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Initial stage of sodium cluster formation in NaCl after intense electron pulse irradiation
Nuclear Instruments and Methods in Physics Research B 91 (1994) 201-204 North-Holland
NOMB
Beam Interactions with Materials &Atoms
Initial stage of sodium cluster formation in NaCl after intense electron pulse irradiation K. Inabe * and N. Takeucbi Faculty
ofTechnology,
Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920, Japan
S. Owaki University of Osaka Prefecture, 4-804 Mozu-ume-cho, Sakad, Osaka 591, Japan
Optical absorption spectra of electron-irradiated NaCl after consecutive heating steps are measured. Thermal annealing of the localized electron centers and growth of the band (peaking at 2.10 eV> due to sodium clusters are examined using computer analysis of the optical absorption spectrum. It has revealed that the sodium clusters may be nucleated from the N, centers, and that the 2.10 eV band is initially composed of two peaks appearing around 1.70 and 2.30 eV, respectively. The oscillation mode of metal aggregates on the basis of dipole-dipole interaction suggests that these two peaks are attributed to the surface plasma oscillation of planar aggregates being formed at the initial stage.
1. Introduction Formation of sodium clusters in NaCl is characterized by an optical absorption band peaking around 2.10 eV, which is interpreted as the energy absorption due to the surface plasma resonance ]1,2]. The free electron effect of the 2.10 eV band is, in fact, revealed by the transient observation indicating that no saturation of the band occurs even under nanosecond pulse laser excitation; i.e., the Iifetime of excited states of the centers responsible for the 2.10 eV band is so short that the electron population is very small even under the intense excitation [3]. Then it is interesting to know what size of the clusters reveals the free electron effect. Positron annihilation measurements have also done to confirm the free electron effect of sodium clusters since free electrons are expected to reduce the lifetime of positrons. Recently, we have shown that the sodium clusters can be formed in electron-irradiated NaCl single crystal during heat treatment at high temperatures [4]. This provides a unique method to investigate, in particular, the initial stage of cluster formation since the optical absorption band is easily available to trace the color centers responsible for sodium clusters. In this paper,
* Corresponding author. Tel. + 81762 612101 (ext. 3931, fax +81 762 64 1047.
we examine annihilation of localized electron centers and sodium cluster formation at the initial stage by analyzing the optical absorption spectrum of NaCl after intense electron pulse irradiation.
2. Experimental Specimens (5 x 5 X 0.5 mm3) were obtained by cleaving a single crystal block of NaCl purchased from the Nihon Kesshou Kougaku Co., Japan. Irradiation was carried out using a high current electron pulse (energy: 20 MeV; pulse width: 1.5 ps; peak current: more than 240 ti, repetition: 2 Hz) from the electron linear accelerator Qinac) of the Institute of Scientific and Industrial Research‘ @SIR), Osaka University. The absorbed dose at the specimen position, estimated using a chemical dosemeter, was about 100 Gy/pulse. The specimen, wrapped with aluminum foil, was placed on an aluminum plate which was cooled by water and located at the electron beam. The temperature of the specimen during irradiation was less than 300 K. The optical absorption spectrum was measured using a Hitachi U-2000 spectrophotometer attaching a home made cryostat with a controller available for the temperature range from 77 to 500 K. Thus, thermal annealing at high temperatures (more than 400 K) and the measurements at low temperatures (100 K) could be repeated without changing the specimen position.
0168-583X/94/~07.~ 0 1994 - Elsevier Science B.V. All rights reserved &WI 0168583X(931ElOZO-M
III. HALIDES
K Inabe et al./Nucl. Instr. and Meth. in Phys.Res. B 91 (1994) 201-204
202
Wavelength
76 1111I 9
5 I
2.0 Energy
3.1. Thermal annealing of F and F-aggregates
(nm)
4xl00 I
I
3.0 (eV)
Fig. 1. Optical absorption spectra of NaCl after electron irradiation of 2 X lo6 Gy and subsequent heating at the temperatures shown for 1 min and cooled down to 100 K within 10 mm. The specimen thickness is 0.380 mm and each spectrum is observed at lOOK.
