- Email: [email protected]
On photoacoustic spectra of X-irradiated NaCl:Y crystals
Volume 156, number 5
On photoacoustic
PHYSICS LETTERS A
17 June 1991
spectra of X-irradiated NaCl : Y crystals
Ch. Gopal Reddy, K. Narasimha Reddy, M. Mutha Reddy, K. Rama Reddy Department ofPhysics, Osmania University, Hyderabad 500 007, Andhra Pradesh, India
and Ch. Mohan Rao Centerfor Cellular and Molecular Biology, Hyderabad 500 007, Andhra Pradesh, India
Received 20 September 1990; accepted for publication 14 April 199 1 Communicated by J.I. Budnick
A correlation between the photoacoustic (PA) and optical absorption (OA) spectra of X-irradiated NaCl:Y crystals is reported. The F-band spectra observed through PA spectroscopy is found to be sharper than that of OA spectroscopy. This sharpening is attributed to the reduced probability of nonradiative decay processes of F-centres over the radiative processes.Two bands observed in the UV region of both the spectra are attributed to the Y + impurity transitions in the host matrix.
Due to renewed interest in the colour centre lasers and to understand the ground state electronic structure of yttrium impurity in NaCl crystals, an attempt has been made to investigate the optical properties of NaCl : Y crystals through thermoluminescence (TL), optical absorption (OA) and photoacoustic spectroscopy (PAS). The photoacoustic F-centre band in alkali halides is known to be broader than their optical absorption band [ 11. This broadening was attributed to the effect of periodic thermal expansion of the sample [2] as suggested by McDonald and Wetsel [ 3 1. Later Da Silva and Pizzarro [ 41 attributed this broadening to the presence of K-band. The present work is a report of photoacaustic and optical absorption spectra of X-irradiated yttrium-doped NaCl samples where a narrowing, and not broadening, of the F-band in PA spectra is observed. The samples used in the present investigation were grown from melt as follows: Analar grade NaCl powder was intimately mixed with 1 mol% of yttrium chloride and the mixture was heated in a crucible to melt and the melt was carefully cooled (at a cooling rate of 10°C per hour) form crystals. The crystals thus obtained were cleaved into pieces of required
size and annealed at 600°C for 5 h and brought to room temperature (RT). The powder sample was prepared by mechanical crushing of these crystals. Before recording the spectra, all the samples were irradiated at RT for 30 min with X-rays eminated from a copper target operated at 30 kV and 10 mA. The OA spectrum of X-irradiated single crystal of NaCl : Y was recorded at RT by using a Becman DU6 spectrophotometer. The PA spectra at RT were obtained by using the EDTs (OAS 400 model) PA spectrometer to which several modifications were incorporated [ 5 1. Light beam from a source (300 W Xe arc lamp) is intensity-modulated by using a mechanical chopper, whose chopping frequency is fixed at 20 Hz. This beam is monochromated and passed onto the sample cell to which a microphone is attached. Microphone signal, through a pre-amplifier (EG&G 113 ) and a lock-in-amplifier (EG&G 5206) is sent to a microcomputer (IBM PC AT) over an interface through A/D converter. The microcomputer controls the spectrometer, processes the data and plots the spectrum. Normalization of the PA spectra to constant input light intensity was achieved by using the PA spectrum of carbon black. Fig. 1 shows the OA spectrum of X-irradiated NaCl
0375-9601/91/$ 03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
253
Volume 156, number
0
01 220
PHYSICS
5
I
1
I
320
520
420 WAVELENGTH
Fig. 1. Optical absorption gle crystal.
o-o
250
300
spectrum
350
WAVELENGTH
620
(nm)
of X-irradiated
NaCl: Y sin-
I
400
450
-0.0
500
lnm)
Fig. 2. Photoacoustic spectra of X-irradiated crystal and (2) microcrystalline powder.
