Optical properties of cubic and tetragonal Cu- centers in alkali halides

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Optical properties of cubic and tetragonal Cu- centers in alkali halides

W. Kleemann, E. Grawe, L. Becker

To cite this version:

W. Kleemann, E. Grawe, L. Becker. Optical properties of cubic and tetragonal Cu- centers in alkali halides. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-199-C6-202. �10.1051/jphyscol:1980651�.

�jpa-00220089�

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JOURNAL DE PHYSIQUE Colloque C6, supple'ment au no 7 , Tome 4 1 , Juillet 1980, page C6-199

Optical properties of cubic and tetragonal Cu- centers in alkali halides

W. Kleemann, E. Grawe and L. Becker Fachbereich 6 , Gesamthochschule Paderborn, F.R.G.

K&um6. - Dans le spectre d'excitation de KC1 : Cu- la bande A, due a 'A1,(4s2) --, 3 T l u ( 4 ~ 4p), est dttectke.

Sa forme aplatie rtvile un couplage pareil aux modes e, et t2,, lorsque la forme presque gaussienne de la bande C signifie, que l'ktat 'TI, soit prkferablement couplk au mode tz, Selon une analyse des moments de plusieurs bandes C le couplage quadratique est ntgligeable. Une nouvelle bande d'absorption dans la rtgion des bandes D et le splitting de la bande D des centres Cu, tetragonaux semblent confirmer l'attribution de Tsuboi des bandes D a 3d10 -+ 3d9 4p. L'analogie des centres C U ~ et A g i est prouvke par le splitting des bandes C , par le dichroi'srne induit, et par la polarisation de I'kmission.

Abstract. - In the excitation spectrum of KC1 :Cu- the A band due to 'A,,(4s2) + 3Tlu(4s 4p) is detected. Its flat-topped shape reveals equal coupling to e, and tZg modes, whereas the near-gaussian shape of the C band signifies preponderant coupling of the ITlu state to the t2, mode. Negligible quadratic coupling is deduced from a moment analysis of several C bands. A newly detected absorption band in the D band region and the splitting of the D, band of tetragonal Cu, centers seem to confirm Tsuboi's D band assignment to 3d1° + 3d9 4p. C band splitting, induced dichroism, and polarized emission of Cu, centers prove their close analogy with A g i centers.

In contrast with the usual behavior of s2-type cen- 41. For excitation spectra a 1000 W xenon lamp and ters in alkali halides only one of the known absorp- a double monochromator have been used.

tion bands of Cu- centers is due to a s2 + sp type Figure 1 shows some of the spectra obtained on transition (C band : 'Alq-+ 'TI ), whereas at least cubic and tetragonal Cu- centers in KC1 and CsBr.

two absorption bands at higher energies (Dl and D2) belong to transitions into other electron configura- tions [I]. Based on the qualitative agreement of the D band spectrum with the energy level diagram of free Cu+, Tsuboi [Z] proposed the attribution to transitions of the type 3d1° 4s2--r 3d9 4s2 4p.

In this paper we shall contribute some further arguments to support this new assignment. In its main part, however, we shall tackle some open ques- tions concerning the 4s 4p states :

1. The location of the hitherto unknown A and B bands using excitation spectrometry because of the low center concentration available in single crystals (- 1016 cm-3 [I]) and of the low expected oscillator strengths ( 1 0 - ~ - 1 0 - ~ 111).

2. The absence of any structure within the C bands despite the Jahn-Teller effect (JTE ; here : T-t pro- blem [3]), which manifests itself in a trigonal distortion of the relaxed excited Cu- center in KC1 [I].

3. The investigation of perturbed Cu- centers (e.g. Cu-(Na) in KI-,Na,Cl with x -- 0.02), which are expected to behave analogously to the corresponding Agi centers [I, 41.