3. Results and discussion Fig. 1 shows several representative absorption spectra of NaCl after electron pulse irradiation at room temperature (RT) and subsequent heat treatment at high temperatures. The F and M(F,)-band intensities cannot be determined since their peak heights exceed the detecting limit of log(l,/l) around 2.5. The spectrum at lower energy side of the F-band was resolved into several Gaussian bands, F, M(F,), R(F,),, R,, N(F&, and N,-bands, respectively, using a personal computer (NEC PC-9801DX). Here, the F-(F’)-band was not considered since measurements were carried out at high temperatures where the F--band was annealed out. Gaussian peak analysis was performed using known peak energies, leaving the values of widths and intensities as parameters, which were determined by comparing the calculated curve with data. Fig. 2 shows a calculated curve and several components resolved from the spectrum observed. It should be noted that we must assume the new bands peaking around 1.70 and 2.30 eV, showing by dashed curves, in order to get the best fit with experimental data. Although these bands are located at almost the same energy as the M and R-bands, the band widths are broader and annealing temperatures are higher compared with those of the M and R-bands. We show later that these bands grow in intensity and approach the X-band (2.10 eV) with increase of the annealing temperature. Thus, they seem to correspond to the sodium clusters at their initial stage.
The thermal annealing behavior of several bands of electron centers was examined by plotting the peak height of each component (Gaussian band) and is shown in Fig. 3. It is seen that R,, R,, and N,-bands are annealed out below 425 K where the X-band starts to grow. In particular, the R, and R,-bands are likely to be masked by the broad bands growing at the same wavelength range. At this temperature, however, no decrease of the N,-band is observed, and the decrease of both M and N,-bands occurs at higher temperatures corresponding to the increase of the X-bands. This indicates that the M and N,-bands are closely associated with formation of the X-band, though it is difficult to obtain a strict correlation between the decrease of these bands and the increase of the X-band because of ambiguity due to the recombination loss of these electron centers. It is noted that the N, center is believed to be a planar aggregate of four F centers at the nearest neighbor site in (111) plane [5,6]. 3.2. Two peaks of the X-band The absorption spectrum resolved into several Gaussian peaks has revealed that two broad bands are needed to get the best fit with experimental data (Fig. 2). These two bands are likely to correspond to the initial stage of sodium cluster formation, since these bands are found to grow into a single peak around 2.10
Wavelength
9
76
(nm)
5
4xl00
0 1.0
2.0 Energy
3.0
4.0
(eV)
Fig. 2. Representative absorption spectrum resolved into several Gaussian bands of localized electron centers. A dotdashed line: sum of the Gaussian bands. Solid line: spectrum observed at 100 K for NaCl after electron irradiation of 2~ lo6 Gy and subsequent heating at 423 K for 1 min. New bands are shown by the dashed lines.
IL Inabe et al. /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 201-204
ters of sodium metal depending on wavelength cannot explain the presence of two prominent peaks. When metal particles are small, however, it is known that the peak position of the extinction band of the Mie theory is identical with that of the surface plasma resonance [2]. In the following, we try to examine the peak positions of the bands on the basis of the surface plasma resonance. The metal sphere replaced by a point dipole is represented by an oscillator with the surface plasma frequency [Sl,
. -3
rM
2,o-
350
.
450
400
Temperature (K) Fig. 3. Temperature dependence of the peak height of individual band corresponding to the localized electron centers. Peak positions of the two X-bands are shown in the figure.
eV with increasing annealing temperatures as shown in Fig. 4. Although the optical absorption band due to small metal particles is usually explained using the Mie theory [7], the theory that utilizes the optical parame-
Wavelength
9 llllI
76 !
