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UV absorption edge shift by doping alkali fluorides in fluoroaluminate glass
Journal of Physics and Chemistry of Solids 63 (2002) 691±694
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UV absorption edge shift by doping alkali ¯uorides in ¯uoroaluminate glass N. Kitamura a,*, K. Fukumi a, J. Nishii a, M. Makihara a, T. Sasaki b, N. Ohno b a
Department of Optical Materials, Osaka National Research Institute, AIST, 1-8-31 Midorigaoka, Ikeda-shi, Osaka 563-8577, Japan b Osaka Electro-Communication University, 18-8 Hatcho, Neyagawa, Osaka 572-8530, Japan Received 13 October 2000; received in revised form 27 July 2001; accepted 24 August 2001
Abstract The effect on transmittance and re¯ectance in the vacuum ultraviolet (VUV) region by doping alkali ¯uorides (LiF, NaF and KF) has been investigated in a ternary ¯uoroaluminate (18BaF2 ±37CaF2 ±45AlF3) glass. The absorption edge of the glass obeys the Urbach rule and was shifted monotonically towards higher energies by increasing the concentration of each alkali ¯uoride. The VUV re¯ection peak at 11.3 eV was not sensitive to the change of the concentration of dopants. The magnitude of the edge shift was 10±20 times larger than that expected from the additive law on the magnitude of absorption coef®cients of the glass and alkali ¯uorides. An excitonic interaction similar to that observed in mixed crystals of alkali halides is suggested from the monotonic shift toward the absorption edges of the dopants. The weak sensitivity of the re¯ection peak upon doping supports that the excitonic state lies predominantly around the edge energies. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Glasses; A. Optical materials; D. Optical properties
1. Introduction Fluoride glasses consisting of wide gap ¯uorides are of great interest as bulk or thin ®lm optical materials utilizing vacuum ultraviolet (VUV) light. Over the last 20 years, ¯uoroaluminate glasses have been studied in terms of the glass formation [1,2], glass structure [3±5], and optical and chemical properties [6±10]. We have found that ternary AlF3-based glass shows an excellent transmittance in the VUV region [8], while ZrF4-/HfF4-based glasses give a cutoff above 200 nm [6,10]. The absorption edge of the ¯uoride glass is understood to have an excitonic nature because of its high-energy shift upon cooling [8], from the analogy with alkali halide crystals [11±13]. It is well-known that mixed crystals of alkali halides give a variety of absorption energies by interactions between excitons of each halide [14±17]. Therefore, it is expected that the addition of another ¯uoride in the glass causes an interaction between excitonic states of the glass and additives, which leads to an energy shift of the absorption edge similarly to mixed * Corresponding author. Tel.: 181-727-519647; fax: 181-727519637. E-mail address: [email protected] (N. Kitamura).
crystals. In the present paper, we dealt with a ternary ¯uoroaluminate (18BaF2 ±37CaF2 ±45AlF3) glass and have measured absorption and re¯ection spectra of the glass doped with alkali ¯uorides (LiF, NaF and KF) in the VUV region. Effects on absorption edge and on re¯ection band in the VUV region by doping alkali ¯uorides are discussed.
2. Experimental 18BaF2 ±37CaF2 ±45AlF3 glass was prepared by a conventional liquid-quench method. AlF3, BaF2 and CaF2 (High Purity Chemical Co., 99.9%) were used as starting materials. Ammonium bi¯uoride was used as ¯uorinating agent. Preparation of the glass has been described in detail elsewhere [8]. Five and 10 mol% of LiF or NaF (High Purity Chemical Co., 99.9%), and 2.5 and 5 mol% of KF (High Purity Chemical Co., 99.9%) were mixed into the mixture of starting materials. The glass plate, which had a thickness of about 1.0 mm, was polished for optical measurements. Transmittance and re¯ectance of the samples were measured at the beam line BL-7B and BL-1B in the ultraviolet synchrotron orbital radiation (UVSOR) facility of the Institute for Molecular Science, Okazaki, Japan. In the
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(01)00215-3
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N. Kitamura et al. / Journal of Physics and Chemistry of Solids 63 (2002) 691±694
Fig. 1. Absorption spectra of 18BaF2 ±37CaF2 ±45AlF3 (BCA) glass and (a) LiF-doped, (b) NaF-doped and (c) KF-doped BCA glasses at 300 K. Broken lines indicate the spectra at 100 K.
re¯ectance measurement, the incident angle was 108 off the normal incidence. These measurements were performed at 300 and 100 K with an energy resolution of 0.03 eV. Absorption coef®cient was determined from absorption spectra after the correction for re¯ection loss.
