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X-ray luminescence of solid noble gases
JOURNAL OF LUMINESCENCE 1,2 (1970) 842-850 © North-I-lolland Publishing Co., Amsterdam
X-RAY LUMINESCENCE OF SOLID NOBLE GASES* K. J. SWYLER and M. CREIJZBURG Universilv of Rochester, Rochester, N. Y., LISA
Crystals of Ar, Kr, and Xe, grown from the melt in plastic or quartz tubes, show weak light emission during irradiation with X-rays. This luminescence consists of impurity and intrinsic contributions. It is called intrinsic if It is not sLippressed b~reduction of the impurity concentration. This Consists mostly of broader hands hetween 2000 and 5000 A which are temperature and radiation-dose dependent. We attribute luminescence in solid Kr to two different centers. A tentative model for one of them is the V~center, since noble gas molecular ions are stable in free space and are not much affected by the crystal. Extensions of these luminescence measurements will be discussed.
We are going to give a brief report on the present state of our measurements of X-ray induced luminescence from rare gas solids. Our specific interest lies in determining whether there are stable intrinsic color centers in rare gas crystals, analogous to those in alkali halides, for instance. Such intrinsic centers are formed by local distortions of a regular lattice. In a perfect rare gas lattice we expect that the most easily produced stable center could be a VK center. This is illustrated in fig. I. In alkali halides the Vk
CENTER
in KCi
in Ar
~
~ Fig. I. The Va center in alkali halides and solid rare gases.
VK center can be thought of as a hole trapped between two halide ions. resembling a negatively charged halide molecule ion, Cli. Since a neutral rare gas atom is isoelectronic to a negative halide ion, a VK center in the rare gas crystal would resemble a positively charged rare gas molecular ion, *
Work supported by the Air Force Office of Scientific Research.
J.8
842
J.8
X-RAY LUMINESCENCE OF SOLID NOBLE GASES
843
Art. Such rare gas molecular ions have been observed in free spacet), and recent calculations indicate that the VK center should be stable in Ar, Kr and Xe, with a hole trapping energy of about 0.5 to 1 eV2). To produce such a center, an electron must be removed from an atom; due to the large band gap of the rare gas crystals, we chose to irradiate the samples with X rays. Since the color center concentration is usually very low, it was felt necessary to obtain bulk samples. Fig. 2 shows the experimental setup schematiGAS SUPPLY
DIFFUSION
VARIABLE VOLUME CHAMBER
PUMP
L He OEWAR (JANIS SUPER VARI TEMP)
PLASTIC GROWING——---~CHAMBER
TEMPERATURE CONTROLLED Cu BLOCKS
X-RAY TUBE MONOCROMATOR AND PM TUBE
Fig. 2. Experimental set-up.
cally. The crystals are grown from the melt in a thin-walled plastic chamber in a variable temperature cryostat. The method is similar to that used by Simmons’ group3). The temperatures of the top and bottom of the chamber are thermocouple controlled. The gas handling system shown at the top of fig. 2 permits evacuation of the growing chamber to less than 106 torr. The variable volume chamber allows pressure control during gas condensation and crystal growth. In our more recent work a nickel strip coated with a getter material* was mounted in the variable volume chamber to further * CerAlloy 400 Getter, supplied by Ronson Metals Corp., 45—65 Manufacturers Place, Newark, N.J.
844
J.S
K. J. SWYLER ANI) M. (REUZBUR;
purify the rare gases. When heated, the getter strongly absorbs impurities such as N7 and 02. while having little effect on the inert rare gases. The samples were irradiated by a copper X-ray tube rated at 50 keV and 15 mA from a distance of about 2 inches. Luminescence is observed at right ang)es to the X rays with a monochromator and photomultiplier. It was considered sufficient t’or our purposes to grow crystals 20 mm long and 5 mm in diameter in about one hour, although such samples are almost certainly polycrystalline. In our initial experiments, our samples were quite Impure. We observed primarily sharp line emission during X-irradiation, such as seen in fig. 3.
