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Electron stimulated desorption of positive ions from alkali halide surfaces
Surface Science 186 (1987) 191-200 North-Holland, Amsterdam
191
ELECTRON STIMULATED DESORPTION OF POSITIVE IONS F R O M ALKALI HALIDE S U R F A C E S Tsuneo YASUE, Ayahiko I C H I M I Y A
Department of Applied Physics, Nagoya University, Chikusa-ku, Nagoya 464, Japan and Shunsuke O H T A N I
Institute of Plasma Physics, Nagoya University, Chikusa-kt~ Nagoya 464, Japan Received 18 November 1986; accepted for publication 28 February 1987
Electron stimulated desorption (ESD) of positive ions from LiF, LiBr and RbF is observed in the incident electron energy region below 800 eV. In ESD from LiF, the experimental results are mostly explained in terms of the Knotek-Feibelman (KF) mechanism. On the other hand the KF mechanism is not satisfactory to understand the desorption yield spectra from LiBr, because the threshold energy of desorption was not necessarily observed at binding energies of core levels of LiBr. Since the yield spectrum of Li § ions showed quite different profile from that of Br + ions in ESD from LiBr, it is possible that the desorption mechanism for cations is not the same as that for anions. In the case of ESD from RbF the enhancement of the desorption yields was observed at about 80 eV of the incident energy, which does not agree with any interband transition or ionization energy. We observed the desorption of Rb2+ ions besides that of Rb § and F + ions, which are always associated with the desorption of H2O+ ions. The role of H2O+ ions is also discussed.
1. Introduction D e s o r p t i o n p h e n o m e n a i n d u c e d b y electronic transitions are well studied for various materials [1]. As to alkali halides, the K n o t e k - F e i b e l m a n mechan i s m (the K F m e c h a n i s m ) [2] is often used i n order to explain the results of m e a s u r e m e n t s of the positive i o n yields [3-5]. We previously reported p h o t o n s t i m u l a t e d desorption (PSD) of Li + a n d F § ions from L i F with p h o t o n energies between 30 a n d 70 eV [3]. The yield spectrum of Li § ions was quite different from that of F § ions above 60 eV, b u t the yield spectra of Li + a n d F § ions o b t a i n e d b y Parks et al. [4] do n o t show such difference in the same energy region. I n b o t h cases the decay of the core exciton of L i F i n d u c e d b y p h o t o n s is considered to play a n i m p o r t a n t role in the ion d e s o r p t i o n processes. 0 0 3 9 - 6 0 2 8 / 8 7 / $ 0 3 . 5 0 9 Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
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T. Yasue et al. / ESD of cations from alkali halide surfaces
Electron stimulated desorption (ESD) of positive ions and excited neutral species from alkali halide surfaces was observed by Pian et al. [5-7]. In the yield spectra of positive ions and excited neutrals structures were seen at the binding energies of core levels. This means that the core excitation due to the incident electrons leads to the desorption of positive ions and excited neutral species. The Coulomb repulsive force, however, does not work on neutral species, hence the M G R mechanism [8,9] might be used to explain the desorption of neutral species. As mentioned above, it is generally believed that the K F mechanism is capable of explaining the experimental results of the electronically stimulated desorption from alkali halides. However, Walkup and Avouris [10] showed using the classical trajectory theory that desorption of positive ions from the perfect alkali halide surfaces except for that from LiF cannot take place by the Coulomb repulsive force because of the fast lattice relaxation ( - 1 0 -14 s). Therefore it is necessary to examine the ionic desorption from several kinds of alkali halides and to compare each result. In the present work, we measured the relative ionic desorption yields from LiF, LiBr and RbF as a function of incident electron energy. The experimental results of ESD from LiF were mostly explained in terms of the K F mechanism, while those from LiBr and RbF were not understood using only the K F mechanism. Moreover the ionic desorption yield spectra of cations showed different profiles from those of anions in ESD from LiBr. Rb z§ ions were observed in ESD from R b F only when desorption of H 2 0 + ions was observed. A modified mechanism is not given at present, but the role of H 2 0 + ions in ESD from RbF is discussed.
