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Inelastic light scattering of the VK- and H-centre in alkali halides

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Inelastic light scattering of the VK- and H-centre in alkali halides

E. Goovaerts, L. de Schepper, D. Schoemaker

To cite this version:

E. Goovaerts, L. de Schepper, D. Schoemaker. Inelastic light scattering of the VK- and H- centre in alkali halides. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-472-C6-475.

�10.1051/jphyscol:19806123�. �jpa-00220031�

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JOURNAL D E PHYSIQUE Colloque C6, supplément au n° 7, Tome 4 1 , Juillet 1980, page C6-472

Résumé. — On a mesuré les spectres raman du centre VK dans le LiF, le KC1 et le RbCl. Les spectres consistent en une seule ligne étroite à 437 c m "1 de la fréquence du laser dans le LiF, à 241 cm ~ ' dans le KC1 et à 244 cm "1

dans le RbCl [1]. Le mode de vibration fondamental de la molécule X J ( X- : ion halogène) produit le signal raman, ce qui est vérifié entre autres par comparaison avec la fréquence de vibration calculée de ces molécules libres [2], ainsi qu'avec les fréquences mesurées d'autres systèmes ressemblants. Un calcul de la fréquence de vibration du centre VK dans la série des fluorures alkalins ainsi que dans celle des chlorures alkalins est tenté. Il est fondé sur une théorie semi-empirique fournissant l'énergie configurationnelle du défaut. Le centre H a été produit dans des cristaux de KC1, dopés d'impuretés OH~, par excitation ultra-violette. Un signal raman à 266 c m- 1 de la fréquence du laser est attribué à ce défaut. L'expérience est compliquée par l'instabilité du centre H sous le rayonnement laser.

Inelastic light scattering of the VK- and H-centre in alkali halides (*)

E. Goovaerts. L. De Schepper and D. Schoemaker Dept. Physics, Umversity of Antwerp (U.I.A.), B-2610 Wilrijk, Belgium

Abstract. — The inelastic light scattering of the VK-centre was measured in LiF, KC1 and RbCl. The spectra consist of a single peak at 437 cm'1 of the laser frequency in LiF, at 241 cm"1 in KC1 and at 244 c m- 1 in RbCl [1]. The raman active fundamental mode of the XJ molécule (X~ : halogen ion) induces the inelastic light scattering, as is seen by comparison with calculated data on the free X^" molécules [2] as well as with measured frequencies of ana- logous Systems. A tentative calculation of the frequency of the VK mode in the séries of the alkali chlorides and fluondes, based on a semi-empirical theory of the defects configurational energy, is performed and compared with our expérimental results. The H-centre was produced in OH" doped KC1 crystals by UV-excitation, and a raman line at 266 cm" ' is correlated with this process. The experiment was complicated by a fast bleaching of this raman signal by the laser light.

1. Introduction. — The fundamental hole centre in the alkali halides is the self trapped hole or VK-centre. As was determined by ESR [3, 4], absorp- tion [5], and ENDOR [6] studies the hole is shared by a pair of nearest neighbour (n.n.) substitutional halogen ions (X") which form a XJ molécule (see Fig. la).

The main feature of this defect when looking at its vibrational properties, is the strong enhancement of the force constant between the two halogen ions

Fig. 1. — Schematic représentation of : a) the VK-centre in the alkali halides and b) the H-centre in KC1 showing the { 100 } plane that contains the XJ molecnlar axis. The coordinates x, y and z indicated on the VK-centre drawing are used to describe the vibration of the defect.

(*) Work supported by the IIKW projekt « Light Scattering in Solids ».

by the molecular binding. A vibrational mode results in which the stretching vibration of the XJ molécule is strongly involved.

The H-centre, the fundamental interstitial halogen atom centre, consists of an Xj molécule on a substitu- tional X " position. In KC1 the molécule is oriented along < 110 > and is weakly bound to two n.n. substitu- tional Cl ""-ions along this direction [7] (see Fig. Ib).

2. Expérimental results. — 2.1 THE VK-CENTRE MEASUREMENTS. — The VK-centre was produced by X-irradiation of the alkali halide crystals at liquid nitrogen température. The LiF crystals were nomi- nally pure, while the K O and RbCl spécimens were doped with électron trapping impurities (NOJ, Sn + + or P b+ +) in order to enhance the VK-centre production.

The raman scattering was measured with a standard apparatus using laser excitation at 514.5 nm, 488.0 nm (Ar+ laser) and in some of the measurements at 647.1 nm (Kr+ laser). Attempts to measure the VK raman scattering in LiCl, NaCl and NaF were not successful due to a very fast bleaching of the centres by the laser excitation. Further experimeni.i'. détails are given in référence [1].

The raman signais of the VK-centre measured in LiF, K G and RbCl consist of a single narrow line.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19806123

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INELASTIC LIGHT SCATTERING OF THE VK- AND H-CENTRE I N ALKALI HALIDES C6-473

The raman shift in LiF is (437

k

2) cm-' and its linewidth is about 10 cm-'. In KC1 and RbCl the raman lines are situated at (241

+

2) cm-' and (244 _+ 2) cm-' respectively, from the laser line.