Sooo
4000
3000
SoooA
Ar,2o~K
5000
4000
3000
2000
A
Fig.3. Impurity emission in solid argon.
which shows impurity argon spectra. This emission is very complex, and both the absolute and relative intensities depend strongly on the temperature. At 60K (upper curve) the lines between 2000 and 3500 A predominate. 4). At lower Most of these lines are due to molecullar nitrogen impurities temperatulres the visible lines exceed the nitrogen lines. The visible lines originate from molecular oxygen impulrities4). Similar impulrity emission was observed in krypton and xenon. In all three hosts. molecular impurity luminescence exhibited the following characteristics: 1. The luminescence consists of sharp lines. most of them in a series:
J.8
845
X-RAY LUMINESCENCE OF SOLID NOBLE GASES
2. The positions of these lines are nearly independent of the host; 3. Emission intensity increases with impurity concentration; 4. The emission intensity is not strongly affected by prolonged X irradiation. We were able to improve sample purity by refining the gas handling system, and by obtaining purer gas supplies. Fig. 4 shows emission spectra
x[A} 5
4000
4
3000
2500
2000
T~8OK
>-
I‘I)
z
w z
w -J
Co
35
40
4.5
5.0
5.5
6.0
EMITTED PHOTON ENERGY[eV]
25
20
3000
2500
2000
T~2OI(
I-.
(n
z ~
z
IS
Ui
> lO
5.0
EMITTED
PHOTON ENERGY
5~5
6.0
levI
Fig. 4. Emission spectra of solid krypton, at 80°K(upper curve) and at 20°Kafter 1-~ hours X irradiation at this temperature (lower curve). The intensities are corrected for sample chamber transmission and detection system response. The intensity scale is the
same for both spectra.
846
J.8
K. J. SWYLLR AND M, CREVZBURG
of krypton5). In the upper curve (taken at 800K) there are two broad bands, but no sharp lines from molecular impurity luminescence. The intensity of these bands was found to vary with impurity concentration, X-ray dose, and temperature. The emission is strongest when no sharp impurity lines are seen; the broad bands are quenched by impurities. Prolonged X irradiation results in an increase of the intensities of both bands. When the temperature is decreased, the higher energy band increases in intensity, while the lower energy band decreases and disappears at 20°Kand below. The lower spectrum is emission from the same crystal after 13 hours of X irradiation at 200K. The band at 5.5 eV has split into at least five bands. These can be fitted fairly well by Gaulssian bands of equal half widths of 0.20 eV and spacing of 0.26 eV. The structure around 4 eV is strongly X-ray dose dependent and is not recorded before about 10 mm of X irradiation. The center associated with this emission is apparently produced by the X rays at low temperature and remains stable u~pto 50 K. EMITTED PHOTON ENERGY [eV] 40 4.5 50 55
3000 2500 EMISSION WAVELENGTH
[Al
EMITTED PHOTON ENERGY [eV] 40 4.5 50 55 25
~ G fl2O
60
2000
60
KRYPTON, 2O~K X-RAYED 15mm
H
z
uj 15
3000 2500 EMISSION WAVELENGTH
[A]
2000
Fig. 5. Emission spectra of solid krypton, grown from a different gas supply and purified with the getter. The figure shows the emission at 77 ~K (upper curve) and at 20~Kafter 15 mm X irradiation at this temperature (lower curve). The intensities are corrected as in fig. 4. The intensity scale of the lower curve is ten times largerthan that ofthe upper curve.
J.8
847
X-RAY LUMINESCENCE OF SOLID NOBLE GASES
In recent Kr samples grown with gas from a different supply, and in a sample in which the getter was used to further purify the gas, the 4 eV band at higher temperatures was not observed. Fig. 5 shows spectra of a Kr sample in which the getter was used. The 4eV band at 77°Kand the 5.5 eV intensity increase at 20°K,as seen in fig. 4, are not observed. Evidently the structure around 4 eV at 20°K is not related to that at 77°K. The 4 eV band at higher temperatures cannot be considered intrinsic in the sense that it is always seen in pure crystals, but it does not fit our concept of molecular impurity luminescence. If this emission is impurity luminescence, the impurity may interact strongly with the host. At 20°Kit appears that two different centers produce the Kr emission, since the dose dependence of the 4 eV structure differs from the dose dependence of the 5.5 eV bands. We have recently observed somewhat similar emission from purified argon crystals. The getter was quite effective in reducing 02 and N2 impurity EMITTED PHOTON ENERGY [eV] 4.0 45 50 55
35 5:
60
0K
~4RE~j >ARGON. 70
3500
3000 2500 EMISSION WAVELENGTH [A] EMITTED PHOTON ENERGY [eV] 40 45 50 5.5
~35
~ 4
2000
60
ARGON. 5°K X-RAYED 1 HOUR RESOLUTION = H
3500
3000 2500 EMISSION WAVELENGTH [A]
2000
Fig. 6. Emission spectra of solid argon at 70°K(upper curve) and at 5°Kafter 1 hour X irradiation at this temperature (lower curve). The intensities are corrected as in fig. 4. The intensity scale is the same for both spectra.
