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OPTICALLY DETECTED E.P.R. OF SELF-TRAPPED
EXCITONS IN ALKALI
FLUORIDES-LUMINESCENCE IDENTIFICATION
A. Wasiela, D. Block
To cite this version:
JilURNAL DE PHYSIQUE Colloque C7, suppliment au no 12, Tome 37, Dtcembre 1976, page C7-221
OPTICALLY DETECTED E. P. R. OF SELF-TRAPPED
EXClTONS IN ALKALI FLUORIDES-LUMINESCENCE IDENTIFICATION
A. WASIELA and D. BLOCK
Laboratoire de SpectromCtrie Physique, UniversitC Scientifique et MCdicale de Grenoble B. P. 53, 38041 Grenoble-Cedex, France
Rbsumb. - La resonance de l'exciton autopiegk dans NaF, LiF et K F a Bte observee par detection optique. La bande d'kmission a issue de 1'6tat triplet a kt6 identifik et sa duree de vie mesuree. La
structure hyperfine observee est en accord avec le modkle decrivant I'exciton autopikg6 comme un Vk
ayant capture un electron. La decomposition a champ magnetique nu1 de l'etat triplet est principa- lement dkfinie par les interactions dipolaires magnktiques. L'observation d'un croisement de niveaux montre que les sous-niveaux Zeeman ne sont pas en Cquilibre thermique.
Abstract. - The optical-microwave double resonance of the triplet state of the self-trapped exciton has been detected in alkali fluorides. The luminescence originating from this triplet state
(a band) has been identified and the corresponding lifetime measured. The values for the hyperfine interaction parameters are consistent with the model of the Vk having trapped an electron. The contribution of the dipole-dipole interaction to the zero field splitting is dominant. An observed level crossing has shown that the triplet sublevels are not in thermal equilibrium.
1. Introduction. - Thanks to the works of Kabler [I], Murray and Keller 121, it is now well established that luminescence from alkali halides excited by ionizing radiation at low temperatures occurs through the recombination of the electron- hole pairs. The self-trapping of these pairs is attributed to the formation of a-covalent bond between two adjacent halide ions, and the self-trapped exciton (S. T. E.) can be regarded as a (X; -)* molecular ion in a n excited state.
The existence of an emission band (or n band) originating from the lowest triplet state has allowed the optical detection of E. P. R. of the S. T. E. in some alkali halides [3, 4, 51. This detection has been made easy by the long lifetime of this state. The above experiments have confirmed that the S. T. E. is ana- logous to a V, center (self-trapped hole) having trapped an electron in a diffuse orbital.
The luminescence in fluorides under X-ray excita- tion has already been studied by Pooley et al. [6], but the emission bands were not clearly identified. These authors mentioned that their samples contained some impurities and suggested that some of their results are suspect.
The purpose of the present paper is to present results of E. P. R. detection of the lowest triplet state, report the qualitative observation of circular pola- risation, and tentatively identify the n band in LiF, NaF and KF.
2. Experimental procedures.
-
Single undoped crys- tals were obtained from Harshaw Chemical Company and Dr. Karl Korth Monokristalle-Kristalloptik.The K F samples were cleaved and polished in an atmosphere of dry helium gas to minimize the surface deterioration due to absorption of water vapour. The samples were immersed in the helium bath of a metal cryostat.
2.1 LUMINESCENCE.
-
Luminescence was excited by an X-ray tube operating at 45 kV and 20 mA. The tungsten anode was approximately 10 cm from the samples. The luminescence was analysed by a SPEX type 1700-11 grating monochromator and detected by an EM1 type 9635 QB photomultiplier.The spectra were corrected for monochromator and photomultiplier response.
2 . 2 CIRCULAR POLARISATION. - A silica plate
modulator was used to detect the circular polarisation of the luminescence. The emission bands were selected by a set of optical filters.
2.3 RESONANCE.
-
The experimental arrangement used for the optical detection of E. P. R. has been described previously [3]. The changes in luminescence intensity induced by a modulated microwave transition were detected by a standard lock-in detector.3. Results and discussion.
-
3.1 LUMINESCENCE. - The luminescence spectra recorded at 4.2 K are represented in figure 1. The agreement of peak positions with those found previously [6] (at about 5 K) is good ;however, the relative intensities of emission bands are noticeably different, specially in NaF. This is probably due t o the presence of some impurities whose concen- tration can vary from sample to sample. In KF the
C7-222 A. WASIELA AND D. BLOCK
P H O T O N E N E R G Y ( e V )
I I I I I
4.5 4.0 3.5 3.0 2.5
FIG. 1.
