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Phonon broadening of line widths of Auger-free luminescence in wide-gap ionic crystals
JOURNAL OF
LUMINESCENCE Journal of Luminescence 72-74 (1997)762-764
Phonon broadening of line widths of Auger-free luminescence in wide-gap ionic crystals M. Itoh”**, K. Sawada”, H. Haraa, N. Ohnob, M. Kamadac aFaculty of Engineering, Shinshu Universily, Nagano 380, Japan b Faculty of Engineering,
Osaka Electra-Communication
University. Neyagawa
572, Japan
‘Institute for Molecular Science, Okazaki 444, Japan
Abstract
Spectral shapes of the Auger-free luminescence in RbF, CsF, CsCl, CsBr and BaF2 have been studied in the temperature range between 10 and 300 K with use of a time-resolved technique. The significant contribution of phonon broadening to the line widths is verified for all systems, their temperature dependence being expressed well by the [coth(hw/2kT)]“‘-law. This result is in accordance with the model describing the broadening due to lattice relaxation around a hole generated in the outermost-core bands. Keywords:
Core luminescence; Lattice relaxation; Scintillators
1. Introduction
Phonon-relaxation effects are very important in understanding many luminescence phenomena induced by valence-band excitation in condensed matter. However, this is not always true in the case of core-level excitation, where the core-hole lifetime z is mainly governed by the nonradiative Auger process, and is much shorter than the phononrelaxation time tR. About ten years ago, a specific kind of core luminescence was found in the course of searching fast scintillators [ 1). This luminescence, called Auger-free luminescence (AFL) [2], originates from the radiative transition of valence electrons into
*Corresponding author. Fax: + 81(Japan)-26-223-4031; gipwc.shinshuu.ac.jp.
e-mail: itohlab@
0022-2313/97/$17.00 x> 1997 Elsevier Science B.V. All rights reserved PI1 SOO22-23 13(96)002621
outermost-core holes, in which the Auger decay process is energetically forbidden. The AFL spectra are in general composed of three parts; the main band(s), the low-energy tail and the high-energy structure [3]. It is expected that some phononrelaxation effect will occur for AFL because z 9 zR. However, there has been no clear evidence for such effects. With this in mind, the present work has been undertaken on the temperature dependence of AFL line shapes for RbF, CsF, CsCl, CsBr and BaF, in the range from 10 to 300 K.
2. Experimental
procedure
We used synchrotron radiation from the electron storage ring UVSOR in Okazaki as a light source. The samples were mounted on the copper holder in a variable-temperature cryostat of He-flow type.
163
M. Itoh et al. /Journal of Luminescence 72-74 (1997) 762- 764 0.81
PHOTON
ENERGY
I
I
I
I
I
(eV)
Fig. 1. Auger-free luminescence (AFL) spectra of RbF measured at 10, 175 and 295 K under the core-band excitation with 21.4eV photons. The spectra were taken with use of a timeresolved technique. Each curve has been normalized at the maximum.
The spectra of AFL were separated from those originating in the valence-band excitation by using a time-resolved technique [3].
Fig. 2. Temperature dependence of the full-width at half maximum of the main band of RbF. The solid curve is the best fit of Eq. (1) to the experimental data indicated by open circles.