848
K. i. SWYLER AND ~t. CREUZBURG
iN
emission in argon. although some trace of N2 emission was observed in all samples. Fig. 6 shows emission spectra of argon, corrected for sample chamber transmission and detection system response. The upper curve gives the emission spectrum at 70~K.It consists of a series of semi-broad bands near 5.5 eV, of about 0.09 eV half-width and 0.26 eV spacing. The relative intensity of the first band in this series may be somewhat in error. since this band is almost at the transmission limit of the sample chamber. and a large correction is necessary. No strong N, and 07 emissions are observed. These bands are not sulppressed in crystals with lowest impulrily concentration. In this respect the behavior of the argon bands and the 5.5 cV krypton bands is similar. However the band intensity in argon increases slightly as the temperature is decreased to 20 K. There are further significant differences between the argon and krypton bands. In Ar the bands are narrower and do not sharpen appreciably as the temperature is decreased to 20 K. Each argon band is shifted toward lower energy by about 0.07 eV, relative to the nearest krypton band, and in argon the strongest emission is at higher energy. possibly below 2000 A. Fulrthermore, prolonged X irradiation does not increase the intensity of the argon bands. In fact. ulnder prolonged X-ray dose the emission decreased at all temperatulres, buit we are not yet certain that this effect is attributable solely to the irradiation. No new featulres are observed in the argon spectrum when the temperatulre is decreased froni 70°Kto 20K. However. reducing the temperatulre to 5 K decreases the intensity of the 5.5 eV bands and produlces an additional series of bands. The lower curve of fig. 6 shows emission from the same crystal after one hour of X irradiation at 5 K. The new series of bands overlap the 5.5 eV bands. These bands have also been observed at 10K in a crystal with high impurity content, The new bands are not strongly X-ray dose dependent and may be due to an unidentified molecular impurity. They have about the same energy spacing as the highest energy lines in the N, molecular emission series. bult are shifted to lower energy by about 0.05 eV. The bands are also broader than the N, emission and their intensities do not vary directly with the amount of identifiable N2 emission seen at higher temperatulre. These bands areshows not seen in pure 0K also a sharp peakkrypton. at 4.20 eV overlapping one The spectrum at 5 of the new bands. This peak increases strongly with X-ray dose. It also agrees in energy with the peak in the 4 eV structure in krypton. to within experiniental error. In krypton the 4 eV structLire includes a broad band at lower energy than the sharp peak. A similar band is not seen in argon. In view of the different X-ray dose dependences of the 4.2 eV peak and the
J.8
X-RAY LUMINESCENCE OF SOLID NOBLE GASES
849
5.5 eV bands, we assume that two different centers produce these features in argon. The behavior of the 5.5 eV bands and the low temperature 4 eV emissions in both argon and krypton seems characteristic of intrinsic luminescence. These emissions are seen in all pure crystals and are not suppressed when the concentration of identifiable impurities is reduced. Also, prolonged X irradiation increases the intensity of the krypton 4.20 eV peak and the 4.20 eV argon peak. Since the luminescence occurs at nearly the same energy
in argon and krypton, however, we cannot yet be certain that the luminescence is really intrinsic to either host. This is particularly true of the 4.20 eV emission. In view of the similar behavior of this peak in argon and krypton it is likely that the same center is responsible for this emission in both cases. If this center is in fact intrinsic, it is quite surprising that the peak energies correspond so exactly; a fairly pronounced energy shift between hosts would seem more reasonable. The 5.5 eV bands in argon and krypton cannot be attributed to similar centers until we can explain the different behavior of these bands under prolonged X irradiation. We might speculate that either the argon or krypton bands are vibrational states of an intrinsic center with molecular characteristics. II would then be tempting to further assume that this is the ~K center, particularly in krypton, where the sharpening at lower temperature implies a strong coupling of the center to lattice phonons. However, at present such conclusions are clearly premature and strong objections can certainly be raised. For instance, we have no model to explain the observed band spacings, which are as large as those seen in molecular nitrogen emission in the same hosts. Also, we cannot yet adequately describe the upper state of the transition that would result in these bands, particularly with regard to stability. Further measurements over an extended spectral range are planned in both emission and absorption under X irradiation. These results should provide more insight into the nature of the X-ray luminescence reported here.