I
4iXlOO
I
I
1 NO-Q ,....?!::::p’... 2 Q-413K
A
By substituting the dielectric constant of the medium (NaCl), es = 1.40, the high frequency dielectric constant of sodium, E, = 5.70 and the bulk plasma frequency, up = 6.0 eV, we obtain ws = 2.06 eV which agrees approximately with the peak energy observed. A doubly peaked absorption spectrum is expected when two dipoles are assumed to interact with each other [8-101. Then, we tentatively suppose a small aggregate of the N, centers as the dipoles inducing the doubly peaked spectrum. Using the two-point-dipole model, Inoue and Ohtaka have analyzed the doubly peaked absorption qualitatively [8]. When the dipoles oscillate parallel to the sphere axis, the optically active mode is given by 1 - 2( a/o>”
tit=,;
1 - 2A( a/D)”
’
(2)
where, a is the radius of metal sphere replaced by a point dipole and D is the distance between two dipoles. When the dipoles oscillate perpendicular to the sphere axis, the active mode is given by a frequency
3 Q-4233 Q.433K 0.4433 Q.453K
4.0 (eV)
Fig. 4. Growth of the X-band with annealing temperatures. Arrows in the figure correspond to the peaks obtained from Eqs. (2) and (4). Solid and open circles denote peak positions of closely packed planar aggregates and linear quadruplet, respectively, according to the results of Kreibig and Genzel
ml.
(1)
(nm)
5
Energy
6l,/gTTg.
W, =
OF
.
0
203
2
_
WI-%
2
1+
(am3
1 +A( a/D)”
’
(4)
By substituting (D/a) = 2.10 (minimum value of D) and the parameters used above, we obtain w,, = 0.942w,, and w L = l.O4w,, which are marked by the arrows in Fig. 4. Separation of two peaks by Eqs. (2) and (4) is, in this case, smaller than that of the peaks observed at the initial stage of the X-band growth. Clippe et al. have calculated the splitting of the single resonance peak into several eigenmodes of the aggregates [9]. Kreibig and Genzel have applied their results to gold particle systems, and have also shown doubly peaked absorption with various separation which depends on geometrical form of aggregation [lo]. Applying their results to sodium particles, i.e., using the surface plasma frequency of sodium (w, = 2.06 eV) instead of gold (ws = 2.28 eV), we found that closely III. HALIDES
204
K. Inabe et al. /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 201-204
packed planar aggregates give the best fit result to the peak separation observed. The obtained absorption peaks are shown by the solid circles in Fig. 4. The approach of the two peaks with growing the bands seems to suggest that the planar aggregates may initially grow along one direction preferentially, since the absorption peaks are, then, considered to approach the linearly aggregated dipole oscillation mode [lo], which is shown by the open circles in Fig. 4. Planar aggregates at the initial stage of sodium cluster formation are deduced only from the peak positions of two absorption bands but the following reports may support the result. Observation using an electron microscope suggests that the centers responsible for the X-band are attributed to plate shaped defects elongating parallel to one of the (100) plane [ll]. Planar aggregates are also comparable with the result of the small-angle X-ray scattering, which indicates the flat shaped CuCl microcrystalline (less than 10 nm in size) being formed in NaCl doped heavily with Cuf ions [12].
Acknowledgements
We wish to thank the members of the machine group at the Radiation Laboratory, ISIR, Osaka Uni-
versity for their help in electron irradiation using the linac. A part of this work is supported by a Grant-in-Aid for scientific research (C) from the Ministry of Education, Science and Culture, Japan.
References [ll F. Fujimoto and K. Komaki, J. Phys. Sot. Jpn. 25 (1968) 1679. Dl U. Kreibig and P. Zacharias, Z. Phys. 231 (1970) 128. 131K. Inabe, M. Adachi and N. Takeuchi, Jpn. J. Appl. Phys. 32 (1993) Part 2, L1429. 141K. Inabe, S. Owaki and N. Takeuchi, Jpn. J. Appl. Phys. 28 (1989) L2242. [51 J.H. Schuhnan and W.D. Compton, Color Centers in Solids (Pergamon, Oxford, 1963) p. 160. t61 A.E. Hughes, Proc. Phys. Sot. 87 (1966) 535. [71 G. Mie, Ann. Phys. 25 (1908) 377. [81 M. Inoue and K. Ohtaka, J. Phys. Sot. Jpn. 52 (1983) 3853. 191 P. Clippe, R. Evard and A.A. Lucas, Phys. Rev. B 14 (1976) 1715. [lOI U. Kreibig and L. Genzel, Surf. Sci. 156 (1985) 678. [ill K. Izumi, J. Phys. Sot. Jpn. 26 (1969) 1451. [121 T. Itoh, Y. Iwabuchi and M. Kataoka, Phys. Status Solidi B 145 (1988) 567.
