom. Previously [ 1.21, we reported experimental and theoretical results for H- formation in 1 to 4 keV H’ collisions on Mg, Al and Ag surfaces over a large domain of scattering conditions, for which the final charge state distribution is determined at very different atom-surface separations. It was shown that the non perturbative Cou-
* Corresponding author. Email: esaulov@veofl .lcam.u-psud.fr.
pled Angular Mode (CAM) method [7], in conjunction with a semiclassical rate equation approach [3,4] gave a good description of this charge transfer process over a large range of scattering conditions. In this work we studied O- formation in oxygen ion/atom scattering. Oxygen is one of the most common adsorbates and the study of its interaction with metal surfaces is of much practical interest. From a heuristic point of view, the oxygen case is a very interesting one since we deal with a multichannel problem. Indeed the ground state electronic configuration of oxygen 2p4 corresponds to three states: ‘P, ‘D and ‘S. In considering electron capture we then have to consider capture and loss processes corresponding to three atom t3P, ‘D and IS)metal continua as sketched in Fig. la. There exist several theoretical studies of oxygen adsorption on metals and positions and widths of the relevant negative ion affinity level have been calculated [g-12]. Negative ion formation in singly and more recently multiply charged oxygen ion scattering on low workfunction cesiated tungsten 1131 and on a much higher workfunction Au surface [14] have been investigated previously. In the latter work the parallel velocity dependence was in particular explored. Here we present the results for some of the simplest free electron metals like Al, for which some of the theoretical investigations were performed. We also investigated positive ion production. Fig. lb presents a schematic diagram of the characteristics of the metal-oxygen atom systems considered, as well as the
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOl68-583X(96)00807-5
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0
0(3P)+e)
E(eV
Mg
Al
Ag
0 ’
0
0 +f3s)
I
*+
I
i
7
I
Io**(3s 2)
I I , I
-I
o+*
I 15
I!(b)
‘9
Fig. I. (a) Schematic diagram of the energy levels of the multichannel metal-oxygen atom/ion system, relevant to O- formation. (b) Schematic diagram of the energy levels of the various metal-oxygen ion systems considered here. The work functions are C$= 3.64 eV (Mg), C#J = 4.4 eV (Al) and 4 = 4.3 eV (Ag) while the Fermi energies arc: cF = 7.08 eV (Mg), Ed = 11.65 eV (Al) and Ed = 5.49 eV (Ag). The electron affinity of O- with respect to the ground ‘P oxygen state is 1.45 eV [8].
general disposition of levels of oxygen positive ions relevant to this work as will be discussed later.
2. Experiment The apparatus used for the present experiments is described elsewhere [ 1.51.It is equipped for measurement of
electron and ion energy spectra using tandem parallel plate energy analysers, as well as time of flight scattering and direct recoil spectroscopy. O- ions are produced in a discharge source running with a N,O and Ar mixture. They are mass selected and deflected through 90” before entering into the main UHV chamber. The pressure in the chamber is less than 5 X lo-I0 Torr. Some measurements were also performed using a neutral 0” atom beam, obtained by passing the O- beam through a cell containing Ne. This procedure yields a 0” beam in the 3P and ‘D states [ 161. As in our previous study of H- production [ 1,2], polycrystalline Mg, Al and Ag samples were used. These are hand polished to 0.05 pm. In situ preparation consisted in repeated cycles of small angle (less than 10“) Ar+ sputtering and annealing. Because surface flatness is an important consideration in these measurements, in the final stage of preparation the samples were subjected to prolonged more grazing incidence (circa 4”) Ar bombardment. Our previous measurements of the angular distributions of scattered H atoms incident at a 3.5” angle on the surface gave a distribution, which had a FWHM of 6” with a tail extending to large angles [l]. This distribution was wider than reported in some other H grazing incidence studies [ 171. However the residual roughness allows us to study scattering over a wide angular range. The surface cleanliness was ascertained by measuring time of flight (TOF) spectra of scattered and recoiled particles under Ar bombardment. The surface was assumed to be clean when statistically significant peaks of recoiled 0 and H were no longer observed. O- ion fraction measurements were made for ion energies in the 1 keV to 4 keV range. The ion fractions (@,‘/-) for a given scattering angle are defined as the ratio of the scattered positive or negative ion ((N+/-(r))) flux over the total scattered flux (N(JI)). Measurements were made for fixed scattering angles (0) of 7” and 38” using position sensitive 30 mm diameter channelplate detectors set at a distance of 2.2 m from the scattering centre at the end of time of flight analysis tubes. These detectors are equipped with three discrete anodes. Deflector plate assemblies set before the channelplates allowed to separate and simultaneously count the incoming positive and negative ions and neutrals. The sample orientation was varied and incidence angies (o) changed in the 2” to 36” range (as measured with respect to the surface plane), allowing us to sample a similar exit angle (I)) range with the 38” detector. Measurements with the 7” detector allowed determinations of @- for both grazing incidence and exit angles. Initially charge fractions were determined by counting scattered ions and neutrals using a continuous beam in order to minimise data acquisition times and eliminate any effects due to oxygen implantation. Time of flight spectra for each charge state were then recorded for various angular settings and the charge fractions were determined from these spectra. In all these measurements