2. Experimental LiF, LiBr and RbF powder were dissolved in distilled water, and spread on CuBe substrates. After drying for a few minutes in air, the specimens were mounted in a U H V chamber and baked at about 180 ~ for ten hours using a halogen lamp. Since the samples are insulators, the halogen lamp was also lit during measurements in order to avoid charging. An electron gun was operated with incident electron energies between 25 and 800 eV. The incident electron current, which was determined by measuring the current through the sample, was less than 10 /~A. Desorbed positive ions were detected using a quadrupole mass spectrometer (ANELVA, AGA-360) whose ion source was removed. The signals for a fixed mass-to-charge ratio ( m / q ) were counted by a ORTEC 772 type counter for a few seconds, and the counting rates for three consecutive runs were averaged. The deviation of each counting rate from the averaged one was less than 10%. The measured ion intensity was normalized
T. Yasue et al. / ESD of cations from alkafi halide surfaces
193
with the incident electron current, and the obtained relative ion yield was plotted as a function of primary electron energy (the ion yield spectrum). The yield spectra of alkali metal ions and halogen ions were obtained a few times. For alkali metal ions nearly the same yield spectra were reproduced, while for halogen ions the yield spectra were poorly reproduced.
3. Results
3.1. LiF Fig. 1 shows the ordinary mass spectrum of ESD from LiF, whose horizontal coordinate is the mass-to-charge ratio. The incident electron energy was 100 eV and the current was 305 nA. The peaks of Li § and F § ions were clearly seen together with a few impurity peaks ( m / q = 1, 14, 16, 28, 35 and 37). The peaks of m / q = 1, and 35, 37, which are due to H § and C1 + ions, respectively, were relatively strong, and the others were much weaker than those peaks. The yields of Li + and F § ions were measured as a function of the incident electron energy with m / q fixed at 7 and 19 respectively. Fig. 2 shows the relative yield spectra of Li § and F § ions. In fig. 2a the yield increased rapidly from about 50 eV and had a maximum at about 225 eV. Weak structures were obtained at about 525 and 650 eV. The shape of the yield spectrum of F § ions in fig. 2b was slightly different from that of Li § ions. The primary electron energy at which the yield of F § ions was maximized is about 200 eV, which was less than that for Li § ions. The relative yield of Li § ions was smaller than
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m/q Fig. 1. Typical mass spectrum of ESD from LiF. The incident electron energy is 100 eV, and the electron current is 305 nA.
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T. Yasue et al. / ESD of cations from alkafi halide surfaces
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that of F § ions b e l o w 100 eV, while a b o v e 100 eV that of Li § ions was larger t h a n that of F § ions. The d e p e n d e n c e of the relative yields of Li + and F § ions on the electron energy below 100 eV is shown in fig. 3. T h e threshold a n d the r a p i d increase of the yield for Li § ions are at n e a r l y the same energy as that for F § ions. T h e s h a p e of the yield s p e c t r u m of Li § ions, however, was different f r o m that of F § ions: firstly the rate of the increase o f the yield of Li § ions b e t w e e n 30 a n d 55 eV was m u c h smaller t h a n that of F § ions, s e c o n d l y the r a t i o of the g r a d i e n t a b o v e 55 eV to t h a t b e l o w 55 eV in the yield s p e c t r u m o f Li § ions was larger t h a n that of F § ions, a n d thirdly in the yield s p e c t r u m o f F § ions there were a d d i t i o n a l structures between 55 a n d 70 eV. 3.2. LiBr I n the mass s p e c t r u m of E S D of positive ions from LiBr, the Li + and Br + p e a k s were seen as well as relatively strong i m p u r i t y p e a k s due to h y d r o -
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196
T. Yasue et al. / E S D of cations from alkali halide
surfaces
carbons. The peak of H § ions, however, was weaker than that from LiF shown in fig. 1. The relative yield spectra of Li § and Br -~ ions in fig. 4 were obtained with m/q = 7 and 79, respectively. The yield spectrum of Li + ions was considerably different from that of Br § ions. The relative yield of Li + ions was about ten times greater than that of Br § ions. However the yield of Li + ions from LiBr was an order of magnitude smaller than that from LiF shown in fig. 2a. In fig. 4a the changes of the gradient of the yield spectrum were seen at about 50 and 100 eV. Although these changes in slope are also seen in fig. 4b, the direction of the changes is opposite to that of Li + ions: the yield of Li + ions between 50 and 100 eV increased more rapidly than that below 50 and above 100 eV, and vice versa for Br § ions. The peak position in the yield spectrum of Li + ions is at 350 eV and is different from that of Br + ions, which is at about 180 eV. The yield spectrum of Li § ions in fig. 4a is broad in comparison with that of Br § ions. In fig. 4b the scattering of the yields was comparatively large, since the counting rate was small.
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Fig. 5. Relative ion yield spectra from RbF. (a) The spectrum of Rb + ions. (b) That of F + ions.
