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Temperature dependence of photon stimulated desorption of ground state and excited state Na from NaCl
Surface Science 169 (1986) 267-274 North-Holland, Amsterdam
267
TEMPERATURE DEPENDENCE OF PHOTON DESORPTION
OF GROUND
STIMULATED STATE AND EXCITED STATE Na
F R O M NaCI E, T A G L A U E R
* a n d N. T O L K
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
R. R I E D E L ,
E. C O L A V I T A
a n d G. M A R G A R I T O N D O
Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, USA
N. GERSHENFELD
and N. STOFFEL
Bell Communications Research, 600 Mountain Avenue, Murray Hill, New Jersey 07974, USA
J.A. K E L B E R
a n d G. L O U B R I E L
Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
and
A.S. B O M M A N A V A R ,
M . B A K S H I a n d Z. H U R I C
University of Illinois, Chicago, Illinois, USA Received 2 October 1985; accepted for publication 26 December 1985
Photon stimulated desorption (PSD) of ground state and excited Na from NaC1 has been detected by observing the sodium resonance line emitted from atoms leaving the surface as a consequence of soft X-ray (synchrotron) irradiation. Synchrotron light in the spectral range of 14-73 eV was used for PSD. Sodium atoms in the ground state were detected using laser induced fluorescence (LIF). The temperature dependence of the desorption yield was measured between 300 and 750 K. The yield of ground state atoms is more than two orders of magnitude higher than the yield of excited state atoms, A remarkable anticorrelation was found, showing an exponential increase of the ground state signal and a simultaneous exponential decrease of the excited state signal with increasing temperature. This observation can be partly explained by assuming a model which involves primary exciton formation and subsequent diffusion of defect centers to the surface. Additional observed structures may be related to annihilation of various types of defects. * Permanent address: Max-Planck-lnstitut ffir Plasmaphysik, D-8046 Garching, Fed. Rep. of Germany.
0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © E l s e v i e r S c i e n c e P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)
268
E. Taglauer et al. / Desorption of Na frorn NaCI
1. Introduction In ionic crystals, such as NaC1, electronic excitations, either by photons or electrons, lead to the release of atoms and ions from the surface. The physical processes of photon and electron stimulated desorption (PSD and ESD) or sputtering connect to two major fields of research, to the desorption induced by electronic excitation (DIET) [1] in general and to the creation of lattice defects in ionic crystals, so-called color centers [2]. Although considerable research has been done in both fields, the processes leading to PSD from ionic crystals are as yet not understood. The present paper is intended as a contribution to shed light on some of the physical processes, in particular on the temperature dependence of ground state and excited Na atom emission due to soft X-ray (synchrotron) irradiation. These results represent first measurements of ground state neutrals desorbed by synchrotron radiation. The observed temperature behavior suggests a mechanism which includes the creation of localized defects which subsequently diffuse to the surface and release their energy into desorbing surface atoms. Self-trapped excitons (equivalent to Vk centers with an excited electron) have been discussed as important precursor states of such defects [3]. As the diffusing species the Vk center (a C12 ion along a (110) direction) [4,5] maybe through intermittent capture at point defects [6] or the H center [7,8] have been considered. The H center (a C1 z ion occupying one lattice site) can be created together with an F center by the decay of the Vk center [3]. The result of the diffusion process is the emission of a halogen atom and the remaining Na then can desorb thermally. The desorption of excited N a atoms appears to be inhibited by the same process, but more information is needed to explain the underlying mechanism.
2. Experimental The experimental sebup is schematically shown in fig. 1. The measurements were done in an ultrahigh vacuum system at a base pressure of 2 × 10 10 mbar. The sample is a NaC1 single crystal with (100) surfaces which could be conduction heated through the Cu sample holder, the temperature was measured using a chromel-alumel thermocouple. Synchrotron radiation from the storage ring T A N T A L U S at the University of Wisconsin Synchrotron Radiation Center was used for the PSD experiments. All experiments were done using zero order light reflected from the Grasshopper Mark II monochromator and applying an 800 A thick A1 filter. The photons impinging on the sample thus had energies between 14 and 73 eV with a total flux of about 2 × 1011 p h o t o n s / s at an electron current of 100 mA in the storage ring. The synchrotron beam spot size on the crystal was about 1 × 2 mm 2. An electron beam could be directed to the same spot for reasons of comparison and alignment.