3. Results Fig. 1(a)±(c) shows absorption spectra of 18BaF2 ± 37CaF2 ±45AlF3 (BCA) glass and LiF-, NaF- and KF-doped
BCA glasses at 300 and 100 K. Absorption edges were observed in the region above 7.5 eV. The spectral pro®le and the absorption coef®cient in this region were independent of the surface condition of samples as well as the glass preparation. On the other hand, the spectral pro®le below 7.5 eV depended on both of them. The absorption edge of all the glasses followed exponential function in relation to photon energy. The edge energies at 100 K were higher than those at 300 K by about 0.1 eV for all the glasses as shown by broken lines in the same ®gures. The absorption edge shifted toward higher energies with
N. Kitamura et al. / Journal of Physics and Chemistry of Solids 63 (2002) 691±694
693
were observed at around 11.3 and 13.0 eV. Exact peak position at around 11.3 eV was obtained by ®tting a Gaussian curve to the experimental re¯ection spectra. The position shifted toward higher energies by about 0.1 eV at most with the 10 mol% doping of LiF. No shift was found in the peak position within the NaF- and KF-doped glasses. 4. Discussion
Fig. 2. Absorption edge positions (a 20 cm 21) of LiF-, NaF- and KF-doped BCA glasses at 300 K against the concentration of alkali ¯uorides.
increasing concentrations of LiF, NaF and KF. The absorption edge energies of the glasses at 300 K are plotted in Fig. 2 against the concentration of alkali ¯uorides. In this ®gure, the photon energy at which the absorption coef®cient is equal to 20 cm 21 was adopted as the energy of absorption edge. The edge energy increased monotonically with increasing amount of dopant and increased in the order of LiF . KF . NaF. Fig. 3 shows re¯ection spectra of BCA glass and 5 and 10 mol% LiF-doped BCA glasses. Peaks
Fig. 3. Re¯ection spectra of 18BaF2 ±37CaF2 ±45AlF3 (BCA) glass and LiF-doped BCA glasses at 300 K.
Absorption edges were observed above 7.5 eV in all the glasses as shown in Fig. 1. The spectral pro®le above 7.5 eV was reproducible and independent of the surface condition and glass batch, indicating that the absorption edges observed in this experiment are intrinsic to the LiF-, KFand NaF-doped BCA glasses. The absorption edge behaved in an exponential manner and shifted toward higher energies upon cooling. Therefore, this edge should be the Urbach fundamental absorption edge [13], that is, this edge has an excitonic nature. Spectral pro®le in the region below 7.5 eV depended on the surface condition and glass batch. Since absorption below 7.5 eV is greatly affected by extrinsic factors, such as transition-metal impurities [18], defects, surface damage and so on, we will not discuss this further. Two peaks were found at 11.3 and 13.0 eV in the re¯ection spectra for all the glasses as shown in Fig. 3. The re¯ection peak, which is assigned to the electronic transition in bridging ¯uoride ions in the octahedral structure unit [AlF6] 32, was observed at 11.2 eV in ¯uoride± phosphate glass [19]. Since the 11.3 eV peak of the present glasses is close to the 11.2 eV peak, the 11.3 eV peak is attributed to an electric transition, which may have an excitonic nature, in bridging ¯uoride ions in the structure unit. On the other hand, it is known that a peak due to nonbridging ¯uoride ions is present at energies lower than the peak due to bridging ¯uoride ions. For example, absorption peaks due to non-bridging ¯uoride ions terminated by Ca 21 and Ba 21 ions in CaF2 and BaF2 crystals, which show excitonic behavior, appear at around 11.1 [12,20] and 10.3 eV [12,21], respectively. However, peaks or shoulders due to the non-bridging ¯uoride ions were not observed at energies lower than the peak at 11.3 eV in this study, even though non-bridging ¯uoride ions were present in the glasses. The energy of the electric transition in the nonbridging ¯uoride ions might be distributed widely around the low energy side of the peak at 11.3 eV owing to the disordered glassy structure. Therefore, the absorption in energies lower than the peak at 11.3 eV consists of the electronic transitions in bridging and non-bridging ¯uoride ions having the excitonic nature. The re¯ection peak at 11.3 eV shifted slightly toward higher energies with the increase of LiF concentration as shown in Fig. 3. Although the absorption edge also shows a high-energy shift by the LiF doping, the magnitude of the shift of the absorption edge is over four times larger than that of the re¯ection peak in the LiF-doped glass. Moreover, no shift was found in the