-
X-ray excited luminescence spectra of fluorides at 4.2 K. Solid curve LiF, dashed curve NaF, alternate dash-dot curve KF. The intensity scales for different crystals are not equivalent. The spectra have been corrected for monochromator and photomultiplier response (monochromator resolution : 28 A).shape of the emission bands can be somewhat modified by the absorption by F centers (absorption band at 2.8 eV) and the dip occurring near about 5000 i( is not very significant.
By taking advantage of the half-wave rectified X-ray excitation, the lifetimes (only those which are longer that 0.5 milliseconds) of the same bands were measured, and are reported in table I with the principal peak band positions.
X-ray excited luminescence bands
in alkali Jiruorides at 4.2 K
Peak band Lifetime Crystal
-
1
- (eV> (ms1 - LiF 3520 3.5 14.5NaF 2 900 4.27 < 0.5
(=) From Pooley et al. [6].
( b ) From Call P. J., Thesis, Oxford 1974, unpublished.
This value has been measured for an emission band located at 3.14 eV.
3 . 2 RESONANCE. - The resonance spectra obtain- ed in the alkali fluorides present some similarities with those observed for the triplet state of the S. T. E. in bromides and chlorides. The E. P. R. spectrum recorded in the case of KF with unpolarised light is shown in figure 2. The lines corresponding to transi- tions AM, =
t.
1 of the S. T. E. with its molecular axis parallel to magnetic field, are easely recognizable by their three-line structure. This structure is due toFIG. 2.
-
E. P. R. spectrum of the S. T. E. in KF at 4.2 K. The magnetic field was along a [I101 direction of the crystal. Themicrowave frequency was 9,247 MHz.
the hyperfine interaction with the nuclear moments of the two central fluorine ions (I, = 1, =
3).
The E. P. R. spectra can be described for D,,
symmetry in alkali halides by the spin hamiltonian :
where S = 1. The principal axes Ox, Oy, Oz are oriented respectively parallel to [OOl], [ l ' i ~ ] and [I101 directions. The results are reported in table 11. The optical selection rules can give the sign of the fine structure parameter D . We have done an experiment showing that the transition M, = 0 o M, = I which
corresponds to an increase of cr, polarized light, occurs at higher magnetic field than the transition
Ms = 0 o M, =
-
1. So we can conclude that the sign of D is negative. This fact confirms the recent observations made of the S. T. E. in the chlorides [5]. This fine structure term is defined by :where Dss is the contribution of the spin-spin inter-
action and Ds0 is given by [7] :
where R and A are respectively the spin-orbit interaction and the exchange energy. In the alkali halides Ds0 is
always positive. Using the free exciton value for the exchange energy [8] and the V, values for the other parameters 191, we found Dso 1.100 G in NaF.
In fact, the value of A for the S. T. E. is expected to be lower than for the free exciton, as has been shown for the bromides 131. From our experimental data we
OPTICALLY DETECTED EPR OF S. T. E. IN ALKALI FLUORIDES Crystal gz
-
-
LiF 1.983 f 0.005 NaF 1.993 f 0.005 (1.98) ("1 K F 1.987 f 0.005Spin hamiltonian parameters of the S. T . E. in alkali jiuorides. TheJine and hyperjne parameters are expressed in gauss.
(a) From D. Schoemaker [9].
( b ) There still remains some ambiguity upon the assignment for the lines of the [001] spectrum. So this value is still doubtful.
( c ) From Call P. J., Thesis, Oxford 1974, unpublished.
The hyperfine structure is well resolved along the molecular axis. The Az values for the S. T. E. are nearly half the values for the corresponding V, center, which shows that the contribution of the excited electron is small. Although the resonance spectra are well understood in NaF and KF, there are some features in the case of LiF which are not :
- Usually the light emitted by a given center is polarised along both
x
and y axis. In lithium floride,the E. P. R. has shown that the x polarisation is predominant.
- Some E. P. R. lines observed for [OOl] orientation of the magnetic field are not yet identified.