269
E. Taglauer et al. / Desorption of Na from NaCI
~ NoCICrystal ~
Electron Beam--\\\ ~ / X-Rays from StorageRing
SampleHolder and Heater
\x ' ~ \
~.
Detection Optics Spectrometer Slit
"~-,~
Laser Beamy ........
- -
~ThermoCouple ReferenceJ1[~ Cell
Fig. 1. Schematic representation of the experimental set-up. Ground state (3s) Na atoms in front of the surface which have been desorbed can be excited into the 3p state by shining a 589 nm laser beam through the transparent crystal from the backside. The laser beam passes through the same spot on the surface as the synchrotron radiation and emerges collinear with that beam from the surface. The laser system consists of a 20 W Argon ion pump laser and a tunable CW dye laser with a maximum output power of 2 W. For the present experiments a dye laser power of about 100 mW~ reduced by a filter with optical density 3.0 was sufficient to reach saturation, creating signal count rates of 10 a s-1. The laser light was guided to the UHV system by means of a 40 m long fiber optic line. The laser was tuned in single mode to the 589 nm N a D resonance line by using a Na vapor reference cell through which part of the beam was diverted. A condensor lens focused the light emitted by desorbed atoms onto the entrance slit of a spectrometer. In the case of ground state atom detection the condenser lens imaged a detection volume about 5 mm in front of the NaC1 surface in order to reduce the background from scattered laser light. Excited state Na atoms were imaged immediately in front of the surface according to their short radiation decay length of about 10-3 cm.
3. Results and discussion The desorption of ground state neutrals could clearly be detected in our experiment with both photon and electron stimulation. Electrons were used in
E. Taglauer et al. / Desorption of Na from NaCI
270 10.10 5
a
SR
Zero Order, AI Filter l& eV < hv < 73eV ISR:123 8 m A
CRYSTAL NaCI (100) 397 K
lul LLI
SPECTROMETER 5890/~ 150pro Slits
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Fig. 2. (a) Fluorescence signal from Na desorbed by synchrotron radiation. (b) Fluorescence s i g n a l from Na desorbed by e l e c t r o n b o m b a r d m e n t .
an initial alignment experiment, in that case the fluorescence light emitted by the beam of desorbed atoms is visible even by eye. The size of the effect and the background can be seen from figs. 2a and 2b. The background count rates arise mainly from scattered laser light, proper experimental measures to improve the background situation are in progress [9]. The photon flux from
E. Taglauer et al. / Desorption of Na from NaCI
271
those neutrals which leave the surface in the excited 3p state is orders of magnitude lower. Therefore for excited neutral detection the spectrometer slights are fully opened (2 mm), which is possible because there is no background laser radiation. Optical detection was made at a wavelength setting of 589 nm at the spectrometer. The linewidth of neutrals detected by L I F is determined by the spectrometer resolution, the effective dye laser band width being about 50 MHz. The spectral distribution from originally excited Na atoms taken with open spectrometer slits has a half width of 3 nm. This is an important indication that the radiation originates from excited free atoms and is not luminescence from the bulk material. The count rates in the peak are of the order of 103 s 1, sitting on a broad background of about 2 × 103 s -1. The temperature dependence for both yields is shown in fig. 3. Note that both count rates have been normalized to the storage ring current, the ground state neutral signal also to the (approximate) dye laser power at the crystal. In fact, the ground state count rate is about two orders of magnitude higher under the given experimental conditions. The overall ratio of ground state to excited state is expected to be much larger. The most striking feature in the temperature dependence is the obvious anticorrelation between both signals, the ground state signal increasing strongly above 300 K to a saturation at about 500 K, whereas the excited state signal falls off at the same temperature around 300 K. Evaluation of the initial slopes of the temperature dependence shown in fig. 3 in Arrhenius plots gives activation energies of 0.27 _+ 0.02 eV for the excited state and 0.25 + 0.05 eV for the ground state atoms. This is in good agreement with the migration energy of the self-trapped exciton or Vk center in NaC1 [10]. In fact, the same temperature dependence was found for the sputtering yield of NaC1 with 1700 eV electrons [5] and a similar anticorrelation with the luminescence yield was observed. For the diffusing species for halogen desorption in alkali halides also an activation energy near that of Vk centers was found, their lifetime being longer than that for Vk centers; intermittent capture at point defects was therefore suggested [6]. For excited Na desorption by electron b o m b a r d m e n t a quite different temperature dependence was reported, which was discussed in terms of a combination of processes such as defect transport, defect annealing and evaporation from the surface [1l]. There are additional more or less pronounced features in the temperature dependence of the ground state atoms at temperatures around 400, 450 and 500 K. The maxima coincide with minima in the excited state signal. These features occur at temperatures which are characteristic for the annealing of various absorption bands in NaC1 X-rayed at room temperature, namely the R band (400 K), the N~ band (450 K) and the M band (500 K) [12]. Apparently, the annealing of these centers results in an increased release of ground state Na atoms from the surface.