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we assumed an equal detector efficiency for ions and neutrals. This assumption seems reasonable on the basis of our earlier studies of negative ion scattering [16] for keV energies.
3. Results Typical results of ion fraction measurements for the Mg surface are shown in Fig. 2 for 1 and 3 keV incident ion energies as a function of the exit angle I) to the surface. Both negative and positive backscattered ions are observed. The overall trend of these results is that the ion fractions are found to increase as a function of increasing 1,4and ion energy. In order to verify the infIuence of the incoming ion trajectory on @+I- we performed measurements with both O- and 0” beams. Similar results were obtained using the neutral and negative ion beam indicating that the memory of the incident ion charge state is lost. Our
measurements performed for small exit angles for both grazing incidence and large angle incidence were also in good mutual general agreement, once one takes into account differences in final energy. These are large for Mg but small for the case of scattering on Ag considered below. These measurements correspond to a sum over energy losses of scattered particles and could also contain a
A
0
%A
0
A
CwreCWd
%I
I
0
.
IO
0
3 keV
X
I keV
I
.
20
I
30
,
40
Exit angle (deg.) Fig. 2. Ion fractions for oxygen scattering on Mg. The TOF corrected data is explained in the text. The final scattering energies corresponding to scattering through 38” are of 2.2 keV and 0.74 keV.
20
25
30
35
40
Time(p sec.) Fig. 3. Time of flight spectra for 3 keV oxygen scattering on Mg for specular 19” incidence and scattering conditions. The main peak for a flight time of less than 25 ps is due to 0, while the structure at 30 ~LSis due to sputtered Mg.
contribution from recoiled surface atoms. In order to assess these, TOF spectra for scattered ions and neutrals were measured for some scattering configurations. Typical results for 3 keV incident ions for scattering with (Y= 19” for I)= 19” are shown in Fig. 3. The spectrum shows structures due to single and multiple scattering. One can also observe a structure corresponding to recoiled Mg atoms. The spectra of scattered ions had a similar general shape, but Mg- ions are not observed. This is not surprising, since for Mg there only exists a metastable Mg (3~3~~) ‘P excited state bound by 0.32 eV [8]. Using these spectra, ion fractions corresponding to the scattered oxygen alone were evaluated, by integrating the peak areas as indicated by the vertical bars in the figure. The resulting fraction was normalised to the summed fraction reported above. The results are indicated in Fig. 2. where the error bars correspond to statistical errors in the evaluation of peak integrals in the TOF spectra after background subtraction and also the scatter of the original data used in the normalisation. Note that in the following when presenting the charge fractions the value of the final energy for single scattering will be indicated. Since the energy width of the TOF spectrum is not large the ion fraction for various values of the energy in the spectrum do not vary significantly. The positive ion fractions @+ were smaller than @-. They increase with increasing exit angles and grow with increasing incident energy. These O+ must be produced in inelastic binary collisions with surface atoms. Several inelastic processes may lead to the observation of the O+, including both single and double ionisation processes leading to O+, O+* or 02+ production. To identify the ionisation processes leading to O+ formation we measured electron spectra and searched for any evidence of structures that could be attributed to 0 * * autoionising state VI. NEUTRALISATION AND CHARGE EXCHANGE
M. Maazouz et al./Nucl. Instr. and Meth. in Phys. Res. B I25 (1997) 283-287
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formation. Such structures were observed in our previous study of inelastic scattering of He and Ne ions [I&19]. These may be considered signatures of doubly charged ion production. As discussed previously [l&20] the prcductionofO** may be considered a result of a sequence of electron capture events: O*+ + 0+ * -+ 0 * * The lowest lying relevant excited states are schematised in Fig. lb. Decay by autoionisation of 0’ * could be responsible for
10
0
20
10
30
40
5.0
5.5
Exit angle (deg.) A
. t
0.1 /
x 10
A
A
* 4 1.