temperature in Fig. 2. The experimental data can be expressed by the well-known formula [4]: W(T) = W(O)[coth(hw/2kT)]“*,
3. Results and discussion Fig. 1 shows the AFL spectra of RbF measured at three different temperatures under the core-band excitation at 21.4 eV. The spectra consist of an intense band peaking at 5.35 eV, with a hump on the low-energy side and a weak structure on the high-energy side. It is confirmed that the decay time does not change throughout the spectrum. This indicates that the overall structure of Fig. 1 originates from the same initial state with a core hole. The same conclusion has been obtained for all the AFL spectra in CsF, CsCl, CsBr and BaF,. The main band of RbF shows a Gaussian line shape, although such an analysis of the line shape is difficult for other systems because of the large overlap with the low-energy tail. From Fig. 1 we can recognize the thermal broadening of the main band in RbF. The fullwidth at half-maximum is plotted as a function of
(1)
where w is the effective angular frequency of the phonons involved in the relaxation process. From this fit we get o = 4.1 x 1013 s- ’ for RbF, which is somewhat smaller than the LO(I)-phonon frequency. Thermal variation of the AFL line width in other systems is also expressed by Eq. (1). These results strongly suggest that some lattice relaxation is induced around a core hole generated on metal ions before the radiative transition takes place. In Ref. [3] we have measured the time-resolved spectra of AFL at RT, and have explained the existence of the low-energy tail as being due to the lattice relaxation effect. Furthermore, it has been found that the Cl--impurity associated AFL band in mixed CsF 1_ $1, shows a board line width comparable to that of pure CsCl [S], which suggests that the line width of AFL does not depend sensitively on whether the upper filled band is derived from the valence state or the impurity state. The present experiment provides more favorable
764
M. Itoh et al. 1 Journal of Luminescence
evidence for that the lattice relaxation effect plays an important role for the broadening of the AFL bands. As in the case of the photoluminescence from localized electronic states, the configuration coordinate model may be valid for deep understanding of the emitting process of AFL in widegap ionic crystals [6].
Acknowledgements This work was performed under the Joint Studies Program of the Institute for Molecular Science (IMS).
72-74 (1997) 762- 764
References Cl1 M.
Lava], M. Moszynski, R. Allemand, E. Cormoreche, P. Guinet, R. Odru and J. Vacher, Nucl. Instr. and Meth. 206 (1983) 169. VI M. Itoh, S. Kubota, J. Ruan(Gen) and S. Hashimoto, Rev. Solid State Sci. 4 (1990) 467. c31 T. Matsumoto, K. Kan’no, M. Itoh and N. Ohno, J. Phys. Sot. Japan 65 (1996) 1195. M W.B. Fowler, Physics of Color Centers, ed. W.B. Fowler (Academic Press, New York, 1968) ch. 2. 151M. Itoh, H. Hara, N. Ohno, H. Yoshida, S. Hashimoto, K. Kan’no and M. Kamada, UVSOR Activity Report 1995 (IMS, Okazaki, 1996) p. 56. 161Y. Kayanuma and A. Kotani, J. Electron Spectrosc. Relat. Phenom. 79 (1996), 219.
LUMINESCENCE Journal of Luminescence 72-74 (1997)762-764
Phonon broadening of line widths of Auger-free luminescence in wide-gap ionic crystals M. Itoh”**, K. Sawada”, H. Haraa, N. Ohnob, M. Kamadac aFaculty of Engineering, Shinshu Universily, Nagano 380, Japan b Faculty of Engineering,
Osaka Electra-Communication
University. Neyagawa
572, Japan
‘Institute for Molecular Science, Okazaki 444, Japan
Abstract
Spectral shapes of the Auger-free luminescence in RbF, CsF, CsCl, CsBr and BaF2 have been studied in the temperature range between 10 and 300 K with use of a time-resolved technique. The significant contribution of phonon broadening to the line widths is verified for all systems, their temperature dependence being expressed well by the [coth(hw/2kT)]“‘-law. This result is in accordance with the model describing the broadening due to lattice relaxation around a hole generated in the outermost-core bands. Keywords:
Core luminescence; Lattice relaxation; Scintillators
1. Introduction
Phonon-relaxation effects are very important in understanding many luminescence phenomena induced by valence-band excitation in condensed matter. However, this is not always true in the case of core-level excitation, where the core-hole lifetime z is mainly governed by the nonradiative Auger process, and is much shorter than the phononrelaxation time tR. About ten years ago, a specific kind of core luminescence was found in the course of searching fast scintillators [ 1). This luminescence, called Auger-free luminescence (AFL) [2], originates from the radiative transition of valence electrons into
*Corresponding author. Fax: + 81(Japan)-26-223-4031; gipwc.shinshuu.ac.jp.
e-mail: itohlab@
0022-2313/97/$17.00 x> 1997 Elsevier Science B.V. All rights reserved PI1 SOO22-23 13(96)002621
outermost-core holes, in which the Auger decay process is energetically forbidden. The AFL spectra are in general composed of three parts; the main band(s), the low-energy tail and the high-energy structure [3]. It is expected that some phononrelaxation effect will occur for AFL because z 9 zR. However, there has been no clear evidence for such effects. With this in mind, the present work has been undertaken on the temperature dependence of AFL line shapes for RbF, CsF, CsCl, CsBr and BaF, in the range from 10 to 300 K.