NOMB
Beam Interactions with Materials &Atoms
Initial stage of sodium cluster formation in NaCl after intense electron pulse irradiation K. Inabe * and N. Takeucbi Faculty
ofTechnology,
Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920, Japan
S. Owaki University of Osaka Prefecture, 4-804 Mozu-ume-cho, Sakad, Osaka 591, Japan
Optical absorption spectra of electron-irradiated NaCl after consecutive heating steps are measured. Thermal annealing of the localized electron centers and growth of the band (peaking at 2.10 eV> due to sodium clusters are examined using computer analysis of the optical absorption spectrum. It has revealed that the sodium clusters may be nucleated from the N, centers, and that the 2.10 eV band is initially composed of two peaks appearing around 1.70 and 2.30 eV, respectively. The oscillation mode of metal aggregates on the basis of dipole-dipole interaction suggests that these two peaks are attributed to the surface plasma oscillation of planar aggregates being formed at the initial stage.
1. Introduction Formation of sodium clusters in NaCl is characterized by an optical absorption band peaking around 2.10 eV, which is interpreted as the energy absorption due to the surface plasma resonance ]1,2]. The free electron effect of the 2.10 eV band is, in fact, revealed by the transient observation indicating that no saturation of the band occurs even under nanosecond pulse laser excitation; i.e., the Iifetime of excited states of the centers responsible for the 2.10 eV band is so short that the electron population is very small even under the intense excitation [3]. Then it is interesting to know what size of the clusters reveals the free electron effect. Positron annihilation measurements have also done to confirm the free electron effect of sodium clusters since free electrons are expected to reduce the lifetime of positrons. Recently, we have shown that the sodium clusters can be formed in electron-irradiated NaCl single crystal during heat treatment at high temperatures [4]. This provides a unique method to investigate, in particular, the initial stage of cluster formation since the optical absorption band is easily available to trace the color centers responsible for sodium clusters. In this paper,
* Corresponding author. Tel. + 81762 612101 (ext. 3931, fax +81 762 64 1047.
we examine annihilation of localized electron centers and sodium cluster formation at the initial stage by analyzing the optical absorption spectrum of NaCl after intense electron pulse irradiation.
2. Experimental Specimens (5 x 5 X 0.5 mm3) were obtained by cleaving a single crystal block of NaCl purchased from the Nihon Kesshou Kougaku Co., Japan. Irradiation was carried out using a high current electron pulse (energy: 20 MeV; pulse width: 1.5 ps; peak current: more than 240 ti, repetition: 2 Hz) from the electron linear accelerator Qinac) of the Institute of Scientific and Industrial Research‘ @SIR), Osaka University. The absorbed dose at the specimen position, estimated using a chemical dosemeter, was about 100 Gy/pulse. The specimen, wrapped with aluminum foil, was placed on an aluminum plate which was cooled by water and located at the electron beam. The temperature of the specimen during irradiation was less than 300 K. The optical absorption spectrum was measured using a Hitachi U-2000 spectrophotometer attaching a home made cryostat with a controller available for the temperature range from 77 to 500 K. Thus, thermal annealing at high temperatures (more than 400 K) and the measurements at low temperatures (100 K) could be repeated without changing the specimen position.
0168-583X/94/~07.~ 0 1994 - Elsevier Science B.V. All rights reserved &WI 0168583X(931ElOZO-M
III. HALIDES
K Inabe et al./Nucl. Instr. and Meth. in Phys.Res. B 91 (1994) 201-204
202
Wavelength
76 1111I 9
5 I
2.0 Energy
3.1. Thermal annealing of F and F-aggregates
(nm)
4xl00 I
I
3.0 (eV)
Fig. 1. Optical absorption spectra of NaCl after electron irradiation of 2 X lo6 Gy and subsequent heating at the temperatures shown for 1 min and cooled down to 100 K within 10 mm. The specimen thickness is 0.380 mm and each spectrum is observed at lOOK.
