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1、1462T.SEETTRUPANDI.BALSLEVplainedbyincorporatingnonlocallossmecha-tallineresonanceswefindthatP=0atthesurfacenisms.isanadequateconditionthroughoutthespectrum.TheinfluenceofthethicknessaofthesurfacelayerVII.SUMMARYhasbeentreatedstrictlyinthelong-wavelimit.InthispaperwehaveintroducedasimplemodelThu
2、s,ifapolaritonmodecarryingaconsiderablewhichisbelievedtodescribetherelevantinfluencepartoftheenergyhasawavevectorcomparableofaboundaryonthepolaritonwaves.Incontrastto1/a,thenamoreelaboratecalculationisneces-totheconsiderationsinRef.2,wedonotneglectsary.Wehavealsoextendedthetheoryofbulkthecouplin
3、gtolightintheboundary-conditionprob-wavesandboundaryconditionstoincludetwoorlem.Generally,ourtreatmentleadstothecondi-moredispersiveresonances.Finally,wehavetionthatanappropriatelinearcombinationofPdemonstratedtheagreementwithexperimentoftheandsP/axvanishesatthesurface[seeEq.(16)j.boundarycondit
4、ionincaseoftheexcitonlinesinHowever,wheninsertingactualparametersofcrys-ZnO.'S.I.Pekar,Zh.Eksperim.iTeor.Fiz.33,1022&pMCardona,J.Phys.Chem.Solids24,1543(1963)(1957)[Sov.Phys.JETP6,785(1958)].(M=0.9mp).J.J.HopfieldandD.G.Thomas,Phys.Bev.132,~~D.E.Dietz,J.J.Hopfield,andD.G.Thomas,J.563(1963),Appl.
5、Phys.32Suppl.2282(1961).(M=1.5mp,if3J.J.Hopfield,J.Phys.Soc.Japan,Suppl.~21777correctedforthenewvalueoftheelectronmass,(1966).me=0.25mp).V.M.AgranovichandV.L.Ginzburg,SpatialDis-'B.Segall,Phys.Rev.163,769(1967)(M=2mp).persionincrystalOpticsandtheTheoryofExcitonsThetime-savingapproximationsintrod
6、ucedinpre-(Interscience,JohnWileyandSons,London,1966).viouscomputationsofexcitonspectrainZnO(seeRef.13)C.W.DeutscheandC.A.Mead,Phys.Rev.158,havenotbeenappliedhere.Consequently,weobtainA63(1965).somewhatdifferentvaluesforoscillatorstrengthsand6J.J.Hopfield,Phys.Rev.112,1555(1958).resonancefrequen
7、cies.D.G.Thomas,J.Phys.Chem.Solids15,86(1960).T.Skettrup,SolidStateCommun.7,869(1969).Y.S,ParkandJ.R.Schneider,J.Appl.Phys.39,'5T.SkettrupandI.Balslev,Phys.Stat.Solidi40,3049(1968).93(1970).J.FilinskiandT.Skettrup(unpublishe
