STUDIES OF VK CENTERS IN CsI BY OPTICAL AND EPR TECHNIQUES

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STUDIES OF VK CENTERS IN CsI BY OPTICAL AND EPR TECHNIQUES

T. Sidler, J. Pellaux, A. Nouailhat, M. Aegerter

To cite this version:

T. Sidler, J. Pellaux, A. Nouailhat, M. Aegerter. STUDIES OF VK CENTERS IN CsI BY OP- TICAL AND EPR TECHNIQUES. Journal de Physique Colloques, 1973, 34 (C9), pp.C9-89-C9-91.

�10.1051/jphyscol:1973914�. �jpa-00215389�

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JOURNAL DE PHYSIQUE

Colloque C9, sirpplf'nzent

uu t z O

11-12, Tonze 34, Nouetnbre-Dkcembre 1973, page C9-89

STUDIES OF V, CENTERS IN CsI BY OPTICAL AND EPR TECHNIQUES

T. S I D L E R , J. P. PELLAUX, A . N O U A I L H A T (*) and M. A. A E G E R T E R Institut de Physique, Universitk de Neuchgtel

2000 Neuchiitel, Switzerland

RBsurnC.

-

Les centres V I ~ ont eli: observes daiis Csl

:

Na et Csl

:

TI apres irradiation X a la temperature de I'hClium liquide par des nlethotlcs optiques et de RPE. Leur transition principale est situee a 410 nm et ils sont oricntes dans les directions [100]. Nous avons trouve deux types de migration thermique

:

un deplacemcnt lineaire le long d'un axe du cube (saut de 0°) et une migra- tion par saut de 900. Ces deux migrations sont associees a deux pics de therrnoluminescence a 60 K et 90 K respectivement et correspondent a une recombinaison radiative des centres V I ~ mobiles a I'endroit des trappes electroniques (Na"

i =

420 nm, TI0

i .=

560 nm). Des experiences similaires ont ete effectuees dans CsI colore additivenient. La reconibinaison radiative Vlr-F donne lieu a une nouvelle luminescence situie

h

495 nm.

Abstract. - V n

centers have been observcd in Csl doped with Na+ and TI+ after X ray irra- diation at LHeT using optic~il and E P R techniques. Their main optical transition is located at 410 nm and they are oriented in [I001 directions. Two types of thermal migration have been found

:

a linear displacement along the cubic axis (Oo jumps) and a migration with 900 jumps. They are related to two glow peaks at 60 K and 90 K respectively and which correspond to the radiative recombination of mobile VJ; centers at the site of the electron traps (Nao

=

420 nm, TI"

i,

- 560 nm). Similar expcririients have been done in Csl additively colored. The radiative recombinatio~i V I ; - F gives rise to a new luminescence at 495 nm.

The existence of V, centers in alkali halides with CsCl structure has never been explicitly demonstrated although it can be inferred indirectly. In pure Csl for instance the fact that the intrinsic luminescent compo- nents a t 290 n m and 338 nni excited by U V light in the excitonic bands can also be observed under ionising irradiation (X, 8) shows that the relaxed exciton states produced directly (without the isolated 1; molecular- ions) is identical to the metastable states formed by an electron in presence of

;L

preexisting V, center [ I ] . The formation of stable selr-trapped holes iuust necessarily be accompanied by the trapping of a n equal amount of electrons on specific traps. Tliese traps may either be created duriiig the ionising irra- diation (anionic vacancies for instance) or may preexist in the crystal a s impuritie.s. The first possibility is ruled out for Csl, since no structure defect has been ever observed in a stable configuration under tlie most severe ionisi~lg irradiation conditions. We have tliere- fore used CsI doped with N a i (200 to 300 ppm in the melt, Teledyne Isotopes) or T I + ( 1 000 ppm in the melt, K. Korth) [2].

Figure I shows the absorption spectra observed after X irradiation (20 min for Csl

:

TI and I

11iin

for CsI

:

Na) at 10 I<. Tlic band centered at 410 nrn is found independent of tlie n ~ t u r c of t h e impurity and has been attributed to tlic U V transition

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Visiting scientist

on Icavc

from

the I n \ l i t l ~ l

Nalional

des

Sciences

A p p l i q ~ ~ c e s ( I N S A ) , Lyon, I'rancc.

FIG.

1. -

Optical absorption after X-ray irradiation

at

LHeT.

N )

CSI:

N a ' ,

b) Csl: TI'.

'c;

-

'c: of tlie self-trapped holes. The bands appearing a t longer wavelengths are related to tlie specific impurity introduced in the crystal. It is

\vorthwhile to mention that N a " beliaves in this matcrial as a n cscellent electron t r ~ ~ p . TIic new absorption observed at 720 nni is probably rel:~tcd to N a O centcrs. These defects appcxr

:IS

tlic first isoelectronic ticl'ccts ever- createti i n alk;~li halicics.

