Solid State Communications,Vol. 6, pp. 505- 507, 1968. PergamonPress. Printed in GreatBritain

MAGNETIC CIRCULAR DICHROISM OF THE R’ CENTER IN L1F*

J.A. Davis and D.B. Fitchen

Laboratoryof Atomic andSolid StatePhysics, CornellUniversity

Ithaca, New York 14850, U.S.A.

(Received 24 March 1968)

A temperature-dependentmagneticcircular dichroismpropor-tional to the derivativeof the 8328 A absorptionline hasbeenmeasuredfor the R’ center in LiF in the range1. 6°� T � 4. 2°Kand H � 24 kG. The resultsimply that the transition is3A

2 — ~E andthat the orbital momentin the excitedstateisstrongly reducedby dynamicJahn-Tellerdistortion.

THE ABSORPTION andemissionbandsassocia- four-electron R’ centerare ‘A1 — ‘E ortedwith the R’ zero-phononline at 8328A in

3A2 -.

3E, with Hund’s rule favoringthe latter.LiF showclearly severalfeaturespredictedfor The presentexperimentis able to distinguisha trigonal color centersubjectedto adynamic betweenthese.Jahn-Teller distortion in Its E excitedstate.1This Jahn-Tellerdistortion shouldalso affect The theoryandresults of magneticcir-the magneticbehaviorof the center, causinga cular dichroism measurementsof R-type centerslargeandpredictablereductionof the orbital with Jahn-Tellerdistortion havebeendiscussedmagneticmomentin the excitedstate. Prelimi- by Shepherd4 for E — A transitions. The tech-nary attemptsto observeorbital Zeemaneffects nique relies on the differencein absorptionforin emissionwereunsuccessfulandonly indicated right- andleft-circularly polarizedlight, eitherthat the reductionwasa largeone.2 We report becauseof a Boltzmarmpopulationdifferenceinherea measurementof the magneticcircular the Zeeman-splitinitial levels favoringonepo-dichroism of the 8328A line, this time in absorp- larization, or becausetransitionsfor the twotion, from which wehavebeenable to identify polarizationsare no longerdegenerate. In thethe transitionas 3A

2 -.

2E andfind an orbital- caseof the A -. E transitionof the R’ center,momentreductionevenlarger thanpredicted. only the latter conditionshouldapply. If the

splitting is small comparedto the llnewidth, theThe R’ line was known to be an A -. E circular dichroism shouldjust be proportional

transition on the basisof uniaxial stressmea- to the derivativeof the absorptionline,surements,3 but its spin stateswereunknown.It is assumedthat spin is conservedin the tran- ~sition sincethe short (— 10 nsec2) radiativelife- atime of the 8328A resonancefluorescenceline a.~(E,H)-a_(E,H)~- y . . (1)implies that this is an electric dipole transition. ~ EThe two alternativesfor the transition at the_______________ The proportionality constant, y, is a measure*Work supportedprimarily by the U. S. Atomic of the magneticinteractionin the final state. ItEnergyCommissionundercontractAT (30-1)- consistsof a temperature-independentorbital3464, TechnicalReport No. NYO-3464-11. The Zeemantermanda temperature-dependenttermAdvancedResearchProjectsAgency is alsoack- which canbe ascribedto the effectof spinpolari-nowledgedfor the useof the Central Facilities zation, if S ~ 0.of the MaterialsScienceCenterat Cornell Uni-versity, MSC ReportNo. 909. = (~bAH+ bAX <S(H, T)>) , (2)

505

506 MAGNETIC CIRCULAR DICUROISM OF THE R’ CENTERIN LiF Vol.6, No. 7

(a) (b)

when H is appliedalong [100] andthe absorption ois measuredfor light travelingalong [100].Here bA is the reducedorbital moment,<I, L, 1 ~,>, in the excitedstateof the dynami-cally distortedcenter (A is the orbital momentumof anundistortedcenterand b is the Jahn-Tellerreductionfactor). A is the spin-orbit interactionconstantand <S> is the expectationvalue of thespin in the [100] direction. Shepherd4hasnotedthat theseexpressionsstill apply evenwhenthe interactionwith static strain fields is largecomparedto the magneticinteraction. bA