Experimental details like sample preparation and orientation, photochemical Cu--Cu, conversion, and optical techniques have been described previously [I,

Fig. 1. - Absorption (C, D l , D,), emission (A', C'), and excitation spectra (A", C , D'i, D';) at LHeTof (a) Cu- (-), Cu-(Na) (---), and Cu-(Li) (. . . .) centers in KC1, and of (b) Cu- (-1 and Cu-(K) (---) centers in CsBr, respectively.

0

6 5

f C

9 c

0

2

The peak positions of all absorption (excitation) and emission bands, included those obtained on RbCl and KBr, are compiled in table I.

The most important new feature, in our opinion, is the A excitation band on the high energy side of the A emission band in KCI. Its shape is remarkably

2.5 3.0 3.5 4.0 4 5 5.0 5.5

Photon Energy /eV

A r DZ I b)

+ ,

1; l7 1 , I 1 1 1

; r

I

? ' I

r n

CsBr

I $

,; , \.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980651 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980651

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C6-200 W. KLEEMANN, E . GRAWE AND L, BECKER

Table I. -Peak positions in eV of rhe absorption, excitation (*), and emission bands (in brackets) of Cu- centers at LHeT.

KC1 CsBr RbCl KBr

Band Cu - Cu - (Na) Cu - (Li) Cu - Cu - (K) Cu - Cu -

- - - - - - - -

(A) (2.85) (2.77) (2.56) (2.71) (2.52) (2.82) (2.79)

A 3.08 (*) - - - - -

( c ) (3.90) (3.78) (3.69) (3.65) - (3.82) (3.75)

C 4.10 3.9814.1 1 4.00/4.16(*) 3.86 3.7313.87 3.98 3.97

4.34 - - 4.04 4.00 4.33 4.29

D ^I 4.43 4.2814.56 (*) 4.3014.48 (*) 4.23 4.1014.36 (*) 4.40 (*) 4.49 (*)

4.63 (*) - - 4.40 (*) - 4.59 (*) 4.61

4.76 (*) 4.73 (*) 4.67 (*) 4.51 4.48 4.70 (*) -

4.90 - - 4.65 4.65 4.76 4.69 (*)

D ² 5.10 5.09 4.96 4.80 4.80 4.92 4.89

5.44 (*) 5.38 - 5.08 (*) 5.12 5.16 (*) 5.13 (*)

D3 6.36 ^- ^- ^- ^- - -

(a)' ( b ) mean values

3

⁼(SA

+

^S,

+

ST)/3 obtained from

1. A Band ( LHeT I

2. CaO:F+ (LHeT I the second moment

-

W_ 3 S A = l S,=S,=10 T * = 0 0 2

L ( ^E2) (O) N @A

+

^SE

+

^{1.5 ST)}( b 0 l 2

and from the Stokes shift 2(SA

+

S,,,) ho [5]:

9

= 2.3 and 2.9, respectively.

Phonon structure and a zero phonon line could not yet be resolved because of the inherent intensity

-10 - 5 0 5 10 -10 -5 0 5 10 problems, which also inhibited the detection of the

Reduced Photon Energy I E -<E,>) /h 0, vibration-allowed B band up to now.

Fig. 2. - Shape functions of the A and C bands of KC1 : Cu- versus the reduced photon energy with ( E, = 3.079(4.117) eV and o,,, = 160(108) cm-' (calculated from

<

^E,⁾(T)) compared with the F+ band in CaO (( El ) = 3.682 eV, w , = 265 cm-' [ 5 ] ) and with model calculations for corresponding T* = 2 kT/fioo [3].

flat-topped (Fig. 2a) and becomes rounded and symmetric at temperatures above 200 K. This behavior reminds strongly of the F' band in CaO [5] and of the A band in KC1 : Au- 161. In both systems the excited T I states are known to couple equally with e, and t2, modes. Furthermore, if coupling with the al, mode is weak, the absorption band becomes ff at-topped with a moment ratio ( E,

