Sodium deexcitation spectra at ion-bombarded sodium halide surfaces

Section III Excitation processes

Nuclear Instruments and Methods in Physics Research I378 (1993) 129-133 North-Holland

Sodium deexcitation spectra at ion-bombarded

NOMB

Beam Interactions with Materials 8 Atoms

sodium halide surfaces

Joseph Fine ‘, M. Szymonski ‘, J. Kolodziej ‘, M. Yoshitake ‘9’ and K. Franzrcb a7d a Surface und Microanalysis Science Diuision, Naiional Imtitute of Standards and Techno&y, Gaithersburg MD 20899,

’ Institute of Physics, Jagellonian University, 30-549 Krakow, Poland ’ National Research Institute for Metals, Tsukuba, Ibaraki 305, Japan ’ Unicersity of Kaiserslautern, W-6750 Kulserskrutern, Germany

USA

Electron emission spectra observed on collisionally excited sodium halide surfaces indicate that the sodium dcexcitation process in ionic solids is more complex than that in metallic sodium. The spectra for the sodium halides can be interpreted in the context of a collisional deexcitation model in which the charge state of the excited sodium determines the deexcitation process. Electron capture. which can only occur when an excited sodium ion collides with a static lattice ion, dominates this deexcitation process and is directly responsible for the observed spectra.

1. Introduction Inner-shell atomic excitation which takes place at ion-bombarded surfaces is known to result from orbital interactions and electron promotion processes [1,2] that occur during energetic binary encounters. Following such collisional excitation, electron emission due to inner-shell deexcitation can occur. Some of these collisionally excited atoms promptly eject from the solid, can remain excited as they leave the surface, and then dcexcite in the gas-phase if the characteristic free-atom, inner-shell lifetime is sufficiently long. Such ejection and electron deexcitation processes are well known for metals and have been extensively invcstigatcd [3-111. Inside the solid, however, dcexcitation can occur, not only as a consequence of this basic lifctime-dcpendent decay mechanism, but also as a result of subscqucnt collisional interactions which can significantly affect the decay process itself. Fast moving, inner-shell excited atoms that collide with nearby target atoms may experience further collisional perturbation and interaction of their electronic levels. This additional interaction can lead to new decxcitation mechanisms in which collisions of excited, moving atoms arc involved. Such new dcexcitation channels result from direct electron capture processes, in which new excited states are formed that can have different decay schcmcs, as well as interatomic Auger dccxcitation processes where electrons from both colliding atoms participate [12]. Even though this concept of collisional deexcitation seems straightforward, it has not been specifically considered in descriptions of inelastic ion-surface collisions nor has it been previously investigated. We bclicve that the measurements which we will describe of collisional excitation in ionic solids should provide con0168-583X/93/$06.00

elusive evidence that such collisional decxcitation processes do take place and therefore are critical to the understanding of inelastic ion-surface collisions in solids.

2. Experimental Electron energy spectra, produced by low-energy bombardment with NC+ and Arf ions, have been measured on sodium halide (100) surfaces of NaF, NaCI, and NaI. The singly-charged inert-gas ions were produced in an electron-impact-ionization type ion gun which was differentially pumped. Ion beam energies ranged from 0.4 to 5 keV at beam currents of a few nA; the focused ion beam irradiated an area on the target of 1-3 mm* and was incident at an angle of 50 with respect to the surface normal. Single crystal surfaces were prepared by cleaving sodium halide crystals in air prior to mounting them in the Auger spectrometer. These surfaces were cleaned by heating them in vacuum (< lo-’ Torr) for several hours at 650 K; such a procedure is known to produce clean, stoichiometric surfaces on these materials [ 131. Specimens were mounted on a heated target holder so that substrate temperatures could be varied from 300 to 700 K. Emitted clcctron energy distributions were measured using a single-pass cylindrical mirror analyzer that contained a concentric electron gun for generating conventional electron-impact excited Auger spectra. Electron spectra were obtained with an energy resolution of 0.25 eV under computer control in an EN(E) mode but were not corrected for the transmission function of the spectrometer. The spectrometer energy

