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Magnetic circular polarization of the F center emission
H. Ohkura, K. Imanaka, K. Iwahaka, T. Wada, M. Tanaka
To cite this version:
H. Ohkura, K. Imanaka, K. Iwahaka, T. Wada, M. Tanaka. Magnetic circular polarization of the F center emission. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-42-C6-44.
�10.1051/jphyscol:1980611�. �jpa-00220016�
JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 7 , Tome 41, Juillet 1980, page C6-42
Magnetic circular polarization of the F center emission
H. Ohkura, K. Imanaka, K. Iwahana, T. Wada and M. Tanaka
Department of Applied Physics, Osaka City University, Sumiyoshi-ku, Osaka, Japan 558.
Rksumk. - La propriete magndtique de 1'Btat excitk relax6 ( R E S ) du centre F s'est rktlee de l'ktude experimentale et thkorique de la polarisation circulaire magnktique (AMcp) de 1'Cmission du centre F. Les composants para- et diamagnetique ( A , et A d ) de AMcp aussi bien que le facteur g de R E S ont CtC determines expkrimentalement. La comparaison entre ces valeurs et celles obtenues par la thbrie vibronique permet de determiner la constante du couplage spin-orbite, le facteur g de I'orbite et la polarisation de spin de R E S . On a trouve la dkpendance de A, a l'knergie accordante du photon incident, qui indique que la perte de la mkmoire de spin pendant le processus non radiant est significative. La dkpendance de A , a la tempkrature est exprimCe.
Abstract. - The magnetic property of the relaxed excited state ( R E S ) of the F center has been manifested by the experimental and theoretical study of the magnetic circular polarization (AMcp) of the F center emission. The para- and diamagnetic components ( A , and A d ) of AMcp as well as the g-shift of the R E S have been experimentally deter- mined. From the comparison of these values with those computed values on the vibronic theory, the spin-orbit interaction constant, the orbital g-factor, and the spin polarization of the R E S are determined. The tuning photon energy dependence of A , is observed. The spin-memory-loss during radiationless decay is found to be significant.
The temperature dependence of Ad is mentioned.
The study of the magnetic circular polarization (AMcp) of the F center emission manifested fundamen- tally important information on the magnetic structure of the relaxed excited state (RES) of the F center, which has been successfully described on the basis of the vibronic scheme [I]. We have derived the formu- lae of A,,, as well as the g-factor of the RES or the g-shift from the g-factor of the free electron (Ag), by taking into account both the spin-orbit interaction and the vibronic interaction with 2 r:, r:,. and
&-mode phonon interaction in the 2s-2p space [2, 31. It is found that AMcp is represented by a sum of para- and diamagnetic components (A, and Aa), where A, is proportional to the spin polarization in the R ES (P*), and Ad is proportional to applied mnyne- tic field (H,). Both proport'ionality constants are computed consistently with all other experimental data of the RES [2, 31.
The formula of P*, under the saturated optical pumping condition, has been derived [4] from the steady state solutions of rate equations of the optical cycle which consists of the following successive processes; (i) optical excitation from the ground state (GS) to the unrelaxed excited state (URES), (ii) radiationless decay to the RES, (iii) emission from the RES, (iv) radiationless decay to the GS.
The spin-mixing occurs mainly in the preceding three processes. We define the spin-mixing parameters by each of above-mentioned processes : (i) E + and E-(E,), (ii) E,, and (iii) E * . The anisotropy in E, was firstly proposed by Winnacker et al. to precisely
explain the M C D in the F absorption [5]. Now, the formula of P* is derived as [4],
Notice that P * is represented only by the spin-mixing parameters. After symmetry argument in P*, we have pointed out that A M c p is relevant to the polariza- tion of the F pumping light, n, o + , or o-(o*), respec- tively [4]. These are shown as follows,
Therefore, from the experimental data of the magnetic field dependence of AMcp, when 0
<
H ,<
4.0 kCk, for the fixed polarization of pumping light, both A , and Ad can be separately determined. Moreover, Ag is determined from the ESR of the RES : A new ESR method was proposed and successfully tested by detecting the change of A, caused by the resonance microwave transition [6].From the comparison of thus obtained A , and Ad as well as Ag that were observed by us [6] and other authors [7, 81 with their theoretically computed values in the vibronic scheme, the values of the spin-orbit interaction constant
(A),
the orbital g-factor (g,,,), and P* are determined. They are tabulated in table I.These values provide essentially important informa- tion on the magnetic nature of the RES ; namely :
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980611
MAGNETIC CIRCULAR POLARIZATION O F T H E F CENTER EMISSION C6-43
Table I. - The spin-orbit interaction constant
(A),
the orbital g-factor (go,,), and the spin polarization (P*) of the RES are shown. Parenthesis in the 4th column shows the pumping light wavelength. The upper and lower rows for RbCl are due to diferent Ag taken from references [6] and [8], respectively.
KBr 21 .O 1.03 3.90 (6 328 A) 1.9 2 o 2.1 2.2
PHOTON ENERGY [eV]
RbCl 25.1 0.97 4.26 (6 328 A)
14.9 0.97 6'90 (6 328 A) Fig. I. - dp versus tuning photon energy of the F band (dotted
RbBr 30.6 0.76 lines). I E + - E - I and E +
+
E - (solid lines) are taken from refe- rence [12]. E,, calculated from equation (1) are shown.(i) The observed negative large g-shift in the RES is explained in terms of positive large ,I-values shown in table I ; (ii) The fact that the go,,-values are close to unity gives the recovery of reduced quantity due to the vibronic contribution (the Ham effect [9]) ; (iii) Small magnitudes of P * in table I reflect the fact that the Kramers doublet in the RES is not thermalized.
