- Email: [email protected]
Kerr effect of CN- molecules in KCl crystals
Vol. 14, pp. 923—927, 1974.
Solid State Communications,
Pergaxnon Press.
Printed in Great Britain
KERR EFFECT OF CW MOLECULES IN KCI CRYSTALS Antonio Diaz.Géngorat and Fritz LUty Physics Department, University of Utah, Salt Lake City, Utah 84112, U.S.A. (Received 25 October 1973 by A. G. Chynoweth)
The electric field alignment of substitutional CW molecules in KC1 crystals was studied by measurements of the field-induced optical birefringence (Kerr.effect). The electric field, temperature, wavelength and concentration dependence of the Ken-effect and its pronounced anisotropy is found in quantitative agreement with a paraelectric alignment model for <111) oriented CW molecules, which have electronic absorption transitions hidden under the fundamental crystal absorption, and are characterized by an anisotropic polarizability of a11 —aj = + 1.60 A
SUBSTITUTIONAL CW molecules in KCI are model examples for paraelectric defects in alkali halides, which are hindered in their orientational motion by a very weak crystal potential, and thus have a large tunnelling splitting (~ 1.2 cm~).This system has 1 been specific extensively heat,2 dielectric,3’4 investigatedacoustical,5 with thermal andconductivity, Rarnan techniques6 as well as with elasto-2 and electro-optical7 techniques (using the CN stretching vibration absorption at 5p). While several of these expern~ientswere inconclusive about the CN defect symmetry, the elasto-optical and acousticalwork yielded contradictory results on the elastic and electric dipole symmetry of the defect. In essentially all treatments of this system including a recent comprehensive review article,8 a (100) dipole model indicated by the elasto-optical experiment has been adopted as the only basis for discussion and interpretation of the KCI: CW properties. —
—
interpreted elasto-optical measurements yield definitely a (111) symmetry of the CN elastic dipole tensor. l’his new elastic result opens the question for a similarly conclusive paraelectric experiment on the CN electricmoment dipole symmetry. the small electric dipole (p = 0.3 d),Due thetoelectric polarization cannot be saturated, so that no anisotropy effects can be detected with dielectric techniques.3 Previous electro.optical experiments on the CW stretching vibration7 were inconclusive about the dipole symmetry. The apparently simplest experimental approach, to measure the electro-dichroism of the electronic absorption and its anisotropy (which was used successfully with 0H molecules10”1) seems inapplicable due to the fact that all electronic absorptions of the CN defect are ‘hidden’ under the fundamental crystal absorption. Field-induced absorption changes ~K(E) = K 11 K1 in these inaccessible absorptions, however, are expected to produce Kramers—Kronig related refractive index changes ~.n(E) = n 11 n1, which due to the extended spectral pattern of the effect should be detectable far away from the M defect absorp. tion in the transparent crystal range. This electrobirefringence method has been tested recently for the 0H model defect with its well-known u.v. absorption, symmetry and electro-dichroism by measurements of the 0H Kerr-effect.’2 We apply this method here to —
9 it will be shown that the In a separate article theoretical and experimental basisthe of (100) the early elasto2 which established defect optical model, work, is not valid, and that properly performed and
—
Supported by NSF Grant No. GH33704-Xl. Helium gas was provided by a departmental grant with ONR. t Supported by a fellowship from E.S.F.M. 1~N.) Mexico, D.F. *
923
KERR EFFECT OF CN MOLECULES IN KCI CRYSTALS
924
a defect system with inaccessible electro.dichroism and questionable dipole symmetry.
The measurements were done with monochromatic
light (polarized under to the electric field) traversing the crystal I to the field, using the Kerr-effect arrangement, described in reference 12. Two methods were used: 450
The KC1 crystals, doped with 4 different CN concentrations and grown in pure argon atmosphere, were analyzed spectroscopically, revealing besides the CW content small and rather constant admixtures of OW and CNCI impurities. -
(a) The field-induced birefringence An(E) was directly measured (with the crystal between the crossed polarizers) as a field-induced intensity change, using
—
—
Impurity
either d.c. application or a.c. field modulation techniques. (b) A Babinet-compensator was introduced additionally between the crossed polarizers, tuned to a
CrystalA CrystalB Crystal C Crystal D
10’° 1.1 10” 4.6- 10” 2.7 1020 NOH — (cm°) 1.3- 10” 2.2~1~ 2.1 - ~ 1.9NNCO—(cnr’) 2.2~10” 47. 10~.3.5 - 1~ 5.4-10” N~N— (cin°) 2.5
phase-shift of~= ir/4,’thus producing circularly polarized light.