References I) 2) 3) 4) 5)
R. E. Hoffman and D. H. Katayama, J. Chem. Phys. 45(1966)138. S. J. Druger, Doctoral Thesis, University of Rochester, Rochester, N.Y. (1969). G. G. Peterson, D. H. Batchelder and R. 0. Simmons, J. AppI. Phys. 36 (1966) 2682. L. J. Schoen and H. P. Broida, J. Chem. Phys. 32(1960)1184. M. Creuzburg and K. Teegarden, Phys. Rev. Letters 20 (l968) 593.
850
K. J. SWYLER AND M. CREUZBURG
J.8
Discussion on paper J8 Question 1:
R. B. Murray
(I) Have you made measuirements of the absorption spectrulm resulting
from X-ray irradiation? (2) Could the appearance of sharp structure in krypton at 20~Kbe due to absorption bands resulting from X irradiation? Answer: K. J. Swyler (I) No. These measurements are planned after we improve the sample
configuration. (2) 1 do not think so. The krypton bands are not sharpened by prolonged irradiation buit are sharpened when the temperatuire is lowered. Comment I:
K. S. Song
We recently made a calculation on self-trapping of holes in solid argon by a semi-empirical method. We found that the VK-like self-trapped hole is the stable hole state. The transition energies of VK centers for a —~eV. 7tg transitions are estimated to be approximately 4.5 eV and0 1.5 and a~ The VK-~ center is known to migrate by hopping motion at high temperatures. the activation energy being given by S/tw/4 at the high temperature limit.
S is the Huang-Rhys factor. As S is proportional to the effective phonon frequency w. the activation energy is proportional to w2. Rare gas solids, in particular solid argon. have an order of magnitude smaller elastic constants than for a typical alkali halide. This indicates that the activation energy for YR-center hopping solid argon is at least an order of magnitilde smaller than in an alkali halides (which is 0.2 0.6 eV). Hence one should operate at very low temperature, say < 20K. to “freeze” the created VK centers in solid argon.
X-RAY LUMINESCENCE OF SOLID NOBLE GASES* K. J. SWYLER and M. CREIJZBURG Universilv of Rochester, Rochester, N. Y., LISA
Crystals of Ar, Kr, and Xe, grown from the melt in plastic or quartz tubes, show weak light emission during irradiation with X-rays. This luminescence consists of impurity and intrinsic contributions. It is called intrinsic if It is not sLippressed b~reduction of the impurity concentration. This Consists mostly of broader hands hetween 2000 and 5000 A which are temperature and radiation-dose dependent. We attribute luminescence in solid Kr to two different centers. A tentative model for one of them is the V~center, since noble gas molecular ions are stable in free space and are not much affected by the crystal. Extensions of these luminescence measurements will be discussed.
We are going to give a brief report on the present state of our measurements of X-ray induced luminescence from rare gas solids. Our specific interest lies in determining whether there are stable intrinsic color centers in rare gas crystals, analogous to those in alkali halides, for instance. Such intrinsic centers are formed by local distortions of a regular lattice. In a perfect rare gas lattice we expect that the most easily produced stable center could be a VK center. This is illustrated in fig. I. In alkali halides the Vk
CENTER
in KCi
in Ar
~
~ Fig. I. The Va center in alkali halides and solid rare gases.
VK center can be thought of as a hole trapped between two halide ions. resembling a negatively charged halide molecule ion, Cli. Since a neutral rare gas atom is isoelectronic to a negative halide ion, a VK center in the rare gas crystal would resemble a positively charged rare gas molecular ion, *
Work supported by the Air Force Office of Scientific Research.
