原子物理讲义 amo_9new

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1、Contents9Two-PhotonExcitation19.1Introduction....................................19.2CalculationoftheTwo-PhotonRate......................29.3CasesofTwo-PhotonAbsorption........................39.3.1Two-photonratewithasingleintermediatestate...........39.3.2T

2、wo-photonabsorptionfromasingleradiationsource........49.4Two-PhotonDoppler-FreeSpectroscopy....................59.5RamanProcesses.................................69.5.1StimulatedRamanscattering......................69.5.2SpontaneousRamanscattering...........

3、..........7iiiCONTENTSChapter9Two-PhotonExcitation9.1IntroductionEveryradiativeprocessdescribedsofarhasinvolvedasinglephoton,whetheritbeahy-per¯netransitioninmagneticresonance,opticalexcitation,orspontaneousemission.How-ever,processescanoccurinwhichanatomsim

4、ultaneouslyabsorbstwoormorephotons.Suchmulti-photonprocessescanleadtoionizationofatomsordissociationofmoleculesinintenselaser¯elds,andphenomenasuchasfree-to-freetransitionsinwhichanelectronabsorbssuccessivequantaasit°iesoutfromthe¯eldofanatom.Themostfrequent

5、lyencounteredmulti-photonprocessistwo-photonabsorptionoremission.Two-photonpro-cesseshavebecomeoneofthestandardtoolsinatomicphysicsforexcitingatomstostateswhoseenergiesaretoohightoachievewithasinglephoton,andalsotostatesofthesameparitythatwouldnormallybeinac

6、cessible.Inaddition,anumberofultra-highresolu-tionspectroscopictechniquesarebasedontwo-photonprocesses.Ourapproachwillbetousesecond-orderperturbationtheory,extendingthe¯rstorderdevelopmentusedinear-lierchaptersinastraightforwardfashion.Analternativeapproachi

7、nvolvessolvingthedynamicalequationsinthesamemannerasweanalyzedthetwo-levelsystem.However,theperturbationapproachisappropriateinmanycases,andissimplerthanthedynamicalapproach.Theaimistocauseatransitiona!bbyapplyingtwo¯elds:E~(t)=E1^e1cos!1t+E2^e2cos!2t(9.1)wh

8、ere¹h(!1+!2)=(Eb¡Ea)(9.2)Statesjaiandjbihavethesameparity,soasinglephotontransitionisforbidden.TheprocessisshowninFig.9.1,left.AmorerealisticviewisshowninFig.9.1right,wherejfirepresentssomeinter