By thermal treatment of small crystal samples in chlorine gas, the OW content could be reduced to unmeasurable amounts (NØff < 1014 cnf3), while the CN and CNO content remained nearly unchanged. The fInally shaped crystal samples (with gold electrodes) were carefully thermally annealed again, to reduce background strain (which seriously affects the Kerreffect13). By this procedure the ratio of transmitted light intensity between crossed and parallel polarizers for a crystal thickness 9 mm was typically reduced from about 10 to 0.1%.
Figure 1 shows the field-induced intensity change ~ between crossed polarizers, (normalized to the transmitted intersity J~for parallel polarizers), obtained with method (a) and (b) for crystals of different CN content in (111) and (100) orientation, measured as a function of field and temperature. The effect shows a F’ and E2 dependence for method A and B respectively, a T~and T2 dependence for method A and B (the latter not shown here), and very much larger values for (111) compared to (100) fields.
—
KCI: CN
~‘
Kerr-Effect c
1 m~ / ~
Ii
/
41
/
!
I;
0’
f I
I!
¶7 /
/ /
e”~’ ‘~
25
Method A
/
—,•1~
‘
-,
//I ‘11 /I
/
/‘
I
I
~,
/
/
I0’~e,,•’
i
Vol. 14, No. 10
/
-,
0 I I E~,
2 5 IO~’en,’ 0
Cl
/
//I~
/
0’ en,’
~,
/
I
r ~
•6~’0n,’ ~“
/•
\
-,
\
d /
/
7
44
O’e
071 ~. 1; I Method A
___
,‘
/
without Compn~tor•0’I,,0’ £I.ctric F .1* (Volt/c,,)
I
I
I0’cm’
Method B with X/4 Copsnsotoc
___________ 02 5-t0”e,n’50
0
T.mpvotur. 1K)
FIG. 1. Field-induced relative intensity change AI~fi~measured with method A and B for KC1 crystals with different CW content under electric fields applied in (111) or (100) direction, as a function of field and temperature (double logarithmic plot).
Vol. 14, No. 10
KERR EFFECT OF CW MOLECULES IN KC1 CRYSTALS
In Fig. 2 is displayed the spectral dependence of ‘the field-induced birefringence M(E), obtained from AJ(E)measurements under constant field and temperature conditions. The results (which are identical
925
KCI :CN, Kerr-Effect iø-’
U
I
~
Tt
for method A and B) show a dispersion-free (111> effect, scaling with the CW concentration. The weak observed (100> effect which has a strong dispersion is found to become reduced after chlorine treatment by more thanan order of magnitude.
4,~ E,,, io’
—
‘~
— ‘
‘‘‘‘‘‘‘
1.1
In Fig. 3 the Kerr-effect (measured with method A and B) is plotted as a function of CW content for crystals with and without chlorine treatment. For CW concentrations NCN < 3 . iO’~cm’’ the effect scales
io”cm’
(0’
—
with the CW content in a quadratic and linear way for methods A and B respectively.
\
I~.io~j
I i. rx
\6~00
+ —.-—.
An optical birefringence ~An= n I!
n1 introduces for light of frequency ~ and optical path length da phase shift of 6 = wd/2c ~Anbetween II and I pola-
+—
+ —
—
200
2
46(0* cm’ I
400
600
-
*ovsl.ngth (m~&)
rization. Thus for our light, polarized under 450, an intensity change between crossed polarizers ~‘1 (nor-
FIG. 2. Spectral dependence of the field-induced birefringence ~n(E), measured for crystals of different CW content with method A (open symbols) and method B (filled symbols) for identical field- and
malized to the intensity for parallelpolarizers I~~) of = sin2 5/2 will occur. The field-induced ~ effect is Kramers—Kronig related to the electrodichroism ~K(E), so that both should have the same field and temperature dependence and anisotropy.
temperature-conditions.