3. Results and discussion Fig. 1 shows several representative absorption spectra of NaCl after electron pulse irradiation at room temperature (RT) and subsequent heat treatment at high temperatures. The F and M(F,)-band intensities cannot be determined since their peak heights exceed the detecting limit of log(l,/l) around 2.5. The spectrum at lower energy side of the F-band was resolved into several Gaussian bands, F, M(F,), R(F,),, R,, N(F&, and N,-bands, respectively, using a personal computer (NEC PC-9801DX). Here, the F-(F’)-band was not considered since measurements were carried out at high temperatures where the F--band was annealed out. Gaussian peak analysis was performed using known peak energies, leaving the values of widths and intensities as parameters, which were determined by comparing the calculated curve with data. Fig. 2 shows a calculated curve and several components resolved from the spectrum observed. It should be noted that we must assume the new bands peaking around 1.70 and 2.30 eV, showing by dashed curves, in order to get the best fit with experimental data. Although these bands are located at almost the same energy as the M and R-bands, the band widths are broader and annealing temperatures are higher compared with those of the M and R-bands. We show later that these bands grow in intensity and approach the X-band (2.10 eV) with increase of the annealing temperature. Thus, they seem to correspond to the sodium clusters at their initial stage.
The thermal annealing behavior of several bands of electron centers was examined by plotting the peak height of each component (Gaussian band) and is shown in Fig. 3. It is seen that R,, R,, and N,-bands are annealed out below 425 K where the X-band starts to grow. In particular, the R, and R,-bands are likely to be masked by the broad bands growing at the same wavelength range. At this temperature, however, no decrease of the N,-band is observed, and the decrease of both M and N,-bands occurs at higher temperatures corresponding to the increase of the X-bands. This indicates that the M and N,-bands are closely associated with formation of the X-band, though it is difficult to obtain a strict correlation between the decrease of these bands and the increase of the X-band because of ambiguity due to the recombination loss of these electron centers. It is noted that the N, center is believed to be a planar aggregate of four F centers at the nearest neighbor site in (111) plane [5,6]. 3.2. Two peaks of the X-band The absorption spectrum resolved into several Gaussian peaks has revealed that two broad bands are needed to get the best fit with experimental data (Fig. 2). These two bands are likely to correspond to the initial stage of sodium cluster formation, since these bands are found to grow into a single peak around 2.10
Wavelength
9
76
(nm)
5
4xl00
0 1.0
2.0 Energy
3.0
4.0
(eV)
Fig. 2. Representative absorption spectrum resolved into several Gaussian bands of localized electron centers. A dotdashed line: sum of the Gaussian bands. Solid line: spectrum observed at 100 K for NaCl after electron irradiation of 2~ lo6 Gy and subsequent heating at 423 K for 1 min. New bands are shown by the dashed lines.
IL Inabe et al. /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 201-204
ters of sodium metal depending on wavelength cannot explain the presence of two prominent peaks. When metal particles are small, however, it is known that the peak position of the extinction band of the Mie theory is identical with that of the surface plasma resonance [2]. In the following, we try to examine the peak positions of the bands on the basis of the surface plasma resonance. The metal sphere replaced by a point dipole is represented by an oscillator with the surface plasma frequency [Sl,
. -3
rM
2,o-
350
.
450
400
Temperature (K) Fig. 3. Temperature dependence of the peak height of individual band corresponding to the localized electron centers. Peak positions of the two X-bands are shown in the figure.
eV with increasing annealing temperatures as shown in Fig. 4. Although the optical absorption band due to small metal particles is usually explained using the Mie theory [7], the theory that utilizes the optical parame-
Wavelength
9 llllI
76 !