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N. Kitamura et al. / Journal of Physics and Chemistry of Solids 63 (2002) 691±694
11.3 eV peak for NaF- or KF-doped BCA glasses. Therefore, the shift of the 11.3 eV peak is not directly responsible for the absorption edge shift in this glass system. The absorption edge shift by doping is mainly due to a change of electronic state at the low energy side of the peak at 11.3 eV. Assuming that the additive law is valid for the absorption coef®cient of the glass and the dopants, the edge shift by doping is expected to be only 0.01 eV at most. Since the high-energy shift of the absorption edge observed is over ten times larger than the expected value, the absorption edge shift in the glass cannot be explained by a simple mixing of alkali ¯uorides into the glass. Absorption edge energies at which the absorption coef®cient becomes 20 cm 21 for LiF, NaF and KF single crystals were estimated to be 11.9, 9.0 and 8.6 eV respectively from their optical constants [12,22±25] and were plotted at a concentration of full substitution as shown in Fig. 2. The energy increased monotonically towards the edge energy of each alkali ¯uoride. Peak energy of the exciton absorption band in mixed crystals varies monotonically with the ratio of two components as is well-known as a persistent type behavior in mixed crystals of alkali halide RbCl±KCl [14] and KBr±KCl [16,17] and as an amalgamation type behavior in KCl±NaCl [14] and KCl±KI [15]. The shift due to the change in composition is not explained by a simple mixing of each exciton band but by the interaction between two excitonic states in the mixed crystals. Since it was found that the electronic state has an excitonic nature around absorption edges in the glasses, and that the edge energy is changed monotonically with a change in composition of the glasses, as mentioned above, it was deduced that the absorption edge was shifted by interaction between two excitonic states of the base glasses and dopants from the analogy of mixed crystals. In the present study, no speci®c exciton bands were observed in either the absorption or re¯ection spectra of the glasses. Therefore, it was impossible to determine which type of interaction occurred in the glasses. It should be noted, however, that the magnitude of the energy shift was increased in the order of LiF . KF . NaF in the glasses, when the concentration increased, as shown in Fig. 2. This suggests that the excitonic interaction in KF-doped glass is different from that in LiF- and NaF-doped glasses. 5. Conclusions The absorption edge of the ternary ¯uoroaluminate glass shifted toward higher energies by doping with LiF, NaF or KF. The re¯ection peak which is assigned to an electronic transition of the bridging ¯uoride ion in [AlF6] 32 was found at around 11.3 eV for all glasses. The absorption edge showed an excitonic nature that is explained by the Urbach fundamental absorption edge. The absorption edge energy increased monotonically with the increase of concentration of dopants toward the absorption edge energy of the dopants. The magnitude of the shift of re¯ection peak at
11.3 eV is far smaller than that of the absorption edge due to the doping. An excitonic interaction similar to that observed in mixed crystals of alkali halides is deduced from the large monotonic shift toward the absorption edges of the dopants.
Acknowledgements The authors are indebted to Professor T. Kinoshita, Mr M. Hasumoto and the other staff of the UVSOR facility for their support in this experiment under the Joint Studies Program of the Institute for Molecular Science.
