DICHROIC EFFECTS IN THE F-CENTER LUMINESCENCE IN KBr

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DICHROIC EFFECTS IN THE F-CENTER

LUMINESCENCE IN KBr

G. Baldacchini, U. Grassano, A. Tanga

To cite this version:

G. Baldacchini, U. Grassano, A. Tanga. DICHROIC EFFECTS IN THE F-CENTER

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C7-154 JOURNAL DE PHYSIQUE Colloque C7, supplément au n° 12, Tome 37, Décembre 1976

DICHROIC EFFECTS IN THE F-CENTER LUMINESCENCE IN KBr

G. BALDACCHINI, U. M. GRASSANO (*) and A. TANGA(*) Laboratori Nazionali di Frascati del CNEN,

CP 70, 00044 Frascati, Italy

Résumé. — La polarisation magnétique circulaire de l'émission du centre F a été mesurée dans le KBr à la température de 1*9 K et dans un champ magnétique jusqu'à 80 kG.

L'effet diamagnétique, linéaire avec le champ jusqu'à 60 kG, donne la valeur Cd = (17 ± 2) X 10-s G - i .

L'effet paramagnétique est dû à la modulation du pompage optique entre <x+ et a~. Celle-ci

produit une polarisation modulée des spins du niveau excité relaxé. La dépendance de la fréquence de modulation de l'effet paramagnétique est en accord avec les prévisions théoriques dérivées des équations du cycle du pompage optique du centre F.

Abstract. — Magnetic field induced circular polarization of the F-center emission has been measured in KBr at 1.9 K and in fields up to 80 kG. The diamagnetic effect, linear in the field up to 60 kG yields the value of Cd = (17 ± 2) X 10~8 G~i. The paramagnetic effect is due to the

modu-lation between a+ and o- of the optical pumping. This induces a modulated spin polarization in the relaxed excited state. The frequency dependence of the paramagnetic effect is in agreement with the prevision deduced from the rate equations of the optical pumping cycle of the F-center.

1. Introduction. — The importance of the small magnetic circular dichroic effects in the emission of the F center was recently stressed by Ham and Grev-smiihl [1, 2] in connection with a theoretical approach to the problem of the relaxed excited state (RES). Indeed intense experimental [3, 4, 5] and theoreti-cal [6, 7, 8] efforts have not been able to give a com-plete picture of the RES [9, 10, 11].

The first measurement of the dichroic properties in the"'luminescence was made by Fontana and Fit-cKe'n iti K F [12] and KC1 [13]. They discovered an effect,-linear in the magnetic field, that is known as

Mafh&gnetic effect. Baldacchini and Mollenauer [14]

observed successively the same effect in other alkali halides and for the first time detected a dichroic emission which is associated with an electron spin polarization in the RES. This second effect is known as

paramagnetic effect.

Recently experimental data on the diamagnetic and paramagnetic effects have been obtained for the F-center in KI (G. Baldacchini, U. M. Grassano and A. Tanga, to be published). In this paper we present the results of both effects for the F center in KBr, at 1.9 K and in magnetic fields up to 80 kG.

As pointed out above the so called paramagnetic effect is closely connected to the electron-spin pola-rization Pp, of the RES. It has been shown [15] that a pumping beam with steady polarization cannot produce simultaneously a finite polarization and a significant population of the RES.

However a spin polarization can be obtained if the beam is modulated between right and left circular

(*) Istituto di Fisica, Universita di Roma and Gruppo Nazio-nale di Struttura della Materia del C. N. R.

polarization, a+ and a". The amplitude of the oscil-lating polarization, as calculated from the rate equa-tions [15], is strongly dependent on the modulating frequency. An almost exact solution for the ampli-tude, Pp, has been presented elsewhere (G.

Baldac-chini, U. M. Grassano and A. Tanga, to be publish-ed). The values of Pp in particular ranges of frequency are as: follows :

I. co x T- 1

\Pp\ = {l-2e)Ps(l+co2x2ril2

II.T'1 < co < TT1

| P J = ( l - 2 e ) Ps ( 1 )

III. cox Tf1, eVXIITi), | P„ | = (1 - 2 8) Ps coTp(l + co2 If)-"1 VI. co -> 0

| P „ | - ( 1 - 2 £ ) P8( l + £ C 7 Tir1

where s is the electron-spin memory during the opti-cal cycle, Ps the ground state polarization induced by

a steady intense pump of <x+ or a~ light, Tr the spin-lattice relaxation time of the ground state, 1/TP = sU,

