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Auger-free luminescence due to interatomic transitions of valence electrons into core holes in BaF2
Solid State Communications, Vol. 65, No. 6, pp. 523-526, 1988. Printed in Great Britain.
0038--1098/88 $3.00 + .00 ~) 1988 Pergamon Journals Ltd.
A U G E R - F R E E L U M I N E S C E N C E DUE TO I N T E R A T O M I C T R A N S I T I O N S OF VALENCE E L E C T R O N S INTO CORE HOLES IN BaF: Minoru Itoh Department of Applied Science, Faculty of Engineering, Shinshu University, Nagano 380, Japan Satoshi Hashimoto Kyoto University of Education, Fushimi-ku, Kyoto 612, Japan Shiro Sakuragi Union Material Inc., Tone-machi, Ibaraki 27(~12, Japan and Shinzou Kubota Department of Physics, Rikkyo University, Nishi-lkebukuro 3, Tokyo 171, Japan
(Received 10 August 1987 by H. Kamimura) The nature of BaF z emission is investigated by measuring excitation spectra using synchrotron radiation for the energy range up to 40 eV, Intense emission bands, appearing at 5.6 and 6.4eV in the Auger-transparent region, are ascribed to a radiative decay of valence electrons into Ba 2+ 5p core holes. This luminescence process of a new type is discussed. 1. I N T R O D U C T I O N INTRINSIC L U M I N E S C E N C E processes in BaF2 crystals have been investigated with increasing interest in recent years. Two different types of emission are observed at room temperature [I-4]: one peaking at 5.6eV with a decay time shorter than 1 ns, the other at 4.1 eV with a decay time of ~ 0.6/ts. The latter (slow component) is excited with photons in the fundamental absorption region [5], similarly to the intrinsic luminescence of alkali halides. From microwave-optical double-resonance experiments [6], Kabler et al. attributed this to a radiative annihilation of the self-trapped exciton having a nearest-neighbor F H pair configuration. The former (fast component) is (i) seen only in BaF: crystal among alkaline-earth fluorides, (ii) not quenched thermally up to 400K, and (iii) excited efficiently by high-energy electrons but not by :t-particles [4]. Although much attention has been paid to this component from the view point of application to detectors for high-energy physics, its nature is still under discussion [2~4]. In order to clarify the luminescence process of the fast component in BaF2, we measured emission spectra and excitation spectra using synchrotron radiation as a light source, The obtained results confirm that the fast component is due to interatomic radiative transition of electrons from the F - 2p valence band to the 523
deeper-lying Ba 2+ 5p core level, in which holes are created with high-energy photons or electrons. An important feature of this transition in BaF 2 is that the Auger decay process is energetically impossible, leading to high yield of the luminescence. 2. E X P E R I M E N T A L Experiments were performed using synchrotron radiation from the 750 MeV electron storage ring at Institute for Molecular Science, Okazaki. The vacuum-ultraviolet synchrotron radiation was monochromatized with a l-m Seya-Namioka type monochromator equipped with a 2400-grooves mm grating. Its bandpassveas held to be about 0.8 nm for all measurements. As an order-sorting filter, crystal plate of LiF was used. Luminescence emitted from the sample was viewed at a right angle to the exciting light through a sapphire window, and was focused by a fused-silica lens onto the entrance slit of a Bausch and Lomb grating monochromator with a Hamamatsu R585 photomultiplier. The emission spectra were not corrected for the spectral response of the detecting system. The BaF~ crystal used in the present experiment was grown from the melt by the vertical Bridgman method using graphite crucible in vacuum. Freshly cleaved sample (about 2 x 10 x 10ram) was moun-
524
AUGER-FREE LUMINESCENCE IN BaF2 PHOTON ENERGY (eV) 6.0 4.0
8.0 i
I
--I
i
,
~0.3 - -
Z l.0 Z
f
o 0.5 z0.5
,g, 0
200
300 WAVELENGTH (nm)
400
Fig, I. Emission spectrum of BaF 2 crystals measured at 295 K. Excitation was made with 21.4eV photons, corresponding to the interband transition from the Ba 2' 5p core band to the conduction band.