References [1] M. Imaoka, Yogyo-Kyokai-shi 62 (1954) 24. [2] J.J. Videau, J. Portier, B. Piriou, Rev. Chim. Min. 16 (1979) 393. [3] Y. Kawamoto, A. Kono, J. Non-Cryst. Solids 85 (1986) 335. [4] D. Ehrt, C. Erdmann, W. Vogel, Z. Chem. 23 (1983) 37. [5] D. Ehrt, W. Vogel, Z. Chem. 23 (1983) 111. [6] T. Izumitani, S. Hirota, K. Tanaka, H. Onuki, J. Non-Cryst. Solids 86 (1986) 361. [7] D. Ehrt, M. Krauû, C. Erdmann, W. Vogel, Z. Chem. 22 (1982) 315. [8] N. Kitamura, J. Hayakawa, H. Yamashita, J. Non-Cryst. Solids 126 (1990) 155. [9] A.B. Seddon, in: A.E. Comyns (Ed.), Fluoride Glasses, Wiley, Chichester, 1989, p. 158. [10] R.N. Brown, B. Bendow, M.G. Drexhage, C.T. Moynihan, Appl. Opt. 21 (1982) 361. [11] J.E. Eby, K.J. Teegarden, D.B. Dutton, Phys. Rev. 116 (1959) 1099. [12] A.H. Laufer, J.A. Pirog, J.R. McNesby, J. Opt. Soc. Amer. 55 (1965) 64. [13] F. Urbach, Phys. Rev. 92 (1953) 1324. [14] M. Watanabe, Y. Nakamura, Y. Nakai, T. Murata, J. Phys. Soc. Jpn. 26 (1969) 1014. [15] N. Nagasawa, H. Nakagawa, Y. Nakai, J. Phys. Soc. Jpn. 24 (1968) 1403. [16] H. Mahr, Phys. Rev. 122 (1961) 1464. [17] T. Murata, Y. Nakai, J. Phys. Soc. Jpn. 23 (1967) 904. [18] Y. Ohishi, S. Mitachi, T. Kanamori, T. Manabe, Phys. Chem. Glasses 24 (1983) 135. [19] T. Izumitani, S. Hirota, J. Non-Cryst. Solids 73 (1985) 255. [20] D.F. Bezuidenhout, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, 1991, p. 815. [21] M.E. Thomas, W.J. Tropf, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, 1991, p. 683. [22] I. Ohlidal, K. Navratil, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, 1991, p. 1021. [23] T. Tomiki, T. Miyata, H. Tsukamoto, J. Phys. Soc. Jpn. (1973) 35495. [24] D.M. Roessler, W.C. Walker, J. Opt. Soc. Am. 57 (1967) 835. [25] T. Tomiki, T. Miyata, J. Phys. Soc. Jpn. 27 (1969) 658.
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UV absorption edge shift by doping alkali ¯uorides in ¯uoroaluminate glass N. Kitamura a,*, K. Fukumi a, J. Nishii a, M. Makihara a, T. Sasaki b, N. Ohno b a
Department of Optical Materials, Osaka National Research Institute, AIST, 1-8-31 Midorigaoka, Ikeda-shi, Osaka 563-8577, Japan b Osaka Electro-Communication University, 18-8 Hatcho, Neyagawa, Osaka 572-8530, Japan Received 13 October 2000; received in revised form 27 July 2001; accepted 24 August 2001
Abstract The effect on transmittance and re¯ectance in the vacuum ultraviolet (VUV) region by doping alkali ¯uorides (LiF, NaF and KF) has been investigated in a ternary ¯uoroaluminate (18BaF2 ±37CaF2 ±45AlF3) glass. The absorption edge of the glass obeys the Urbach rule and was shifted monotonically towards higher energies by increasing the concentration of each alkali ¯uoride. The VUV re¯ection peak at 11.3 eV was not sensitive to the change of the concentration of dopants. The magnitude of the edge shift was 10±20 times larger than that expected from the additive law on the magnitude of absorption coef®cients of the glass and alkali ¯uorides. An excitonic interaction similar to that observed in mixed crystals of alkali halides is suggested from the monotonic shift toward the absorption edges of the dopants. The weak sensitivity of the re¯ection peak upon doping supports that the excitonic state lies predominantly around the edge energies. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Glasses; A. Optical materials; D. Optical properties
1. Introduction Fluoride glasses consisting of wide gap ¯uorides are of great interest as bulk or thin ®lm optical materials utilizing vacuum ultraviolet (VUV) light. Over the last 20 years, ¯uoroaluminate glasses have been studied in terms of the glass formation [1,2], glass structure [3±5], and optical and chemical properties [6±10]. We have found that ternary AlF3-based glass shows an excellent transmittance in the VUV region [8], while ZrF4-/HfF4-based glasses give a cutoff above 200 nm [6,10]. The absorption edge of the ¯uoride glass is understood to have an excitonic nature because of its high-energy shift upon cooling [8], from the analogy with alkali halide crystals [11±13]. It is well-known that mixed crystals of alkali halides give a variety of absorption energies by interactions between excitons of each halide [14±17]. Therefore, it is expected that the addition of another ¯uoride in the glass causes an interaction between excitonic states of the glass and additives, which leads to an energy shift of the absorption edge similarly to mixed * Corresponding author. Tel.: 181-727-519647; fax: 181-727519637. E-mail address: [email protected] (N. Kitamura).
