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1、4M¨ossbauerSpectroscopy(MS)4.1IntroductionM¨ossbauerspectroscopyisaversatiletechniquethatisusefulinmanyareasofsciencesuchasPhysics,Chemistry,Biology,andMetallurgy.Ityieldsverypreciseinformationaboutthechemical,structural,magneticpropertiesofamaterial.Thebasicfeatureofthete
2、chniqueisthediscoveryofrecoillessγ-rayemissionandabsorption,nowreferredtoasthe“M¨ossbauerEffect,”afteritsdiscovererRudolphM¨ossbauer,whoreportedtheeffectin1958(M¨ossbauer1958)andwasawardedtheNobelPrizein1961forhispioneerwork.Whenabeamofelectromagneticradiationwithacontinuous
3、frequencydistributionismadetopassthroughagaseouselementormetallicvapor,certainfrequencieswillgetabsorbed.Thesefrequenciescorrespondtotheallowedexcitedstates.Similarlytheatomicnucleiwillabsorbtheγ-raysastheatomicexcitedstatesfallintheγ-region.Theimportantaspectofsuchabsorpt
4、ionisthatitisverysensitivetotheγ-rayenergyinthesensethatiftheγ-rayhasfrequencydifferentfromresonancebyonepartin1012,itwillnotbeabsorbed.Suchsensitivitywillnotberealizedunlessthenaturalfrequencyspread(linewidthforatomicsystems)oftheγ-rayissmallwhichwillhappenifthelifetimeoft
5、heexcitedstateemittingtheγ-rayislong(≥10−7s).Whenanisolatedexcitednucleusemitsaγ-ray,itrecoilsinordertocon-servethemomentum.Asaresultofthemotion,theenergyoftheemittedγ-rayisDopplershiftedtoasomewhatlowervalue,whichmakestheabsorp-tionofthatγ-rayimpossiblebysomeothernucleiof
6、thesamespecies.Therecoilmotionofthesourcecan,however,becompensatedbymovingeitherthesourceortheabsorberinadirectionsuchthatthetotalmomentumofthenucleusplusγ-rayiszero.RudolfM¨ossbauershowedthatiftheexcitednucleusisfree,therecoilenergyandmomentumaretakenbythenucleusitself.Ho
7、wever,inasolidwheretheatomsareboundintoacrystallattice,momentumandenergygointolatticevibrationsi.e.,phonons.Sincetheentirelatticewillabsorbthe1784M¨ossbauerSpectroscopy(MS)recoilmomentum,therefore,therecoilenergyisverysmallandtheDopplershiftislikewiseverysmall.Thetemperatu
8、redependenceoftheabsorptioncross-sectionof191IrledM¨ossbauertobethefirsttorealizethataphot