J.8
842
J.8
X-RAY LUMINESCENCE OF SOLID NOBLE GASES
843
Art. Such rare gas molecular ions have been observed in free spacet), and recent calculations indicate that the VK center should be stable in Ar, Kr and Xe, with a hole trapping energy of about 0.5 to 1 eV2). To produce such a center, an electron must be removed from an atom; due to the large band gap of the rare gas crystals, we chose to irradiate the samples with X rays. Since the color center concentration is usually very low, it was felt necessary to obtain bulk samples. Fig. 2 shows the experimental setup schematiGAS SUPPLY
DIFFUSION
VARIABLE VOLUME CHAMBER
PUMP
L He OEWAR (JANIS SUPER VARI TEMP)
PLASTIC GROWING——---~CHAMBER
TEMPERATURE CONTROLLED Cu BLOCKS
X-RAY TUBE MONOCROMATOR AND PM TUBE
Fig. 2. Experimental set-up.
cally. The crystals are grown from the melt in a thin-walled plastic chamber in a variable temperature cryostat. The method is similar to that used by Simmons’ group3). The temperatures of the top and bottom of the chamber are thermocouple controlled. The gas handling system shown at the top of fig. 2 permits evacuation of the growing chamber to less than 106 torr. The variable volume chamber allows pressure control during gas condensation and crystal growth. In our more recent work a nickel strip coated with a getter material* was mounted in the variable volume chamber to further * CerAlloy 400 Getter, supplied by Ronson Metals Corp., 45—65 Manufacturers Place, Newark, N.J.
844
J.S
K. J. SWYLER ANI) M. (REUZBUR;
purify the rare gases. When heated, the getter strongly absorbs impurities such as N7 and 02. while having little effect on the inert rare gases. The samples were irradiated by a copper X-ray tube rated at 50 keV and 15 mA from a distance of about 2 inches. Luminescence is observed at right ang)es to the X rays with a monochromator and photomultiplier. It was considered sufficient t’or our purposes to grow crystals 20 mm long and 5 mm in diameter in about one hour, although such samples are almost certainly polycrystalline. In our initial experiments, our samples were quite Impure. We observed primarily sharp line emission during X-irradiation, such as seen in fig. 3.
Sooo
4000
3000
SoooA
Ar,2o~K
5000
4000
3000
2000
A
Fig.3. Impurity emission in solid argon.
which shows impurity argon spectra. This emission is very complex, and both the absolute and relative intensities depend strongly on the temperature. At 60K (upper curve) the lines between 2000 and 3500 A predominate. 4). At lower Most of these lines are due to molecullar nitrogen impurities temperatulres the visible lines exceed the nitrogen lines. The visible lines originate from molecular oxygen impulrities4). Similar impulrity emission was observed in krypton and xenon. In all three hosts. molecular impurity luminescence exhibited the following characteristics: 1. The luminescence consists of sharp lines. most of them in a series:
J.8
845
X-RAY LUMINESCENCE OF SOLID NOBLE GASES
2. The positions of these lines are nearly independent of the host; 3. Emission intensity increases with impurity concentration; 4. The emission intensity is not strongly affected by prolonged X irradiation. We were able to improve sample purity by refining the gas handling system, and by obtaining purer gas supplies. Fig. 4 shows emission spectra
x[A} 5
4000
4
3000
2500
2000
T~8OK
>-
I‘I)
z
w z
w -J
Co
35
40
4.5
5.0
5.5
6.0
EMITTED PHOTON ENERGY[eV]
25
20
3000
2500
2000
T~2OI(
I-.
(n
z ~
z
IS
Ui
> lO
5.0
EMITTED
PHOTON ENERGY
5~5
6.0
levI
Fig. 4. Emission spectra of solid krypton, at 80°K(upper curve) and at 20°Kafter 1-~ hours X irradiation at this temperature (lower curve). The intensities are corrected for sample chamber transmission and detection system response. The intensity scale is the
same for both spectra.
846
J.8
K. J. SWYLLR AND M, CREVZBURG
of krypton5). In the upper curve (taken at 800K) there are two broad bands, but no sharp lines from molecular impurity luminescence. The intensity of these bands was found to vary with impurity concentration, X-ray dose, and temperature. The emission is strongest when no sharp impurity lines are seen; the broad bands are quenched by impurities. Prolonged X irradiation results in an increase of the intensities of both bands. When the temperature is decreased, the higher energy band increases in intensity, while the lower energy band decreases and disappears at 20°Kand below. The lower spectrum is emission from the same crystal after 13 hours of X irradiation at 200K. The band at 5.5 eV has split into at least five bands. These can be fitted fairly well by Gaulssian bands of equal half widths of 0.20 eV and spacing of 0.26 eV. The structure around 4 eV is strongly X-ray dose dependent and is not recorded before about 10 mm of X irradiation. The center associated with this emission is apparently produced by the X rays at low temperature and remains stable u~pto 50 K. EMITTED PHOTON ENERGY [eV] 40 4.5 50 55
3000 2500 EMISSION WAVELENGTH
[Al
EMITTED PHOTON ENERGY [eV] 40 4.5 50 55 25
~ G fl2O
60
2000
60
KRYPTON, 2O~K X-RAYED 15mm
H
z
uj 15
3000 2500 EMISSION WAVELENGTH
[A]
2000
Fig. 5. Emission spectra of solid krypton, grown from a different gas supply and purified with the getter. The figure shows the emission at 77 ~K (upper curve) and at 20~Kafter 15 mm X irradiation at this temperature (lower curve). The intensities are corrected as in fig. 4. The intensity scale of the lower curve is ten times largerthan that ofthe upper curve.
