物理chap33to光與物質(zhì)相互作用時(shí)可能會(huì)發(fā)生

Differenteffectofphoton ctionsThephotoncanbescatteredfroman(oranucleus)andintheprocesslosessomeenergyanditsfwillbelower——Comptoneffect.Aphotonmayknockanelectronoutofanatomandintheprocessitdisappears——photo-electric3.Aphotonmayknockanatomicelectrontoahigherenergystateintheatom.Intheprocessphotondisappears,andallitsenergyisgiventotheatom.(Suchanatomisinanexcitedstate.)4Aphotoncanactuallycreatemattersuchastheproductionofanelectronandapositron.Thisprocessiscalledpairproduction(正負(fù)電子偶).若干種不同的效應(yīng):有光電效應(yīng)、LightrevealsbothwaveandparticleAccordingtophoton

Accordingtospecial Massof

m=hn=

(m

c2 1-vRestmassof

m0= Momentumof

p=mc

p=lnck’sconstantlinkstheparticleandwaveofalight

p=l33-433-4TheHydrogenEarlymodelsoftheatomTheideathatmatterismadeupofatomswasacceptedbymostscientistsby1900.Withthediscoveryoftheelectroninthe1890s,scientistsbegantothinkoftheatomitselfashavingastructureandelectronsaspartofthatstructure.J.J.Thomson,soonafterhisdiscoveryofelectronsin1897,visualizedatypicalmodeloftheatomin1903asahomogeneoussphereofpositivechargeinsideofwhichthereweretinynegativechargedelectrons.Thomson’sapparatusformeasuringtheratioofmasstochargefortheelectron.Around1911,E.Rutherford(1871—1937)andhiscolleaguesperformedexperimentswhoseresultscontradictedThomson’smodeloftheTheresultsbasedonthe netary”modelaccordwellwiththeonefromscatteringofaparticles. Twomaindifficultiesof netary”Itpredictsthatatomsareunstable—electronsshouldbequicklyspiralintothenucleus—butweknowthatatomsingeneralarestable,sincethematteraroundusisstable;Itpredictsthatlightofacontinuousrangeoffrequencieswillbeemitted,whereasexperimentshowslinespectra.Rutherford’s“ netary”modeloftheatomwasamajorsteptowardhowweviewtheatomtoday.Itwasn’tacompletemodelandshowedsomemajorproblems,asweshallsee.Atomicspectra—keytothestructureof Heatedsolids,liquidsanddensegasesemitlight,duetooscillationsofatomsandmolecules.Theemissionspectrumischaracteristicofthematerialandcanserveasatypeof“fingerprint”foridentificationofthegas.A.J.?ngstr?m(Swedish)observedtheredlinehydrogenlinespectruminthevisibleregionin。Aisfor Theradiationfromexcitedgasesshowedthatthespectrumwasnotcontinuous,butdiscrete.In1885,J.J.Balmershowedthatthefourvisiblelinesinhydrogenspectrumfitfollowingformula:1=R(1-1),n= R=1.0974·107m-1—ThisBalmerseriesoflinesalsoextendedintotheUVregion,endingatl=365nmcorrespondington=￥.LaterexperimentsonhydrogenshowedthatthereweresimilarseriesoflineswhichhadapatternjustliketheBalmerseries,butat ,,

=R 1 1

n2 n

=2,3,...(intheUVPaschen

1 1=R 3

,nnin

=4,5,...hf=hc=hcR( - n2 n

(intheIRTheserieslimitsforBalmer,LymanandPaschenaredifferent.eg.TheserieslimitforBalmerseriesrepresentsatransition￥fi2.Rutherford’smodelneedstobemodified.ItwasNeilsBohrwhoprovideditbyaddinganessentialidea——thequantumhypothesis.Bohr’stheoryoftheHydrogenElectronsmovearoundthenucleusinorbits,butonlycertainorbits(stationarystates)areallowed.Anelectronineachorbitwouldhaveadefiniteenergyandwouldmoveintheorbitwithoutradiatingenergy.Lightisemittedonlywhenanelectronjumpsfromonestationarystatetoanotheroflowerenergy.WhenanelectronjumpsfromstateEutostateEl,asinglephotonoflight(frequencyf)isemittedff=Eu-hBohr’squantumTheelectron’sangularmomentumLis andequaltoanintegerntimesh/2p.L=L=nh=n (n=1,2,3,)m●-mp●EEn

n:quantumnumberof Theradiusofthecircularorbitofelectroninahydrogenatom: betweenprotonandelectron:F

eBasedonN-II

e2

=mr nFromquantumcondition:mv=2πnm

m●- =

=0.53·10-10

p●EEnrn=0Thesmallestrn=0Anelectroncanexistonlyintheorbitsabove.Therearenoallowableorbitsinbetween.Thespeedofelectroninthecircularorbitradius vn

(n=1,2,3,)E1TheenergyE1En=K+U

1nmv1n2

e2-

En=8ehEn=8eh22204E=-14E=-10Forn=Theelectroninahydrogenatomtendstobeinthelowestenergylevel—initsgroundstate.Bohr’smodelofatomgaveusafirstpictureofwhatanatomislike.Itproposedanex forwhythereshouldbeemissionandabsorptionoflightbyatomsatdiscretewavelengths.由能級算出的光譜線頻率和實(shí)驗(yàn)結(jié)果完全一致Fig.33-25ofBohrtheorypredictswellionizationenergy13.6eVforhydrogen,butwasnotsosuccessfulforothers.Example:Ahydrogenatomisatthefirstexcitedstate(n=2).Canitbeionizedifilluminatedbyvisiblelight?Solution：Theenergytoionizeahydrogenatomatthefirstexcitedstateis =0-(-13.6)= umenergyofavisiblelightphoton =hc l4000·10-

= Example:Ahydrogenatomisbombardedbyanelectronof12.2eV.Whatarethepossiblewavelengthsemittedbythehydrogenatom? -13.6+12.2=-TheenergyofthesecondexcitedstateofHisE3=-1.51eV，F(xiàn)orthethirdexcitedstate:E4=-0.85eV.Sothehydrogenatomcanjumptothesecondexcitedstatefromthegroundstate.Therearethreepossibilitieswhenjum E3fiE2,E2fiE1,E3fiE1n1

E3-

l=c=

=

Balmer nE-

=E-

=

Lymanl3