crystals. In the present paper, we dealt with a ternary ¯uoroaluminate (18BaF2 ±37CaF2 ±45AlF3) glass and have measured absorption and re¯ection spectra of the glass doped with alkali ¯uorides (LiF, NaF and KF) in the VUV region. Effects on absorption edge and on re¯ection band in the VUV region by doping alkali ¯uorides are discussed.
2. Experimental 18BaF2 ±37CaF2 ±45AlF3 glass was prepared by a conventional liquid-quench method. AlF3, BaF2 and CaF2 (High Purity Chemical Co., 99.9%) were used as starting materials. Ammonium bi¯uoride was used as ¯uorinating agent. Preparation of the glass has been described in detail elsewhere [8]. Five and 10 mol% of LiF or NaF (High Purity Chemical Co., 99.9%), and 2.5 and 5 mol% of KF (High Purity Chemical Co., 99.9%) were mixed into the mixture of starting materials. The glass plate, which had a thickness of about 1.0 mm, was polished for optical measurements. Transmittance and re¯ectance of the samples were measured at the beam line BL-7B and BL-1B in the ultraviolet synchrotron orbital radiation (UVSOR) facility of the Institute for Molecular Science, Okazaki, Japan. In the
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(01)00215-3
692
N. Kitamura et al. / Journal of Physics and Chemistry of Solids 63 (2002) 691±694
Fig. 1. Absorption spectra of 18BaF2 ±37CaF2 ±45AlF3 (BCA) glass and (a) LiF-doped, (b) NaF-doped and (c) KF-doped BCA glasses at 300 K. Broken lines indicate the spectra at 100 K.
re¯ectance measurement, the incident angle was 108 off the normal incidence. These measurements were performed at 300 and 100 K with an energy resolution of 0.03 eV. Absorption coef®cient was determined from absorption spectra after the correction for re¯ection loss.
3. Results Fig. 1(a)±(c) shows absorption spectra of 18BaF2 ± 37CaF2 ±45AlF3 (BCA) glass and LiF-, NaF- and KF-doped
BCA glasses at 300 and 100 K. Absorption edges were observed in the region above 7.5 eV. The spectral pro®le and the absorption coef®cient in this region were independent of the surface condition of samples as well as the glass preparation. On the other hand, the spectral pro®le below 7.5 eV depended on both of them. The absorption edge of all the glasses followed exponential function in relation to photon energy. The edge energies at 100 K were higher than those at 300 K by about 0.1 eV for all the glasses as shown by broken lines in the same ®gures. The absorption edge shifted toward higher energies with
N. Kitamura et al. / Journal of Physics and Chemistry of Solids 63 (2002) 691±694
693
were observed at around 11.3 and 13.0 eV. Exact peak position at around 11.3 eV was obtained by ®tting a Gaussian curve to the experimental re¯ection spectra. The position shifted toward higher energies by about 0.1 eV at most with the 10 mol% doping of LiF. No shift was found in the peak position within the NaF- and KF-doped glasses. 4. Discussion
Fig. 2. Absorption edge positions (a 20 cm 21) of LiF-, NaF- and KF-doped BCA glasses at 300 K against the concentration of alkali ¯uorides.
increasing concentrations of LiF, NaF and KF. The absorption edge energies of the glasses at 300 K are plotted in Fig. 2 against the concentration of alkali ¯uorides. In this ®gure, the photon energy at which the absorption coef®cient is equal to 20 cm 21 was adopted as the energy of absorption edge. The edge energy increased monotonically with increasing amount of dopant and increased in the order of LiF . KF . NaF. Fig. 3 shows re¯ection spectra of BCA glass and 5 and 10 mol% LiF-doped BCA glasses. Peaks
Fig. 3. Re¯ection spectra of 18BaF2 ±37CaF2 ±45AlF3 (BCA) glass and LiF-doped BCA glasses at 300 K.