E. Taglauer et al. / Desorption of Na from NaCI
272 10
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200
300
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600
700
800
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T(K)
Fig. 3. Temperaturedependenceof the 598 nm radiation from ground state, GS, (excited by LIF) and excited Na atoms, ES, desorbed under synchrotron irradiation.
The anticorrelation in the temperature dependence of ground state and excited state Na is thus observed in the general trend and also in specific features. Due to the fact that the by far dominant Na species released from the surface is always sodium in the ground state (in addition to the corresponding amount of CI) this anticorrelation cannot be explained by population of one state at the expense of the other. The explanation must rather be that the same process which promotes desorption of ground state Na inhibits the release of excited atoms. At this stage the following model can be given to explain our results, in concurrence with most of the related previous work on electronically excited alkali halides [7,8,12,14] and condensed noble gases [15,16]. The incoming synchrotron radiation (or electrons) creates locally excited states, probably excitons, in the crystal within a range from the surf~ce which corresponds to the penetration depth of the radiation, approximately several tens of nanometers. Details of this initial excitation process, e.g. the contribution of core hole excitations [11], cannot be assessed from the present results. This requires measurements of the dependence on synchrotron radiation energy in first order which will be attempted in further experiments [9]. The
E, Taglauer et al. / Desorption of Na from NaCI
273
exciton (electron-hole pair) can relax within 10 -15 s into the stable form of a Vk center or selftrapped exciton [3] which m a y subsequently undergo further transformation ( F - H pair formation). At or below r o o m temperature the diffusing species is immobile, its activation energy for migration being 0.25 eV. Therefore the major a m o u n t of excitation energy deposited in the crystal does not result in the release of N a atoms, the corresponding signal is very low. At elevated temperatures the defect diffuses to the surface, releasing its energy into the emission of a C1 a t o m and subsequently the extra N a a t o m is thermally desorbed. The energy stored in other defect centers can apparently be released by a similar mechanism. At the same time the diffusion of defects to the surface, perhaps the presence of N a atoms on the surface, inhibits the emission of excited Na. But this can only be speculated at this stage, the mechanisms leading to the observed anticorrelation are not yet known.
4. Summary Photon stimulated desorption of N a from NaC1 has been observed using laser induced fluorescence. The desorption yield increases with temperature with an activation energy of about 0.25 eV. This dependence shows an anticorrelation with the emission of excited N a atoms. The temperature dependence is compatible with the assumption that the synchrotron radiation leads to the creation of defect centers which subsequently diffuse to the surface.
Acknowledgements We gratefully acknowledge the assistance of the staff of the University of Wisconsin Synchrotron Radiation Center. One of us (ET) also wants to thank for support from Bell C o m m u n i c a t i o n s Research.
References [1] N.H. Tolk, M.M. Traum, J.C. Tully, T.E. Madey, Eds., Desorption Induced by Electronic Transitions DIET I, Springer Series in Chemical Physics, Vol. 24 (Springer, Berlin, 1983). [2] N. hoh, Advan. Phys. 31 (1982) 491. [3] N. Itoh, Radiation Effects 64 (1982) 161. [4] H. Overeijnder, M. Szymonski, A. Haring and A.E. de Vries, Radiation Effects 38 (1978) 21. [5] P.D. Townsend, R. Browning, D.J. Garlant, J.C. Kelly, A. Mahjoobi, A.J. Michael and M. Saidhoh, Radiation Effects 30 (1976) 55. [6] H. Kanzaki and T. Mori, Phys. Rev. B29 (1984) 3573.