0 1993 - Elscvier Science Publishers B.V. All rights reserved

III. BXCITATION

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J. Fine et al. / Na deexcitation at sodium halide surfaces

scale was calibrated using elastically scattcrcd electrons of known initial energy; the zero point as well as the linearity of the energy scale were verified. This calibration procedure allows measurement of electron energies with respect to the vacuum lcvcl with an estimated accuracy of 0.5 eV. Single crystal alkali halide surfaces may charge under electron or ion bombardment and can make electron spectroscopy measurements difficult to obtain. One technique used to reduce such charging is simply to heat the specimen and so to increase its ionic conductivity. This method is particularly suitable for sodium halide crystals since thcsc materials remain stoichiometric during both electron and ion bombardment at temperatures above 450 K. Even though this method is very cffcctive at reducing surface charging, some residual current-density-dcpendcnt charging is nevertheless present on the sodium halide surfaces. Dctcrminations of the characteristic spectral line energies to about 0.5 CV were made by measuring the line energy position as a function of decreasing ion beam current density and then extrapolating to zero current.

3. Results The one characteristic feature of the ion-excited sodium halide spectra which suggests that collisional processes in ionic solids may be different from those in metals is the set of three distinct transitions in the 25-35 eV region. In contrast to the single line observed on ion-bombarded metallic sodium at about 26 CV [14], the energy distribution of electrons emitted from stoichiometric NaCl due to Ar+ bombardment (at 600 K) consists of three narrow (about 1 cV, FWHM) transitions at 25.3, 27.9 and 30.9 eV [15]. These transitions were also observed for Ar’ bombarded crystals of NaF and Nal; spectra for all three sodium halides are shown in fig. 1. Changing the bombarding ion from Ar+ to NC- did not affect the measured spectra: Ne+ bombardment of NaCl also produced the same three transitions at the same cnergies as did Ar ’ ; both are shown in fig. 2. These two NaCl spectra indicate that the relative line intensities are independent of the bombarding ion. The dependence of the spectral-line intensity on bombarding ion energy was determined for both Ar+ and NC ’ on NaCl as well as for AY on NaF. Spectra obtained on NaCl for Ar+ ion energies between 1 and 5 keV and at constant ion current density (0.4 nA/mm’) are shown in fig. 3. Whereas the intensities of all three 25-35 eV lines increased with increasing energy of the bombarding ions for both Ar- and Ne+, the relative line intensities did remain constant over the entire range of ion energies used (0.4-5 keV). Excitation thresholds (upper limits) for both Ar’ and

A 20

Ar+-

30

25

Electron

NaCl

35

Energy

40

[ev]

Fig. 1. Electron spectra obtained on several stoichiometric sodium halide crystal surfaces (NaCI, NaF. NaI) bombarded ions. The energies of each of the three with 3 keV Ar’ sodium autoionizing transitions arc the same for all of these halide surfaces. A smooth secondary electron background has been subtracted from the measured data to give the spectra shown here and in fig. 2.

NC+ bombardment of NaCl and NaF were observed to occur at about 400-500 eV; it was difficult to dcfinc these low-energy thresholds with bcttcr accuracy using our present ion source. These threshold and energy-dependent intensity measurements show that the three transitions have the same excitation threshold and that therefore they may all originate from the same initial collisional event [ 151. Such collisionally excited transitions, however, were not observed on KC1 indicating that the transitions seen on the sodium halides are definitely associated with the excitation of sodium.

h

Electron

Ne++ NaCl

Energy

l-V]

Fig. 2. Electron spectra obtained on stoichiometric crystal surfaces of NaCl bombarded with 3 kcV Ar’ and Ne’ ions. The set of three sodium transitions is virtually the same for both bombarding ions.

131

J. Fine et al. / Na deercitation at sodium halide surfaces

Ar++

10

20

NaCl

30

40

50

Electron Energy [eV] Fig. 3. The dcpendencc of the electron energy distribution on ion bombardment energy. Energy distributions are shown for stoichiometric surfaces of NaCl bombarded with 1-5 keV Ar’. For the higher ion bombardment encrgics. the energy shift seen in the three sodium lines is due to residual surface charging.

species with lattice halogen ions may thus be involved in the decxcitation spectra that arc observed. The most significant of our results are summarized below. For stoichiometric surfaces of NaF, NaCI, and Nal collisionally excited with 0.4 to 5 keV ions of argon and neon, WC find that: (1) all of the low-energy electron spectra consist of the same three lines at 25.3, 27.9 and 30.9 cV; and (2) electron bombardment of the sodium halides gives no transitions in the 25-35 eV range. On halogen depleted surfaces, however, we find that: (3) the ion-excited line intensities (25-35 eV) arc depcndcnt on the halogen surface concentration: the less halogen present, the lower are the line intensities. And on surfaces of KCI. (4) no ion-excited transitions arc observed in the 25-35 cV region. From thcsc results, it seems quite clear that the three characteristic transitions arc initially related to the collisional excitation of sodium and that their intensitics dcpcnd on the near-surface halogen concentration.

