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1、BasicsofPulsedMagneticResonanceCWvs.FTMagneticResonance.InordertounderstandpulsedEPR,firstthedifferencebetweenCW(continuouswave)andFT(FourierTransform)techniquesmustberealized.Wecancomparethesetechniquesbyusingananalogywhichrelatesthemtomusicalinstruments,whereoursamplei
2、saguitarwhichiscontinuouslyplayingauniquechord,andourmagnetisapiano.ForCWtechniques,wewouldplayeachkeyonthepianoinsuccession,detectingresonancesbetweenthefrequenciesintheguitarchordandfrequenciesofthepianonotesalongtheway.WebasicallyperformthesameexperimentinCWEPR,onlyth
3、emagneticfieldissweptinsteadofitsfrequencyandwedetectanyresonancesinoursample.AnalternativeapproachistostrikeeachkeyonthepianoatthesametimeandFouriertransformtheresultingsoundtoobtainthefrequencyspectrumoftheguitar;thisfactiscalledthemultiplexadvantageandisfundamentaltop
4、ulsedmagneticresonanceexperiments.InpulsedEPR,weapplyashortandintensemicrowavepulseconsistingofafinitebandwidthoffrequencies,digitizethesignalscomingfromoursample,andperformaFouriertransformationtoobtaintheEPRspectruminthefrequencydomain.SpinBehaviorinStaticMagneticField
5、s.Onemustunderstandtheequilibriumnatureofelectronspinsinanexternalmagneticfieldbeforethebehaviorofthespinsduringandafteramicrowavepulsecanbecomprehended.Allelectronshavetheintrinsicpropertyofspin,whichgivesrisetoamagneticmoment.Whenanelectronspinisplacedinamagneticfield,
6、atorqueisexertedonthespin,causingitsmagneticmomenttoprecessaboutthemagneticfieldlikeaspinningtopaboutagravitationalfield.TheangularfrequencyofthisprecessioniscalledtheLarmorfrequencyandisrelatedtothemagneticfieldby:ωL=-γB0whereωListheLarmorfrequency,γisthegyromagneticrat
7、io,andB0istheexternalmagneticfield.Ifweconsideralargenumberofelectronspinsprecessingaboutamagneticfieldalignedalongthezaxis,eachelectronspinwillassumeoneoftwoquantumstatesandaligneitherparallel(spinquantumnumberms=-½)orantiparallel(ms=+½)tothemagneticfield.Theparallelsta
8、tehaslowerenergy,andthusatthermalequilibriumthereisasurplusofelectronspinsintheparallelstateaccordingto