J.8
847
X-RAY LUMINESCENCE OF SOLID NOBLE GASES
In recent Kr samples grown with gas from a different supply, and in a sample in which the getter was used to further purify the gas, the 4 eV band at higher temperatures was not observed. Fig. 5 shows spectra of a Kr sample in which the getter was used. The 4eV band at 77°Kand the 5.5 eV intensity increase at 20°K,as seen in fig. 4, are not observed. Evidently the structure around 4 eV at 20°K is not related to that at 77°K. The 4 eV band at higher temperatures cannot be considered intrinsic in the sense that it is always seen in pure crystals, but it does not fit our concept of molecular impurity luminescence. If this emission is impurity luminescence, the impurity may interact strongly with the host. At 20°Kit appears that two different centers produce the Kr emission, since the dose dependence of the 4 eV structure differs from the dose dependence of the 5.5 eV bands. We have recently observed somewhat similar emission from purified argon crystals. The getter was quite effective in reducing 02 and N2 impurity EMITTED PHOTON ENERGY [eV] 4.0 45 50 55
35 5:
60
0K
~4RE~j >ARGON. 70
3500
3000 2500 EMISSION WAVELENGTH [A] EMITTED PHOTON ENERGY [eV] 40 45 50 5.5
~35
~ 4
2000
60
ARGON. 5°K X-RAYED 1 HOUR RESOLUTION = H
3500
3000 2500 EMISSION WAVELENGTH [A]
2000
Fig. 6. Emission spectra of solid argon at 70°K(upper curve) and at 5°Kafter 1 hour X irradiation at this temperature (lower curve). The intensities are corrected as in fig. 4. The intensity scale is the same for both spectra.
848
K. i. SWYLER AND ~t. CREUZBURG
iN
emission in argon. although some trace of N2 emission was observed in all samples. Fig. 6 shows emission spectra of argon, corrected for sample chamber transmission and detection system response. The upper curve gives the emission spectrum at 70~K.It consists of a series of semi-broad bands near 5.5 eV, of about 0.09 eV half-width and 0.26 eV spacing. The relative intensity of the first band in this series may be somewhat in error. since this band is almost at the transmission limit of the sample chamber. and a large correction is necessary. No strong N, and 07 emissions are observed. These bands are not sulppressed in crystals with lowest impulrily concentration. In this respect the behavior of the argon bands and the 5.5 cV krypton bands is similar. However the band intensity in argon increases slightly as the temperature is decreased to 20 K. There are further significant differences between the argon and krypton bands. In Ar the bands are narrower and do not sharpen appreciably as the temperature is decreased to 20 K. Each argon band is shifted toward lower energy by about 0.07 eV, relative to the nearest krypton band, and in argon the strongest emission is at higher energy. possibly below 2000 A. Fulrthermore, prolonged X irradiation does not increase the intensity of the argon bands. In fact. ulnder prolonged X-ray dose the emission decreased at all temperatulres, buit we are not yet certain that this effect is attributable solely to the irradiation. No new featulres are observed in the argon spectrum when the temperatulre is decreased froni 70°Kto 20K. However. reducing the temperatulre to 5 K decreases the intensity of the 5.5 eV bands and produlces an additional series of bands. The lower curve of fig. 6 shows emission from the same crystal after one hour of X irradiation at 5 K. The new series of bands overlap the 5.5 eV bands. These bands have also been observed at 10K in a crystal with high impurity content, The new bands are not strongly X-ray dose dependent and may be due to an unidentified molecular impurity. They have about the same energy spacing as the highest energy lines in the N, molecular emission series. bult are shifted to lower energy by about 0.05 eV. The bands are also broader than the N, emission and their intensities do not vary directly with the amount of identifiable N2 emission seen at higher temperatulre. These bands areshows not seen in pure 0K also a sharp peakkrypton. at 4.20 eV overlapping one The spectrum at 5 of the new bands. This peak increases strongly with X-ray dose. It also agrees in energy with the peak in the 4 eV structure in krypton. to within experiniental error. In krypton the 4 eV structLire includes a broad band at lower energy than the sharp peak. A similar band is not seen in argon. In view of the different X-ray dose dependences of the 4.2 eV peak and the