274
E. Taglauer et al. / Desorption of Na from NaCI
[7] F.A. Lopez and P.D. Townsend, Phys. Status Solidi (b) 97 (1980) 575. [8] M. Szymonski, Radiation Effects 52 (1980) 9. [9] N.G. Stoffel, R. Riedel, E. Colavita, G. Margaritondo, R.F. Haglund, E. Taglauer and N.H. Tolk, Phys. Rev. B32 (1985) 6805. [10] E. Sonder and W.A. Sibley, in: Point Defects in Solids, Vol. 1, Eds. J.H. Crawford, Jr. and L.M. Slifkin (Plenum, New York, 1972). [1l] T.R. Pian, N. Tolk, J. Kraus, M.M. Traum, J. Tully and W.E. Collins, J. Vacuum Sci. Technol. 20 (1982) 555. [12] W.D. Compton and H. Rabin, in: Solid State Physics, Eds. F. Seitz and D. Turnbull (Academic Press, New York, 1964). [13] T.R. Pian, M.M. Traum, J.S. Kraus, N.H. Tolk, N.G. Stoffel and G. Margaritondo, Surface Sci. 128 (1983) 13. [14] N.H. Tolk, M.M. Traum, J.S. Kraus, T.R. Pian, W.E. Collins, N.G. Stoffel and G. Margaritondo, Phys. Rev. Letters 49 (1982) 812. [15] P. Borgesen, J. Schou, H. Sorensen and C. Clausen, Appl. Phys. A29 (1982) 57. [16] C.T. Reimann, R.E. Johnson and W.L. Brown, Phys. Rev. Letters 53 (1984) 600.
267
TEMPERATURE DEPENDENCE OF PHOTON DESORPTION
OF GROUND
STIMULATED STATE AND EXCITED STATE Na
F R O M NaCI E, T A G L A U E R
* a n d N. T O L K
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
R. R I E D E L ,
E. C O L A V I T A
a n d G. M A R G A R I T O N D O
Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, USA
N. GERSHENFELD
and N. STOFFEL
Bell Communications Research, 600 Mountain Avenue, Murray Hill, New Jersey 07974, USA
J.A. K E L B E R
a n d G. L O U B R I E L
Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
and
A.S. B O M M A N A V A R ,
M . B A K S H I a n d Z. H U R I C
University of Illinois, Chicago, Illinois, USA Received 2 October 1985; accepted for publication 26 December 1985
Photon stimulated desorption (PSD) of ground state and excited Na from NaC1 has been detected by observing the sodium resonance line emitted from atoms leaving the surface as a consequence of soft X-ray (synchrotron) irradiation. Synchrotron light in the spectral range of 14-73 eV was used for PSD. Sodium atoms in the ground state were detected using laser induced fluorescence (LIF). The temperature dependence of the desorption yield was measured between 300 and 750 K. The yield of ground state atoms is more than two orders of magnitude higher than the yield of excited state atoms, A remarkable anticorrelation was found, showing an exponential increase of the ground state signal and a simultaneous exponential decrease of the excited state signal with increasing temperature. This observation can be partly explained by assuming a model which involves primary exciton formation and subsequent diffusion of defect centers to the surface. Additional observed structures may be related to annihilation of various types of defects. * Permanent address: Max-Planck-lnstitut ffir Plasmaphysik, D-8046 Garching, Fed. Rep. of Germany.