4. Spcctrdl assignments Measurcmcnts of electron dcexcitation spectra due only to electron bombardment excitation also have been made and no characteristic Auger transitions were observed in this low-energy 25-35 eV region for any of the three sodium halide surfaces investigated. This unexpected result is characteristic of stoichiomctric sodium halide surfaces and strongly suggests that the valence electrons are highly localized at static ionic lattice sites and do not participate in inner-shell dcexcitation of the sodium. The fact that WC do not obscrvc any low-energy electron-excited Auger lines under static-lattice conditions does indicate that the three ion-induced, low energy lines must involve excitation and/or decxcitation of moving, displaced sodium atoms. It is possible to modify the stoichiomctry of sodium halide surfaces in a controlled manner by electron stimulated dcsorption &SD) so that the surface is only partially depleted of halogen atoms, yet is not metallic. Ion-bombardment-excited Auger spectra were obtained on such sodium halide surfaces partially depleted of halogen atoms by sequential electron irradiation (i.e., by ESD) [15]. Intensity measurements made of the three ion-excited Auger transitions as a function of ESD irradiation time show that the intensity dccrcascs with increasing irradiation time for Ar L cxcited NaCI. Since halogen depletion increases with the irradiation time, it seems clear that the three ion-cxcited line intensities decrcasc due to a dccrcasing halogen concentration. Collisions of the excited Na

for sodium halides

The free-atom electron spectra for collisionally-cxcited neutral sodium in the gas phase [16,17] can bc used to identify the three low-energy transitions which WC have obscrvcd on sodium halide surfaces. There is very good agreement bctwccn our three transitions at 25.3, 27.9 and 30.9 eV with the following autoionizing transitions measured on free-atom neutral sodium Na”*: (1) 2~~3s’ + 2p” + c-(25.7 (2) 2pj3s3p

+ 2p” + c-(28.0

cV), eV),

(3) 2pS3s3d --) 2p6 + c- (30.9 cV). This good agreement suggests that the energy levels associated with a moving, 2p core-excited Na”* atom in an alkali halide crystal, where the sodium atom is no longer bound to the ionic lattice, are not very different from those of a fret sodium atom in the gas phase.

5. Discussion: halides

collisional

excitation

in ionic sodium

Although collisional excitation proccsscs at surfaces have been extensively studied on metallic targets [3-t 11, the excitation spectrum in conductive materials is much more limited than it would be in wide band gap insulators such as ionic solids. In metals, the inner shell electrons that have been collisionally promoted to unfilled states (analogous to autoionizing levels in a free atom) are no longer associated with the excited atom 111.EXCITATION

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J. Fine et al. / Na deercitation at sodium halide surfaces

but find themselves delocalized in the conduction band. This situation suggests that the deexcitation spectrum, which involves electrons from the conduction band, would reflect merely the occupied density of states in the valence band (but not any specific excited state). Electron deexcitation spectra obtained on thin metallic films of sodium demonstrate this limited range of excited levels accessible in metals (see ref. [15]). The situation is quite different in wide band gap ionic solids where, because of the highly localized nature of the valence electrons, there arc no available conduction electrons. Sodium and chlorine, for example, are both essentially closed-shell structures in NaCI: Na’(2p6) and Cl (3~“). It is then possible to excite to a number of localized states and to obtain discrete deexcitation transitions from this set of excited levels. Our spectra obtained on ion-bombarded surfaces of sodium halides arc characterized by three intense transitions, rather than only the one observed on metallic sodium, and may reflect the wider range of cxcitcd states available in ionic solids as well as the more complex deexcitation processes that can occur. Decxcitation of 2p core-excited sodium states that results in the emission of 25-35 eV electrons can only occur if the excited particle is a neutral atom: for example, Na “* 2~~3s’ can decay to Na+ 2ph and emit a 25.7 cV electron [15]. The excited ion, Na ’ * (2~~3s or higher excited states) can decxcite to a 2p” state but the energy gained (33.3 cV) is not sufficient to eject one of the Icast-bound electrons, a 2p electron whose free-particle binding energy is 47.3 eV. Two outer-shell electrons are necessary for a Na2p vacancy to dccxcite and emit an electron. Neutral excited sodium atoms do have enough electrons and can decay nonradiativcly (by electron emission); excited sodium ions, however, do not and can only decay radiativcly (by photon emission). It follows that transitions observed in the electron spectra of ion-bombarded sodium halides must be due to the decxcitation of neutral Na’*. Since the collisionally excited sodium in a sodium halide is initially an excited lattice ion Na+*, nonradiative deexcitation then implies that electron capture processes play