ted on the aluminum holder of a cryostat. The sample chamber was evacuated below 9 x 10 6Pa by a turbomolecular pump. 3. R E S U L T S A N D D I S C U S S I O N Figure 1 shows the emission spectrum of BaF~ crystal excited with 21.4eV photons at 295 K. In the low-energy region, broad luminescence arising from sell-trapped excitons is observed around 4.1 eV, which is the slow component. The high-energy band, which is the fast component, is composed of two resolved bands peaking at 5,6 and 6.4 eV. These peak positions of emission bands are in good agreement with those obtained under excitation by an electron beam [3, 4]. Valbis et al. [3] have l\~und a weak additional band at about 7.0 eV in the wlcuum-ultraviolet region. Such a band, however, cannot be observed in our experimental arrangement for luminescence measurements, if any. In Fig. I, it is worth noting that the intensity of the fast component is larger than that of the slow component. This is quite different from the case of electron excitation [3, 4], in x~hich the slow component is more intensely excited. In Fig. 2(b) are shown the excitation spectra for the 5.6eV band (solid curve) and the 4.1 eV band (broken curve) at 2~5 K. The result obtained for the 6.4eV band was practically the same as the solid curve. For reference, the reflectance spectrum of BaF, taken at 300 K by Rubloff [7] is depicted in Fig. 2(a). The dips observed in the excitation spectra coincide well with the peak positions of the reflectance spect r u n i , at least partly arising l¥om reflection loss of the incident light at sample surface. According to the band-calculations [8 10], BaF, has the conduction band originating from the excited 6s and 5d states of Ba:' ions and the valence band from the filled 2p state of F ions. The s-like conduc-
-
r-
r F'excilon ~ Ba'*5p(:~i)exciton r " exciton
°0ii ~> 1.0~
Z
~
Vol. 65, No. 6
-
(b) '
fl
i 'v' l
IO
(a)
eaFi'
',r, _--
,
TL.._.
20 30 PHOTON ENERGY (eV)
"-..
40
k i g . 2. ( a ) : Reflectance spectrum of BaF2 crystals at 300 K [7]. (b): Excitation spectra for the 5.6eV band (solid curve) and the 4.1eV band (broken curve) of BaF: crystals measured at 295 K.
Iion band has a minimum at F,, while the minimum of the d-like band is at X~, somewhat higher in energy than F~. It should be noted that the Ba 2÷ 5p core level is located only several electron volts below tile bottom of the valence band. Rubloff [7] assigned the lowest reflectance peak at 9.8 eV to the optical creation of V--excitons. The band gap of BaF~ was found to be E = 10.5eV [11]. In the high-energy region the reflectance spectrum exhibits two strong peaks at 17.1 and l~.8eV~ These peaks were assigned to the core excitons associated with Ba 2+ 5p (j = 3/2) ---, i' Eand Ba :+ 5p (j = I/2) --+ X~. respectively [7]. The 4. I eV band (slow component) can be excited in the whole range of fundamental absorption of BaF,, indicating that it is a result of radiative annihilalion of self-trapped excitons, The most outstanding feature of the excitation spectrum of the fast component ( 5 . c , and 6.4eV) is its onset at hv = 18. l ± 0.2eV. This threshold is clearly above the energy of Ba :+ 5p (j = 3/2) core excitons and corresponds to the onset of interband transitions from the Ba :~ 5p core band to the conduction band. Thus a radiative transition of the F 2/) valence electrons to the Ba ~' 5p core holes becomes possible, as suggested by Alcksandrov et al. [2]. The fast component can theret\~re be ascribed to such luminescence processes. The doublet structure of the fast component should not be associated with the 2.0eV spin-orbit splitting of the Ba-'- 5p core levels, since both bands at 5.6 and 6.4 eV have the same excitation threshold. It may be explained in terms of the density-of-states of the valence band, because the core levels are almost fiat. Furthermore, the valence electron-core hole luminescence mentioned above is expected to be located in the xicinity of hv = Ej.c - 0.5AE~, where E , c is the difference in energy between the tops of the valence and core bands and AE, is the total width of the valence band. These situations are compared with the
