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Threshold of F-center formation of alkali halide crystals by two-photon absorption
Volume 60, number 5
OPTICS COMMUNICATIONS
1 December 1986
THRESHOLD OF F-CENTER FORMATION OF ALKALI HALIDE CRYSTALS BY T W O - P H O T O N A B S O R P T I O N
C.H. C H E N and M.P. M c C A N N ~ Chemical Ph],sics Section, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Received 19 August 1986
Thresholds of F-center formations for various alkali halide crystals with accuracy better than 5 meV are first reported by twophoton absorption process of a tunable dye laser.
It is known that F-centers can be p r o d u c e d in most alkali halide crystals by X-ray excitation, v a c u u m ultraviolet ( V U V ) excitation, or electron excitation [ 1 - 3 ] . The mechanism involves the p r o m o t i o n o f electrons from the valence b a n d to the conduction band. When an electron-hole recombines, an F-center can be produced [4]. F-centers can also be produced by the decay o f excitons [5]. Since exciton states often lie close to the conduction band, it is not easy to measure the threshold o f the conduction band by a one-photon process. F-centers can also be p r o d u c e d by a two-photon absorption process [6]. In this work, a wavelength tunable N d - Y A G p u m p e d dye laser was used to obtain the first precise m e a s u r e m e n t o f the thresholds for F-center formation in various alkali halide crystals at r o o m temperature. F o r a typical experiment, the laser energy density is fixed a r o u n d 4 mJ/cm 2 with a 10 ns pulse length. The color pattern of an F-center can be easily observed by the naked eye. When the energy o f two laser p h o t o n s is smaller than the threshold energy, the F-center color patterns disappear completely. Thus, the accuracy o f the threshold of F-center formation is only limited by the b a n d w i d t h o f the laser used. The laser b a n d w i d t h easily can be better than 0.001 eV for a commercially available laser. Since the two-photon transition to the first excitation b a n d is f o r b i d d e n due to parity consideration, the confusion due to exciton bands can be Graduate Student, University of Tennessee, Knoxville, Tennessee, USA 296
Table 1 Numerical values of threshold of F-center formation of alkali halide crystals Crystal
Threshold (eV)"
Band gap (eV) b
Color of F-center pattern
NaCI KCI NaBr KBr Nal KI KCI(TI)
7.739 + 0.002 7.501 +_0.002 6.704 +_0.002 6.928 +_0.004 5.352 + 0.001 5.667 _+0.001 7.431 +_0.002
8.2 8.1 6.8 6.9 5.6 5.8
yellow purple purple blue blue green purple
a) From this work. ~ Estimated from ref. [7]. eliminated. The numerical values o f energy thresholds for F-center production in various crystals o b t a i n e d from this work are shown in table 1. These results are somewhat lower than the energy gap estim a t e d from threshold o f one-photon V U V absorption spectra. This difference can be attributed to e l e c t r o n - p h o n o n interaction. If this m e t h o d is applied to crystals which are cooled down close to 0 K, the b a n d gaps for alkali halide crystals can be d e t e r m i n e d very accurately ( > 0.0001 eV). D u e to the color pattern o f F-centers, most alkali halide crystals can be used to m o n i t o r the b e a m distribution o f ultraviolet ( U V ) lasers. F r o m table 1, a laser b e a m with a wavelength shorter than 463.3 n m can be m o n i t o r e d by two-photon absorption o f alkali halide crystals. Since it is a two-photon process, alkali halide crystals can be used to m o n i t o r laser beams in three d i m e n s i o n s because o f the small percentage o f 0 030-401/86/$03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D i v i s i o n )
Volume 60, number 5
OPTICS COMMUNICATIONS
a b s o r p t i o n o f the laser p h o t o n s . By d o p i n g with extra alkali m e t a l atoms, the n u m b e r o f F - c e n t e r s produced c a n be increased. Thus, it c a n be very c o n v e n i e n t to use t h e m as U V laser b e a m m o n i t o r s [ 8 ]. We acknowledge the very helpful d i s c u s s i o n with J.C. Wang. Research s p o n s o r e d by the Office o f H e a l t h a n d E n v i r o n m e n t a l Research, U.S. D e p a r t m e n t o f Energy u n d e r c o n t r a c t D E - A G 0 5 8 4 O R 2 1 4 0 0 with M a r t i n M a r i e t t a Energy Systems, Inc.
1 December 1986
References [ 1] S. Wakita and M. Ueta, J. Phys. Soc. Jpn. 19 (i964) 924. [ 2 ] J.N. Bradford, R.T. Williams and W.L. Faust, Phys. Rev. Lett. 35 (1975) 300. [ 3 ] Y. Suzuki, M. Okumura and M. Hirai, J. Phys. Soc. Jpn. 41 (1979) 184. [4] H.N. Hersch, Phys. Rev. 148 (1965) 928. [ 5 ] R.S. Knox, Theory ofexcitons, Solid State Phys. Suppl., Vol. 5 (Academic Press, New York, 1963). [6] J.J. Hopfield and J.M. Worlock, Phys. Rev. A 137 (1965) 1455. [7] J.E. Eby, K.J. Teegarden and D.B. Button, Phys. Rev. 116 (1959) 1099. [8] M.P. McCann, C.H. Chen and S.D. Kramer, Optical Engineering, to be published.