a critical role in determining the electronic state of the excited sodium and hence its decay channels. Our results on electron-impact excitation of NaCl indicate that Na ’ *, so excited, dots not dcexcite by emitting an electron; ion-bombardment excitation, however, does. The significant difference between these two excitation processes is that, in the ion-bombardment case, the collisionally-excited sodium is moving with hundreds of electron volts of kinetic energy; the electron-excited sodium, however, remains static in its lattice site. This difference indicates that energetic collisions arc certainly involved in the dcexcitation process and suggests that electron capture can take place in subsequent collisions between a moving Na+* ion and stationary lattice ions. Such collisional electron capture can produce the inner-shell excited neutral Na”* precursor state that is necessary for nonradiative decay to occur.

6. Collisional

deexcitation

kinetics

A new collisional decxcitation model has been dcveloped to explain the electron spectra obtained by ion-impact excitation on sodium halide surfaces [15]. This model takes into account those collisional processes involved in the excitation, electron capture, and deexcitation sequence which are consistent with our measurements and results. Even though a large number of collisional processes are considered within this extensive evaluation of the collision kinetics, there is but one sequence of events that can realistically account for the sodium halide electron spectra. This sequence will be presented here in some detail; a scheme of the events is given in fig. 4. We consider the impact of an inert-gas positive ion projectile (P.’ ) with a sodium halide surface; WC will use NaCl as a representative halide for this discussion. It is very unlikely that such an incident, high-velocity projectile will be neutralized before colliding with surfact or bulk atoms of an ionic solid. For NaCl (and other sodium halides) which has a large band gap

Fig. 4. Collisional deexcitation sequence for inert-gas ion (P-) bombardment of NaCI. This is basically a two-step sequence in which a neutral, excited Nan* atom is produced which can then decay nonradiatively. In the first step. the projectile energetically collides with a lattice Na’ ion, excites it and sets it into motion; moving particles are shown as shaded circles. In the second step, the excited Na ’ * ion collides with a stationary lattice Cl- ion and attaches an electron; this collision results in the formation of a dcexcitation of the Nan* to ground state Na+ can occur by electron emission. neutral Cla and an excited Na “* atom. Subsequent

J. Fine et al. / No deexcitution at sodium halide surfaces

(about 8 eV) and no conduction band electrons, the probability of a Hagstrum-type neutralization (which would involve tunnelling directly from the valence band) must be very low and, thercforc, the projectile charge-state would remain unaltered near the surface. In this case, the primary collision, in which the sodium becomes excited, is a collision between the positive projectile ion and the positive sodium lattice ion. This collision results in an excited, moving Na ’ * (in, for example, the 2~~3s state) that can only decay radiatively; it can, however, collide with nearby lattice ions before it decxcites (radiative lifetimes are B lo-” s [lOI) and so attach an electron. Capture by the excited, moving Na ’ * ion of an electron from a lattice Na ’ ion in a subsequent collision is energetically very unlikely because the lattice Na- is highly ionic and is therefore similar to a free Na ’ ion which has a very large electron binding energy (47.3 cV). This then leaves collisions with lattice Cl ions as the only likely possibility for electron capture by the moving Nat * to form a neutral Na”* precursor which can then decxcite by emitting a characteristic electron. Energetically such electron capture during a Na i *-Cl collision is very favorable since the chlorine negative-ion electron binding energy in NaCl is only 10.9 eV [18]; the Cl- free-atom electron affinity is even less at 3.6 eV. This sequence, shown in fig, 4, would suggest that electron capture takes place during the Na -*-Clcollision but that dcexcitation occurs after this collision, once the excited, neutral Na”* is formed. Direct electron capture from a lattice Cl ion can result in various inner-shell excited states of Na’*. The NaCl electron dcexcitation spectra, as WC have already indicated, do show transitions involving such Na”* states (2~’ 3s2, 2p5 3s3p and 2p’ 3s3d) and arc consistent with this direct electron capture process. 7. Conclusion Collisionally-excited electron-emission spectra obtaincd on sodium halide surfaces arc characterized by three intense sodium transitions in the 2.5-3.5 eV region. This set of three lines suggests that the collisional cxcitation/deexcitation processes in sodium halides are considerably different than those in metallic sodium where only one intense transition is observed. A new kinetic model has been developed for ionic solids in which electron capture plays a critical role in the dcexcitation process. In this two-step collisional deexcitation model, excitation takes place in a collision separate from the one in which electron capture occurs. After electron capture, nonradiative decxcitation can follow depending on the new decxcitation channels which are available. These new autoionizing transitions and their interpretation indicate that new collisional processes do occur in ionic solids and that they arc