Vol. 65, No. 6
AUGER-FREE LUMINESCENCE IN BaF2
525
b,and parameters determined from photoelectron spec- around 9 eV and then decreases gradually, but it turns troscopy (Eve, = 7.8eV and k E y = 3.4eV) [12] and to increase above 18 eV, corresponding to the onset of the calculation of electronic energy-band structures the fast component. Finally, we comment on the absence of the fast [9]. The agreements are found to be satisfactory [2, 4], showing the adequacy of our luminescence mechan- component for the ~x-particle excitation [4]. The 0~-particle is characterized by its very high ionization power ism for the fast component. In several ionic crystals [13, 14], electronic tran- (about 1000 times) compared with that of high-energy sitions from the valence band to the deep-lying core electrons. Therefore it is expected that high concentralevels have been observed in the soft X-ray emission tions of free electrons and holes are created in the core spectra, where competing processes (i.e. reabsorption of 0~-particle track structure. These carriers recombine and/or Auger effect) are very efficient. A distinctive quickly to form self-trapped excitons, which can abfeature of the valence electron-core hole luminescence sorb the ultraviolet light [16]. Thus the absence of the in BaF2 is that the Auger process resulting in a valence fast component under c~-particle excitation may be electron excitation cannot occur since E~,c < Ex. This explained in terms of reabsorption effects due to selfexplains why the fast component is seen as an intense trapped excitons at high density in the track core. emission band. Such Auger-free luminescence between the valence and core bands has also been found Acknowh, dgements - - The authors would like to in cesium halides and R b F which are characterized by thank Professor M. Watanabe and Dr K. Fukui for large band gaps and low ionization energies of the their close cooperation during the course of this research. We are also very grateful to Professor Y. cations [15]. The details will be reported elsewhere. Attention is called to no contribution of core Toyozawa for his helpful discussions. The present work was supported by the Joint Studies Program excitons to excitation of the fast component, as seen in (1987) of the Institute for Molecular Science. Fig. 2(b). This fact is somewhat surprising at first Note added #~ p r o q f -- Photoluminescence studies sight, When the incident light is at resonance with similar to the ones described here have recently been core-exciton states, a hole will be created in core levels. done by (_;.S. Shi, T. Kloiber and G. Zimmerer, J, having a bound electron around itselE Hence valence Lumin. (in press). Besides the emission bands at 4.1, electrons may have a chance of undergoing radiative 5.6 and 6.4eV, they observed a weak band at 7.0eV decay to the core holes. Such a decay process, how- and attributed it to the same origin as the 5.6 and ever, competes with the direct decay of core excitons: 6.4eV bands. the latter would be a more predominant process than the former, presumably because of the v~-law of the REFERENCES transition probability. From the present results, we I. M. Laval, M. Moszynski, R. Allemand, E. Corcan estimate the binding energy of core excitons in moreche, P. Guinet, R. Odru & J. Vacher, Nucl. BaF, as 1.0 ± 0.2eV. This value is somewhat larger Instrum. Methods 206, 169 (1983). than that of the valence excitons (0.7 eV) [1 I]. It seems 2. Yu.M. Aleksandrov, V.N. Makhov, P.A. Rodnreasonable as a result thai the valence electrons are yi, T.I. Syreishchikova & M.N. Yakimenko, almost entirely on the F ions: thus the Coulomb Soy. P/n's. Solid State 26, 1734 ([984). potential of a hole m core levels of the Ba > ions is 3. Ya.A. Valbis, Z.A. Rachko & Ya.L. Yansons. essentially not screened and attracts the electron J E T P Lett. 42, 172 (1985). 