Bleaching with light propagating ~1lo11g tllc [IOO]

direction ; ~ n d polarized a 1 0 1 1 tlic [ O O I ] clircction

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973914

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C9-90

T.

SIDLER,

J. P. PELLAUX, A. NOUAILHAT

AND

M.

A. AEGERTER

(CsI

:

Na i .,,,,,,, ,,, =406 nnl, Csl :TI 1 606 nm) renders dichroic the V, absorption at 410 nni. We report in figure 2 the dichroic spectrum

u,,,-a,,,

in this spectral region. EPR nleasuren~ents [3] confirm the presence of stable V, centers in these crystals. Figure 3 shows the resonance spectrum of non aligned V, centers. The eleven lines marked on the figure suggest indeed an interaction with a total nuclear spin of 5 (nuclear spin of '271 is 5, natural abundance 100 %).

Similar polarized bleaching results in an almost completely aligned V, center system along a [loo]

direction.

FIG.

2. - Dichroic absorption spectra acolo,-acooll for V I ~ centers obtained by bleaching with (001) polarized light propagating along (100) direction.

a ) CsI : Na+ &,leaching = 406 nm, 6 ) CsI :

TI+

Iblcnching = 606 nm.

FIG. 3. -

EPR

spectrum of unaligned Vli centers in CsI :

TI+

H 11

(loo), v ⁼9.523 GHz.

Thermoluminescence measurements of these crys- tals X or /3 irradiated at LHeT show two peaks at 60 K and 90 K for a heating rate of 1.4 K/min (Fig. 4).

The emission spectrum of both bands is located at

420 nm for Csl

:

Na and corresponds to the emission attributed to the radiative recombination of localized excitons near this impurity [4]. For CsI

:

TI we observe an emission at 560 nm which is specifically related to the nature of this impurity. It is worthwhile to mention that in alkali halides of NaCl structure only one glow peak is observed [ 5 ] .

FIG.

4. - Glow curve for CsI :

Na+

after

t

rnin irradiation

B

at LHeT.

These properties can be explained by a double migration of thernially activated V, centers followed by a radiative recombination with the trapped elec- trons. The first peak is associated to a linear migra- tion of V, centers (0° jumps). The dichroic optical absorption and EPR spectrum of aligned V, centers measured at LHeT after a thermal annealing at 70 K present only a global reduction of their intensity.

This means that tlie remaining V, centers have conserved their alignement. The second glow peak is however associated with V, migration with reorien- tation by 900 ju~nps.

Preliminary experiments give evidence that the same double mobility mechanisni 1s common to VK centers in all alkali halides with CsCl structure.

Similar experiments [6] in crystal additively colored i. e. containing F centers confirm tlie above ideas.

They give rise however to a new emission band at 2.5 eV which cmi be observed in tl~ermolunii~iesce~ice and radioluminescence experiments at temperature at which the V, centers are mobile. This new lumi- nescence is attributed to the radiative recombination of mobile V, centers with F centers.

This work has been supported by the Swiss Natio- nal Foundation for Scientific Research (FNSRS).

References

[I]

LAMATSCH,

H., ROSSEL, J. and SAURER,

E.,

Plij*.r. Stnt. [4]

LAMATSCH,

H., ROSSEL, J. and

S A U I ~ E R ,

E., PIIJ'S. SI(1f.

Sol. ( 6 ) 48 (1971) 31 1. Sol. ( h ) 46 (1971 ⁾ 687.

[2]

SIDLER, T., PELLAUX,

J. P., NOUAILHAT, A. and

AEGERTER,

[5] KELLER, F. .J. and

M U R R A Y ,

R . B., Pl~ys. Rev. 150 (1966)

M . A., Solid Stntc, Cotizti11111. 13 (1973) 479. 670.

[3]

PILLOUD,

J. J., JACCAKD, C. and AEGERTEK,

M.

A,, private [6]

PELLAUX,

J.

P.,

SIDLER, T.,

NOUAILHAT,

A. and

AEGERTI:K,

communication, to be published.

M. A.,

to appear in Solirl Stnte Conzt?~ro~. (1973).

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STUDIES O F

Va

CENTERS IN CSI

BY

OPTICAL A N D

EPR

TECHNIQUES

DISCUSSION

G. REUTER. - The V, band in CsI shows dichroism after bleaching with polarized light. Is this behavior due to reorientation of the centers or to recombina- tion with electron traps ?

M. AEGERTER.

^-

It depends on the wavelength of bleaching. In CsI : Na we have bleached with polarized light at 406 nm, i. e. in the UV transition of the V, centers. In this case the dichroism is due t o a destruction of V, centers aligned in the direction of the electric field of the polarized light. For CsI