U the total pumping rate out of the ground state, T the

radiative lifetime of the RES, and 7V"1 = 7 T1 + T~x. The various solutions given in (1) have been obtain-ed by making the approximations T (-=• + —-) -4, 1, and T/T1 P -4 1. The first inequality is well

satisfi-ed at the pumping level ussatisfi-ed in the experiments,

U ^ 102-104 s_ 1, since z m 1(T6 s and Tf1 < 102 s

for the highest field used [16]. The second one, where

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DICHROIC EFFECTS IN THE F-CENTER LUMINESCENCE IN KBr C7-155

T is the spin-lattice relaxation time in the RES, is '/I

satisfied at least at low magnetic fields, B

<

30 kG [IS]. From solution (I) we deduce a polarization, which goes to zero both at high frequencies (1, I) and low frequencies (1, IV). In fact, at low frequency P, depends strongly on the pumping power, cc U, but for EUT, % 1 the polarization goes pratically to zero. In region (1,II) P, assumes a well defined value, while the frequency at which P , decays towards low fre-

quencies is a linear function of the incident power,

cc U.

In the following, beside the result of the diamagnetic effect, we will show that the paramagnetic effect is strictly related to the P,, values described above.

2. Experimental.

-

Samples of KBr, used in the course of the experiments, were home grown by the Kyropulos method, additively colored

(NF

cz 4.5 x loi6 cmF3), quenched in liquid nitrogen, cleaved to

-

1 mm thickness and mounted in the bore of a superconducting magnet in an optical dewar 1171.

2.1 DIAMAGNETIC EFFECT.

-

Figure 1 shows the block diagram of the experimental apparatus, as used

S A M P L E

l-4

OSCILLATOR I - V

LOCK- IN _CONVERTrn

FIG. 1. -Block diagram of the experimental apparatus for measuring the diamagnetic effect. By a simple exchange, see

text, the paramagnetic effect can also be measured.

to detect the diamagnetic effect. A simple substitu-

tion enables the same apparatus to measure the paramagnetic effect, as we will show later. An He-Ne laser, which delivers N 18 mW in the TM,, mode is used

for exciting the crystal. Its wavelength, 6 328 A, cor- responds closely to one of the two dichroic peaks of the F-center absorption 1151. This condition is essen- tial in order to have a polarization in the RES and hence the results (1). The laser beam is linearly pola- rized, x, filtered by a KG-3 Schott color filter to block any IR emission from the laser, and focused on the sample inside the optical dewar. The direction of the

beam is axial to the magnetic field. The luminescence at 1.35 p is focused on a germanium diode cooled at 0 OC.

A

RG-1 000 Scott color filter removes from the luminescence beam any trace of the 6 328

A

light.

The diamagnetic effect is defined as :

where the superscript

+

or

-

refers to the a+ or a-

polarization of the luminescence. The dichroic difference in (2) is analyzed by a combination of a

+

4 4 stress plate modulator [18], which oscillates a t

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C7-156 G . BALDACCHINI, U. M. GRASSANO AND A. TANGA

N 20,kHz, and an IR linear polarizer, HR Polaroid,

with axisgat 450 to the stress plate. This analyzer transmits alternately the emitted I+ and I - light. The

difference signal I+

-

I - is detected by the lock-in and

recorded, while I + f I - is simultaneously measured

after thezcurrent- to- voltage converter, on a different chart recorder. Thus the diamagnetic signal (2) is given directly by the ratio of the two chart recorder outputs.

Figure 2a, shows the signal S,, as a function of the magnetic field. The diamagnetic signal is linear in the field up to

--

60 kG, as found for K F [12], KC1 [13] and KI 1141. Above 2 60 kG a non linear dependence is clearly observable. This unexpected result, analogous but much smaller than in KI, cannot be attributed to a change in the pumping intensity. The luminescence recorded at the same time, figure 2b, does not show any variation at high magnetic field ; the small decrease near zero field is probably due to the Porret- Liity effect [19]. The linear slope in figure 2a, shown at high fields by a dashed line, is given by :

with Cd = (17

+

2) lo-* G-'

.

The value of Cd is the result of an average among many different measurements.

2.2 PARAMAGNETIC EFFECT.

-

TO measure the paramagnetic effect the

1

114 modulator (an electroop- tical device in this case) is inserted in the laser beam after the linear polarizer, see figure 1, while a fixed A14 is mounted in front of the linear polarizer in the luminescence beam. We define the quantity :

where the superscript

fi

is

+

or

-

depending on whether the analyzer in the luminescence beam is set to transmit a" or a- light and the subscript refers to the

pumping light that is switched between a f and a-

polarization. The electronic detection of the signal

S, is analogous to that of Sd.