4. S. Kubota, N. Kanai & J. Ruan(Gen), Phys. strongly. Status Solidi (he' 139, 635 (1987). It must be emphasized that both of the valence 5. S. Kubota, H. lto, J. Ruan(Gen) & S. Asaoka, and core bands are completely filled with electrons in Activ. Rep. o f S vn. Rad. Lab. Inst. Solid State the ground state. This fact provides interesting featPhl'.v., Univ. T o k y o (1984) p. 12. ures for the valence electron-core hole luminescence in 0. P.J. Call, W. Hayes & M.N. Kabler, J. Phys. C8, ionic materials. First, a population reversion between L60 (1975). these levels is easily realized when electrons are excited 7. G.W. Rubtoff, Phrv. Rev. BS, 662 (1972). 8. J.P. Albert, C. Jouanin & C. Gout, Phys, Rev. from the core band into the conduction band. so that BI6, 925 f1977). optical amplification may be achieved at any excita9. R.A. Heaton & C.C. Lin, Phys. Rev. B22~ 36_')"c tion density. Furthermore, the holes lifted into the (1980). valence band subsequently recombine with conduc10. N.V. Starostin. M.P. Shepilov & A.B. Alekseev, tion electrons. That is, one photon produces two kinds Phv,s. Status Solidi (b.) 103, 717 ( 1981 ). of luminescent states: valence electron-core hole pairs 11. T. Tomiki & T. Miyata, J. Phys. Soc, Japan 27, and self-trapped excitons. This is clearly seen in Fig. 658 (1969). 12. R.T. Poole, J. Szajman, R.C.G. Leckey, J.G. 2(b): luminescence yield of the slow component rises
526
13. 14.
AUGER-FREE LUMINESCENCE IN BaF2 Jenkin & J. Liesegang, Phys. Rev. BI2, 5872 (1975). A.A. Maiste, A.M.-E, Saar & M.A. l~lango, Soy. Phys. - - Solid State 16, 1118 (1974). E,T. Arakawa & M.W. Williams, Phys. Rev.
15. 16.
Vol. 65, No. 6
Lett. 36, 333 (1976). S. Kubota, J. Ruan(Gen), S. Sakuragi, M. Itoh & S. Hashimoto, J. Lumin (in press). R.T. Williams, M.N. Kabler, W. Hayes & J.P. Scott, Phys. Rev. BI4, 725 (1976).
0038--1098/88 $3.00 + .00 ~) 1988 Pergamon Journals Ltd.
A U G E R - F R E E L U M I N E S C E N C E DUE TO I N T E R A T O M I C T R A N S I T I O N S OF VALENCE E L E C T R O N S INTO CORE HOLES IN BaF: Minoru Itoh Department of Applied Science, Faculty of Engineering, Shinshu University, Nagano 380, Japan Satoshi Hashimoto Kyoto University of Education, Fushimi-ku, Kyoto 612, Japan Shiro Sakuragi Union Material Inc., Tone-machi, Ibaraki 27(~12, Japan and Shinzou Kubota Department of Physics, Rikkyo University, Nishi-lkebukuro 3, Tokyo 171, Japan
(Received 10 August 1987 by H. Kamimura) The nature of BaF z emission is investigated by measuring excitation spectra using synchrotron radiation for the energy range up to 40 eV, Intense emission bands, appearing at 5.6 and 6.4eV in the Auger-transparent region, are ascribed to a radiative decay of valence electrons into Ba 2+ 5p core holes. This luminescence process of a new type is discussed. 1. I N T R O D U C T I O N INTRINSIC L U M I N E S C E N C E processes in BaF2 crystals have been investigated with increasing interest in recent years. Two different types of emission are observed at room temperature [I-4]: one peaking at 5.6eV with a decay time shorter than 1 ns, the other at 4.1 eV with a decay time of ~ 0.6/ts. The latter (slow component) is excited with photons in the fundamental absorption region [5], similarly to the intrinsic luminescence of alkali halides. From microwave-optical double-resonance experiments [6], Kabler et al. attributed this to a radiative annihilation of the self-trapped exciton having a nearest-neighbor F H pair configuration. The former (fast component) is (i) seen only in BaF: crystal among alkaline-earth fluorides, (ii) not quenched thermally up to 400K, and (iii) excited efficiently by high-energy electrons but not by :t-particles [4]. Although much attention has been paid to this component from the view point of application to detectors for high-energy physics, its nature is still under discussion [2~4]. In order to clarify the luminescence process of the fast component in BaF2, we measured emission spectra and excitation spectra using synchrotron radiation as a light source, The obtained results confirm that the fast component is due to interatomic radiative transition of electrons from the F - 2p valence band to the 523