KCI:CW,Kerr-Effect Concentration-Dependence Mithod A
0
/
withoul Compsesotlon
0’
Xii
IC’’
,.
/
Method B with )y4
£ 0~~~~~~
—1’
• ~.eO0m,
I
o ~Ioo IC I0~
0°
~
I
10°
N
I
£104
Color/n.
I S
.1 ‘‘‘‘I,,,
Ct’
10°
/000
05.. [cm’)
FIG. 3. Concentration dependence of the field-induced intensity change Al1 /l~obtained with method A and B under identical field and temperature conditions for E111 and E100. The filled symbols refer to measurements of
crystals after chlorine treatment.
926
KERR EFFECT OF CN’ MOLECULES IN KC1 CRYSTALS
In the linear range of dipole polarisation (pE/kT< 1), both electro.optical effects (which are second ~rder effects in terms of pE/kT)” thus vary like ~K(ff) ~ M(E) ~ N~(pE/kT).2 For small phase-shifts, sin2 5/2
(5/2)2 holds, so that we expect (for method A): ~Ii
/pE\21 ~I1i~xI N(~_J~,)J
~
~ \ 2 ‘pE \4 (~-N) (1)
2
~
=
For method B, with an additional fixed phase shift 6 = i~/4,we have til(E) sin2 (ir/4 + 1/26(E)) which for 5(E) < ir/4 becomes 0:
ix
5(E).
~i
fp~
~
~-~-
(2)
2(E/fl4 and N(E/T)2 dependence, The measured obtained with N method A and B respectively in Figs. I and 3, are in exact agreement with the above expressions (1) and (2). The spectral dependence of the refractive index change ~.n(w) is related to the electro-dichroism of all CN absorption bands by the dispersion relation c C ~KE(w1) dw’ ~12E(W)
I IT ~
=
W
‘2
(3)
—
The measured absence of any ~.n dispersion for hw < 5.3 eV (Fig. 2) shows that these frequencies w are small compared to the frequencies w’ of the CW absorption transitions so that the expression (3) becomes inde. pendent of w in this range. This absence of dispersion (which is in contrast to the OW case)’2 prevents any conclusion about position and strength of the CN absorption transitions aside of the fact that they must lie in the far u.v. range at energies more than 7 eV.
Vol. 14, No. JO
Using the measured dipole moment value (p = 0.3 d) to correct for the alignment ratio, we obtain ct~ = 3 Contrary to the OW’ defect, the CN’ + 1.60 (A). molecule is more polarizable along its axis than in the perpendicular directions. Using the mean value of the —
electronic polarizability & = 1/3 (a~+ 2a 3 a CWmolecules,’4 for 3.Thesizeofthisa-athsotropy~a/&= we obtaina~= 3.861)A3 2.8 and A 1 =2.26A 0.6 is typical for dipole gas molecules, and is in marked contrast to the nearly spherical a behavior of the OW defect in KCI.12 Most significant is the very pronounced anisotropy of the Kerr-effect. The relatively small E 100 effect is mostly due to the admixed (100) oriented OH” defects: The size, sign and strong dispersion of this (100) effect is in agreement with the observed OW Kerr-effect;’2 its strong reduction together with that of the OW absorption by chlorine treatment is strong support for this assignment. The remaining (100) effect, which flOW may be due to the true CN (100) Kerr-effect, is more than 3 orders of magnitude weaker than the (111> effect. This gives conclusive proof for a (111) symmetry of the electric dipole, contrary to previous work,8 but in agreement with the (ill) elastic dipole symmetry, derived from the new elasto-optical work.9 These new results affect the interpretation of numerous earlier works,18 which mostly based their interpretation on a (100) dipole model. Besides determining dipole symmetry and polar. izability anisotropy, the Kerr-effect has potentials for at least two more aspects of this dipole system:
—
Applying the Lorentz—Lorenz formula, we can relate the measured ~.n(E) effect for dilute systems (extrapolated to full dipole alignment) to the anisotropy of the electronic polarizabiity z~aof the ifl dividual CN defect: =
-
a~—a 1=
I. 2.
2 + 2)2 M
(4)
(a) It allows one to study dipole interaction effects by the deviations from the normal Curie-law behavior, as indicated by the two high CN concentrations measured. (b) It provides a simple and accurate way to study dipole relaxation behavior with optical Kerr-technique. Both these aspects, as well as Kerr-effect studies on CN dipoles in various other host materials, are in the process of investigation.