.00: 4keV 4keV 1 keV
A oo+: AllAg
i
TOF corrected
1
0,01 0
40
10 Exit angle (deg.) I
’
I
’
1
.
(b)
4.0
4.5
Workfunction (ev)
.
lr .
3.5
.
d
i
.
for 1 keV oxygen scattering on the different surfaces studied. (b) Ion fractions for different metals plotted for grazing incidence conditions as a function of the metal workfunctions.
Fig. 5. (a) Comparison of ion fractions measured
A
0
A *
1
A A
0
*
*
h
.
&A
0+
O-
* A
4keV
0
1keV
A
(0+x 10)
Exit angle (deg.) Fig. 4. Ion fractions for scattering on (a) Al and (b) Ag surfaces. The nature of the TOF corrected data is explained in the text. The final scattered particle energies are of 0.77 keV and 3.08 keV for Al and 0.94 keV and 3.96 keV for Ag.
O+ production. Preliminary calculations 1211show that the lowest lying autoionising level of 0 2p23s2 should lie at an energy of 17.75 eV above the ground state of 0 2p4 (3P). No such structure was observed. This indicates that though from the point of view of molecular orbital correlation diagrams for the diatomic 0-Mg collisional system these states or their parents could be excited, they are not efficiently populated near the surface. Indeed the binding energy of the electron with respect to the parent 2~~3s state is of 5.25 eV. Near the surface the energy level of this state is shifted and lies above the Fermi level and hence it can suffer resonant ionisation. As the excited atom in the 2p23s state moves away from the surface the 2p23s2 state can be produced by resonant capture at atom-surface
M. Maazouz et al./Nucl.
Instr. and Meth. in Phys. Res. B 125 (1997) 283-287
greater than about Sa,. In the Ne case, states with a similar binding energy (e.g. Ne’ * 2p43s3p) are produced with a significantly lower efficiency (by a factor of 100) than the lowest states Ne * * 2p43s2 bound by about 7eV [l&-20]. Such a low efficiency would render them difficult to observe in this experiment, where the incident O- current was smaller than that of Ne by an order of magnitude. distances
3.1. Results for Al and Ag surfaces and a general comparison A similar study of scattering on Al and Ag surfaces was performed. The ion fractions obtained in the continuous beam mode as for Mg, are shown in Fig. 4a and b for 1 and 4 keV incident ion energies as a function of the exit angle to the surface t,!~.The overall trend of these results is similar to the Mg case. The negative ion fraction is found to increase as a function of I,!Jand attains a value of close to 20% for large angles at 4 keV for Al. An analysis of the TOF spectra was performed and the ion fractions were corrected taking into account the contribution from recoiled surface atoms. These results are indicated in Fig. 4a, b as for the Mg case. As for Mg we observed some positive ion production. A search for any signs of autoionising states of 0” was performed by measuring electron spectra, but again none were observed. Fig. 5a presents a comparison of the ion fractions for the Mg, Al and Ag targets for a 1 keV incident energy. As may be seen the ion fraction is the largest for the Mg case. This may be intuitively related to the smaller value of the work function of Mg, though as was previously demonstrated for H- this is not the only important factor, when parallel velocity effects are important. However at our low
keV energies these should not play a very important role. It is interesting to compare this data with the ion fractions obtained for grazing incidence scattering for a Au target [14], with a workfunction of 5.4 eV. The negative ion fractions for the various targets deduced from our data and that of [14] for a parallel velocity of 0.1 a.u. (4 keV) and an exit angle of 2”, are plotted as a function of the workfunction in Fig. 5b. The ion fractions nicely follow a general trend of a strong increase with a decrease in the workfunction. At present, parameter free, theoretical calculations of O- production for the extended range of scattering conditions presented here for the various metal surfaces are not available. Calculations in good agreement with O- formation in grazing incidence scattering on Au [14] have been recently performed [22]. We hope that our results will
287
stimulate further theoretical investigations tems.