NaCl: Y; ( 1) single
single crystal containing yttrium, whereas those in fig. 2 are the PA spectra of the same sample in the form of single crystal (curve 1 ) and microcrystalline powder (curve 2 ). The OA spectrum (fig. 1) shows three bands peaking at 255,273 and 465 nm. The 465 nm band is the well known F-band [ 6 1, whereas the two UV bands at 255 and 273 nm are attributed to yttrium impurity in the NaCl host lattice [ 71, whose electronic states originate from the atomic states of Y+ given as 5s2( ‘So) and 5s’5p’( ‘Pi, 3P2,1,0). These bands are tentatively assigned to iS,+‘P, transition of Y+ ions in X-irradiated host. The two bands may be due to the crystal field splitting of ‘Pi excited state. In the PA spectrum (fig. 2 ) of NaCl : Y single crystal, three bands occur around 255,270 and 468 nm. However, in powdered sample only two bands have been ob254
LETTERS A
17 June 1991
served, one broad band at 270 nm and another at 468 nm. Comparing PA and OA spectra, it is clear that the 468 nm band corresponds to F-centres and the 255 and 270 nm bands to yttrium impurity. By observing both the PA and OA spectra of NaCl:Y, one can notice that the band width of Fband in PA spectra is much less than that in the OA spectrum, which is contrary to the earlier reports [ 1,2,4]. This sharpening of F-band in PA spectra may be explained as follows: In the case of optical absorption, the spectrum represents the total absorption by the sample giving rise to optical excitation, and it will not give any information about the subsequent decay processes. On the other hand, PA spectrum is due to the nonradiative decay process that occurs in a system after it has been optically excited. Therefore, during the decay, if nonradiative de-excitation dominates over the radiative transition, one expects the PA spectrum to closely correspond with OA spectrum [ 81. The observed sharpening of the F-band in the present case may, therefore, be attributed to the reduced nonradiative de-excitation probability of the F-centre decay. From this, it can be concluded that the decay probability of F-centres in the present system by means of fluorescence or phosphorescence (radiative decay) is more than that by heat producing process (nonradiative decay). From fig. 2, it is seen that, in the PA spectra of powdered sample, the impurity bands of the UV region are broadened to appear like a single broad band. This could be due to the effect of lattice distortions introduced during the mechanical crushing of the single crystal into a powdered sample. Further investigations are needed to correlate the microsize dependence of this distortion broadening through PA spectroscopy. Ch.G.R. wishes to thank U.G.C., New Delhi for awarding a research fellowship. This work was also partially supported by D.S.T., Govt. of India through a research project.
Volume
156, number
5
PHYSICS
References [ 1] K. Inabe, S. Nakamura
and N. Takeuchi, J. Phys. Sot. Japan 53 (1984) 1621. [ 21 T. Miyanaga, S. Nakamura, T. Ohgaku and N. Takeuchi, J. Phys.D18(1985)L113. [ 131 F.A. McDonald and G.C. Wetsel Jr., J. Appl. Phys. 49 (1978) 2313.
LETTERS
A
17 June 1991
[4] L.F. Da Silva and J. Pizarro, J. Phys. Sot. Japan 56 (1987) 835. [ 51Ch. Mohan Rao, J. Instrum. Sot. India 17 ( 1987) 2 19. [6] F. Seitz, Rev. Mod. Phys. 18 (1946) 384. [ 71 Ch. Gopal Reddy, Ph.D. Thesis, Osmania University, India (1989). [8] A. Rosencwaig, Photoacoustics and photoacoustic spectroscopy (Wiley, New York, 1980).
255
On photoacoustic
PHYSICS LETTERS A
17 June 1991
spectra of X-irradiated NaCl : Y crystals
Ch. Gopal Reddy, K. Narasimha Reddy, M. Mutha Reddy, K. Rama Reddy Department ofPhysics, Osmania University, Hyderabad 500 007, Andhra Pradesh, India
and Ch. Mohan Rao Centerfor Cellular and Molecular Biology, Hyderabad 500 007, Andhra Pradesh, India
Received 20 September 1990; accepted for publication 14 April 199 1 Communicated by J.I. Budnick
A correlation between the photoacoustic (PA) and optical absorption (OA) spectra of X-irradiated NaCl:Y crystals is reported. The F-band spectra observed through PA spectroscopy is found to be sharper than that of OA spectroscopy. This sharpening is attributed to the reduced probability of nonradiative decay processes of F-centres over the radiative processes.Two bands observed in the UV region of both the spectra are attributed to the Y + impurity transitions in the host matrix.
Due to renewed interest in the colour centre lasers and to understand the ground state electronic structure of yttrium impurity in NaCl crystals, an attempt has been made to investigate the optical properties of NaCl : Y crystals through thermoluminescence (TL), optical absorption (OA) and photoacoustic spectroscopy (PAS). The photoacoustic F-centre band in alkali halides is known to be broader than their optical absorption band [ 11. This broadening was attributed to the effect of periodic thermal expansion of the sample [2] as suggested by McDonald and Wetsel [ 3 1. Later Da Silva and Pizzarro [ 41 attributed this broadening to the presence of K-band. The present work is a report of photoacaustic and optical absorption spectra of X-irradiated yttrium-doped NaCl samples where a narrowing, and not broadening, of the F-band in PA spectra is observed. The samples used in the present investigation were grown from melt as follows: Analar grade NaCl powder was intimately mixed with 1 mol% of yttrium chloride and the mixture was heated in a crucible to melt and the melt was carefully cooled (at a cooling rate of 10°C per hour) form crystals. The crystals thus obtained were cleaved into pieces of required
size and annealed at 600°C for 5 h and brought to room temperature (RT). The powder sample was prepared by mechanical crushing of these crystals. Before recording the spectra, all the samples were irradiated at RT for 30 min with X-rays eminated from a copper target operated at 30 kV and 10 mA. The OA spectrum of X-irradiated single crystal of NaCl : Y was recorded at RT by using a Becman DU6 spectrophotometer. The PA spectra at RT were obtained by using the EDTs (OAS 400 model) PA spectrometer to which several modifications were incorporated [ 5 1. Light beam from a source (300 W Xe arc lamp) is intensity-modulated by using a mechanical chopper, whose chopping frequency is fixed at 20 Hz. This beam is monochromated and passed onto the sample cell to which a microphone is attached. Microphone signal, through a pre-amplifier (EG&G 113 ) and a lock-in-amplifier (EG&G 5206) is sent to a microcomputer (IBM PC AT) over an interface through A/D converter. The microcomputer controls the spectrometer, processes the data and plots the spectrum. Normalization of the PA spectra to constant input light intensity was achieved by using the PA spectrum of carbon black. Fig. 1 shows the OA spectrum of X-irradiated NaCl
0375-9601/91/$ 03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
253
Volume 156, number
0
01 220
PHYSICS
5
I
1
I
320
520
420 WAVELENGTH
Fig. 1. Optical absorption gle crystal.