2irN(n
REFERENCES SEWARD W.D., Low Temperature Phsics, LT-9, p. 1130 (edited by DAUNT W. et a!.) Plenum Press, New York (1965). SEWARD W.D. and NARAYANAMURTI V.,Phvs. Rev. 148,463 (1966).
Vol. 14, No. 10 3. 4. 5. 6. 7. 8.
KERR EFFECT OF CN MOLECULES IN KC1 CRYSTALS
SACK H.S. and MORIARTY M.C., Solid State Commun. 3,93 (1965). LAKATOS A. and SACK HS., Solid State Cornmun. 4, 315 (1965). BYER N.E. and SACK HS.,Physica Status Solidi 30, 569, 579 (1966). CALLENDER R. and PERSHAN P.S., Phys. Rev. A2, 672 (1970). POMPI R.L. and NARAYANAMURTI V.,Solid State Commun. 6,645 (1968). NARAYANAMURTI V. and POHL R.O.,Rev. Mod. Phys. 42,201(1970).
9. 10.
L(JTY F.,Phys. Rev. (to be published). KU}{N U. and LOTY F., Solid Stare Commun. 2,281 (1964).
11. 12.
KAPPHAN S.and LOTY F.,J. Phys. Chem Solids 34, 969 (1973). ZIBOLD G. and LIJTY F.,J. Nonrnetals 1, 1(1972).
13. 14.
ZIBOLDG.andLIJTYF.,J.Nonmetals 1,17(1972). TESSMANN J.R., KAHN AR. and SHOCLEY W.,Phys. Rev. 92,4 (1953).
‘Die Ausrichtung von substitutionellen CN MolekUlen in KC1 durch elek-
trische Felder wurde durch Messung der feld-induzierten optischen Doppel. brechung (Kerr-Effekt) untersucht. Die Feld., Temperature-, Wellenlangenund Konzentrations.abhangigkeit sowie die Anisotropie des Kerr-effekts stimmen quantitativ Uberein mit einem paraelektrischen Ausricht-modell fur (ill) orientierte CN Molekule, deren optische Absorptionen unter der Fundamental-absorption des Kristalles lie~en,und die durch eine Polarisations-anisotropie von = 1.60 A charakterisiert sind.’ —
927
Solid State Communications,
Pergaxnon Press.
Printed in Great Britain
KERR EFFECT OF CW MOLECULES IN KCI CRYSTALS Antonio Diaz.Géngorat and Fritz LUty Physics Department, University of Utah, Salt Lake City, Utah 84112, U.S.A. (Received 25 October 1973 by A. G. Chynoweth)
The electric field alignment of substitutional CW molecules in KC1 crystals was studied by measurements of the field-induced optical birefringence (Kerr.effect). The electric field, temperature, wavelength and concentration dependence of the Ken-effect and its pronounced anisotropy is found in quantitative agreement with a paraelectric alignment model for <111) oriented CW molecules, which have electronic absorption transitions hidden under the fundamental crystal absorption, and are characterized by an anisotropic polarizability of a11 —aj = + 1.60 A
SUBSTITUTIONAL CW molecules in KCI are model examples for paraelectric defects in alkali halides, which are hindered in their orientational motion by a very weak crystal potential, and thus have a large tunnelling splitting (~ 1.2 cm~).This system has 1 been specific extensively heat,2 dielectric,3’4 investigatedacoustical,5 with thermal andconductivity, Rarnan techniques6 as well as with elasto-2 and electro-optical7 techniques (using the CN stretching vibration absorption at 5p). While several of these expern~ientswere inconclusive about the CN defect symmetry, the elasto-optical and acousticalwork yielded contradictory results on the elastic and electric dipole symmetry of the defect. In essentially all treatments of this system including a recent comprehensive review article,8 a (100) dipole model indicated by the elasto-optical experiment has been adopted as the only basis for discussion and interpretation of the KCI: CW properties. —
—
interpreted elasto-optical measurements yield definitely a (111) symmetry of the CN elastic dipole tensor. l’his new elastic result opens the question for a similarly conclusive paraelectric experiment on the CN electricmoment dipole symmetry. the small electric dipole (p = 0.3 d),Due thetoelectric polarization cannot be saturated, so that no anisotropy effects can be detected with dielectric techniques.3 Previous electro.optical experiments on the CW stretching vibration7 were inconclusive about the dipole symmetry. The apparently simplest experimental approach, to measure the electro-dichroism of the electronic absorption and its anisotropy (which