T. Yasue et al. / ESD of cations from alkali halide surfaces
197
3.3. R b F Three strong peaks were observed in the mass spectrum from R b F . T w o peaks were due to Rb § and F § ions and the other was due to H § ions. Although the peaks of hydrocarbons were weak, the peaks of C § N § O § and C1 + were relatively strong. The yields of Rb § and F + ions (m/q--85 and 19 respectively) were measured as a function of the incident energy, and the yield spectra are shown in fig. 5. The onset energy, which was located at about 30 eV, was nearly the s a m e in every yield spectra of Rb + and F + ions. W h e n peaks of H 2 O + ions were observed in the mass spectrum, a w e a k Rb 2+ peak was observed. The yield of Rb 2+ ions (m/q= 42.5) was also measured as a function of the electron energy (fig. 6). The shape of the yield spectra of Rb + and Rb 2 § ions as shown in figs. 5a and 6 are similar to each other, while that of F § ions in fig. 5b is m o r e broad. The peak position in the spectra of Rb 2§ and F + was shifted slightly f r o m that of Rb § ions toward the high energy side. Structures were observed in the yield spectra of Rb § and F § ions at about 80 eV. For Rb 2+ ions, however, structures were seen at 125 and 180 eV. The ratio of the relative yield of Rb § ions to that of Rb 2 § ions was about one half. The relative yield of F + ions from R b F was smaller than that from LiF shown in fig. 2b.
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T. Yasue et aL / E S D of cations from alkafi halide surfaces
4. Discussion
In order to analyze the experimental results of the electronically stimulated desorption, it is important to consider what is the initial excitation [11]. It is considered that core excitation results in a rapid increase of the ionic desorption yields from LiF as observed previously [3-7]. Since charge transfer processes are involved in the processes which lead to the desorption of F § ions, non-radiative decay processes are meaningful in the desorption processes. It is necessary to consider two different decays, K(exciton)-V process and K(exciton)-VV process [12], as shown schematically in fig. 7. The decay probability of the K(exciton)-VV process is greater than that of the K(excit o n ) - V process [13]. The final configuration of the K(exciton)-V process, which is a F atom and four Li § ions around the F atom on the surface, does not result in the ejection of F § ions. But it is possible that the ionization of the F atom near the surface by the incident or secondary electrons contributes to the desorption yields of F § ions. If the ionization rate is sufficiently large, the K(exciton)-V process can be important in the desorption processes. The shape of the desorption yield spectrum due to the K(exciton)-V process should be similar to that due to the K(exciton)-VV process. Then the ionization efficiency of the F atom induced by the primary electrons and the secondary electrons near the surface can be estimated. Using the ionization cross section obtained by Lotz [14], the efficiency is calculated as less than 10 -4. On the other hand the final configuration of the K(exciton)-V process is the same as that of the direct valence ionization. In the incident electron energy region below 30 eV, in which the direct valence ionization can take place, the desorption of F § ions was hardly observed. Therefore it is considered that the contribution of the K(exciton)-V process to the desorption processes is negligible, and that the K(exciton)-VV process is dornihant as widely observed [3-7].
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T. Yasue et al. / ESD of cations from alkali halide surfaces
199
For LiBr and RbF a similar discussion may be available. The structure observed at about 50 eV in the spectra shown in fig. 4 is related to the excitation of the Li + ls level of LiBr. The onset energy of about 30 eV for R b F is nearly the same as the binding energy of the Rb + 4s level (30.8 eV) or the F - 2s level (28.8 eV) [15]. However the shape of the yield spectrum of Li + ions is considerably different from that of Br + ions as shown in fig. 4. The threshold energy of desorption from LiBr is not located at binding energies of core levels of LiBr. There is an enhancement of the desorption yields at about 80 eV in fig. 5, which does not agree with any interband transition or ionization energy. On the other hand the shape of the yield spectra of Li + and F* ions from LiF are similar to each other, and all the structures seen in the spectra are mostly explained in terms of the K F mechanism. It is considered that the experimental results of ESD from LiBr and R b F cannot be explained sufficiently using the K F mechanism [10]. Furthermore it is considered that the mechanism of ESD of alkali metal ions is different from that of halogen ions, especially in the systems