133

fundamental to understanding inelastic ion-surface COIlisions. The basic dccxcitation process identified here in the sodium halides needs to be considered in further efforts to model inelastic collisions in these, as well as, other types of solids.

Acknowledgements This work is part of the Joint Collaboration Project (MEN/NIST-89-6), sponsored by the Polish-American Maria Sklodowska-Curie Fund Il. The authors would like to thank L. Johnson for her assistance and Drs. C.J. Powell and J.W. Gadzuk for helpful comments. Two of us (M.Y. and K.F.) arc also grateful to the Science and Technology Agency of Japan and the Alexander von Humboldt-Foundation, respectively, for financial support during their stay at NIST. References [l] M. Barat and W. Lichten, Phys. Rev. A6 (1972) 211. [2] U. Wille and R. Hippler, Phys. Rep. 132 (1986) 129. [3] T.D. Andreadis, J. Fine and J.A.D. Matthew, Nucl. Instr. and Meth. 209/210 (1983) 495. 141 M.H. Shapiro and J. Fine, Nucl. Instr. and Meth. B44 (lY89) 43; M.H. Shapiro, T.A. Tombrcllo and J. Fine, Nucl. Instr. and Meth. H (in press). 151 E.W. Thomas, Vacuum 34 (1984) 1031. [6] J. Mischler and N. Benazeth, Scan. Electr. Microsc. II (1986) 351. 171 V.U. Kitov and ES. Parilis, Surf. Sci. 138 (1984) 203. [8J A. Ronanno, N. Mandarino, A. Oliva and F. Xu, Nucl. Instr. and Mcth. B71 (1992) 161. [9] 0. Grizzi and R.A. Baragiola, Phys. Rev. A35 (1987) 135. [IO] J.A.D. Matthew, Physica Scripta T6 (1983) 79. [II] R. Baragiola, these Proceedings (9th Int. Workshop on Inelastic Ion-Surface Collisions, Aussois, France, 1992), Nucl. Instr. and Meth. B78 (1093) 223. [I21 M.L. Knotek and P.J. Feibelman, Phys. Rev. Lett. 40 (1978) 964; P.H. Citrin, J.E. Rowe and S.H. Christman, Phys. Rev. B14 (1976) 2642. 1131 L.S. Cota Araiza and RD. Powell, Surf. Sci. 51 (1975) 504; M. Szymonski, A. Poradzisz, P. Czuba, J. Kolodziej, P. Piatkowski, J. Fine, L. Tanovic and N. Tanovic. Surf. Sci. 260 (1992) 295. 1141 1. Terzic, D. Ciric and M. Matic, Phys. Letters A61 (1977) 259. 1151 J. Fine. M. Szymonski, J. Kolodzicj, M. Yoshitake and K. Franzreb, Phys. Rev. (in press). [161 P. Dahl, M. Rodbro, G. Hcrmann, H. Fastrup and ME Rudd, J. Phys. BY (lY76) 1581. 1171 J. Gstgaard Olsen, T. Andersen, M. Barat, C. CourbinGaussorgues, V. Sidis, J. Pommier, J. Agusti, N. Andcrscn and A. Russek, Phys. Rev. AlY (lY79) 1457. [I81 P.H. Citrin and T.D. Thomas, J. Chem. Phys. 57 (1972) 4446. III. EXCITATION