o-o
250
300
spectrum
350
WAVELENGTH
620
(nm)
of X-irradiated
NaCl: Y sin-
I
400
450
-0.0
500
lnm)
Fig. 2. Photoacoustic spectra of X-irradiated crystal and (2) microcrystalline powder.
NaCl: Y; ( 1) single
single crystal containing yttrium, whereas those in fig. 2 are the PA spectra of the same sample in the form of single crystal (curve 1 ) and microcrystalline powder (curve 2 ). The OA spectrum (fig. 1) shows three bands peaking at 255,273 and 465 nm. The 465 nm band is the well known F-band [ 6 1, whereas the two UV bands at 255 and 273 nm are attributed to yttrium impurity in the NaCl host lattice [ 71, whose electronic states originate from the atomic states of Y+ given as 5s2( ‘So) and 5s’5p’( ‘Pi, 3P2,1,0). These bands are tentatively assigned to iS,+‘P, transition of Y+ ions in X-irradiated host. The two bands may be due to the crystal field splitting of ‘Pi excited state. In the PA spectrum (fig. 2 ) of NaCl : Y single crystal, three bands occur around 255,270 and 468 nm. However, in powdered sample only two bands have been ob254
LETTERS A
17 June 1991
served, one broad band at 270 nm and another at 468 nm. Comparing PA and OA spectra, it is clear that the 468 nm band corresponds to F-centres and the 255 and 270 nm bands to yttrium impurity. By observing both the PA and OA spectra of NaCl:Y, one can notice that the band width of Fband in PA spectra is much less than that in the OA spectrum, which is contrary to the earlier reports [ 1,2,4]. This sharpening of F-band in PA spectra may be explained as follows: In the case of optical absorption, the spectrum represents the total absorption by the sample giving rise to optical excitation, and it will not give any information about the subsequent decay processes. On the other hand, PA spectrum is due to the nonradiative decay process that occurs in a system after it has been optically excited. Therefore, during the decay, if nonradiative de-excitation dominates over the radiative transition, one expects the PA spectrum to closely correspond with OA spectrum [ 81. The observed sharpening of the F-band in the present case may, therefore, be attributed to the reduced nonradiative de-excitation probability of the F-centre decay. From this, it can be concluded that the decay probability of F-centres in the present system by means of fluorescence or phosphorescence (radiative decay) is more than that by heat producing process (nonradiative decay). From fig. 2, it is seen that, in the PA spectra of powdered sample, the impurity bands of the UV region are broadened to appear like a single broad band. This could be due to the effect of lattice distortions introduced during the mechanical crushing of the single crystal into a powdered sample. Further investigations are needed to correlate the microsize dependence of this distortion broadening through PA spectroscopy. Ch.G.R. wishes to thank U.G.C., New Delhi for awarding a research fellowship. This work was also partially supported by D.S.T., Govt. of India through a research project.
Volume
156, number
5
PHYSICS
References [ 1] K. Inabe, S. Nakamura
and N. Takeuchi, J. Phys. Sot. Japan 53 (1984) 1621. [ 21 T. Miyanaga, S. Nakamura, T. Ohgaku and N. Takeuchi, J. Phys.D18(1985)L113. [ 131 F.A. McDonald and G.C. Wetsel Jr., J. Appl. Phys. 49 (1978) 2313.
LETTERS
A
17 June 1991
[4] L.F. Da Silva and J. Pizarro, J. Phys. Sot. Japan 56 (1987) 835. [ 51Ch. Mohan Rao, J. Instrum. Sot. India 17 ( 1987) 2 19. [6] F. Seitz, Rev. Mod. Phys. 18 (1946) 384. [ 71 Ch. Gopal Reddy, Ph.D. Thesis, Osmania University, India (1989). [8] A. Rosencwaig, Photoacoustics and photoacoustic spectroscopy (Wiley, New York, 1980).
255