0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © E l s e v i e r S c i e n c e P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)
268
E. Taglauer et al. / Desorption of Na frorn NaCI
1. Introduction In ionic crystals, such as NaC1, electronic excitations, either by photons or electrons, lead to the release of atoms and ions from the surface. The physical processes of photon and electron stimulated desorption (PSD and ESD) or sputtering connect to two major fields of research, to the desorption induced by electronic excitation (DIET) [1] in general and to the creation of lattice defects in ionic crystals, so-called color centers [2]. Although considerable research has been done in both fields, the processes leading to PSD from ionic crystals are as yet not understood. The present paper is intended as a contribution to shed light on some of the physical processes, in particular on the temperature dependence of ground state and excited Na atom emission due to soft X-ray (synchrotron) irradiation. These results represent first measurements of ground state neutrals desorbed by synchrotron radiation. The observed temperature behavior suggests a mechanism which includes the creation of localized defects which subsequently diffuse to the surface and release their energy into desorbing surface atoms. Self-trapped excitons (equivalent to Vk centers with an excited electron) have been discussed as important precursor states of such defects [3]. As the diffusing species the Vk center (a C12 ion along a (110) direction) [4,5] maybe through intermittent capture at point defects [6] or the H center [7,8] have been considered. The H center (a C1 z ion occupying one lattice site) can be created together with an F center by the decay of the Vk center [3]. The result of the diffusion process is the emission of a halogen atom and the remaining Na then can desorb thermally. The desorption of excited N a atoms appears to be inhibited by the same process, but more information is needed to explain the underlying mechanism.
2. Experimental The experimental sebup is schematically shown in fig. 1. The measurements were done in an ultrahigh vacuum system at a base pressure of 2 × 10 10 mbar. The sample is a NaC1 single crystal with (100) surfaces which could be conduction heated through the Cu sample holder, the temperature was measured using a chromel-alumel thermocouple. Synchrotron radiation from the storage ring T A N T A L U S at the University of Wisconsin Synchrotron Radiation Center was used for the PSD experiments. All experiments were done using zero order light reflected from the Grasshopper Mark II monochromator and applying an 800 A thick A1 filter. The photons impinging on the sample thus had energies between 14 and 73 eV with a total flux of about 2 × 1011 p h o t o n s / s at an electron current of 100 mA in the storage ring. The synchrotron beam spot size on the crystal was about 1 × 2 mm 2. An electron beam could be directed to the same spot for reasons of comparison and alignment.
269
E. Taglauer et al. / Desorption of Na from NaCI
~ NoCICrystal ~
Electron Beam--\\\ ~ / X-Rays from StorageRing
SampleHolder and Heater
\x ' ~ \
~.
Detection Optics Spectrometer Slit
"~-,~
Laser Beamy ........
- -
~ThermoCouple ReferenceJ1[~ Cell
Fig. 1. Schematic representation of the experimental set-up. Ground state (3s) Na atoms in front of the surface which have been desorbed can be excited into the 3p state by shining a 589 nm laser beam through the transparent crystal from the backside. The laser beam passes through the same spot on the surface as the synchrotron radiation and emerges collinear with that beam from the surface. The laser system consists of a 20 W Argon ion pump laser and a tunable CW dye laser with a maximum output power of 2 W. For the present experiments a dye laser power of about 100 mW~ reduced by a filter with optical density 3.0 was sufficient to reach saturation, creating signal count rates of 10 a s-1. The laser light was guided to the UHV system by means of a 40 m long fiber optic line. The laser was tuned in single mode to the 589 nm N a D resonance line by using a Na vapor reference cell through which part of the beam was diverted. A condensor lens focused the light emitted by desorbed atoms onto the entrance slit of a spectrometer. In the case of ground state atom detection the condenser lens imaged a detection volume about 5 mm in front of the NaC1 surface in order to reduce the background from scattered laser light. Excited state Na atoms were imaged immediately in front of the surface according to their short radiation decay length of about 10-3 cm.
3. Results and discussion The desorption of ground state neutrals could clearly be detected in our experiment with both photon and electron stimulation. Electrons were used in
E. Taglauer et al. / Desorption of Na from NaCI
270 10.10 5
a
SR
Zero Order, AI Filter l& eV < hv < 73eV ISR:123 8 m A
CRYSTAL NaCI (100) 397 K
lul LLI
SPECTROMETER 5890/~ 150pro Slits
05
rY
"EFFECT"
O (.9
38
10L' c t s / s
SR OFF LASER OFF I
1
I
I
I0
20
30
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TIME i s )
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Electrons
b
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800eV.
