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STUDY OF RAMAN SCATTERING BY KI, NaI AND
CsBr DOPED WITH F CENTERS
L. Taurel, J. Buisson, M. Ghomi, S. Lefrant, A. Sadoc, J. Chapelle, M.
Billardon
To cite this version:
G7-106 JOURNAL DE PHYSIQUE Colloque C7* supplément au n° 12, Tome 37, Décembre 1976,
STUDY OF RAMAN SCATTERING BY KI,
Nal AND CsBr DOPED WITH F CENTERS
L. TAUREL, J. P. BUISSON, M. GHOMI, S. LEFRANT, A. SADOC, J. P. CHAPELLE
Laboratoire de Physique Cristalline (*) Universite de Paris-Sud, bailment 490, 91405 Orsay Cedex, France
M. BILLARDON
Laboratoire d'Optique Physique de 1'ESPCI (**) 10, rue Vauquelin, 75231 Paris Cedex 05, France
Résumé. — L'interprétation des spectres Raman de Kl, Nal et CsBr dopés avec des centres F par excitation lumineuse dans la bande F met en évidence la rupture des règles de sélection usuelles établies en dehors des conditions de résonance. L'étude sous champ magnétique du mode résonant (96 cm-1) dans Kl montre que la dépolarisation de ce mode provient du couplage spin-orbite relatif
à l'état excité « 2p » du centre F.
Abstract. — Interpretation of Raman spectra of KI, Nal and CsBr doped with F centers under
light excitation in the F band involves the breaking of the usual selection' rules established in off
resonance approximation. A study of the 96 cm- 1 resonant mode in KI under magnetic field shows
that the observed anomalous depolarization of this mode comes from the spin-orbit coupling in the 2p-like excited state of the F centre.
The main interest of the study of Raman spectra stimulated by F centers in alkali halides under exci-tation in the F-band is the investigation of the break-ing of the usual selection rules established in the off-resonance approximation. We report some new results about first-order Raman scattering in KI, Nal and CsBr doped with F centers concerning this property. It is well known that in these doped crystals first-order Raman scattering results mainly from the modu-lation of the F-center electronic polarizability induced by phonons throughout the Brillouin zone. The usual selection rules show that the Raman active modes are of the symmetry types Al g, Eg and T2g [1 ]. In all the
following experiments the incident and scattered light directions were at right angle, and three obser-vation cases allowed the spectra due to the modes Al g + 4 Eg, 3 E , T2 g to be separated [2]. Calculations
of first-order Raman peak frequencies have been done assuming that the variation of the polarizability is due only to the vibrations of the nearest neighbours of the F centre. Therefore, in the case of KI and Nal three type modes (1 Al g, 1 Eg, 1 T2g) are Raman
active while CsBr [3] involves four type modes (1 Al g,
1 Eg, 1 T^t, 1 T ^ ) . Moreover, calculations of the
perturbed Green's function matrix were performed (*) Equipe de Recherche Associee au C. N. R. S. n° 13. (**) Equipe de Recherche n° 5 du C. N. R. S.
using the parameters [4] which describe the modifi-cation of the longitudinal force constant between the F-center and its 1 nn's and between the 1 nn's and the 4 nn's
/ . Af,(1) Af<4)\
Experimental results. — 1. KI AND Nal : (F
CEN-TERS). — Raman spectra, studied under excitation
with He-Ne Laser light (A = 632.8 nm) which falls in the F band (AF = 666 nm in KI, X¥ = 595 nm in Nal
at 80 K), have been previously published [5, 6, 7, 8]. The main result is the existence, in both crystals, of a very strong sharp resonant line (coR = 96.5 c m- 1
in KI, coK = 113 c m- 1 in Nal) just above the
well-resolved gap region. Calculations in the off-resonance approximation with X = — 0.67, y — 0.05 in KI and A = — 0.67, y = — 0.20 in Nal account for expe-rimental peak frequencies and for the strong reso-nant mode [6, 8]. However, while theory predicts an Al g symmetry type for this mode, experiment shows
that it is observed also for orientations allowing the modes Eg and T2g to be observed. A study of the
depolarization ratio p of the resonant mode was done according to the wavelength of the light emitted from a dye-laser which could be tuned between 590 and 650 nm. This study shows that the p value is
STUDY OF RAMAN SCATTERING BY KI, NaI AND CsBr DOPED, WITH F CENTERS C7-107
mum when the wavelength of the laser light appro- aches to that of the F band maximum
(NaI.: p
-
22%,
I,,,,, =A,),
and almost vanishes in the off-resonance excitation (p 7
%
in NaI, I,,,,, = 650 nm).2. CsBr (F CENTERS).