297
OPTICS COMMUNICATIONS
1 December 1986
THRESHOLD OF F-CENTER FORMATION OF ALKALI HALIDE CRYSTALS BY T W O - P H O T O N A B S O R P T I O N
C.H. C H E N and M.P. M c C A N N ~ Chemical Ph],sics Section, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Received 19 August 1986
Thresholds of F-center formations for various alkali halide crystals with accuracy better than 5 meV are first reported by twophoton absorption process of a tunable dye laser.
It is known that F-centers can be p r o d u c e d in most alkali halide crystals by X-ray excitation, v a c u u m ultraviolet ( V U V ) excitation, or electron excitation [ 1 - 3 ] . The mechanism involves the p r o m o t i o n o f electrons from the valence b a n d to the conduction band. When an electron-hole recombines, an F-center can be produced [4]. F-centers can also be produced by the decay o f excitons [5]. Since exciton states often lie close to the conduction band, it is not easy to measure the threshold o f the conduction band by a one-photon process. F-centers can also be p r o d u c e d by a two-photon absorption process [6]. In this work, a wavelength tunable N d - Y A G p u m p e d dye laser was used to obtain the first precise m e a s u r e m e n t o f the thresholds for F-center formation in various alkali halide crystals at r o o m temperature. F o r a typical experiment, the laser energy density is fixed a r o u n d 4 mJ/cm 2 with a 10 ns pulse length. The color pattern of an F-center can be easily observed by the naked eye. When the energy o f two laser p h o t o n s is smaller than the threshold energy, the F-center color patterns disappear completely. Thus, the accuracy o f the threshold of F-center formation is only limited by the b a n d w i d t h o f the laser used. The laser b a n d w i d t h easily can be better than 0.001 eV for a commercially available laser. Since the two-photon transition to the first excitation b a n d is f o r b i d d e n due to parity consideration, the confusion due to exciton bands can be Graduate Student, University of Tennessee, Knoxville, Tennessee, USA 296
Table 1 Numerical values of threshold of F-center formation of alkali halide crystals Crystal
Threshold (eV)"
Band gap (eV) b
Color of F-center pattern
NaCI KCI NaBr KBr Nal KI KCI(TI)
7.739 + 0.002 7.501 +_0.002 6.704 +_0.002 6.928 +_0.004 5.352 + 0.001 5.667 _+0.001 7.431 +_0.002
8.2 8.1 6.8 6.9 5.6 5.8
yellow purple purple blue blue green purple
a) From this work. ~ Estimated from ref. [7]. eliminated. The numerical values o f energy thresholds for F-center production in various crystals o b t a i n e d from this work are shown in table 1. These results are somewhat lower than the energy gap estim a t e d from threshold o f one-photon V U V absorption spectra. This difference can be attributed to e l e c t r o n - p h o n o n interaction. If this m e t h o d is applied to crystals which are cooled down close to 0 K, the b a n d gaps for alkali halide crystals can be d e t e r m i n e d very accurately ( > 0.0001 eV). D u e to the color pattern o f F-centers, most alkali halide crystals can be used to m o n i t o r the b e a m distribution o f ultraviolet ( U V ) lasers. F r o m table 1, a laser b e a m with a wavelength shorter than 463.3 n m can be m o n i t o r e d by two-photon absorption o f alkali halide crystals. Since it is a two-photon process, alkali halide crystals can be used to m o n i t o r laser beams in three d i m e n s i o n s because o f the small percentage o f 0 030-401/86/$03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D i v i s i o n )
Volume 60, number 5
OPTICS COMMUNICATIONS
a b s o r p t i o n o f the laser p h o t o n s . By d o p i n g with extra alkali m e t a l atoms, the n u m b e r o f F - c e n t e r s produced c a n be increased. Thus, it c a n be very c o n v e n i e n t to use t h e m as U V laser b e a m m o n i t o r s [ 8 ]. We acknowledge the very helpful d i s c u s s i o n with J.C. Wang. Research s p o n s o r e d by the Office o f H e a l t h a n d E n v i r o n m e n t a l Research, U.S. D e p a r t m e n t o f Energy u n d e r c o n t r a c t D E - A G 0 5 8 4 O R 2 1 4 0 0 with M a r t i n M a r i e t t a Energy Systems, Inc.
1 December 1986
References [ 1] S. Wakita and M. Ueta, J. Phys. Soc. Jpn. 19 (i964) 924. [ 2 ] J.N. Bradford, R.T. Williams and W.L. Faust, Phys. Rev. Lett. 35 (1975) 300. [ 3 ] Y. Suzuki, M. Okumura and M. Hirai, J. Phys. Soc. Jpn. 41 (1979) 184. [4] H.N. Hersch, Phys. Rev. 148 (1965) 928. [ 5 ] R.S. Knox, Theory ofexcitons, Solid State Phys. Suppl., Vol. 5 (Academic Press, New York, 1963). [6] J.J. Hopfield and J.M. Worlock, Phys. Rev. A 137 (1965) 1455. [7] J.E. Eby, K.J. Teegarden and D.B. Button, Phys. Rev. 116 (1959) 1099. [8] M.P. McCann, C.H. Chen and S.D. Kramer, Optical Engineering, to be published.
297