)I(

E2 ) rather near the value 1.8 (rectangular) than near 3.0 (gaussian band shape). For the A band in KC1 : Cu- we find the value 2.4, which interpolates between those of CaO : F + (2.1) and KC1 : Au- (2.8). It can thus be inferred that the ratio of the Huang-Rhys factors SAJSE,, must lie between 0.65 (CaO : F+ [5]) and 5.2 (KC1 : Au- [6]). Taking into account that the shape functions of the Cu- and the F f bands nearly coincide in the normalized representation of figure 2a, we tentatively adopt the sum S,

+

^S,

+

ST = 8.4 of CaO : F f , which determines the band width, and assuming SA/SbT = 1, propose SA = SE = ST = 2.8 for KC1 : Cup. This is in good agreement with the

In contrast with the A band, the C band shape is rather gaussian (Figs. l a and 2b). The only anomaly is its low temperature skewness

Y = ( E3

)/(

E2 )312

-

^0.4^,

which drops to zero at T* > 1 (Fig. 3). This behavior is predicted in the case of negligible quadratic coupling [3], which can be expected for weak (Cu-) and intermediate linear coupling (Ag- [3]). Taking into account S, = 0, emerging from stress experiments [I], the C band shape in KC1 : Cu- is reasonably approxi- mated by the computed curve for SA = 1, S, = 0,

0 1 2 3 4

Reduced Temperature liw,/2kBT

Fig. 3. -Skewness of the C bands of Cu- centers versus IIT*

in KCl, RbCl, KBr, and CsBr [w, = 108, 95, 75, and 70 cm-', respect~vely). The eye-guiding solid lines are compared with a typical computed curve (dashed line [3]).

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OPTICAL PROPERTIES OF CUBIC AND TETRAGONAL Cu- CENTERS IN ALKALI HAL1Dt:S C6-201

ST = 5 (Fig. 2b [3]). The remaining faint triplet structure may be smoothed out by setting SA = 2 and ST = 4. The mean value

3

⁼(SA

+

^ST)/2⁼^{3 agrees}

satisfactorily with the results obtained from the second moment and from the Stokes shift,

S

⁼2.5 and 3.7, respectively.

The preponderance of the t2, mode may also explain the trigonal ST distortion in the relaxed excited C state [I]. In CsBr, however, despite the compara- ble band shape, also coupling with the e, mode is considerable. Polarization measurements of the C emission reveal a tetragonal JT distortion at LHeT (P(q = 0) = 0.16). Obviously, as for Ag- centers [I], the ratio of the coupling strengths to e, and t2, modes seems to reflect the different ion arrangements around the impurity in CsCl and NaCl type lattices, respectively.

In the D band region a new absorption shouldet- at the high energy side of the D2 bands (Fig. 1 ; Table I) seem to confirm the prediction of the spin-orbit allowed transition 's0(d1O s2) + 3 ~ , ( d 9 s2 p) [2]. In mixed crystals the Dl band seems to split into a doublet after Cu--Cu; conversion. According to the assignment of 3 ~ 1 to the corresponding dp state [2], the splitting appears reasonable. It is expected that the VUV band D3(lT,J will split in the same way under C4" symmetry as the D l and the C band (IT,,).

The relative insensitivity to the change of symmetry of the other D bands might be due to the fact that they are already composed of two (three) overlapping components emerging from the D(F) states in Oh crystal field.

The CA doublets in general exhibit a low energy shift of their centers of gravity with respect to the unperturbed C bands (Fig. 1; Table I, and RbCl : Cu-(Na) : 3.8213.99 eV). This may be explain- ed by a significant spherically symmetric part of the perturbing potentiaI [7], i.e. the change of the Made- lung potential, being due to the enlargement of the anion vacancy by the small perturbing cation. This will cause a lowering of the binding energies, being smaller for the more diffuse 4s 4p than for the 4s2 ground state. The absence of this effect in most of the Ag, centers [I] is consistent with the idea that the Cu- ion must be smaller than Ag-, in accordance with their different electron-lattice coupling. Unexpec- tedly, however, the band shift effect is absent in KC1 : Cu-(Li). Presumably, in this extreme case, the very small Lif ion causes a local lattice contrac- tion thus restoring the Madelung potential of the unperturbed anion vacancy.