of these sys-
Acknowledgements The authors are grateful to V. Sidis for performing the theoretical estimate of the 0 ‘. state energy. Our thanks for interesting discussions are due to A. Borisov, D. TeilletBilly and J.P. Gauyacq. The participation of T.S. in these experiments was made possible through funding via the European Community HCM network.
References
[II
M. Maazouz, R. Baragiola, A. Borisov, D. Teillet-Billy, V.A. Esaulov, S. Lacombe, J.P. Gauyacq and L. Guillemot, Surf. Sci. 364 ( 1996)L568. M M. Maazouz. R. Baragiola, A. Borisov, D. Teillet-Billy, V.A. Esaulov, S. Lacombe, J.P. Gauyacq and L. Guillemot, Phys. Rev. B (19%), submitted. [31 J.N.M. van Wunnick, R. Brako, K. Makoshi and D.M. Newns Surf. Sci. 126 (1983) 618. t41 R. Brako and D.M. Newns, Rep. Prog. Phys. 52 (1989) 655. t51 1. Los and J.J.C. Geerlings, Phys. Rep. 190 (1990) 133. I61 R. Zimny, H. Nienhaus and H. Winter, Radiat. Eff. Def. in Solids 109 (1989) 9. [71 D. Teillet-Billy and J.P. Gauyacq, Surf. Sci. 229 (1990) 343. [81 V. Esaulov, Ann. Phys. Fr. 11 (1986) 493. [91 N.D. Lang, Phys. Rev. B 27 (1983) 2019. [lOI P. Nordlander, Phys Rev. B 46 (1992) 2584. t111 B. Bahrim, D. Teillet-Billy and J.P. Gauyacq, Surf. Sci. 316 (1994) 189. [121 B. Bahrim, D. Teillet-Billy and J.P. Gauyacq, Phys. Rev. B 50 (1994) 7860. (131 H.M. van Pixteren, C.F.A. van OS, R.M.A. Heeren, R. Rodnik, J.J.C. Geerlings and J. Los, Europhys. Let. 10 (1989) 715. [141 L. Folkerts, S. Scippers, D.M. Zehner and F.W. Meyer, Phys Rev. Lett. 74 (1995) 2204. [ISI V. Esaulov, L. Guillemot, 0. Grizzi, M. Huels, S. Lacombe and Vu Ngoc Tuan, Rev. Sci. Ins@. 67 (1996) 135. [I61 V. Esaulov, D. Dhuicq and J.P. Gauyacq, J. Phys. B 11 (1978) 1049. [171 F. Wyputta, R. Zimny and H. Winter, Nucl. Instr. and Meth. B 58 (1991) 379. 1181 V. Esaulov, S. Lacombe, L. Guillemot and Vu Ngoc Tuan, Nucl. Instr. and Meth. B 100 (1995) 232. [I91 L. Guillemot, S. Lacombe, V.N. Tuan, V.A. Esaulov, E. Sanchez, Y.A. Bandurin, A.I. Dashchenko and V.G. Drobnich, Surf. Sci. 365 (1996) 353. 1201V. Esaulov, 1. Phys: Cond. Matter C 6 (1994) L699. [Zll V. Sidis, private communication. t221 A. Borisov, D. Teillet-Billy and J.P. Gauyacq, private communication; to be published.
VI. NEUTRALISATION
AND CHARGE
EXCHANGE