was used successfully with 0H molecules10”1) seems inapplicable due to the fact that all electronic absorptions of the CN defect are ‘hidden’ under the fundamental crystal absorption. Field-induced absorption changes ~K(E) = K 11 K1 in these inaccessible absorptions, however, are expected to produce Kramers—Kronig related refractive index changes ~.n(E) = n 11 n1, which due to the extended spectral pattern of the effect should be detectable far away from the M defect absorp. tion in the transparent crystal range. This electrobirefringence method has been tested recently for the 0H model defect with its well-known u.v. absorption, symmetry and electro-dichroism by measurements of the 0H Kerr-effect.’2 We apply this method here to —
9 it will be shown that the In a separate article theoretical and experimental basisthe of (100) the early elasto2 which established defect optical model, work, is not valid, and that properly performed and
—
Supported by NSF Grant No. GH33704-Xl. Helium gas was provided by a departmental grant with ONR. t Supported by a fellowship from E.S.F.M. 1~N.) Mexico, D.F. *
923
KERR EFFECT OF CN MOLECULES IN KCI CRYSTALS
924
a defect system with inaccessible electro.dichroism and questionable dipole symmetry.
The measurements were done with monochromatic
light (polarized under to the electric field) traversing the crystal I to the field, using the Kerr-effect arrangement, described in reference 12. Two methods were used: 450
The KC1 crystals, doped with 4 different CN concentrations and grown in pure argon atmosphere, were analyzed spectroscopically, revealing besides the CW content small and rather constant admixtures of OW and CNCI impurities. -
(a) The field-induced birefringence An(E) was directly measured (with the crystal between the crossed polarizers) as a field-induced intensity change, using
—
—
Impurity
either d.c. application or a.c. field modulation techniques. (b) A Babinet-compensator was introduced additionally between the crossed polarizers, tuned to a
CrystalA CrystalB Crystal C Crystal D
10’° 1.1 10” 4.6- 10” 2.7 1020 NOH — (cm°) 1.3- 10” 2.2~1~ 2.1 - ~ 1.9NNCO—(cnr’) 2.2~10” 47. 10~.3.5 - 1~ 5.4-10” N~N— (cin°) 2.5
phase-shift of~= ir/4,’thus producing circularly polarized light.
By thermal treatment of small crystal samples in chlorine gas, the OW content could be reduced to unmeasurable amounts (NØff < 1014 cnf3), while the CN and CNO content remained nearly unchanged. The fInally shaped crystal samples (with gold electrodes) were carefully thermally annealed again, to reduce background strain (which seriously affects the Kerreffect13). By this procedure the ratio of transmitted light intensity between crossed and parallel polarizers for a crystal thickness 9 mm was typically reduced from about 10 to 0.1%.
Figure 1 shows the field-induced intensity change ~ between crossed polarizers, (normalized to the transmitted intersity J~for parallel polarizers), obtained with method (a) and (b) for crystals of different CN content in (111) and (100) orientation, measured as a function of field and temperature. The effect shows a F’ and E2 dependence for method A and B respectively, a T~and T2 dependence for method A and B (the latter not shown here), and very much larger values for (111) compared to (100) fields.
—
KCI: CN
~‘
Kerr-Effect c
1 m~ / ~
Ii
/
41
/
!
I;
0’
f I
I!
¶7 /
/ /
e”~’ ‘~
25
Method A
/
—,•1~
‘
-,
//I ‘11 /I
/
/‘
I
I
~,
/
/
I0’~e,,•’
i
Vol. 14, No. 10
/
-,
0 I I E~,
2 5 IO~’en,’ 0
Cl
/
//I~
/
0’ en,’
~,
/
I
r ~
•6~’0n,’ ~“
/•
\
-,
\
d /
/
7
44
O’e
071 ~. 1; I Method A
___
,‘
/
without Compn~tor•0’I,,0’ £I.ctric F .1* (Volt/c,,)
I
I
I0’cm’
Method B with X/4 Copsnsotoc
___________ 02 5-t0”e,n’50
0
T.mpvotur. 1K)
FIG. 1. Field-induced relative intensity change AI~fi~measured with method A and B for KC1 crystals with different CW content under electric fields applied in (111) or (100) direction, as a function of field and temperature (double logarithmic plot).