Crystal 6LO K
EI11
Loser: 130 mW Filter O D - 3 0
i11 p-
n~
I
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Slits 250 ~m 210 pm
\
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1
I
|0
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30
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TIME
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"Effect" e - A N D LASER OFF
I
I
I
60
70
80
12
105 c t s / s
Fig. 2. (a) Fluorescence signal from Na desorbed by synchrotron radiation. (b) Fluorescence s i g n a l from Na desorbed by e l e c t r o n b o m b a r d m e n t .
an initial alignment experiment, in that case the fluorescence light emitted by the beam of desorbed atoms is visible even by eye. The size of the effect and the background can be seen from figs. 2a and 2b. The background count rates arise mainly from scattered laser light, proper experimental measures to improve the background situation are in progress [9]. The photon flux from
E. Taglauer et al. / Desorption of Na from NaCI
271
those neutrals which leave the surface in the excited 3p state is orders of magnitude lower. Therefore for excited neutral detection the spectrometer slights are fully opened (2 mm), which is possible because there is no background laser radiation. Optical detection was made at a wavelength setting of 589 nm at the spectrometer. The linewidth of neutrals detected by L I F is determined by the spectrometer resolution, the effective dye laser band width being about 50 MHz. The spectral distribution from originally excited Na atoms taken with open spectrometer slits has a half width of 3 nm. This is an important indication that the radiation originates from excited free atoms and is not luminescence from the bulk material. The count rates in the peak are of the order of 103 s 1, sitting on a broad background of about 2 × 103 s -1. The temperature dependence for both yields is shown in fig. 3. Note that both count rates have been normalized to the storage ring current, the ground state neutral signal also to the (approximate) dye laser power at the crystal. In fact, the ground state count rate is about two orders of magnitude higher under the given experimental conditions. The overall ratio of ground state to excited state is expected to be much larger. The most striking feature in the temperature dependence is the obvious anticorrelation between both signals, the ground state signal increasing strongly above 300 K to a saturation at about 500 K, whereas the excited state signal falls off at the same temperature around 300 K. Evaluation of the initial slopes of the temperature dependence shown in fig. 3 in Arrhenius plots gives activation energies of 0.27 _+ 0.02 eV for the excited state and 0.25 + 0.05 eV for the ground state atoms. This is in good agreement with the migration energy of the self-trapped exciton or Vk center in NaC1 [10]. In fact, the same temperature dependence was found for the sputtering yield of NaC1 with 1700 eV electrons [5] and a similar anticorrelation with the luminescence yield was observed. For the diffusing species for halogen desorption in alkali halides also an activation energy near that of Vk centers was found, their lifetime being longer than that for Vk centers; intermittent capture at point defects was therefore suggested [6]. For excited Na desorption by electron b o m b a r d m e n t a quite different temperature dependence was reported, which was discussed in terms of a combination of processes such as defect transport, defect annealing and evaporation from the surface [1l]. There are additional more or less pronounced features in the temperature dependence of the ground state atoms at temperatures around 400, 450 and 500 K. The maxima coincide with minima in the excited state signal. These features occur at temperatures which are characteristic for the annealing of various absorption bands in NaC1 X-rayed at room temperature, namely the R band (400 K), the N~ band (450 K) and the M band (500 K) [12]. Apparently, the annealing of these centers results in an increased release of ground state Na atoms from the surface.
E. Taglauer et al. / Desorption of Na from NaCI
272 10
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I
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I
I
I
I
I
200
300
~00
S00
600
700
800
01
T(K)
Fig. 3. Temperaturedependenceof the 598 nm radiation from ground state, GS, (excited by LIF) and excited Na atoms, ES, desorbed under synchrotron irradiation.
