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Emission spectra of excitons created in a single crystal of KBr by irradiation of heavy ions at 4.2 K
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Volume 67A, number 2
I
24 July 1978
PHYSICS LETTERS
EMISSION SPECTRA OF EXCITONS CREATED IN A SINGLE CRYSTAL OF KBr BY IRRADIATION OF HEAVY IONS AT 4.2 K Kazuie KIMURA and Masashi IMAMURA The Institute of Physicaland Chemical Research, Wako, Saitama351, Japan Received 22 March 1978
An intensity ratio of the I-J-to rr-emission higher than that in X-irradiation was observed for a heavy-ion irradiated single crystal of KBr at 4.2 K. The result was interpreted in terms of the interaction of free excitons at higher energy states.
Although energetic heavy ions are currently in increasing use in solid-state physics and radiation chemistry, experimental information regarding the primary physicochemical processes in irradiated matter is still scarce; several theories published so far [ 1 ] are not so elaborate for predicting actual processeseither. Measurement of the emission spectra from the electronic excited states in irradiated matter may be one of the useful experimental approaches to this end. A KBr crystal was chosen in the present study because spectral assignments for the excitons and color centers formed in alkali halides by X-, electron-, and photoirradiation are well established. He-, C-, and N-ions of 8 MeV per nucleon from the cyclotron at the Institute of Physical and Chemical Research were used and, for pulse irradiation, they were chopped into 40 /JS pulses with a repetition of 416 s-l. The beam current was mostly between 0.1 and 1 nA, and between 0.1 and 100 nA for experiments of dose rate effects. Emission spectra were measured under (1) stationary irradiation and under (2) pulse irradiation,followed by strobe measurements to obtain the time-resolved spectra. In the former, a photo-multiplier (Hamamatsu R758) and Optical Multichannel Analyser (OMA; Princeton applied research) were used; in the latter, the gate of the OMA was synchronized with each pulse and the aperture time of the gate was 3 ps. The photomultiplier, a grating monochromator (Ritsu MC-25N) and filters were calibrated using an NBS standard lamp. A single crystal of KBr obtained from Harshaw Chemical Co. was cleaved into
thin plates (10 X 10 X 1 mm3). The crystal was cooled to 4.2 K using a cryostat (Oxford Instruments, CF-240). Emission spectra with peaks at 280 and 512 nm were observed for KBr irradiated with C-ions at 4.2 K (fig. 1). They may be assigned to the emission due to u- and n-excitons which are self-trapped excitons known in X- and electron-irradiated KBr [2,3]. An additional peak other than the u- and n-emissions, reported by Ukai et al. for RbI irradiated at 77 K [4] could not be recognized under the present experimental conditions. The intensity ratio of the u-
Energy/eV 2.0
4.0
WavalanQth
I “m
Fig. 1. Emission spectrum of KBr at 4.2 K under irradiation of 85 MeV, 1 nA C-ions. Intensity scale of the right-hand spectrum is expanded 5 times.