Vol. 14, No. 10
KERR EFFECT OF CW MOLECULES IN KC1 CRYSTALS
In Fig. 2 is displayed the spectral dependence of ‘the field-induced birefringence M(E), obtained from AJ(E)measurements under constant field and temperature conditions. The results (which are identical
925
KCI :CN, Kerr-Effect iø-’
U
I
~
Tt
for method A and B) show a dispersion-free (111> effect, scaling with the CW concentration. The weak observed (100> effect which has a strong dispersion is found to become reduced after chlorine treatment by more thanan order of magnitude.
4,~ E,,, io’
—
‘~
— ‘
‘‘‘‘‘‘‘
1.1
In Fig. 3 the Kerr-effect (measured with method A and B) is plotted as a function of CW content for crystals with and without chlorine treatment. For CW concentrations NCN < 3 . iO’~cm’’ the effect scales
io”cm’
(0’
—
with the CW content in a quadratic and linear way for methods A and B respectively.
\
I~.io~j
I i. rx
\6~00
+ —.-—.
An optical birefringence ~An= n I!
n1 introduces for light of frequency ~ and optical path length da phase shift of 6 = wd/2c ~Anbetween II and I pola-
+—
+ —
—
200
2
46(0* cm’ I
400
600
-
*ovsl.ngth (m~&)
rization. Thus for our light, polarized under 450, an intensity change between crossed polarizers ~‘1 (nor-
FIG. 2. Spectral dependence of the field-induced birefringence ~n(E), measured for crystals of different CW content with method A (open symbols) and method B (filled symbols) for identical field- and
malized to the intensity for parallelpolarizers I~~) of = sin2 5/2 will occur. The field-induced ~ effect is Kramers—Kronig related to the electrodichroism ~K(E), so that both should have the same field and temperature dependence and anisotropy.
temperature-conditions.
KCI:CW,Kerr-Effect Concentration-Dependence Mithod A
0
/
withoul Compsesotlon
0’
Xii
IC’’
,.
/
Method B with )y4
£ 0~~~~~~
—1’
• ~.eO0m,
I
o ~Ioo IC I0~
0°
~
I
10°
N
I
£104
Color/n.
I S
.1 ‘‘‘‘I,,,
Ct’
10°
/000
05.. [cm’)
FIG. 3. Concentration dependence of the field-induced intensity change Al1 /l~obtained with method A and B under identical field and temperature conditions for E111 and E100. The filled symbols refer to measurements of
crystals after chlorine treatment.
926
KERR EFFECT OF CN’ MOLECULES IN KC1 CRYSTALS
In the linear range of dipole polarisation (pE/kT< 1), both electro.optical effects (which are second ~rder effects in terms of pE/kT)” thus vary like ~K(ff) ~ M(E) ~ N~(pE/kT).2 For small phase-shifts, sin2 5/2
(5/2)2 holds, so that we expect (for method A): ~Ii
/pE\21 ~I1i~xI N(~_J~,)J
~
~ \ 2 ‘pE \4 (~-N) (1)
2
~
=
For method B, with an additional fixed phase shift 6 = i~/4,we have til(E) sin2 (ir/4 + 1/26(E)) which for 5(E) < ir/4 becomes 0:
ix
5(E).
~i
fp~
~
~-~-
(2)
2(E/fl4 and N(E/T)2 dependence, The measured obtained with N method A and B respectively in Figs. I and 3, are in exact agreement with the above expressions (1) and (2). The spectral dependence of the refractive index change ~.n(w) is related to the electro-dichroism of all CN absorption bands by the dispersion relation c C ~KE(w1) dw’ ~12E(W)
I IT ~
=
W
‘2
(3)
—
The measured absence of any ~.n dispersion for hw < 5.3 eV (Fig. 2) shows that these frequencies w are small compared to the frequencies w’ of the CW absorption transitions so that the expression (3) becomes inde. pendent of w in this range. This absence of dispersion (which is in contrast to the OW case)’2 prevents any conclusion about position and strength of the CN absorption transitions aside of the fact that they must lie in the far u.v. range at energies more than 7 eV.