Absorption edges were observed above 7.5 eV in all the glasses as shown in Fig. 1. The spectral pro®le above 7.5 eV was reproducible and independent of the surface condition and glass batch, indicating that the absorption edges observed in this experiment are intrinsic to the LiF-, KFand NaF-doped BCA glasses. The absorption edge behaved in an exponential manner and shifted toward higher energies upon cooling. Therefore, this edge should be the Urbach fundamental absorption edge [13], that is, this edge has an excitonic nature. Spectral pro®le in the region below 7.5 eV depended on the surface condition and glass batch. Since absorption below 7.5 eV is greatly affected by extrinsic factors, such as transition-metal impurities [18], defects, surface damage and so on, we will not discuss this further. Two peaks were found at 11.3 and 13.0 eV in the re¯ection spectra for all the glasses as shown in Fig. 3. The re¯ection peak, which is assigned to the electronic transition in bridging ¯uoride ions in the octahedral structure unit [AlF6] 32, was observed at 11.2 eV in ¯uoride± phosphate glass [19]. Since the 11.3 eV peak of the present glasses is close to the 11.2 eV peak, the 11.3 eV peak is attributed to an electric transition, which may have an excitonic nature, in bridging ¯uoride ions in the structure unit. On the other hand, it is known that a peak due to nonbridging ¯uoride ions is present at energies lower than the peak due to bridging ¯uoride ions. For example, absorption peaks due to non-bridging ¯uoride ions terminated by Ca 21 and Ba 21 ions in CaF2 and BaF2 crystals, which show excitonic behavior, appear at around 11.1 [12,20] and 10.3 eV [12,21], respectively. However, peaks or shoulders due to the non-bridging ¯uoride ions were not observed at energies lower than the peak at 11.3 eV in this study, even though non-bridging ¯uoride ions were present in the glasses. The energy of the electric transition in the nonbridging ¯uoride ions might be distributed widely around the low energy side of the peak at 11.3 eV owing to the disordered glassy structure. Therefore, the absorption in energies lower than the peak at 11.3 eV consists of the electronic transitions in bridging and non-bridging ¯uoride ions having the excitonic nature. The re¯ection peak at 11.3 eV shifted slightly toward higher energies with the increase of LiF concentration as shown in Fig. 3. Although the absorption edge also shows a high-energy shift by the LiF doping, the magnitude of the shift of the absorption edge is over four times larger than that of the re¯ection peak in the LiF-doped glass. Moreover, no shift was found in the
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N. Kitamura et al. / Journal of Physics and Chemistry of Solids 63 (2002) 691±694
11.3 eV peak for NaF- or KF-doped BCA glasses. Therefore, the shift of the 11.3 eV peak is not directly responsible for the absorption edge shift in this glass system. The absorption edge shift by doping is mainly due to a change of electronic state at the low energy side of the peak at 11.3 eV. Assuming that the additive law is valid for the absorption coef®cient of the glass and the dopants, the edge shift by doping is expected to be only 0.01 eV at most. Since the high-energy shift of the absorption edge observed is over ten times larger than the expected value, the absorption edge shift in the glass cannot be explained by a simple mixing of alkali ¯uorides into the glass. Absorption edge energies at which the absorption coef®cient becomes 20 cm 21 for LiF, NaF and KF single crystals were estimated to be 11.9, 9.0 and 8.6 eV respectively from their optical constants [12,22±25] and were plotted at a concentration of full substitution as shown in Fig. 2. The energy increased monotonically towards the edge energy of each alkali ¯uoride. Peak energy of the exciton absorption band in mixed crystals varies monotonically with the ratio of two components as is well-known as a persistent type behavior in mixed crystals of alkali halide RbCl±KCl [14] and KBr±KCl [16,17] and as an amalgamation type behavior in KCl±NaCl [14] and KCl±KI [15]. The shift due to the change in composition is not explained by a simple mixing of each exciton band but by the interaction between two excitonic states in the mixed crystals. Since it was found that the electronic state has an excitonic nature around absorption edges in the glasses, and that the edge energy is changed monotonically with a change in composition of the glasses, as mentioned above, it was deduced that the absorption edge was shifted by interaction between two excitonic states of the base glasses and dopants from the analogy of mixed crystals. In the present study, no speci®c exciton bands were observed in either the absorption or re¯ection spectra of the glasses. Therefore, it was impossible to determine which type of interaction occurred in the glasses. It should be noted, however, that the magnitude of the energy shift was increased in the order of LiF . KF . NaF in the glasses, when the concentration increased, as shown in Fig. 2. This suggests that the excitonic interaction in KF-doped glass is different from that in LiF- and NaF-doped glasses. 5. Conclusions The absorption edge of the ternary ¯uoroaluminate glass shifted toward higher energies by doping with LiF, NaF or KF. The re¯ection peak which is assigned to an electronic transition of the bridging ¯uoride ion in [AlF6] 32 was found at around 11.3 eV for all glasses. The absorption edge showed an excitonic nature that is explained by the Urbach fundamental absorption edge. The absorption edge energy increased monotonically with the increase of concentration of dopants toward the absorption edge energy of the dopants. The magnitude of the shift of re¯ection peak at
11.3 eV is far smaller than that of the absorption edge due to the doping. An excitonic interaction similar to that observed in mixed crystals of alkali halides is deduced from the large monotonic shift toward the absorption edges of the dopants.