Volume 67A, number 2
PHYSICS LETTERS
to K-emission (Z,,/Zn) was found to be lo-12 which is 5-6 times higher than 2, as reported in electronirradiation [5]. Self-absorption at the absorption edge of the F-center (peak at 600 nm) was negligibly small since the emission spectra were measured in the region where the optical density at 512 nm was less than 0.05. The effect of temperature increase caused by irradiation does not account for the high value of Z,/I,, since Z,, is known to decrease faster than Z, with increasing temperature. In addition, the currents (1 nA) employed were not so high to raise the temperature of the sample significantly. Consequently, the possible mechanisms that may be responsible for the higher value of Z,.,/Zn are: (1) a larger probability of radiationless annihilation of the n-excitons, (2) a larger probability of radiative decay of the a-excitons, and (3) preferential formation of the a-excitons. Both Z, and Z, increased linearly with increasing dose in the low dose range, but in the higher dose range they decreased with increasing dose; the rate of the decrease was larger for Z, than for Z,. Plots of Z,/I, versus doses, in the irradiation with He-, C-, and N-ions, are shown in fig. 2. Fig. 2 shows a distinct LET effect for Z,/I,: 2 for X-rays, 5.8 for He-ions, 10 for C-ions, and 15 for N-ions. It is also shown in fig. 2 that the value of Z,/I, in heavy-ion irradiation is almost constant for doses less than 3000 nC, above this value they decrease. This fact indicates that the u-excitons are deactivated easily by the color centers such as H, H’, and F, more than the n-excitons. Therefore, the value of Z,/I, in heavy-ion irradiation cannot be interpreted in terms of annihilation of the r-excitons
!5-
N IO" _____
24 July 1978
by the centers which may be formed densely by heavyion irradiation. Dense excitation by heavy-ion irradiation would cause interaction between excitons, resulting in a shorter lifetime of the exciton. This interaction may occur to the n-exciton, since its lifetime (0.1 ms at 4.2 K) [5] is much longer than that of the u (l-5 ns at 5 K) [7]. The time-resolved spectra of the n-emission were measured at 0, 1.5 X 10e4, 3.0 X 10-4, and 1.32 X lop3 s after the heavy-ion pulses (fig. 3) and a decay curve was obtained by plotting peak intensities, as shown in fig. 4. Although the decay curve appears nonlinear, this is consistent with the result of Karasawa and Hirai [6] who found that the decay curve consists of a fast and a slow component. The lifetime of the fast component, 0.2 ms in the present experiment, cannot be regarded as being shorter than the value reported by Pooley and Runciman [5] for the electron pulse radiolysis of KBr, 0.1 ms at 4.2 K and Karasawa and Hirai [6] for photoirradiation using an N2-laser, 20 ZJSat 4.2 K. This result leads to the conclusion that the lifetime of the rrexciton is not decreased. Since both u- and n-excitons in alkali halides are self-trapped, collisional interaction between them is not likely to take place. If there is interaction, the excitons may either decay radiatively as known in semiconductors or form excitonic molecules which decay with strong characteristic emissions; neither is the case. Process (2) should be also ruled out because of the self-trapping of
__ -
Ol
102
/
I03 DOSe/nC Fig. 2. Dose dependence of IO/In.
160
,
104
I
500
550
600
Wavelength I "m
Fig. 3. Time-resolved n-emission spectra of a KBr single crystal irradiated with 85 MeV C-ions at 4.2 K.
_/*....
-r-
PHYSICS LETTERS
Volume 67A, number 2
24 July 1978
case of the spontaneous intersystem crossing in X-, electron-, and photo-irradiation. 4
The authors are grateful for helpful discussions with Professor N. Itoh, Professor M. Hirai, and Dr. Karasawa. They would also like to acknowledge technical assistance of Mr. H. Endo and the operation group of the cyclotron. References [ 1] Reviewed by U. Fano, Ann. Rev. Nucl. Sci. 13 (1963) 1; 5
10 SEC x 104
Fig. 4. Decay of Iremission.
the o-excitons. Process (3) is, therefore, most plausible. Recently, Nishimura [8] and Hayashi et al. [9] reported that free excitons exist for a short time in a photo-irradiated single crystal of KI before relaxing into the self-trapped state. In heavy-ion irradiation, the free excitons formed densely in the vicinity of the tracks collide with each other, so that the u- and rr-excitons may be formed in a ratio that differs from the
C. Crispin and G.N. Fowler, Rev. Mod. Phys. 42 (1970) [2] ?Itoh, J. Physique Colloq. 37 (1976) C7-27. [3] M. Hirai, Intern. Conf. on Color centers in ionic crystals (Sendai, 1974) Abstract D36. [4] T. Ukai, N. Matsunami, K. Morita and N. Itoh, Phys. Lett. 56A (1976) 127. [ 51 D. Pooley and W.A. Runciman, J. Phys. C3 (1970) 1815. [6] T. Krasawa and M. Hirai, J. Phys. Sot. Japan 40 (1976) 755, 769. [7] I.M. Blair, D. Pooley and D. Smith, AERE-R6906 (1971). [8] H. Nishimura, J. Phys. Sot. Japan 43 (1977) 157. [ 91 T. Hayashi, T. Ohata and S. Koshino, J. Phys. Sot. Japan 42!1977) 1647.