4 AHarshawcrystalsof LIF werecolored I

by prolongedX-irradiation at room temperaturesothat the peakoptical densityin the line was FIG. 1aboutoneat 4°K in a samplea few mm thick.The coloredsamplewas immersedin liquid (a) Transmittedintensityat the 8328~helium (1.6°� T � 4. 2°K)in the boreof a R’ absorptionline in LiF for zerosmall superconductingsolenoidwhich produced magneticfield. Vertical arrow mdi-fields up to 24 kG in the [100J direction. The cates8328 A.differencespectrumbetweenright- and left- (b) Circular dichroism signalat 20. 8 kGcircularly polarized light transmittedalong [1OO~ and 2°K. Intensity scaleis enlargedwas analyzedusingeitheran electro-optic light by a factor of 133.modulatoror arotating quarter-waveplate, a1/2 m. gratingmonochromator,andphase-sen-sitive detection. Sincethe signalwas nulled op-tically for zero field andsincefield-inducedchangeswere small, the differencein absorp-tion constantswasrelatedto the measuredquan- A

tities by,T~L6~K

6

14(x,H)-L(x,H) / A

[a4(X,H)- a(X,H).d~ . (3) AI.~(X,0)

A representativespectrumis shownin Fig. 1. 4

2 A

- As predicted, the dichroism is propor-tional to the derivativeof the absorptionconstant. 0 °

The relativesizeof the dichroism dependsonfield andtemperatureas shownin Fig. 2. Since /2 o0

the temperaturedependenceimplies that S ~ 0, 2 T~4.2~K

we assignthe R’ transitionto the3A

2 —

3Ealternative.

The relatively largespectralslit widthusedis also shownin Fig. 1. We havecalcula- I I

ted that its effect is to reducethe measured 00 0 20 30

dichroism by a factor of 2 andhavecorrected H(KG)

accordingly. Using the valueof ~S> appropriate FIG 2to S = 1 in equation(2), we find the bestfit tothe datain Fig. 2 for .

Field andtemperaturevariation ofbAA = + 0 084 ± 0 004 cm1 ~ the magneticcircular dichroism of

/ the R’ absorptionline in LiF.and Curvesindicatethe valuesof 2A cal-

bt = 0. 004 ± 0. 004. culatedfrom equations(2) and(4).

Vol. 6, No. 7 MAGNETIC CIRCULAR DICHROISM OF THE R’ CENTER IN LiF 507

Thus the spin-orbitcoupling, A, is posi- hand, if weassumethat k’ 3. 5, basedon thetive, in contrastto that for the R center.4 The analysisof the R’ vibronic structure,’ then thevalueof the reducedorbital moment, bA, is predictedvaluefor b is -.. 0. 050, implying thatsmaller thanexpected. Child and Longuet- A 0. 08. The significanceof this discrepancyHiggins5 havecalculatedthe reductionfactor b is unclear.asa functionof the strengthk2 of the Jahn-Teller distortion. If we makethe usualassump- Acknowledgments- We wish to thank Drs. H. R.tion that A is approximatelyunity, thenwe find Fetterman,A. E. Hughes, andI.W. Shepherdb 0. 004 implying k2 6.. 5. On the other for helpful discussionsandassistance.

References

1. FETTERMAN H.R. and FITCHEN D.B., precedingpaper.

2. FETTERMAN H.R. and FITCHEN D.B., Bull. Am. phys. Soc. 12, 410 (1967);

FETTERMAN H.R., Ph.D. thesis, Cornell University, 1968, unpublished.

3. FITCHEN D.B., FETTERMAN H.R. and PIERCE C.B., Solid StateCommun. 4, 205 (1966).

4. SHEPHERDI.W., Phys. Rev. 165, 985 (1968).

5. CHILD M.S. and LONGUET-HIGGINS H.C., Phil. Trans. Roy. Soc. Lond. A254, 259 (1961).

Wir habeneinentemperatur-abhangigenmagnetischenzirkularenDichroismusgemessenfür R’ Zentrenin LiF im Bereich1.6° � T � 4.2°Kund H � 24kG. Der Effekt ist proportionalzur differentiellenAbleitungder 8328 AbsorptionsBande. AusdenErgebnissenfolgt, dassderoptische Ubergangvom type2A

2 —

3E2 ist und dassder Bahndrehimpulsim angeregtenZus-

tand durch eirien dynamischenJahn—TellerEffekt stark reduziertist.