The anticorrelation in the temperature dependence of ground state and excited state Na is thus observed in the general trend and also in specific features. Due to the fact that the by far dominant Na species released from the surface is always sodium in the ground state (in addition to the corresponding amount of CI) this anticorrelation cannot be explained by population of one state at the expense of the other. The explanation must rather be that the same process which promotes desorption of ground state Na inhibits the release of excited atoms. At this stage the following model can be given to explain our results, in concurrence with most of the related previous work on electronically excited alkali halides [7,8,12,14] and condensed noble gases [15,16]. The incoming synchrotron radiation (or electrons) creates locally excited states, probably excitons, in the crystal within a range from the surf~ce which corresponds to the penetration depth of the radiation, approximately several tens of nanometers. Details of this initial excitation process, e.g. the contribution of core hole excitations [11], cannot be assessed from the present results. This requires measurements of the dependence on synchrotron radiation energy in first order which will be attempted in further experiments [9]. The
E, Taglauer et al. / Desorption of Na from NaCI
273
exciton (electron-hole pair) can relax within 10 -15 s into the stable form of a Vk center or selftrapped exciton [3] which m a y subsequently undergo further transformation ( F - H pair formation). At or below r o o m temperature the diffusing species is immobile, its activation energy for migration being 0.25 eV. Therefore the major a m o u n t of excitation energy deposited in the crystal does not result in the release of N a atoms, the corresponding signal is very low. At elevated temperatures the defect diffuses to the surface, releasing its energy into the emission of a C1 a t o m and subsequently the extra N a a t o m is thermally desorbed. The energy stored in other defect centers can apparently be released by a similar mechanism. At the same time the diffusion of defects to the surface, perhaps the presence of N a atoms on the surface, inhibits the emission of excited Na. But this can only be speculated at this stage, the mechanisms leading to the observed anticorrelation are not yet known.
4. Summary Photon stimulated desorption of N a from NaC1 has been observed using laser induced fluorescence. The desorption yield increases with temperature with an activation energy of about 0.25 eV. This dependence shows an anticorrelation with the emission of excited N a atoms. The temperature dependence is compatible with the assumption that the synchrotron radiation leads to the creation of defect centers which subsequently diffuse to the surface.
Acknowledgements We gratefully acknowledge the assistance of the staff of the University of Wisconsin Synchrotron Radiation Center. One of us (ET) also wants to thank for support from Bell C o m m u n i c a t i o n s Research.
References [1] N.H. Tolk, M.M. Traum, J.C. Tully, T.E. Madey, Eds., Desorption Induced by Electronic Transitions DIET I, Springer Series in Chemical Physics, Vol. 24 (Springer, Berlin, 1983). [2] N. hoh, Advan. Phys. 31 (1982) 491. [3] N. Itoh, Radiation Effects 64 (1982) 161. [4] H. Overeijnder, M. Szymonski, A. Haring and A.E. de Vries, Radiation Effects 38 (1978) 21. [5] P.D. Townsend, R. Browning, D.J. Garlant, J.C. Kelly, A. Mahjoobi, A.J. Michael and M. Saidhoh, Radiation Effects 30 (1976) 55. [6] H. Kanzaki and T. Mori, Phys. Rev. B29 (1984) 3573.
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[7] F.A. Lopez and P.D. Townsend, Phys. Status Solidi (b) 97 (1980) 575. [8] M. Szymonski, Radiation Effects 52 (1980) 9. [9] N.G. Stoffel, R. Riedel, E. Colavita, G. Margaritondo, R.F. Haglund, E. Taglauer and N.H. Tolk, Phys. Rev. B32 (1985) 6805. [10] E. Sonder and W.A. Sibley, in: Point Defects in Solids, Vol. 1, Eds. J.H. Crawford, Jr. and L.M. Slifkin (Plenum, New York, 1972). [1l] T.R. Pian, N. Tolk, J. Kraus, M.M. Traum, J. Tully and W.E. Collins, J. Vacuum Sci. Technol. 20 (1982) 555. [12] W.D. Compton and H. Rabin, in: Solid State Physics, Eds. F. Seitz and D. Turnbull (Academic Press, New York, 1964). [13] T.R. Pian, M.M. Traum, J.S. Kraus, N.H. Tolk, N.G. Stoffel and G. Margaritondo, Surface Sci. 128 (1983) 13. [14] N.H. Tolk, M.M. Traum, J.S. Kraus, T.R. Pian, W.E. Collins, N.G. Stoffel and G. Margaritondo, Phys. Rev. Letters 49 (1982) 812. [15] P. Borgesen, J. Schou, H. Sorensen and C. Clausen, Appl. Phys. A29 (1982) 57. [16] C.T. Reimann, R.E. Johnson and W.L. Brown, Phys. Rev. Letters 53 (1984) 600.