Volume 67A, number 2
I
24 July 1978
PHYSICS LETTERS
EMISSION SPECTRA OF EXCITONS CREATED IN A SINGLE CRYSTAL OF KBr BY IRRADIATION OF HEAVY IONS AT 4.2 K Kazuie KIMURA and Masashi IMAMURA The Institute of Physicaland Chemical Research, Wako, Saitama351, Japan Received 22 March 1978
An intensity ratio of the I-J-to rr-emission higher than that in X-irradiation was observed for a heavy-ion irradiated single crystal of KBr at 4.2 K. The result was interpreted in terms of the interaction of free excitons at higher energy states.
Although energetic heavy ions are currently in increasing use in solid-state physics and radiation chemistry, experimental information regarding the primary physicochemical processes in irradiated matter is still scarce; several theories published so far [ 1 ] are not so elaborate for predicting actual processeseither. Measurement of the emission spectra from the electronic excited states in irradiated matter may be one of the useful experimental approaches to this end. A KBr crystal was chosen in the present study because spectral assignments for the excitons and color centers formed in alkali halides by X-, electron-, and photoirradiation are well established. He-, C-, and N-ions of 8 MeV per nucleon from the cyclotron at the Institute of Physical and Chemical Research were used and, for pulse irradiation, they were chopped into 40 /JS pulses with a repetition of 416 s-l. The beam current was mostly between 0.1 and 1 nA, and between 0.1 and 100 nA for experiments of dose rate effects. Emission spectra were measured under (1) stationary irradiation and under (2) pulse irradiation,followed by strobe measurements to obtain the time-resolved spectra. In the former, a photo-multiplier (Hamamatsu R758) and Optical Multichannel Analyser (OMA; Princeton applied research) were used; in the latter, the gate of the OMA was synchronized with each pulse and the aperture time of the gate was 3 ps. The photomultiplier, a grating monochromator (Ritsu MC-25N) and filters were calibrated using an NBS standard lamp. A single crystal of KBr obtained from Harshaw Chemical Co. was cleaved into
thin plates (10 X 10 X 1 mm3). The crystal was cooled to 4.2 K using a cryostat (Oxford Instruments, CF-240). Emission spectra with peaks at 280 and 512 nm were observed for KBr irradiated with C-ions at 4.2 K (fig. 1). They may be assigned to the emission due to u- and n-excitons which are self-trapped excitons known in X- and electron-irradiated KBr [2,3]. An additional peak other than the u- and n-emissions, reported by Ukai et al. for RbI irradiated at 77 K [4] could not be recognized under the present experimental conditions. The intensity ratio of the u-
Energy/eV 2.0
4.0
WavalanQth
I “m
Fig. 1. Emission spectrum of KBr at 4.2 K under irradiation of 85 MeV, 1 nA C-ions. Intensity scale of the right-hand spectrum is expanded 5 times.
