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COLOUR CENTRES IN HALIDES (1).Electron spin resonance in different states of the relaxed configuration
of F-centres
A. Winnacker, K. Hahn, H. Reyher, Th. Vetter
To cite this version:
A. Winnacker, K. Hahn, H. Reyher, Th. Vetter. COLOUR CENTRES IN HALIDES (1).Electron spin resonance in different states of the relaxed configuration of F-centres. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-39-C6-41. �10.1051/jphyscol:1980610�. �jpa-00220005�
JOURNAL DE PHYSIQUE Colfoque C6, supplkment au no 7, Tome 41, Juillet 1980, page C6-39
COLOUR CENTRES IN HALIDES (7).
Electron spin resonance in different states of the relaxed configuration
A. Winnacker, K. Hahn, H. J. Reyher and Th. Vetter
Physikalisches Institut der Universitat Heidelberg, Philosphenweg 12, D 69 Heidelberg, W.-Germany
RBsumC. - La rCsonance paramagnktique Clectronique dans 1'Ctat excitC des centres F dans les halogknures alcalins, a Cte observee par la mCthode du pompage optique saturC. Les facteurs electroniques de Land6 de ces Ctats ont CtC dCterminh pour un grand nombre de centres F et Fa. Les donnCes sont interprttkes sur la base d'une fonction d'onde trts Ctendue de 1'Ctat excitC. Sous des conditions experimentales bien choisies, on observe un signal supplkmentaire dans le spectre de resonance paramagnktique qui peut &tre attribuC A un nouvel Ctat dans la confi- guration relaxte du centre F.
Abstract. - Electron spin resonance in the relaxed excited state (RES) of F- and Fa-centres in alkali halides is observed via the change of the electronic groundstate polarization achieved under saturated optical pumping.
ESR data on the RES have been measured systematically for a number of crystals. The data are interpreted in terms of a very extended wavefunction of the RES. Under appropriate conditions an additional signal is observed in the RES spectrum that seems to be related to a new state in the relaxed configuration of the F-centre. ESR data of this state are given and other experimental evidence for its existence is discussed.
In previous publications [I, 2, 31 we presented a method for detection of electron spin resonance (ESR) in optically excited states of color centers. It has been applied to a number of F- and FA-centers in alkali halides [4]. The method can be summarized as follows (for details see [3, 41) : Under irradiation of circularly polarized light into the F-band the electronic ground state polarization is changed to a value different from the one at thermal equilibrium (opticalpumping). This change of the ground state polarization is monitored via the magnetic circular dichroism (MCD) of the crystal. If ESR in the relaxed excited state (RES) of the centers is performed, the dynamics of the pumping cycle are changed which results in an addi- tional change of the ground state polarization. A scheme of the apparatus is given in figure 1. Figure 2 shows the optically detected ground state and RES spin resonances for the case of KBr. In this way ESR data of the RES of F- and FA-centers were taken for a large number of alkali halides. The results were interpreted in terms of a RES wavefunction of large extension. This interpretation can be summarized as follows : i) The RES wavefunction is the same for the different crvstals when scaled with the lattice constant; ii) the envelope function is given by +(r) = (q5/3 7cd3)'I2.
(a) , i )
with q-
0.4for all crystals. We will not enter into a discussion of these results here but rather focus on a particular feature of these measurements [5] : Under favorable conditions an additional electron spin resonance signal shows up that is usually - as in figure 2 -
I
Be power current
~supph H c o n t r o ,
M
Fig. 1. - Scheme of apparatus. The dye laser can be tuned on the optimum wavelength for detection of ESR in the RES [3]. The monitor beam from the Xenon lamp is rapidly switched between right and lefthanded polarized light. The difference in transmiss~on is proportional to the electronic ground state polarization. This signal changes under ESR in the RES.
hidden under the slopes of the ground state signal.
It can be made visible in the following way : At appropriate wavelength and polarization of the pump- ing light the spin system can be optically pumped to a ground state polarization close to zero. The ground state signal (which depends on a population difTe- rence in the ground state) then becomes small and the usually hidden signal appears. The signals shown in figure 3 were taken under these conditions. The figure clearly shows a third spin resonance signal for the F-center in NaC1. Similar signals were observed so
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980610
C6-40 A. WINNACKER, K. HAHN, H. J. REYHER AND TH. VETTER
Table I. - g-factors and ESR:linewidths for ground-state porn [12]) and excited states RES 1 and RES 2 (this expeuim en t)
.
Groundstate RES 1 RES 2
Crystal 9 ABI, z/mT 9
- - - - AB ,,,ImT
- 9
- AB 1,zImT
-
NaCl 1.997 17.0 1.966 (9) 7.6 (1.9) 1.982 (2) 9.0 (1.0)
KC1 1.995 5.5 1.969 (4) 8.3 (2.0) 1.984 (2) 4.3 (0.6)
KBr 1.984 14.7 1.866 (5) 23.4 (2.0) 1.995 (3) 13.8 (2.0)
CsBr 1.958 72.2 1.638 (15) 48.0 (3.0) 2.184 (16) 48.8 (1.4)
KBr o = ~ . ~ G H z far in KC1, KBr and CsBr. Table I gives the g-factor and linewidths corresponding to these signalr. For
2
lo
comparison the ground state ESR data and the ones__t_ 1.?S 120 & I T
C for the conventional RES (called RES 1) are also given. The essential features of this additional ESR signal are the following : i) The signal is only visible when the system is optically pumped, thus it seems related to a state that forms part of the optical cycle of the F-center (tentatively called RES 2). ii) Increase of the F-center concentration from a normal 10 l 6 cm- to > 1017 c m L 3 results in a strong decrease of both non-ground state signals indicating that the RES 2 belongs to the isolated F-center. iii) The g-factor of the RES 2 is closer to the one of the free electron than
Fig. 2. - ESR signals from groundstate and RES in KBr. The the one ofthe RES 1. iv) Comparison ofthe saturation
magnetic induction B, is sweeped at fixed microwave frequency,
the electronic ground state polarization P,, under saturated optical behavior of the and 2- signa1s
pumping is recorded. The signal at lower field is the groundstate indicates that the lifetime of the RES 2 is about one
signal, the one at higher fieldis from the RES, conesp&ding to a order of magnitude larger than the one of the RES 1.
lower g-factor. It should finally be pointed out that the RES 1 is the
state usually observed in F-center emission. This has only recently been definitely proved by Ohkura and coworkers [6] : Their ESR data in KBr measured via
Na Cl luminescence agree with the RES 1-data of KBr.
