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1、Physics290QuantumMechanics10/17/04ElectronDiffractionInthisexperiment,youwillobservethewavephenomenonofdiffractionwithabeamofparticles;anexperimentalverificationofdeBroglie’spostulate.ThearrangementissimilartoG.P.Thomson'sexperiment,donein1927,forwhichhesharedtheNobelPrizewithDavissonandGerm
2、er.Youwillmakebothqualitativeobservationsaboutthenatureoftheinterferencepatternandquantitativemeasurementsofthetargetstructure.TheapparatusTheelectrondiffractometerhasthreemaincomponents:anelectrongun,atarget,andaviewingphosphorescentscreen.Allofthesearehousedinanevacuatedglasstube.Thepowers
3、upplythatweuseprovidesanacceleratingvoltageforthegun.Thewiringisalreadydoneforyou;thereisnoneedforyoutodisconnectandconnectcables.Recallhowanelectrongunoperates.Aheatedfilamentprovidesasourceofelectrons.IftheelectronsareinthepresenceofastrongE-fieldthenthereisaforceontheelectronsandtheyaccel
4、erate.Inthiscase,theelectronsareacceleratedthroughapotentialdifferenceofseveralkV.Conservationofenergyallowsustowritedowna"gunequation"eVacc=mv2=(1)Whereeisthechargeofanelectron,Vaccistheacceleratingvoltage,misthemassofanelectron,andv(notthefrequencyn)isthespeedwithwhichtheelectronsleavetheg
5、un.WecansolveforthespeedoftheelectronsandthereforecalculatetheirdeBrogliewavelengthintermsofVacc.Recall(andmemorize)that?c=hc/2p≈200eVnm(orhc≈1240eVnm)andme=511keV/c2.Theelectronbeampassesthroughatargetthatisathinlayerofmulti-crystallinegraphite.Picturethetargetasmanyatomiclayersofcarbonatom
6、diffractiongratingsinrandomorientations.Withsucha"grating"theinterferencepatternbecomesaseriesofconcentriccircles.Wewillsimulatethisinlabwithanopticaldemonstration.Physics290QuantumMechanics10/17/04Thescreenwhereweviewtheinterferencepatternis13.5cmfromthetarget.Aselectronshitthephosphor,thei
7、renergyisconvertedtovisiblelightthatwecansee.Youwillbemakingmeasurementsonthediametersofthecircularinterferencepatterns.Asalways,thinkcarefullywhenyourecorddataabouttheuncertaintyassociatedwitheverymeasurement.QualitativeexperimentSwitchontheheater