which contain anions with large atomic number. But a modified mechanism cannot be given at present. In all spectra additional structures were observed in the high energy region above 400 eV. It is considered that these structures are due to secondary electron emission. The secondary electron yield is generally maximized at an incident electron energy of a few hundreds eV [16]. Since the incident electron current was measured as the sample current, an increase of the secondary electron yield causes the sample current to decrease. Consequently the ion yield becomes greater outwardly, because the ion yield was obtained in terms of normalization of the measured ion intensity with the sample current. Moreover an increase of the secondary electron yield might lead to charging of specimens, and results in scattering of the ion yield. Desorption of Rb 2+ ions from RbF was observed only when a peak of HEO§ ions was observed in the mass spectrum. The presence of H20 leads to the solution of RbF and results in the dissociation of RbF to Rb + and F ions. When the incident electrons excite the core electrons of Rb + ions, the following process is not associated with interatomic Auger decay but with intraoatomic Auger decay. Then Rb 2* ions can be created. The structures at 125 and 180 eV in the yield spectrum of Rb 2+ ions, which are not seen in those of Rb § and F* ions, might be due to the above mentioned mechanism. According to this mechanism, desorption of Rb 3+ ions is expected. However in the mass spectrum from R b F the peaks of Rb 3+ ions were not observed. Other possibilities might be necessary to explain ESD of Rb 2+ ions. As shown in the mass spectrum of ESD from LiF (fig. 1), several kinds of impurity ions have desorbed as well as Li + and F + ions. H - ions can form an U-center [3], in which H - ions exist in the vacancies of F - ions, and C1- ions can be contained instead of F - ions. Therefore it is considered that the influences of desorption of H + and C1 + ions on that of Li + and F + ions will be
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T. Yasue et al. / ESD of cations from alkali halide surfaces
negligible, b u t it is necessary to consider the effects of the other ions such as N +, O § etc. o n the d e s o r p t i o n of Li § a n d F § ions, because these ions are due to ESD of adsorbates o n the L i F surface a n d / o r d u e to the i o n i z a t i o n of residual gases. The intensities of these ions i n the mass spectrum are m u c h smaller t h a n those of Li § a n d F § ions, hence the effects of i m p u r i t y ions c a n be ignored. F o r LiBr a n d R b F the above m e n t i o n e d discussion is also applicable except for H 2 0 + ions from R b F .
5. Conclusion Electron stimulated d e s o r p t i o n of positive ions from alkali halides is mostly explained using the K F m e c h a n i s m as well as p h o t o n stimulated desorption. However, there are structures which are n o t explained b y the K F m e c h a n i s m i n the yield spectra from LiBr a n d R b F . This m e a n s that it is possible that the d e s o r p t i o n m e c h a n i s m from alkali halides is different from the K F mechanism, except for d e s o r p t i o n from LiF.
References [1] See, for example, N.H. Tolk, M.M. Traum, J.C. Tully and T.E. Madey, Eds., Desorption Induced by Electronic Transitions, DIET I (Springer, Berlin, 1983); W. Brenig and D. Menzel, Eds., Desorption Induced by Electronic Transitions, DIET II (Springer, Berlin, 1985). [2] M.L. Knotek and P.J. Feibelman, Phys. Rev. Letters 40 (1978) 964. [3] T. Yasue, T. Gotoh, A. Ichimiya, Y. Kawaguchi, M. Kotani, S. Ohtani, Y. Shigeta, S. Takagi, Y. Tazawa and G. Tominaga, Japan. J. Appl. Phys. 25 (1986) L363. [4] C.C. Parks, D.A. Shirley and G. Loubriel, Phys. Rev. B29 (1984) 4709. [5] T.R. Pian, M.M. Traum, J.S. Kraus, N.H. Tolk, N.G. Stoffel and G. Margaritondo, Surface Sci. 128 (1983) 13. [6] T.R. Pian, N. Tolk, J. Kraus, M.M. Traum, J. Tully and W.E. Collins, J. Vacuum Sci. Technol. 20 (1982) 555. [7] T.R. Pian, N.H. Tolk, M.M. Traum, J. Kraus and W.E. Collins, Surface Sci. 129 (1983) 573. [8] D. Menzel and R. Gomer, J. Chem. Phys. 41 (1964) 3311. [9] P.A. Redhead, Can. J. Phys. 42 (1964) 886. [10] R.E. Walkup and Ph. Avouris, Phys. Rev. Letters 56 (1986) 524. [11] D.R. Jennison, in: Desorption Induced by Electronic Transitions DIET I, Eds. N.H. Tolk, M.M. Tram, J.C. Tully and T.E. Madey (Springer, Berlin, 1983) p. 26. [12] M. Kamada, K. Ichikawa and K. Tsutsumi, Phys. Rev. B28 (1983) 7225. [13] K. Ichikawa, M. Kamada, O. Aita and K. Tsutsumi, Phys. Rev. B32 (1985) 8293. [14] W. Lotz, Astrophys. J. Suppl. Ser. 14 (1967) 207. [15] V.V. Nemoshkalenko and V.G. Aleshin, Electron Spectroscopy of Crystals (Plenum, New York, 1979) p. 251. [16] O. Hachenberg and W. Brauer, Advan. Electron. Electron Phys. 11 (1959) 413.