Let us discuss the anisotropy in E*. In early days, it was assumed that E + are isotropic [7]; E + = E-.
Therefore, as seen in equation (I), it has been consi- dered that there exists no spin polarization in the RES under the saturated optical pumping, and thus no A , can be expected to be observable in the measurement of A,,,. This implies that the magnetic field depen- dence of A,,,. would be irrelevant to the polarization of pumping light. So that, when the dependence of A,,, on the polarization of pumping light was observ- ed, no explanation had been achieved. The dependence had remained as a mystery [lo], before we explained it using equations (1) and (2) based on the assumption
E + # E - [4]. The anisotropy in E , is originated from the difference in spin structure in the URES, which consists of two manifolds of states of Ti and Ti symmetry with different energies [ll], reflecting the difference in the selection rule in the optical transition to the URES from the Kramers doublet in the GS denoted as m , = f (112). Therefore, the amount of anisotropy depends on the photon energy of the F absorption as was calculated for KBr by Mauser et al. [12]. So, from equation (I), we predict that both P * and A , are dependent on the tuning photon energy of the F band.
The predicted photon energy dependence of A , has been observed for KBr using a tunable dye laser, SP Model 375 with R6G, pumped by an Arf ion laser, SP Model 165. The observed A , are plotted in figure 1 as a function of tuning photon energy, in which the F band and the computed values of
E + f E- [12] are shown. Observing figure 1 and
equation (I), one may suspect that E , ~ should be dependent on tuning energy ; the values of E,, comput- ed from equation (1) with the values of E + f E-
is also plotted in the same figure. Observing that the magnitudes of 8,'s are so large, we emphasize that the optical cycle cannot be di-cussed without taking into account the spin-mixing during the radiationless decay process. Notice that this spin-memory-loss may be frozen in the unthermalized RES to reduce P * consequently. The fact that the values of E,, are enlarg- ed at higher energy side may be understood if consider- ing that more levels are involved in the radiationless decay process in the case of higher tuning photon energy excitation.
Fontana could not observe A , for KC1 in the measu- rement of AMcp using 5 145 A line of a cw Ar+ ion laser [13]. Observing figure 1, we predict that A , for KC1 can be observable using pumping light of longer wavelengths. The prediction has been really observed in KC1 at 4.2 K by adopting tuning light of 5 750 A. The result shows A , = 1.2 x Using this value, P* is calculated as listed in table I.
The value is rather large of a few percent, and is ten times larger than that obtained from A , which was observed by different method using 5 145 A line of a cw Arf ion laser [lo].
Finally, we have measured the temperature depen- dence of Ad in KBr from 8 to 100 K. It shows abrupt decrease of about 15
%
till 20 K, above which no change is observed. Moreover, no such a characteristic change was observed for KCl. Notice that such a characteristic temperature depe~dence of Ad in KBr cannot be explained by a weak coupling model of the vibronic theory [14]. A physical origin to describe plausibly this characteristic difference may be due to the large spin-orbit interaction in the RES ; the value of I is so appreciable as shown in table I. The p r - e - calculation based on the vibronic scheme is under way.H. OHKURA, K. IMANAKA, K. IWAHANA, T. WADA A N D M. TANAKA
DISCUSSION
Question. - D. SCHMLD. Question. - U . M. GRASSANO.
I am surprised about your statement that you found a large spin memory loss in the optical pumping cycle, since this contradicts the previous results of various groups for instance of Mollenauer, Winnacker et a/.
Can you comment on this discrepancy.
Reply. - H. OHKURA.
The discrepancy of spin-memory loss parameter (E) between the Mollenauer school and ours is due to the different methods for determination. They determined it from the MCD in F absorption with some assump- tions which we suspect dubious. Recent theoretical computations of E in the URES for KBr at 6 328
A,
that is approximate equivalence of 0.04 by Mollenauer, are given by Mauser et al. (semi-classical) and Mura- matsu (quantum mechanical) as 0.16 and 0.29, respectively. This result supports our large E. On the same basis, large E,, value, which was proposed in this report, can be approved. The solution of discrepancy is one of future projects to clarify the magneto-optical processes in the F optical pumping cycle.
I would like to add a comment. Analogous results on M.C.P. of F centre luminescence in K1, KBr, KC1 have been published by Baldacchini, Tanga and myself in recent years. Our interpretation, however, based on Ham's theory and Winnacker's spin memory anisotropy leads to sets of parameters for the RES and for the spin memory different from those presented here.
Reply. - H . OHKURA.
Although the experimental data of A, are the same each other, the method of determination of P* is different from your case. As shown in this report, we determined P* by computing the proportionality constant between
A,
and P* on the basis of the intermediate coupling vibronic scheme to be consistent with almost all other experimental data of the RES.The orbital g-factor, 2 , and P* as tabulated in table I are determined on the same bases. Besides, as shown in equation (I), our P* includes E,, that is different c * ~ m your case.
References
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