The linewidths in KC1 and RbCl were about 7 cm-

'

and 8 cm-

',

respectively.

The laser light bleaches the VK-centre in these crystals probably through an optical excitation of the X; molecule to one of its dissociative excited states. Fortunately, the decay time of the raman line was slow enough to allow the measurements.

The following X i data are useful for comparison with the measured frequencies of the V,-centre.

Gilbert and Wahl [2] calculated the vibrational frequencies of the free F, and C1; molecules to be 510 cm-

'

and 260 cm-

',

respectively. The Cl; hole- type centres in alkali halides-alkali borate glasses possess an average vibration frequency of 265 cm-

'

[8].

Mf-X; molecular complexes (Mf = L i f , Naf

,

K f , Rb+ or C s f ) show an intramolecular vibration of the halogen molecular ion. Its frequency was measured by raman scattering [9] and ranged from 452 cm-' to 475 cm-' for the different Mf-F; species, and from 225 cm-' to 260 cm-' for the M+-CI, ones.

2.2 THE H-CENTRE EXPERIMENTS. - Following a procedure described by Kurz [lo] using UV-excitations of OH- doped KC1 crystals (- 4 x mol-%, T = 10 K), the production of H-centres is achieved with very little F-centre formation.

An unidentified defect with an absorption band at about 540 nm is produced in a secondary reaction, as was also mentioned by Lohse et al. [ll]. This centre is possibly the so called wet F-centre. Laser excitation at 488.0 nm or 514.5 nrn at this stage of the experiment bleaches the absorption bands of the H-centre as well as of the unknown defect. At the same time a luminescence signal is observed with a maximum intensity around 604 nm.

After the UV-excitation of the crystal at 325 nm, two sharp raman lines were detected (see Fig. 2).

Both of them are strongly decaying during the laser excitation and could only be recorded by scanning quickly through the interesting region of the raman spectrum. One of the raman signals possesses a shift of 266 cm-'. This is close to the calculated frequency of the free C1; molecule [2] and to the observed frequency of the VK-centre in KC1 and RbCI. Therefore it is plausible that the H-centre induces this raman peak.

The second raman line is situated at 328 cm-'.

From the large intensity of this line we concluded that a resonant raman effect is probably involved.

This could result from the unidentified centre with absorption at 540 nm. The decay of the H-centre during laser excitation can be induced by a direct bleaching in its long wavelength absorption band at 522 nm. Another possibility is that the bleaching of the unidentified centre causes the H-centre decay,

FREQUENCY SHIFT (cm-'1

Flg. 2. - Raman spectrum of KC1 : O H at T = 10 K after H-centre production (laser intens~ty 350 mW, 1 = 514.5 nm, scan tlme

--

40 s). Arrows indicate the 266 cm-' and 328 cm-' raman lines.

for example if electrons are released by the former and trapped by the latter. The strong luminescence at 604 nm might result from this recombination.

3. The vibrational properties of the V,-centre. -

The configurational energy function of the VK-centre in the alkali chlorides and fluorides was calculated on a semi-empirical basis by Jette ef af. [12]. They determined the dependence of the configurational energy U , on three coordinates x, y and z which have the symmetry of the VK-centre (see Fig. la). The energy function U,(x, y, z) is composed of the intra- molecular X i energy, taken from Gilbert and Wahl[2], the electrostatic monopole energy between the hole and the ions of the crystal, the polarization energy of the crystal by the charge of the hole, and the repul- sive energies between n.n. ions. From later studies [I3 and (I)] it was shown that the lattice energy is pro- bably overestimated in the earlier VK-centre theory.

The repulsion between the halogens of the X i molecule and the surrounding alkali ions was described by the Born-Mayer parameters appropriate for the undisturb- ed crystal. This interaction should be softened due to the presence of the hole in the outer orbital of the C1; molecule.

( I ) Unpubhshed work on the V,-centre in lithlum, sodium and

rubidium salts was kindly communicated by A. N. Jette.

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C6-474 E. GOOVAERTS, L. DE SCHEPPER AND D. SCHOEMAKER

We have calculated the eigenmodes and eigen- frequencies of the three coupled harmonic oscillators x, y and z by diagonalization of the 3 x 3 matrix of the mass reduced force constants :

The masses involved are nix = 2 /?I-, 01, = 2 m+

and M I , = 4 I I I + ; ni- and 171, are halogen and alkali

ion masses, respectively. Except for the case of LiF a vibrational mode can be selected from the three eigenmodes having a distinctively large x-amplitude compared to the other two. 8yJ6x and 6z/6x, the ratios of the y and z vibrational amplitudes to the x vibrational amplitude, and coca,,, the eigenfrequency of this V, mode, are listed in table I. The vibrational

Table I. - The nieamred and calculated frequencies (me,,, and coca,,) of the V, vibmtioizul rliode in the alkali halides are tabulated together with the coniputed relative vibrational aniplitudes Sy/Gx und 6 ~ 1 6 ~ . The inJltlence of the vibrational and electrostutic interactions is shown by the frequencies or and we, (see text).