-
The case of CsBr is very interesting because of the existence of a structure in the F band. This band is mainly formed of two bandsFa (616 nm) F, (640 nm) and absorption band shape
has been explained by taking account of the spin- orbit coupling of the F center in its 2p-likeexcited state [9] and a strong dynamic Jahn-Teller effect [lo]. Stokes Raman spectra obtained under excitation in the Fa band (;1,,,,, = 610 nm) are shown in figures 1, 2, 3. The Stokes first-order Raman peaks at 39,
55, 75 cm-I are due to acoustic phonons, while the broad band 90-105 cm-' was attributed to the super- position of first-order scattering by optical phonons and second-order scattering due to additive combi- nation between acoustic modes [ll]. A good agree- ment between calculated and experimental frequencies was found using I = - 0.60 (Fig. 1, 2, 3). However, the 55 cm-I peak in the << E, spectrum )> (Fig. 1)
cannot be explained by the theory. Moreover, Raman spectra show dramatic variations with Laser light wavelength and, in particular the anomalous 55 cm-I
0 40 80 1 20
v (crnl)--+
CsBr +Fcentres (Stokes)
FIG. 1. - CsBr (F-centers). Comparison between experimental (AL = 610 nm), (El// [lie], Ea
//
[I 101 cc Eg )) spectrum in off resonance approximation) and calculated Eg spectrum (T= 10 K). The calculated intensity is adjusted at 40 cm-1. Ei, Ed = polari- zation of incident and scattered light. In the top right : F absorp-tion band (T = 10 K).
v (crn-')-
C s B r +Fcentres (Stokes)
( x / / [ i i o i , v / / [ i i o l , z //[ooil)
FIG. 2.
-
CsBr (F-centers). Comparison between experimental(Ei/ [OOI], E d / [OOl], cc A1.g
+
Eg )> spectrum) and the cal-culated Alg and Eg spectra (in arbitrary units). H E , )) line intensity decreases at the same time than
the 55 cm-' line due t o T$:' phonons. Jahn-Teller effect could perhaps explain this result [12, 141.
3. RAMAN SCATTERING UNDER MAGNETIC FIELDS
(Spin-flip). - A full interpretation of the results obtained in CsBr is not obvious and our aim is now to show that the spin-orbit coupling in the 2p-like excited state can explain the depolarization of A,,
modes as we have previously suggested in the case of the resonant mode in KI (F centers) [8]. Indeed, in this last case, no Jahn-Teller effect exists and the elec- tronic levels of the F center are similar to those of an alkaline atom ; therefore, mixing of the degenerate
C7-108 L. TAUREL, J. P. BUISSON, S. LEFRANT, A. SADOC, J. P. CHAPELLE AND M. BILLARDON
v(cm-')- C s B r + F centres (Stokes)
( x / / [ i i 0 1 , ~ ~ ~ i 1 0 1 , ~ ~ [ 0 0 1 1 1 FIG. 3.
-
CsBr (F-centers). Comparison between experimental(Ei
//
[OOl], Ed// [1 101. a Tzg )) spectrum) and calculated T&!, Ti:) spectra ( T = 10 K). (C coefficient accounts for therelative intensity of the two peaks).
FIG.. 4. - Raman scattering of KI (F-centers) for the experi- mental arrangement sketched top right (x, y, z = four-fold axes)
(rl~aber = 620 nm, laser power : 140 mW). T = 1.7 K : A) Raman scattering without magnetic field ; B) Rarnan scattering under
4.5 T magnetic field.
would lead to the only determination of T2, modes.
When H = 0 the Stokes spectrum is formed of two sharp lines one at 96 cm-I and its first-overtone at 193 cm-l, the large broad band at 130 cm-' was partially attributed to T,, modes [8]. Under magnetic
field, an emerging feature of the experiment turns out to be the splitting of both 96 cm-' and its first-overtone. This experiment shows that, for this observation case, Raman scattering involves a spin-flip [13] and the sepa- ration Aw between the two lines (Am E 7.9 cm-l) is
in good agreement with the expected value
( A o = 2 gPH 2: 8.4 cm-l).
References
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121 COUTURE, L. and MATHIEU, J. P., Ann. Phys. (Paris) 3
(1948) 521.
[3] MARTIN, T. P., J. Phys. C 4 (1971) 2269.
[4] GETHINS, T., TIMUSK, T. and WOLL, J., Phys. Rev. 157 (1967) 744.
[5] BUISSON, J. P. and TAUREL, L., Phys. Status Solidi (b) 63
(1974) K81.
[6] burs so^, J. P., GHOMI, M. and TAUREL, L., Solid Slate
Commun. 18 (1976) 513.
[7] PAN, D. S. and L ~ ~ T Y , F., Light Scattering in Solids- 3th Int. Conf. Campinas (1975). Ed. by M. Balkanski,
R. C. C. Leite, S. P. S. Porto (Flammarion) 1976, p. 539.
181 Idem [7], p. 587.
[9] MARGERIE, J. et ROMESTAIN, R., C. R. Hebd. Sgan. Acad.
Sci. 258 (1964) 4490.
[lo] MORAN, P. R., Phys. Rev. 137 (1965) 1016.
[I11 LEFRANT, S., BUISSON, J. P. and TAUREL, L., C. R. Hebd.
SPan. Acad. Sci. 282 SCrie B (1976) 407. [12] MULAZZI, E. Idem [7], p. 567.
[13] BILLARDON, M., RUSSEL, M. F., BUISSON, J. P., LEFRANT, S.,
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1141 MULAZZI, E. and TERZI, N., Solid Stare Commun. 18