191
ELECTRON STIMULATED DESORPTION OF POSITIVE IONS F R O M ALKALI HALIDE S U R F A C E S Tsuneo YASUE, Ayahiko I C H I M I Y A
Department of Applied Physics, Nagoya University, Chikusa-ku, Nagoya 464, Japan and Shunsuke O H T A N I
Institute of Plasma Physics, Nagoya University, Chikusa-kt~ Nagoya 464, Japan Received 18 November 1986; accepted for publication 28 February 1987
Electron stimulated desorption (ESD) of positive ions from LiF, LiBr and RbF is observed in the incident electron energy region below 800 eV. In ESD from LiF, the experimental results are mostly explained in terms of the Knotek-Feibelman (KF) mechanism. On the other hand the KF mechanism is not satisfactory to understand the desorption yield spectra from LiBr, because the threshold energy of desorption was not necessarily observed at binding energies of core levels of LiBr. Since the yield spectrum of Li § ions showed quite different profile from that of Br + ions in ESD from LiBr, it is possible that the desorption mechanism for cations is not the same as that for anions. In the case of ESD from RbF the enhancement of the desorption yields was observed at about 80 eV of the incident energy, which does not agree with any interband transition or ionization energy. We observed the desorption of Rb2+ ions besides that of Rb § and F + ions, which are always associated with the desorption of H2O+ ions. The role of H2O+ ions is also discussed.
1. Introduction D e s o r p t i o n p h e n o m e n a i n d u c e d b y electronic transitions are well studied for various materials [1]. As to alkali halides, the K n o t e k - F e i b e l m a n mechan i s m (the K F m e c h a n i s m ) [2] is often used i n order to explain the results of m e a s u r e m e n t s of the positive i o n yields [3-5]. We previously reported p h o t o n s t i m u l a t e d desorption (PSD) of Li + a n d F § ions from L i F with p h o t o n energies between 30 a n d 70 eV [3]. The yield spectrum of Li § ions was quite different from that of F § ions above 60 eV, b u t the yield spectra of Li + a n d F § ions o b t a i n e d b y Parks et al. [4] do n o t show such difference in the same energy region. I n b o t h cases the decay of the core exciton of L i F i n d u c e d b y p h o t o n s is considered to play a n i m p o r t a n t role in the ion d e s o r p t i o n processes. 0 0 3 9 - 6 0 2 8 / 8 7 / $ 0 3 . 5 0 9 Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
192
T. Yasue et al. / ESD of cations from alkali halide surfaces
Electron stimulated desorption (ESD) of positive ions and excited neutral species from alkali halide surfaces was observed by Pian et al. [5-7]. In the yield spectra of positive ions and excited neutrals structures were seen at the binding energies of core levels. This means that the core excitation due to the incident electrons leads to the desorption of positive ions and excited neutral species. The Coulomb repulsive force, however, does not work on neutral species, hence the M G R mechanism [8,9] might be used to explain the desorption of neutral species. As mentioned above, it is generally believed that the K F mechanism is capable of explaining the experimental results of the electronically stimulated desorption from alkali halides. However, Walkup and Avouris [10] showed using the classical trajectory theory that desorption of positive ions from the perfect alkali halide surfaces except for that from LiF cannot take place by the Coulomb repulsive force because of the fast lattice relaxation ( - 1 0 -14 s). Therefore it is necessary to examine the ionic desorption from several kinds of alkali halides and to compare each result. In the present work, we measured the relative ionic desorption yields from LiF, LiBr and RbF as a function of incident electron energy. The experimental results of ESD from LiF were mostly explained in terms of the K F mechanism, while those from LiBr and RbF were not understood using only the K F mechanism. Moreover the ionic desorption yield spectra of cations showed different profiles from those of anions in ESD from LiBr. Rb z§ ions were observed in ESD from R b F only when desorption of H 2 0 + ions was observed. A modified mechanism is not given at present, but the role of H 2 0 + ions in ESD from RbF is discussed.
2. Experimental LiF, LiBr and RbF powder were dissolved in distilled water, and spread on CuBe substrates. After drying for a few minutes in air, the specimens were mounted in a U H V chamber and baked at about 180 ~ for ten hours using a halogen lamp. Since the samples are insulators, the halogen lamp was also lit during measurements in order to avoid charging. An electron gun was operated with incident electron energies between 25 and 800 eV. The incident electron current, which was determined by measuring the current through the sample, was less than 10 /~A. Desorbed positive ions were detected using a quadrupole mass spectrometer (ANELVA, AGA-360) whose ion source was removed. The signals for a fixed mass-to-charge ratio ( m / q ) were counted by a ORTEC 772 type counter for a few seconds, and the counting rates for three consecutive runs were averaged. The deviation of each counting rate from the averaged one was less than 10%. The measured ion intensity was normalized
T. Yasue et al. / ESD of cations from alkafi halide surfaces
193
with the incident electron current, and the obtained relative ion yield was plotted as a function of primary electron energy (the ion yield spectrum). The yield spectra of alkali metal ions and halogen ions were obtained a few times. For alkali metal ions nearly the same yield spectra were reproduced, while for halogen ions the yield spectra were poorly reproduced.