J.8
X-RAY LUMINESCENCE OF SOLID NOBLE GASES
849
5.5 eV bands, we assume that two different centers produce these features in argon. The behavior of the 5.5 eV bands and the low temperature 4 eV emissions in both argon and krypton seems characteristic of intrinsic luminescence. These emissions are seen in all pure crystals and are not suppressed when the concentration of identifiable impurities is reduced. Also, prolonged X irradiation increases the intensity of the krypton 4.20 eV peak and the 4.20 eV argon peak. Since the luminescence occurs at nearly the same energy
in argon and krypton, however, we cannot yet be certain that the luminescence is really intrinsic to either host. This is particularly true of the 4.20 eV emission. In view of the similar behavior of this peak in argon and krypton it is likely that the same center is responsible for this emission in both cases. If this center is in fact intrinsic, it is quite surprising that the peak energies correspond so exactly; a fairly pronounced energy shift between hosts would seem more reasonable. The 5.5 eV bands in argon and krypton cannot be attributed to similar centers until we can explain the different behavior of these bands under prolonged X irradiation. We might speculate that either the argon or krypton bands are vibrational states of an intrinsic center with molecular characteristics. II would then be tempting to further assume that this is the ~K center, particularly in krypton, where the sharpening at lower temperature implies a strong coupling of the center to lattice phonons. However, at present such conclusions are clearly premature and strong objections can certainly be raised. For instance, we have no model to explain the observed band spacings, which are as large as those seen in molecular nitrogen emission in the same hosts. Also, we cannot yet adequately describe the upper state of the transition that would result in these bands, particularly with regard to stability. Further measurements over an extended spectral range are planned in both emission and absorption under X irradiation. These results should provide more insight into the nature of the X-ray luminescence reported here.
References I) 2) 3) 4) 5)
R. E. Hoffman and D. H. Katayama, J. Chem. Phys. 45(1966)138. S. J. Druger, Doctoral Thesis, University of Rochester, Rochester, N.Y. (1969). G. G. Peterson, D. H. Batchelder and R. 0. Simmons, J. AppI. Phys. 36 (1966) 2682. L. J. Schoen and H. P. Broida, J. Chem. Phys. 32(1960)1184. M. Creuzburg and K. Teegarden, Phys. Rev. Letters 20 (l968) 593.
850
K. J. SWYLER AND M. CREUZBURG
J.8
Discussion on paper J8 Question 1:
R. B. Murray
(I) Have you made measuirements of the absorption spectrulm resulting
from X-ray irradiation? (2) Could the appearance of sharp structure in krypton at 20~Kbe due to absorption bands resulting from X irradiation? Answer: K. J. Swyler (I) No. These measurements are planned after we improve the sample
configuration. (2) 1 do not think so. The krypton bands are not sharpened by prolonged irradiation buit are sharpened when the temperatuire is lowered. Comment I:
K. S. Song
We recently made a calculation on self-trapping of holes in solid argon by a semi-empirical method. We found that the VK-like self-trapped hole is the stable hole state. The transition energies of VK centers for a —~eV. 7tg transitions are estimated to be approximately 4.5 eV and0 1.5 and a~ The VK-~ center is known to migrate by hopping motion at high temperatures. the activation energy being given by S/tw/4 at the high temperature limit.
S is the Huang-Rhys factor. As S is proportional to the effective phonon frequency w. the activation energy is proportional to w2. Rare gas solids, in particular solid argon. have an order of magnitude smaller elastic constants than for a typical alkali halide. This indicates that the activation energy for YR-center hopping solid argon is at least an order of magnitilde smaller than in an alkali halides (which is 0.2 0.6 eV). Hence one should operate at very low temperature, say < 20K. to “freeze” the created VK centers in solid argon.