deeper-lying Ba 2+ 5p core level, in which holes are created with high-energy photons or electrons. An important feature of this transition in BaF 2 is that the Auger decay process is energetically impossible, leading to high yield of the luminescence. 2. E X P E R I M E N T A L Experiments were performed using synchrotron radiation from the 750 MeV electron storage ring at Institute for Molecular Science, Okazaki. The vacuum-ultraviolet synchrotron radiation was monochromatized with a l-m Seya-Namioka type monochromator equipped with a 2400-grooves mm grating. Its bandpassveas held to be about 0.8 nm for all measurements. As an order-sorting filter, crystal plate of LiF was used. Luminescence emitted from the sample was viewed at a right angle to the exciting light through a sapphire window, and was focused by a fused-silica lens onto the entrance slit of a Bausch and Lomb grating monochromator with a Hamamatsu R585 photomultiplier. The emission spectra were not corrected for the spectral response of the detecting system. The BaF~ crystal used in the present experiment was grown from the melt by the vertical Bridgman method using graphite crucible in vacuum. Freshly cleaved sample (about 2 x 10 x 10ram) was moun-
524
AUGER-FREE LUMINESCENCE IN BaF2 PHOTON ENERGY (eV) 6.0 4.0
8.0 i
I
--I
i
,
~0.3 - -
Z l.0 Z
f
o 0.5 z0.5
,g, 0
200
300 WAVELENGTH (nm)
400
Fig, I. Emission spectrum of BaF 2 crystals measured at 295 K. Excitation was made with 21.4eV photons, corresponding to the interband transition from the Ba 2' 5p core band to the conduction band.
ted on the aluminum holder of a cryostat. The sample chamber was evacuated below 9 x 10 6Pa by a turbomolecular pump. 3. R E S U L T S A N D D I S C U S S I O N Figure 1 shows the emission spectrum of BaF~ crystal excited with 21.4eV photons at 295 K. In the low-energy region, broad luminescence arising from sell-trapped excitons is observed around 4.1 eV, which is the slow component. The high-energy band, which is the fast component, is composed of two resolved bands peaking at 5,6 and 6.4 eV. These peak positions of emission bands are in good agreement with those obtained under excitation by an electron beam [3, 4]. Valbis et al. [3] have l\~und a weak additional band at about 7.0 eV in the wlcuum-ultraviolet region. Such a band, however, cannot be observed in our experimental arrangement for luminescence measurements, if any. In Fig. I, it is worth noting that the intensity of the fast component is larger than that of the slow component. This is quite different from the case of electron excitation [3, 4], in x~hich the slow component is more intensely excited. In Fig. 2(b) are shown the excitation spectra for the 5.6eV band (solid curve) and the 4.1 eV band (broken curve) at 2~5 K. The result obtained for the 6.4eV band was practically the same as the solid curve. For reference, the reflectance spectrum of BaF, taken at 300 K by Rubloff [7] is depicted in Fig. 2(a). The dips observed in the excitation spectra coincide well with the peak positions of the reflectance spect r u n i , at least partly arising l¥om reflection loss of the incident light at sample surface. According to the band-calculations [8 10], BaF, has the conduction band originating from the excited 6s and 5d states of Ba:' ions and the valence band from the filled 2p state of F ions. The s-like conduc-
-
r-
r F'excilon ~ Ba'*5p(:~i)exciton r " exciton
°0ii ~> 1.0~
Z
~
Vol. 65, No. 6
-
(b) '
fl
i 'v' l
IO
(a)
eaFi'
',r, _--
,
TL.._.
20 30 PHOTON ENERGY (eV)
"-..