3. Results
3.1. LiF Fig. 1 shows the ordinary mass spectrum of ESD from LiF, whose horizontal coordinate is the mass-to-charge ratio. The incident electron energy was 100 eV and the current was 305 nA. The peaks of Li § and F § ions were clearly seen together with a few impurity peaks ( m / q = 1, 14, 16, 28, 35 and 37). The peaks of m / q = 1, and 35, 37, which are due to H § and C1 + ions, respectively, were relatively strong, and the others were much weaker than those peaks. The yields of Li + and F § ions were measured as a function of the incident electron energy with m / q fixed at 7 and 19 respectively. Fig. 2 shows the relative yield spectra of Li § and F § ions. In fig. 2a the yield increased rapidly from about 50 eV and had a maximum at about 225 eV. Weak structures were obtained at about 525 and 650 eV. The shape of the yield spectrum of F § ions in fig. 2b was slightly different from that of Li § ions. The primary electron energy at which the yield of F § ions was maximized is about 200 eV, which was less than that for Li § ions. The relative yield of Li § ions was smaller than
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I
i
19
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m/q Fig. 1. Typical mass spectrum of ESD from LiF. The incident electron energy is 100 eV, and the electron current is 305 nA.
194
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Fig. 2. (a) Relative yield spectrum of Li + ions from LiF. (b) That of F + ions.
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T. Yasue et al. / ESD of cations from alkafi halide surfaces
195
that of F § ions b e l o w 100 eV, while a b o v e 100 eV that of Li § ions was larger t h a n that of F § ions. The d e p e n d e n c e of the relative yields of Li + and F § ions on the electron energy below 100 eV is shown in fig. 3. T h e threshold a n d the r a p i d increase of the yield for Li § ions are at n e a r l y the same energy as that for F § ions. T h e s h a p e of the yield s p e c t r u m of Li § ions, however, was different f r o m that of F § ions: firstly the rate of the increase o f the yield of Li § ions b e t w e e n 30 a n d 55 eV was m u c h smaller t h a n that of F § ions, s e c o n d l y the r a t i o of the g r a d i e n t a b o v e 55 eV to t h a t b e l o w 55 eV in the yield s p e c t r u m o f Li § ions was larger t h a n that of F § ions, a n d thirdly in the yield s p e c t r u m o f F § ions there were a d d i t i o n a l structures between 55 a n d 70 eV. 3.2. LiBr I n the mass s p e c t r u m of E S D of positive ions from LiBr, the Li + and Br + p e a k s were seen as well as relatively strong i m p u r i t y p e a k s due to h y d r o -
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196
T. Yasue et al. / E S D of cations from alkali halide
surfaces
carbons. The peak of H § ions, however, was weaker than that from LiF shown in fig. 1. The relative yield spectra of Li § and Br -~ ions in fig. 4 were obtained with m/q = 7 and 79, respectively. The yield spectrum of Li + ions was considerably different from that of Br § ions. The relative yield of Li + ions was about ten times greater than that of Br § ions. However the yield of Li + ions from LiBr was an order of magnitude smaller than that from LiF shown in fig. 2a. In fig. 4a the changes of the gradient of the yield spectrum were seen at about 50 and 100 eV. Although these changes in slope are also seen in fig. 4b, the direction of the changes is opposite to that of Li + ions: the yield of Li + ions between 50 and 100 eV increased more rapidly than that below 50 and above 100 eV, and vice versa for Br § ions. The peak position in the yield spectrum of Li + ions is at 350 eV and is different from that of Br + ions, which is at about 180 eV. The yield spectrum of Li § ions in fig. 4a is broad in comparison with that of Br § ions. In fig. 4b the scattering of the yields was comparatively large, since the counting rate was small.
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Fig. 5. Relative ion yield spectra from RbF. (a) The spectrum of Rb + ions. (b) That of F + ions.