2. Experimental
procedure
We used synchrotron radiation from the electron storage ring UVSOR in Okazaki as a light source. The samples were mounted on the copper holder in a variable-temperature cryostat of He-flow type.
163
M. Itoh et al. /Journal of Luminescence 72-74 (1997) 762- 764 0.81
PHOTON
ENERGY
I
I
I
I
I
(eV)
Fig. 1. Auger-free luminescence (AFL) spectra of RbF measured at 10, 175 and 295 K under the core-band excitation with 21.4eV photons. The spectra were taken with use of a timeresolved technique. Each curve has been normalized at the maximum.
The spectra of AFL were separated from those originating in the valence-band excitation by using a time-resolved technique [3].
Fig. 2. Temperature dependence of the full-width at half maximum of the main band of RbF. The solid curve is the best fit of Eq. (1) to the experimental data indicated by open circles.
temperature in Fig. 2. The experimental data can be expressed by the well-known formula [4]: W(T) = W(O)[coth(hw/2kT)]“*,
3. Results and discussion Fig. 1 shows the AFL spectra of RbF measured at three different temperatures under the core-band excitation at 21.4 eV. The spectra consist of an intense band peaking at 5.35 eV, with a hump on the low-energy side and a weak structure on the high-energy side. It is confirmed that the decay time does not change throughout the spectrum. This indicates that the overall structure of Fig. 1 originates from the same initial state with a core hole. The same conclusion has been obtained for all the AFL spectra in CsF, CsCl, CsBr and BaF,. The main band of RbF shows a Gaussian line shape, although such an analysis of the line shape is difficult for other systems because of the large overlap with the low-energy tail. From Fig. 1 we can recognize the thermal broadening of the main band in RbF. The fullwidth at half-maximum is plotted as a function of
(1)
where w is the effective angular frequency of the phonons involved in the relaxation process. From this fit we get o = 4.1 x 1013 s- ’ for RbF, which is somewhat smaller than the LO(I)-phonon frequency. Thermal variation of the AFL line width in other systems is also expressed by Eq. (1). These results strongly suggest that some lattice relaxation is induced around a core hole generated on metal ions before the radiative transition takes place. In Ref. [3] we have measured the time-resolved spectra of AFL at RT, and have explained the existence of the low-energy tail as being due to the lattice relaxation effect. Furthermore, it has been found that the Cl--impurity associated AFL band in mixed CsF 1_ $1, shows a board line width comparable to that of pure CsCl [S], which suggests that the line width of AFL does not depend sensitively on whether the upper filled band is derived from the valence state or the impurity state. The present experiment provides more favorable
764
M. Itoh et al. 1 Journal of Luminescence
evidence for that the lattice relaxation effect plays an important role for the broadening of the AFL bands. As in the case of the photoluminescence from localized electronic states, the configuration coordinate model may be valid for deep understanding of the emitting process of AFL in widegap ionic crystals [6].
Acknowledgements This work was performed under the Joint Studies Program of the Institute for Molecular Science (IMS).
72-74 (1997) 762- 764
References Cl1 M.
Lava], M. Moszynski, R. Allemand, E. Cormoreche, P. Guinet, R. Odru and J. Vacher, Nucl. Instr. and Meth. 206 (1983) 169. VI M. Itoh, S. Kubota, J. Ruan(Gen) and S. Hashimoto, Rev. Solid State Sci. 4 (1990) 467. c31 T. Matsumoto, K. Kan’no, M. Itoh and N. Ohno, J. Phys. Sot. Japan 65 (1996) 1195. M W.B. Fowler, Physics of Color Centers, ed. W.B. Fowler (Academic Press, New York, 1968) ch. 2. 151M. Itoh, H. Hara, N. Ohno, H. Yoshida, S. Hashimoto, K. Kan’no and M. Kamada, UVSOR Activity Report 1995 (IMS, Okazaki, 1996) p. 56. 161Y. Kayanuma and A. Kotani, J. Electron Spectrosc. Relat. Phenom. 79 (1996), 219.