40
k i g . 2. ( a ) : Reflectance spectrum of BaF2 crystals at 300 K [7]. (b): Excitation spectra for the 5.6eV band (solid curve) and the 4.1eV band (broken curve) of BaF: crystals measured at 295 K.
Iion band has a minimum at F,, while the minimum of the d-like band is at X~, somewhat higher in energy than F~. It should be noted that the Ba 2÷ 5p core level is located only several electron volts below tile bottom of the valence band. Rubloff [7] assigned the lowest reflectance peak at 9.8 eV to the optical creation of V--excitons. The band gap of BaF~ was found to be E = 10.5eV [11]. In the high-energy region the reflectance spectrum exhibits two strong peaks at 17.1 and l~.8eV~ These peaks were assigned to the core excitons associated with Ba 2+ 5p (j = 3/2) ---, i' Eand Ba :+ 5p (j = I/2) --+ X~. respectively [7]. The 4. I eV band (slow component) can be excited in the whole range of fundamental absorption of BaF,, indicating that it is a result of radiative annihilalion of self-trapped excitons, The most outstanding feature of the excitation spectrum of the fast component ( 5 . c , and 6.4eV) is its onset at hv = 18. l ± 0.2eV. This threshold is clearly above the energy of Ba :+ 5p (j = 3/2) core excitons and corresponds to the onset of interband transitions from the Ba :~ 5p core band to the conduction band. Thus a radiative transition of the F 2/) valence electrons to the Ba ~' 5p core holes becomes possible, as suggested by Alcksandrov et al. [2]. The fast component can theret\~re be ascribed to such luminescence processes. The doublet structure of the fast component should not be associated with the 2.0eV spin-orbit splitting of the Ba-'- 5p core levels, since both bands at 5.6 and 6.4 eV have the same excitation threshold. It may be explained in terms of the density-of-states of the valence band, because the core levels are almost fiat. Furthermore, the valence electron-core hole luminescence mentioned above is expected to be located in the xicinity of hv = Ej.c - 0.5AE~, where E , c is the difference in energy between the tops of the valence and core bands and AE, is the total width of the valence band. These situations are compared with the
Vol. 65, No. 6
AUGER-FREE LUMINESCENCE IN BaF2
525
b,and parameters determined from photoelectron spec- around 9 eV and then decreases gradually, but it turns troscopy (Eve, = 7.8eV and k E y = 3.4eV) [12] and to increase above 18 eV, corresponding to the onset of the calculation of electronic energy-band structures the fast component. Finally, we comment on the absence of the fast [9]. The agreements are found to be satisfactory [2, 4], showing the adequacy of our luminescence mechan- component for the ~x-particle excitation [4]. The 0~-particle is characterized by its very high ionization power ism for the fast component. In several ionic crystals [13, 14], electronic tran- (about 1000 times) compared with that of high-energy sitions from the valence band to the deep-lying core electrons. Therefore it is expected that high concentralevels have been observed in the soft X-ray emission tions of free electrons and holes are created in the core spectra, where competing processes (i.e. reabsorption of 0~-particle track structure. These carriers recombine and/or Auger effect) are very efficient. A distinctive quickly to form self-trapped excitons, which can abfeature of the valence electron-core hole luminescence sorb the ultraviolet light [16]. Thus the absence of the in BaF2 is that the Auger process resulting in a valence fast component under c~-particle excitation may be electron excitation cannot occur since E~,c < Ex. This explained in terms of reabsorption effects due to selfexplains why the fast component is seen as an intense trapped excitons at high density in the track core. emission band. Such Auger-free luminescence between the valence and core bands has also been found Acknowh, dgements - - The authors would like to in cesium