T. Yasue et al. / ESD of cations from alkali halide surfaces
197
3.3. R b F Three strong peaks were observed in the mass spectrum from R b F . T w o peaks were due to Rb § and F § ions and the other was due to H § ions. Although the peaks of hydrocarbons were weak, the peaks of C § N § O § and C1 + were relatively strong. The yields of Rb § and F + ions (m/q--85 and 19 respectively) were measured as a function of the incident energy, and the yield spectra are shown in fig. 5. The onset energy, which was located at about 30 eV, was nearly the s a m e in every yield spectra of Rb + and F + ions. W h e n peaks of H 2 O + ions were observed in the mass spectrum, a w e a k Rb 2+ peak was observed. The yield of Rb 2+ ions (m/q= 42.5) was also measured as a function of the electron energy (fig. 6). The shape of the yield spectra of Rb + and Rb 2 § ions as shown in figs. 5a and 6 are similar to each other, while that of F § ions in fig. 5b is m o r e broad. The peak position in the spectra of Rb 2§ and F + was shifted slightly f r o m that of Rb § ions toward the high energy side. Structures were observed in the yield spectra of Rb § and F § ions at about 80 eV. For Rb 2+ ions, however, structures were seen at 125 and 180 eV. The ratio of the relative yield of Rb § ions to that of Rb 2 § ions was about one half. The relative yield of F + ions from R b F was smaller than that from LiF shown in fig. 2b.
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198
T. Yasue et aL / E S D of cations from alkafi halide surfaces
4. Discussion
In order to analyze the experimental results of the electronically stimulated desorption, it is important to consider what is the initial excitation [11]. It is considered that core excitation results in a rapid increase of the ionic desorption yields from LiF as observed previously [3-7]. Since charge transfer processes are involved in the processes which lead to the desorption of F § ions, non-radiative decay processes are meaningful in the desorption processes. It is necessary to consider two different decays, K(exciton)-V process and K(exciton)-VV process [12], as shown schematically in fig. 7. The decay probability of the K(exciton)-VV process is greater than that of the K(excit o n ) - V process [13]. The final configuration of the K(exciton)-V process, which is a F atom and four Li § ions around the F atom on the surface, does not result in the ejection of F § ions. But it is possible that the ionization of the F atom near the surface by the incident or secondary electrons contributes to the desorption yields of F § ions. If the ionization rate is sufficiently large, the K(exciton)-V process can be important in the desorption processes. The shape of the desorption yield spectrum due to the K(exciton)-V process should be similar to that due to the K(exciton)-VV process. Then the ionization efficiency of the F atom induced by the primary electrons and the secondary electrons near the surface can be estimated. Using the ionization cross section obtained by Lotz [14], the efficiency is calculated as less than 10 -4. On the other hand the final configuration of the K(exciton)-V process is the same as that of the direct valence ionization. In the incident electron energy region below 30 eV, in which the direct valence ionization can take place, the desorption of F § ions was hardly observed. Therefore it is considered that the contribution of the K(exciton)-V process to the desorption processes is negligible, and that the K(exciton)-VV process is dornihant as widely observed [3-7].
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T. Yasue et al. / ESD of cations from alkali halide surfaces
199
For LiBr and RbF a similar discussion may be available. The structure observed at about 50 eV in the spectra shown in fig. 4 is related to the excitation of the Li + ls level of LiBr. The onset energy of about 30 eV for R b F is nearly the same as the binding energy of the Rb + 4s level (30.8 eV) or the F - 2s level (28.8 eV) [15]. However the shape of the yield spectrum of Li + ions is considerably different from that of Br + ions as shown in fig. 4. The threshold energy of desorption from LiBr is not located at binding energies of core levels of LiBr. There is an enhancement of the desorption yields at about 80 eV in fig. 5, which does not agree with any interband transition or ionization energy. On the other hand the shape of the yield spectra of Li + and F* ions from LiF are similar to each other, and all the structures seen in the spectra are mostly explained in terms of the K F mechanism. It is considered that the experimental results of ESD from LiBr and R b F cannot be explained sufficiently using the K F mechanism [10]. Furthermore it is considered that the mechanism of ESD of alkali metal ions is different from that of halogen