Figure 3 a shows the signals S: and

Si

as recorded as a function of the magnetic field. The step structure is due to the fixed 214 plate on the luminescence beam which is set to reveal alternately the a+ and a - emis-

sion, while the magnetic field is slowly varying. The paramagnetic signal which is due to the spin polarization, P,, is AS, = (s:

-

Si)/2, while the quantity Sa = (s:

+

S i ) / 2 arises from the differential absorption of the a+ and a- pumping light. Indeed Sa

can be obtained even without the analyzer (214

+

linear polarizer) in the luminescence beam. We will discuss in detail in a future paper the dependence of Sa on the absorption dichroism and on the intrinsic properties of the F-center.

The intensity of the luminescence recorded at the same time as the paramagnetic signal is reported in

FIG. 3. - Paramagnetic signal a, and luminescence intensity, 6,

as recorded at 20 kHz and pump power

-

7.5 mW.

figure 3b. The small decrease at zero field has presu- mably the same origin as that in the diamagnetic measurement. The difference between the a" and a- emission, increasing with B, is a further check for the diamagnetic effect. Indeed the pumping beam modulated between a" and a- is equivalent for the d. c.

luminescence to a stationary incident light. In this condition, when the A/4 plate is alternately rotated by 900, the variation of the d. c. output of the apparatus reveals ( I f - I - ) and hence the diamagnetic signal Sd (2).

The values of Cd evaluated from figure 3b coincide with those obtained by the diamagnetic measurements, figure 2a.

Figure 4 shows the true paramagnetic effect AS, versus the modulation frequency of the

+

A/4 plate. The measurements are taken at 20 kG and at two pumping powers, 7.5 mW and 0.75 mW. Clearly we expect a behaviour of AS, versus frequency similar to that of P,(w) in (1). This would indicate that AS, is linearly related to the spin polarization of the RES.

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DICHROIC EFFECTS IN THE F-CENTER LUMINESCENCE IN KBr (3-157

1

lo2 10' lo4 10 modulation f r e q u e w [HZ) +

FIG. 4. - True paramagnetic effect versus the frequency modulation at B = 20 kG. Dots pumping power 7.5 mW ;

triangles, pumping power 0.75 mW.

[ll], obtained with the technique of single photon delay distribution. The paramagnetic signal is cons- tant at intermediate frequencies as required by (1-11) and increases smoothly to a bigger value at low frequencies. This behaviour shows strong analogies with the results of AS, in KI. Practically instead of having a signal that goes to zero for high pumping power (I-IV), we find in both crystals a well defined limit value of AS, which was independent of U at

low frequencies. Presently we do not have any expla- nation to offer for such behaviour ; further experimental data and discussion will be presented in the near future.

We wish to emphasize that the expected behaviour around o

-

T,-' = EU, (1-111), is well satisfied. Indeed because of the smaller value of E in KBr than in

KI, the frequency at which AS, begin to change towards the low frequencies must be smaller in KBr than in KI.

3. Discussion.

-

A side from the unexpected low

frequency value, the paramagnetic signal AS, behaves according to the theoretical predictions expressed in analytical form by (1). This is a further proof of the origin of the paramagnetic effect which is due to the polarization of the RES, whose value is

I

P,

I

= (1

-

2 E ) P, with P, E 0.15 and e = 0.04 [15].

Several values of AS, in the frequency range (l/Tp)

<

o

<

(l/z) were taken at

--

20 kG. The average value of these measurements is

Knowledge of the diamagnetic effect,

1

C,

I,

and the paramagnetic effect

1

AS,

1,

is important in order to clarify the nature of the RES, as stressed by several theoretical works [l, 2, 6, 7, 8, 121. Indeed knowledge

of these two effects enables one to calculate the spin- orbit coupling 1, the energy separation between the low lying states in the relaxed configuration and other fundamental parameters [l, 21. It is clear that this represents a further check of the proposed theoretical models of the RES. Without going into numerical calculations, which will be done elsewhere, we would like to say that the value of the spin-orbit coupling implied by the experimental results of the diamagnetic and paramagnetic effects is of the order of one meV. This value for ;1 seems to be incredibly low expecially in comparison with the value A =

-

19 meV measured in absorption [20]. An analogous result was obtained for F-centers in KI. On the other hand at the moment, we are reluctant to believe that the rate equations are in error [15] ; indeed they explain several experimental results including the dependence of P , on the modula- tion frequency.

In order to have a more clear understanding of these phenomenawe are extending similar measurements to F-centers in KCI. This system is the most thoroughly investigated and therefore it will be pos- sible to compare the properties of RES deduced from MCD experiments with those obtained from experiments with external perturbations due to electric or stress fields [8, 211.

Acknowledgments. - The authors are deeply indebted to P. Cardoni and I. Giabbai for valuable technical assistance. Two of us, U. M. G. and A. T. are grateful to the laboratories of C . N. E. N. in Fras- cati for the hospitality during the completion of this work.

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