halides and R b F which are characterized by thank Professor M. Watanabe and Dr K. Fukui for large band gaps and low ionization energies of the their close cooperation during the course of this research. We are also very grateful to Professor Y. cations [15]. The details will be reported elsewhere. Attention is called to no contribution of core Toyozawa for his helpful discussions. The present work was supported by the Joint Studies Program excitons to excitation of the fast component, as seen in (1987) of the Institute for Molecular Science. Fig. 2(b). This fact is somewhat surprising at first Note added #~ p r o q f -- Photoluminescence studies sight, When the incident light is at resonance with similar to the ones described here have recently been core-exciton states, a hole will be created in core levels. done by (_;.S. Shi, T. Kloiber and G. Zimmerer, J, having a bound electron around itselE Hence valence Lumin. (in press). Besides the emission bands at 4.1, electrons may have a chance of undergoing radiative 5.6 and 6.4eV, they observed a weak band at 7.0eV decay to the core holes. Such a decay process, how- and attributed it to the same origin as the 5.6 and ever, competes with the direct decay of core excitons: 6.4eV bands. the latter would be a more predominant process than the former, presumably because of the v~-law of the REFERENCES transition probability. From the present results, we I. M. Laval, M. Moszynski, R. Allemand, E. Corcan estimate the binding energy of core excitons in moreche, P. Guinet, R. Odru & J. Vacher, Nucl. BaF, as 1.0 ± 0.2eV. This value is somewhat larger Instrum. Methods 206, 169 (1983). than that of the valence excitons (0.7 eV) [1 I]. It seems 2. Yu.M. Aleksandrov, V.N. Makhov, P.A. Rodnreasonable as a result thai the valence electrons are yi, T.I. Syreishchikova & M.N. Yakimenko, almost entirely on the F ions: thus the Coulomb Soy. P/n's. Solid State 26, 1734 ([984). potential of a hole m core levels of the Ba > ions is 3. Ya.A. Valbis, Z.A. Rachko & Ya.L. Yansons. essentially not screened and attracts the electron J E T P Lett. 42, 172 (1985). 4. S. Kubota, N. Kanai & J. Ruan(Gen), Phys. strongly. Status Solidi (he' 139, 635 (1987). It must be emphasized that both of the valence 5. S. Kubota, H. lto, J. Ruan(Gen) & S. Asaoka, and core bands are completely filled with electrons in Activ. Rep. o f S vn. Rad. Lab. Inst. Solid State the ground state. This fact provides interesting featPhl'.v., Univ. T o k y o (1984) p. 12. ures for the valence electron-core hole luminescence in 0. P.J. Call, W. Hayes & M.N. Kabler, J. Phys. C8, ionic materials. First, a population reversion between L60 (1975). these levels is easily realized when electrons are excited 7. G.W. Rubtoff, Phrv. Rev. BS, 662 (1972). 8. J.P. Albert, C. Jouanin & C. Gout, Phys, Rev. from the core band into the conduction band. so that BI6, 925 f1977). optical amplification may be achieved at any excita9. R.A. Heaton & C.C. Lin, Phys. Rev. B22~ 36_')"c tion density. Furthermore, the holes lifted into the (1980). valence band subsequently recombine with conduc10. N.V. Starostin. M.P. Shepilov & A.B. Alekseev, tion electrons. That is, one photon produces two kinds Phv,s. Status Solidi (b.) 103, 717 ( 1981 ). of luminescent states: valence electron-core hole pairs 11. T. Tomiki & T. Miyata, J. Phys. Soc, Japan 27, and self-trapped excitons. This is clearly seen in Fig. 658 (1969). 12. R.T. Poole, J. Szajman, R.C.G. Leckey, J.G. 2(b): luminescence yield of the slow component rises
526
13. 14.
AUGER-FREE LUMINESCENCE IN BaF2 Jenkin & J. Liesegang, Phys. Rev. BI2, 5872 (1975). A.A. Maiste, A.M.-E, Saar & M.A. l~lango, Soy. Phys. - - Solid State 16, 1118 (1974). E,T. Arakawa & M.W. Williams, Phys. Rev.
15. 16.
Vol. 65, No. 6
Lett. 36, 333 (1976). S. Kubota, J. Ruan(Gen), S. Sakuragi, M. Itoh & S. Hashimoto, J. Lumin (in press). R.T. Williams, M.N. Kabler, W. Hayes & J.P. Scott, Phys. Rev. BI4, 725 (1976).