Volume 67A, number 2
PHYSICS LETTERS
to K-emission (Z,,/Zn) was found to be lo-12 which is 5-6 times higher than 2, as reported in electronirradiation [5]. Self-absorption at the absorption edge of the F-center (peak at 600 nm) was negligibly small since the emission spectra were measured in the region where the optical density at 512 nm was less than 0.05. The effect of temperature increase caused by irradiation does not account for the high value of Z,/I,, since Z,, is known to decrease faster than Z, with increasing temperature. In addition, the currents (1 nA) employed were not so high to raise the temperature of the sample significantly. Consequently, the possible mechanisms that may be responsible for the higher value of Z,.,/Zn are: (1) a larger probability of radiationless annihilation of the n-excitons, (2) a larger probability of radiative decay of the a-excitons, and (3) preferential formation of the a-excitons. Both Z, and Z, increased linearly with increasing dose in the low dose range, but in the higher dose range they decreased with increasing dose; the rate of the decrease was larger for Z, than for Z,. Plots of Z,/I, versus doses, in the irradiation with He-, C-, and N-ions, are shown in fig. 2. Fig. 2 shows a distinct LET effect for Z,/I,: 2 for X-rays, 5.8 for He-ions, 10 for C-ions, and 15 for N-ions. It is also shown in fig. 2 that the value of Z,/I, in heavy-ion irradiation is almost constant for doses less than 3000 nC, above this value they decrease. This fact indicates that the u-excitons are deactivated easily by the color centers such as H, H’, and F, more than the n-excitons. Therefore, the value of Z,/I, in heavy-ion irradiation cannot be interpreted in terms of annihilation of the r-excitons
!5-
N IO" _____
24 July 1978
by the centers which may be formed densely by heavyion irradiation. Dense excitation by heavy-ion irradiation would cause interaction between excitons, resulting in a shorter lifetime of the exciton. This interaction may occur to the n-exciton, since its lifetime (0.1 ms at 4.2 K) [5] is much longer than that of the u (l-5 ns at 5 K) [7]. The time-resolved spectra of the n-emission were measured at 0, 1.5 X 10e4, 3.0 X 10-4, and 1.32 X lop3 s after the heavy-ion pulses (fig. 3) and a decay curve was obtained by plotting peak intensities, as shown in fig. 4. Although the decay curve appears nonlinear, this is consistent with the result of Karasawa and Hirai [6] who found that the decay curve consists of a fast and a slow component. The lifetime of the fast component, 0.2 ms in the present experiment, cannot be regarded as being shorter than the value reported by Pooley and Runciman [5] for the electron pulse radiolysis of KBr, 0.1 ms at 4.2 K and Karasawa and Hirai [6] for photoirradiation using an N2-laser, 20 ZJSat 4.2 K. This result leads to the conclusion that the lifetime of the rrexciton is not decreased. Since both u- and n-excitons in alkali halides are self-trapped, collisional interaction between them is not likely to take place. If there is interaction, the excitons may either decay radiatively as known in semiconductors or form excitonic molecules which decay with strong characteristic emissions; neither is the case. Process (2) should be also ruled out because of the self-trapping of
__ -
Ol
102
/
I03 DOSe/nC Fig. 2. Dose dependence of IO/In.
160
,
104
I
500
550
600
Wavelength I "m
Fig. 3. Time-resolved n-emission spectra of a KBr single crystal irradiated with 85 MeV C-ions at 4.2 K.
_/*....
-r-
PHYSICS LETTERS
Volume 67A, number 2
24 July 1978
case of the spontaneous intersystem crossing in X-, electron-, and photo-irradiation. 4
The authors are grateful for helpful discussions with Professor N. Itoh, Professor M. Hirai, and Dr. Karasawa. They would also like to acknowledge technical assistance of Mr. H. Endo and the operation group of the cyclotron. References [ 1] Reviewed by U. Fano, Ann. Rev. Nucl. Sci. 13 (1963) 1; 5
10 SEC x 104
Fig. 4. Decay of Iremission.
the o-excitons. Process (3) is, therefore, most plausible. Recently, Nishimura [8] and Hayashi et al. [9] reported that free excitons exist for a short time in a photo-irradiated single crystal of KI before relaxing into the self-trapped state. In heavy-ion irradiation, the free excitons formed densely in the vicinity of the tracks collide with each other, so that the u- and rr-excitons may be formed in a ratio that differs from the
C. Crispin and G.N. Fowler, Rev. Mod. Phys. 42 (1970) [2] ?Itoh, J. Physique Colloq. 37 (1976) C7-27. [3] M. Hirai, Intern. Conf. on Color centers in ionic crystals (Sendai, 1974) Abstract D36. [4] T. Ukai, N. Matsunami, K. Morita and N. Itoh, Phys. Lett. 56A (1976) 127. [ 51 D. Pooley and W.A. Runciman, J. Phys. C3 (1970) 1815. [6] T. Krasawa and M. Hirai, J. Phys. Sot. Japan 40 (1976) 755, 769. [7] I.M. Blair, D. Pooley and D. Smith, AERE-R6906 (1971). [8] H. Nishimura, J. Phys. Sot. Japan 43 (1977) 157. [ 91 T. Hayashi, T. Ohata and S. Koshino, J. Phys. Sot. Japan 42!1977) 1647.