Several mechanisms can be proposed to explain these observations, such as the presence of impurities or vacancies near the S. T. E. or more fundamental effects related to the spin-orbit coupling or the nature of the excited state. For the moment none of them is very conclusive. PHOTON ENERGY ( e V ) I I I I I 4.5 4.0 3.5 3.0 2.5
-
-
I \ I I lr ' I I I I I I 300 350 400 450 500 WAVELENGTH ( n m )FIG. 3. - Spectral analysis of the optical detection of the E. P. R. signal of the S. T. E. in NaF (solid curve). For com- parison, the luminescence spectrum is also shown (dashed curve). The magnetic field was along a [I101 direction. The full curve represents the intensity of an optically detected E. P. R. line of the S. T. E. versus the wavelength of the detected
light.
3.3 IDENTIFICATION OF THE LUMINESCENCE FROM
THE TRIPLET STATE (n BAND).
-
The first identification of the intrinsic luminescence in alkali halides were obtained by studying the polarisation properties of the emitted light [I, 21. More recently, the n emission (3Z: +'z.:
transition) of the S. T. E. in SrCl, [lo]have been determined by means of the spectral repartition of the resonance. In this method, the E. P. R. signal is analysed as a function of the wave- length of the emitted light and is compared with the luminescence spectrum.
For this purpose we have used either a monochro- mator, as in the case of the S. T. E. in NaF (Fig. 3) and LiF, or a set of optical filters, as for KF.
From this study, the .n emissions can be located at 4 750
+
100A
in NaF, 3 500 f 100A
in LiF and 5 000 f 200 in KF. These identifications agree with the long lifetimes observed for these emission bands.3 .4 MAGNETIC CIRCULAR POLARISATION (I, + -I, -). -
We have done a qualitative study of the variation of the circular polarisation versus magnetic field. In K F (Fig. 41, I,+
-
I,- shows a maximum for amagnetic field of 700 G. This value is very close to
B ( k G )
Ro. 4. - Magnetic circular polarisation (I,, - I , - ) of the
7c band in KF. The magnetic field was along a [I 101 direction and
C7-224 A. WASIELA AND D. BLOCK
the value of B which corresponds to the level crossing between the states M, = 0 and M, =
+
1(D = - 615 G ) . In the vicinity of this magnetic field there is a population transfer from the non emitting sublevel M, = 5 to sublevel M, =
+
1, which gives rise to an increase in the intensity of theG + light. A similar effect occurs in NaF and LiF,
and shows that the populations of the Zeeman states are not in thermal equilibrium (at last at liquid helium temperatures). This kind of level crossing has been observed in the case of the S. T. E. in the other alkali halides [3] [ l l ] for the excited state of F centers in in CaO [123, and for triplet states in organic molecules.
4. Conclusions.
-
The measurements made in this work show that the behaviour of the triplet states of the S. T. E. in alkali fluorides is very similar to that observed in the other alkali halides. The E. P.R.
results are in agreement with the model which treats the S . T. E. as an electron trapped at a V, center site. They indicate clearly that when the spin orbit cou- pling is small, the fine structure is essentially deter- mined by the spin-spin interaction. The spectral repartition of the resonance has permitted the iden- tification of the luminescence occurring from triplet state.
References
[I] KABLER, M. N., Phys. Rev. 136 (1964) A 1296.
[Z] MURRAY, R. B. and KELLER, F. J., Phys. Rev. 137 (1965) A 942.
[3] WASIELA, A., ASCARELLI, G. and MERLE D'AUBIGN~, Y..
Phys. Rev. Left. 31 (1973) 993 and J. Physique Colloq.
34 (1973) C9-123.
[4] MARRONE, M. J., PATTEN, F. W. and KABLER, M. N.,
Phys. Rev. Lett. 31 (1973) 467.
[5] CALL, P. J., HAYES, W., HUZIMURA, R. and KABLER, M. N.,
J . Phys. C : Solid St. Phys. 8 (1975) L-56.
[6] POOLEY, D. and RUNCIMAN, W. A., J. Phys. C : Solid St. Phys. 3 (1970) 1815.
[7] FOWLER, W. B., MARRONE, M. J. and KABLER, M. N.,
Phys. Rev. B 8 (1 973) 5909.
[8] SANO, R., J. Phys. Soc. Japan 27 (1969) 695. [9] SCHOEMAKER, D., Phys. Rev. B 7 (1973) 786.
[lo] WASIELA, A., DURAN, J. et MERLE D'AUBIGN~, Y., C. R.
Hebd. Sian. Acad. Sci. 278 (1974) 1099.
[ l l ] MARRONE, M. J. and KABLER, M. N., Bull. Amer. Phys. Soc. 20 (1975) 696.
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