1.08 1.10 2 12 Be / Tesh
-
Fig. 3. - ESR spectrum in NaCl. The spectrum is taken at optimum conditions (see text) to show the additional resonance called RES 2 in the text. The RES 2 signal is the one in the middle, the low field signal is the groundstate ESR. The lower part shows a computer fit to three Gaussian curves ( x = experimental points,
-
=computer fit).
On this occasion we will not enter the discussion on the nature of the new state (some arguments within the frame of existing theories have been given in [5]), but will rather concentrate on the following question : Are there other experimental indications to the existence of a second RES of the F-centre ? To discuss this we will consider more closely a group of F-centres where the emission spectra have recently been studied with particularly high sensitivity : The F-centres like the ones in NaBr, NaI, LiCl whose RES should be unstable under radiationless decay [7].
Existing results on these F-centres are summarized in table 11. It is evident that essentially two different sets of observations seem to exist obtained by different experimental techniques : States with lifetimes of about a nanosecond and those with lifetimes of microseconds are observed. The interesting fact is that the emission energy of the nanosecond states fits very well with the phenomenological formula (see last column of table 11) for F-centre emission energies which is very well established from a large group of F-centres [ll]. So, instead of stating a contradiction between the two measurements on NaBr [9, 101, as done in [9], why shoudn't one speculate as follows :
ELECTRON SPIN RESONANCE IN DIFFERENT STATES O F THE RELAXED CONFIGURATION C6-41
Table 11. - Emission energies and lifetimes for F-cen- tres with the RES partially quenched by radiationless transitions. The energies of the last column dre calcu-
10.15 m*
luted from [ l l ] Em = eV, m* = eflective
Em
mass = 0.5, E, = high frequency dielectric constant.
EernIeV EemIeV
Crystal exp. calc.
- - -
LiCl 1.56 [8] 0.657
(Z = 1.9 ps)
NaBr 1 [91
(7
=
PS)0.73 [lo] 0.70
(Z
=
ns)NaI 0.56 [lo] 0.51
(Z
=
ns)There exist two types of states in the relaxed confi- guration of the F-centre : The normal RES that has been seen in emission for a long time in most of the F-centres (like the ones in the Potassium- and the Rubidium-halides), and a second state that has been seen recently in the exceptional F-centres, and has
been seen in the ESR experiments reported in this talk. The reasons why the new state has been seen just in these two types of experiments would be the following : It was seen in emission in LiCl, NaBr, NaI, because fluorescence measurements were done and could be done with particularly high sensitivity due to the large quenching of the radiative decay of the normal RES, and it was seen in our experiments because the ESR detection method does not depend on whether the decay is radiationless or not.
The relatively long lifetime of the new state as inferred from the saturation behaviour of the ESR agrees with the lifetime measurements of the optical observations. Further experiments must be done, however, to clarify the situation. In this context a very interesting object of study for excited state ESR experiments should be the F-centre in NaBr. Here both excited states have been observed optically (see Table 11), the nanosecond state - corresponding to the conventional RES - would be unobservable in our ESR experiment because of its short lifetime, the microsecond state, however, should be visible in ESR and - according to the hypothesis discussed above -
should show the characteristics of the RES 2 spin reso- nance. Careful investigations are under way. The work was sponsored by the Deutsche Forschungsgemein- schaft.
References
[I] WINNACKER, A,, MAUSER, K. E. and NIESERT, B., Z. Phys.
B 26 (1977) 97.
[2] MAUSER, K. E., NIESERT, B. and WINNACKER, A., Z. Phys.
B 26 (1977) 107.
[3] HAHN, K., MAUSER, K. E., REYHER, H. J. and WINNACKER, A., Phys. Lett. 63A (1977) 151.
[4] REYHER, H. J. HAHN, K., VETTER, Th. and WINNACKER, A., Z . Phys. B 33 (1979) 357.
[5] HAHN, K., REYHER, H. J., VETTER, Th. and WINNACKER, A., Phys. Lett. 72A (1979) 363.
[6] IMANAKA, K., WADA, T., TANAKA, M. and OHKURA, H., J. Phys. Soc. Japan Lett. 45 (1978) 2041.
[7] BERTRAM, R. M. and STONEHAM, A. M., Solid State Commun.
17 (1975) 1593.
[8] TAKIYAMA, K., FUJITA, T., FUJII, A. and NISHI, M., Intern.
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[9] BOSI, L., LONGONI, A. and NIMIS, M., Phys. Status Solidi (b) 89 (1978) 221.
[lo] BALDACCHINI, G. and L u n , F., Intern. Conf. Defects in Insulating Crystals, Gatlinburg. October 1977 (unpu- blished).
[ l l ] BOSI, L., PODINI, P. and SPINOLO, G., Phys. Rev. 175 (1968) 1133.
[12] SCHMID, D., Ph. D. Thesis Stuttgart 1966, and : Physics of of Color Centres, Fowler, W . B., ed. (New York, London : Academic Press) 1968, p. 555.