Acknowledgements The authors are indebted to Professor T. Kinoshita, Mr M. Hasumoto and the other staff of the UVSOR facility for their support in this experiment under the Joint Studies Program of the Institute for Molecular Science.
References [1] M. Imaoka, Yogyo-Kyokai-shi 62 (1954) 24. [2] J.J. Videau, J. Portier, B. Piriou, Rev. Chim. Min. 16 (1979) 393. [3] Y. Kawamoto, A. Kono, J. Non-Cryst. Solids 85 (1986) 335. [4] D. Ehrt, C. Erdmann, W. Vogel, Z. Chem. 23 (1983) 37. [5] D. Ehrt, W. Vogel, Z. Chem. 23 (1983) 111. [6] T. Izumitani, S. Hirota, K. Tanaka, H. Onuki, J. Non-Cryst. Solids 86 (1986) 361. [7] D. Ehrt, M. Krauû, C. Erdmann, W. Vogel, Z. Chem. 22 (1982) 315. [8] N. Kitamura, J. Hayakawa, H. Yamashita, J. Non-Cryst. Solids 126 (1990) 155. [9] A.B. Seddon, in: A.E. Comyns (Ed.), Fluoride Glasses, Wiley, Chichester, 1989, p. 158. [10] R.N. Brown, B. Bendow, M.G. Drexhage, C.T. Moynihan, Appl. Opt. 21 (1982) 361. [11] J.E. Eby, K.J. Teegarden, D.B. Dutton, Phys. Rev. 116 (1959) 1099. [12] A.H. Laufer, J.A. Pirog, J.R. McNesby, J. Opt. Soc. Amer. 55 (1965) 64. [13] F. Urbach, Phys. Rev. 92 (1953) 1324. [14] M. Watanabe, Y. Nakamura, Y. Nakai, T. Murata, J. Phys. Soc. Jpn. 26 (1969) 1014. [15] N. Nagasawa, H. Nakagawa, Y. Nakai, J. Phys. Soc. Jpn. 24 (1968) 1403. [16] H. Mahr, Phys. Rev. 122 (1961) 1464. [17] T. Murata, Y. Nakai, J. Phys. Soc. Jpn. 23 (1967) 904. [18] Y. Ohishi, S. Mitachi, T. Kanamori, T. Manabe, Phys. Chem. Glasses 24 (1983) 135. [19] T. Izumitani, S. Hirota, J. Non-Cryst. Solids 73 (1985) 255. [20] D.F. Bezuidenhout, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, 1991, p. 815. [21] M.E. Thomas, W.J. Tropf, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, 1991, p. 683. [22] I. Ohlidal, K. Navratil, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, Boston, 1991, p. 1021. [23] T. Tomiki, T. Miyata, H. Tsukamoto, J. Phys. Soc. Jpn. (1973) 35495. [24] D.M. Roessler, W.C. Walker, J. Opt. Soc. Am. 57 (1967) 835. [25] T. Tomiki, T. Miyata, J. Phys. Soc. Jpn. 27 (1969) 658.