I
4iXlOO
I
I
1 NO-Q ,....?!::::p’... 2 Q-413K
A
By substituting the dielectric constant of the medium (NaCl), es = 1.40, the high frequency dielectric constant of sodium, E, = 5.70 and the bulk plasma frequency, up = 6.0 eV, we obtain ws = 2.06 eV which agrees approximately with the peak energy observed. A doubly peaked absorption spectrum is expected when two dipoles are assumed to interact with each other [8-101. Then, we tentatively suppose a small aggregate of the N, centers as the dipoles inducing the doubly peaked spectrum. Using the two-point-dipole model, Inoue and Ohtaka have analyzed the doubly peaked absorption qualitatively [8]. When the dipoles oscillate parallel to the sphere axis, the optically active mode is given by 1 - 2( a/o>”
tit=,;
1 - 2A( a/D)”
’
(2)
where, a is the radius of metal sphere replaced by a point dipole and D is the distance between two dipoles. When the dipoles oscillate perpendicular to the sphere axis, the active mode is given by a frequency
3 Q-4233 Q.433K 0.4433 Q.453K
4.0 (eV)
Fig. 4. Growth of the X-band with annealing temperatures. Arrows in the figure correspond to the peaks obtained from Eqs. (2) and (4). Solid and open circles denote peak positions of closely packed planar aggregates and linear quadruplet, respectively, according to the results of Kreibig and Genzel
ml.
(1)
(nm)
5
Energy
6l,/gTTg.
W, =
OF
.
0
203
2
_
WI-%
2
1+
(am3
1 +A( a/D)”
’
(4)
By substituting (D/a) = 2.10 (minimum value of D) and the parameters used above, we obtain w,, = 0.942w,, and w L = l.O4w,, which are marked by the arrows in Fig. 4. Separation of two peaks by Eqs. (2) and (4) is, in this case, smaller than that of the peaks observed at the initial stage of the X-band growth. Clippe et al. have calculated the splitting of the single resonance peak into several eigenmodes of the aggregates [9]. Kreibig and Genzel have applied their results to gold particle systems, and have also shown doubly peaked absorption with various separation which depends on geometrical form of aggregation [lo]. Applying their results to sodium particles, i.e., using the surface plasma frequency of sodium (w, = 2.06 eV) instead of gold (ws = 2.28 eV), we found that closely III. HALIDES
204
K. Inabe et al. /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 201-204
packed planar aggregates give the best fit result to the peak separation observed. The obtained absorption peaks are shown by the solid circles in Fig. 4. The approach of the two peaks with growing the bands seems to suggest that the planar aggregates may initially grow along one direction preferentially, since the absorption peaks are, then, considered to approach the linearly aggregated dipole oscillation mode [lo], which is shown by the open circles in Fig. 4. Planar aggregates at the initial stage of sodium cluster formation are deduced only from the peak positions of two absorption bands but the following reports may support the result. Observation using an electron microscope suggests that the centers responsible for the X-band are attributed to plate shaped defects elongating parallel to one of the (100) plane [ll]. Planar aggregates are also comparable with the result of the small-angle X-ray scattering, which indicates the flat shaped CuCl microcrystalline (less than 10 nm in size) being formed in NaCl doped heavily with Cuf ions [12].
Acknowledgements
We wish to thank the members of the machine group at the Radiation Laboratory, ISIR, Osaka Uni-
versity for their help in electron irradiation using the linac. A part of this work is supported by a Grant-in-Aid for scientific research (C) from the Ministry of Education, Science and Culture, Japan.
References [ll F. Fujimoto and K. Komaki, J. Phys. Sot. Jpn. 25 (1968) 1679. Dl U. Kreibig and P. Zacharias, Z. Phys. 231 (1970) 128. 131K. Inabe, M. Adachi and N. Takeuchi, Jpn. J. Appl. Phys. 32 (1993) Part 2, L1429. 141K. Inabe, S. Owaki and N. Takeuchi, Jpn. J. Appl. Phys. 28 (1989) L2242. [51 J.H. Schuhnan and W.D. Compton, Color Centers in Solids (Pergamon, Oxford, 1963) p. 160. t61 A.E. Hughes, Proc. Phys. Sot. 87 (1966) 535. [71 G. Mie, Ann. Phys. 25 (1908) 377. [81 M. Inoue and K. Ohtaka, J. Phys. Sot. Jpn. 52 (1983) 3853. 191 P. Clippe, R. Evard and A.A. Lucas, Phys. Rev. B 14 (1976) 1715. [lOI U. Kreibig and L. Genzel, Surf. Sci. 156 (1985) 678. [ill K. Izumi, J. Phys. Sot. Jpn. 26 (1969) 1451. [121 T. Itoh, Y. Iwabuchi and M. Kataoka, Phys. Status Solidi B 145 (1988) 567.