Vol. 14, No. JO
Using the measured dipole moment value (p = 0.3 d) to correct for the alignment ratio, we obtain ct~ = 3 Contrary to the OW’ defect, the CN’ + 1.60 (A). molecule is more polarizable along its axis than in the perpendicular directions. Using the mean value of the —
electronic polarizability & = 1/3 (a~+ 2a 3 a CWmolecules,’4 for 3.Thesizeofthisa-athsotropy~a/&= we obtaina~= 3.861)A3 2.8 and A 1 =2.26A 0.6 is typical for dipole gas molecules, and is in marked contrast to the nearly spherical a behavior of the OW defect in KCI.12 Most significant is the very pronounced anisotropy of the Kerr-effect. The relatively small E 100 effect is mostly due to the admixed (100) oriented OH” defects: The size, sign and strong dispersion of this (100) effect is in agreement with the observed OW Kerr-effect;’2 its strong reduction together with that of the OW absorption by chlorine treatment is strong support for this assignment. The remaining (100) effect, which flOW may be due to the true CN (100) Kerr-effect, is more than 3 orders of magnitude weaker than the (111> effect. This gives conclusive proof for a (111) symmetry of the electric dipole, contrary to previous work,8 but in agreement with the (ill) elastic dipole symmetry, derived from the new elasto-optical work.9 These new results affect the interpretation of numerous earlier works,18 which mostly based their interpretation on a (100) dipole model. Besides determining dipole symmetry and polar. izability anisotropy, the Kerr-effect has potentials for at least two more aspects of this dipole system:
—
Applying the Lorentz—Lorenz formula, we can relate the measured ~.n(E) effect for dilute systems (extrapolated to full dipole alignment) to the anisotropy of the electronic polarizabiity z~aof the ifl dividual CN defect: =
-
a~—a 1=
I. 2.
2 + 2)2 M
(4)
(a) It allows one to study dipole interaction effects by the deviations from the normal Curie-law behavior, as indicated by the two high CN concentrations measured. (b) It provides a simple and accurate way to study dipole relaxation behavior with optical Kerr-technique. Both these aspects, as well as Kerr-effect studies on CN dipoles in various other host materials, are in the process of investigation.
2irN(n
REFERENCES SEWARD W.D., Low Temperature Phsics, LT-9, p. 1130 (edited by DAUNT W. et a!.) Plenum Press, New York (1965). SEWARD W.D. and NARAYANAMURTI V.,Phvs. Rev. 148,463 (1966).
Vol. 14, No. 10 3. 4. 5. 6. 7. 8.
KERR EFFECT OF CN MOLECULES IN KC1 CRYSTALS
SACK H.S. and MORIARTY M.C., Solid State Commun. 3,93 (1965). LAKATOS A. and SACK HS., Solid State Cornmun. 4, 315 (1965). BYER N.E. and SACK HS.,Physica Status Solidi 30, 569, 579 (1966). CALLENDER R. and PERSHAN P.S., Phys. Rev. A2, 672 (1970). POMPI R.L. and NARAYANAMURTI V.,Solid State Commun. 6,645 (1968). NARAYANAMURTI V. and POHL R.O.,Rev. Mod. Phys. 42,201(1970).
9. 10.
L(JTY F.,Phys. Rev. (to be published). KU}{N U. and LOTY F., Solid Stare Commun. 2,281 (1964).
11. 12.
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‘Die Ausrichtung von substitutionellen CN MolekUlen in KC1 durch elek-
trische Felder wurde durch Messung der feld-induzierten optischen Doppel. brechung (Kerr-Effekt) untersucht. Die Feld., Temperature-, Wellenlangenund Konzentrations.abhangigkeit sowie die Anisotropie des Kerr-effekts stimmen quantitativ Uberein mit einem paraelektrischen Ausricht-modell fur (ill) orientierte CN Molekule, deren optische Absorptionen unter der Fundamental-absorption des Kristalles lie~en,und die durch eine Polarisations-anisotropie von = 1.60 A charakterisiert sind.’ —
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