DISC1 Question. ^-P. EDEL.

The equality of the coupling coefficients to e, and t,, modes seems to be a quite general fact for p elec- trons. Do you think there is a theoretical reason for this ?

The C4 symmetry of Cu-(Na) in KC1 has been checked by photo-induced orientation, which causes 30

%

dichroism within the CA doublet after intense [loo] polarized CA2 irradiation at 230 K.

More information on the symmetry of the excited states is provided by measurements of polarized emission. In KC1 : Cu-(Na) it proves very advantageous that not only AA (as in Agi), but also CA emission is excited after irradiation in either CA subband.

According to the angular dependences of the degree of polarization, P(q) (Fig. 4), we find four different

Photon Energy/eV Angle

3

Fig. 4. - Degree of polarization of the CA ( 0 0 0 ) and A, )a..(

emission of Cu-(Na) centers in KC1 at 5 and 78 K, respectively, (a) versus incident photon energy in the C, absorption range (solid line) at q = 0, (b, c) versus q after CA1 and CAZ excitation, respectively, in comparison with calculated curves (solid lines ; see text).

excitation-emission processes, which correspond to electric dipole transitions of the type o-o(CA2-AA), n-o(CA1 -AA), o-n(CA 2-CA)7 and n-n(CA I -C,), respectively. Hence we deduce the same energy level diagram as for Ag, [7J : the A and C states decompose both into A,,

+

E, of C,, with E,(A,,) as the lowest A(C) sublevel, from which A(C) emission takes place.

The theoretical expressions P(y) [I] as fitted to the experimental data in figure 4 contain a depolariza- tion factor /3 = 0.1, which mainly reflects the mixing of A,, and E, due to the JTE acting on the degene- rate E, level.

It should be noted that the Cu-(K) center in CsBr has C3 symmetry owing to the CsCl lattice structure.

According to polarized emission data obtained on suitably oriented samples we can describe the processes CA2-A, and CAI-A, by o-o and z-a transitions, respectively, of C3 oscillators with

P

⁼0.3.

JSSION

Reply. - W. KLEEMANN.

This inference does not apply to the IT,, state in KC1 : Cu- where Coulomb and exchange interactions just seem to cancel their contributions to SE. Hence the equality of SE and ST in the A transition of KC1 : Cu- and of other systems seems to be fortuitous.

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W. KLEEMANN, E. GRAWE AND L. BECKER

References

[l] KLEEMANN, W., 2. Physik 214 (1968) 285; 234 (1970) 362; [6] LEMOYNE, D., DURAN, J., BILLARDON, M. and LE SI DANG.

249 (1971) 145. Phys. Rev. B 14 (1976) 747 ;

[2] TSUBOI, T., Can. J. Phys. 55 (1977) 1316; Physica 95B (1978) MURAMATSU, S. and SAKAMOTO, N., Phys. Status Sohd~ (b) 91

397. (1979) K 87.

[3] NASU, K. and KOJIMA, T., Progr. Theor. Phys. 51 (1974) 26. [7] KOJIMA, K., SHIMANUKI, S. and KOJIMA, T., J. Phys. Soc.

[4] KLEEMANN, W., 2. Physik 252 (1972) 134. Japan 30 (1971) 1380.

[5] ESCRIBE, C. and HUGHES, A. E., J. Phys. C 4 (1971) 2524;

MERLE D'AUBIGN~, Y. and ROUSSEL, A,, Phys. Rev. B 3 (1971) 1421.