All frequencies are given in cm-

'

.

Calculated ratio of the vibrational

amplitudes

LIF

NaF KF RbF LiCl NaCl KC1 RbCl

amplitudes of the alkali ions decreases strongly for increasing alkali masses, as expected. The fre- quency of the V, mode increases from the Rb to the Na salts for a given halogen. The frequency in LiCl is much lower than that in NaCl and this anomaly is accompanied by a sign reversal of the relative vibrational amplitudes of the alkali and halogen ions (see Table I). LiCl is also the only crystal in which the V, mode has the lowest of the three eigenfrequencies.

For LiF the three eigenmodes have comparable x-amplitudes and all of them are given in table 1.

Question. - J . ROLFE.

What is the third peak in the Raman spectrum of the H-centre ?

The one with lowest frequency, 494 cm-

',

is analogous to the V, mode in LiCl (see the values of 6y/6x and 6zJ6x). The experiments did not yield any V, raman signal in LiF with a frequency shift higher than 437 cm-'. We believe that the two calculated high frequency modes in the lithium salts are due to the overestimations of the repulsive forces between the X; molecule and the n.n. alkali ions. The effect is more important in these two crystals because of the lower inertia of the Li+ ions.

One can separate the V matrix of equation (1) in an intramolecular part V,,, an electrostatic part V,, and a r e p d i v e part Vr. V = V,,

+

V,,

+

V,. The VK mode frequencies obtained

by

diagonalization of

Vx,

+

V,, and of Vx,

+

V,, which are denoted we, and o r , respectively, are given in table I. In the cases of LiF and LiCl the choice of the V, mode is not unambiguous when different interactions are taken into account. The repulsive interactions, which we know to be overestimated, induce a strong enhance- ment of the X; frequency, and also determine the increasing trend from the Rb to the Na salts. The elecrrostatic forces alone cause a small decrease of the frequency, but this decrease is too small to explain the experimental VK-centre frequencies in KC1 and RbCI. The F; frequency in LiF is also much lower than the free molecule value of 510 cm-

'.

4. Conclusion. - Measurements of the vibrational frequency of the VK-centre in a larger set of alkali halides would be necessary to permit a study of the influence of the crystalline surroundings on the vibrational properties of the centre. The raman experiment is however complicated by the strong bleaching of the defects by the laser light. The semi- empirical theory of the V,-centre is not able to give a good description of the localized mode induced by the defect. A more refined calculation is needed, based on an explicit quantum mechanical treatment of the interactions between the C1; molecule and the neighbouring K + ions, and in some cases taking into account a larger set of ions in the V,-centre vibration.

The H-centre raman experiment is intrinsically as interesting as that of the VK-centre, but the experi- mental problems seem to us even harder to resolve.

Acknowledgments. - We wish to thank A. Bouwen for expert experimental support. One of us wishes to thank the I.W.O.N.L. (Instituut ter bevordering van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw) for a scholarship during the early part of this work.

JSSION

Reply. - E. GOOVAERTS.

I t is a feature of the second order phonon spectrum in KCl. At low temperature this is fairly sharp.

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INELASTIC LIGHT SCATTERING OF THE VK- AND H-CENTRE IN ALKALI HALIDES C6-475

Question. - W. HAYES. Reply. - E. GOOVAERTS.

Is it too difficult to line up the V,-centres with Comment only.

polarized light and do polarized Raman scattering ? References

[I] GOOVAERTS, E., SCHOEMAKER, D., Phys. Status Solidi (b) 88 (1978) 615.

121 GILBERT, T. L., WAHL, A. C., J. Chem. Phys. 55 (1971) 5247.

[3] CASTNER, T. G., KANZIG, W., J. Phys. Chem. Solids 3 (1957) 178.

[4] SCHOEMAKER, D., Phys. Rev. B 7 (1973) 786.

[5] D ~ B E C Q , C. J., HAYES, W., YUSTER, P. H., Phy.9. Rev. 121 (1961) 1043.

[6] DALY, D. F., MIEHER, R. L., Phys Rev. 183 (1969) 368.

[7] DELBECQ, C. J., KOLOPUS, J. L., YASAITIS, E. L., YUSTER, P. H., Phys. Rev. 154 (1967) 866.

[8] HASS, M. and GRISCOM, L., J. Chem. Phys. 51 (1969) 5185.

[9] HOWARD, Jr., W. F. and ANDREWS, L., J. Am. Chem. Soc.

95 (1973) 2056 and 3045.

[lo] KURZ, G., Phys. Status Solidi (b) 31 (1969) 93.

1111 LOHSE, F., REUTER, G., SPAETH, J. M., Phys. Status Solidi (b) 89 (1978) 109.

[12] JETTE, A. N., GILBERT, T. L., DAS, T. P., Phys. Rev. 184 (1969) 884.

[13] ADRIAN, F. J., JETTE, A. N., Phys. Rev. B 14 (1976) 3672.

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