ions, especially in the systems which contain anions with large atomic number. But a modified mechanism cannot be given at present. In all spectra additional structures were observed in the high energy region above 400 eV. It is considered that these structures are due to secondary electron emission. The secondary electron yield is generally maximized at an incident electron energy of a few hundreds eV [16]. Since the incident electron current was measured as the sample current, an increase of the secondary electron yield causes the sample current to decrease. Consequently the ion yield becomes greater outwardly, because the ion yield was obtained in terms of normalization of the measured ion intensity with the sample current. Moreover an increase of the secondary electron yield might lead to charging of specimens, and results in scattering of the ion yield. Desorption of Rb 2+ ions from RbF was observed only when a peak of HEO§ ions was observed in the mass spectrum. The presence of H20 leads to the solution of RbF and results in the dissociation of RbF to Rb + and F ions. When the incident electrons excite the core electrons of Rb + ions, the following process is not associated with interatomic Auger decay but with intraoatomic Auger decay. Then Rb 2* ions can be created. The structures at 125 and 180 eV in the yield spectrum of Rb 2+ ions, which are not seen in those of Rb § and F* ions, might be due to the above mentioned mechanism. According to this mechanism, desorption of Rb 3+ ions is expected. However in the mass spectrum from R b F the peaks of Rb 3+ ions were not observed. Other possibilities might be necessary to explain ESD of Rb 2+ ions. As shown in the mass spectrum of ESD from LiF (fig. 1), several kinds of impurity ions have desorbed as well as Li + and F + ions. H - ions can form an U-center [3], in which H - ions exist in the vacancies of F - ions, and C1- ions can be contained instead of F - ions. Therefore it is considered that the influences of desorption of H + and C1 + ions on that of Li + and F + ions will be
200
T. Yasue et al. / ESD of cations from alkali halide surfaces
negligible, b u t it is necessary to consider the effects of the other ions such as N +, O § etc. o n the d e s o r p t i o n of Li § a n d F § ions, because these ions are due to ESD of adsorbates o n the L i F surface a n d / o r d u e to the i o n i z a t i o n of residual gases. The intensities of these ions i n the mass spectrum are m u c h smaller t h a n those of Li § a n d F § ions, hence the effects of i m p u r i t y ions c a n be ignored. F o r LiBr a n d R b F the above m e n t i o n e d discussion is also applicable except for H 2 0 + ions from R b F .
5. Conclusion Electron stimulated d e s o r p t i o n of positive ions from alkali halides is mostly explained using the K F m e c h a n i s m as well as p h o t o n stimulated desorption. However, there are structures which are n o t explained b y the K F m e c h a n i s m i n the yield spectra from LiBr a n d R b F . This m e a n s that it is possible that the d e s o r p t i o n m e c h a n i s m from alkali halides is different from the K F mechanism, except for d e s o r p t i o n from LiF.
References [1] See, for example, N.H. Tolk, M.M. Traum, J.C. Tully and T.E. Madey, Eds., Desorption Induced by Electronic Transitions, DIET I (Springer, Berlin, 1983); W. Brenig and D. Menzel, Eds., Desorption Induced by Electronic Transitions, DIET II (Springer, Berlin, 1985). [2] M.L. Knotek and P.J. Feibelman, Phys. Rev. Letters 40 (1978) 964. [3] T. Yasue, T. Gotoh, A. Ichimiya, Y. Kawaguchi, M. Kotani, S. Ohtani, Y. Shigeta, S. Takagi, Y. Tazawa and G. Tominaga, Japan. J. Appl. Phys. 25 (1986) L363. [4] C.C. Parks, D.A. Shirley and G. Loubriel, Phys. Rev. B29 (1984) 4709. [5] T.R. Pian, M.M. Traum, J.S. Kraus, N.H. Tolk, N.G. Stoffel and G. Margaritondo, Surface Sci. 128 (1983) 13. [6] T.R. Pian, N. Tolk, J. Kraus, M.M. Traum, J. Tully and W.E. Collins, J. Vacuum Sci. Technol. 20 (1982) 555. [7] T.R. Pian, N.H. Tolk, M.M. Traum, J. Kraus and W.E. Collins, Surface Sci. 129 (1983) 573. [8] D. Menzel and R. Gomer, J. Chem. Phys. 41 (1964) 3311. [9] P.A. Redhead, Can. J. Phys. 42 (1964) 886. [10] R.E. Walkup and Ph. Avouris, Phys. Rev. Letters 56 (1986) 524. [11] D.R. Jennison, in: Desorption Induced by Electronic Transitions DIET I, Eds. N.H. Tolk, M.M. Tram, J.C. Tully and T.E. Madey (Springer, Berlin, 1983) p. 26. [12] M. Kamada, K. Ichikawa and K. Tsutsumi, Phys. Rev. B28 (1983) 7225. [13] K. Ichikawa, M. Kamada, O. Aita and K. Tsutsumi, Phys. Rev. B32 (1985) 8293. [14] W. Lotz, Astrophys. J. Suppl. Ser. 14 (1967) 207. [15] V.V. Nemoshkalenko and V.G. Aleshin, Electron Spectroscopy of Crystals (Plenum, New York, 1979) p. 251. [16] O. Hachenberg and W. Brauer, Advan. Electron. Electron Phys. 11 (1959) 413.