Contents - Max-Planck-Institut für Physik komplexer Systeme
Contents - Max-Planck-Institut für Physik komplexer Systeme
Contents - Max-Planck-Institut für Physik komplexer Systeme
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TheFano-ADCformulationfordoublyionizedstates Usingthealgebraicdiagrammaticconstruction<br />
(ADC)abinitiotechniqueofthegrouparoundL.Cederbaum(Heidelberg),wehavegeneralizedour<br />
Fano-ADCcomputationalmethod[6]todecayofdoublyionizedstates[9]incollaborationwithP.<br />
Kolorenč(Prague)andtheHeidelberggroup.ThenewmethodhasbeenappliedtoacascadeAuger-ICD<br />
process[7,8]inheteronucleardiatoms(NeAr,MgNe). Knowledgeofthedecaywidthsofthedoubly<br />
ionizedstatesresultingfromNe 1sAugerdecayinthesesystemsisessentialforthequantitativeprediction<br />
ofthekineticenergyspectraoftheICDelectronsemittedonthesecondstageofthecascade[9].<br />
TheFano-ADCapproachasformulatedin[6]isnotapplicabletosystemsinwhichthedecayingexcitationisdelocalizedduetoinversion(orhigher-order)symmetryasisthecaseforthephysicalsituation<br />
consideredbelow. ForthesesituationswehaveformulatedanextensionoftheFano-ADCscheme[1],<br />
forareviewoftherecentadvancesinthedevelopmentofFano-ADC,seethereview[10].<br />
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10 -2<br />
10 -3<br />
200<br />
150<br />
100<br />
50<br />
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(b)<br />
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Figure1:(a)Survivalprobabilityofthe 2svacancyinAr<br />
+ (H + )12(solid)andinanisolated<br />
Ar + (dashed).(b)Timedependenceofthe r 2<br />
expectationvaluemeasuringthespatialextensionoftheAr<br />
+ (H + )12 electronicwavefunction.<br />
Asmentionedabove,theavailabilityofthehigh-power<br />
FELsourcescallsforathoroughinvestigationofelectronicdecayinmultiplychargedsystems.Indeed,anygiveninner-shellvacancycreatedbytheFELradiationcaninteractwiththeneighboringvacancieswhich<br />
areabundantduetothehighphotonfluxes. These<br />
neighboringvacanciescanparticipateinacollective<br />
electronictransitionor,probablymorefrequently,are<br />
spectatorsthataffectthedecayprocesswithoutundergoingatransitionthemselves.Thesescenariosare<br />
discussedbelow.<br />
Collectiveinter-atomicdecayofinner-shellvacancies<br />
in doubly charged clusters we have identifiedasanewinter-atomicdecayprocess,cICD[2].<br />
Twoinner-shellvacanciesintwodifferentatomsor<br />
moleculescanbefilledsimultaneously,whileaneighboringneutralspeciesisionized.cICDcanberegardedasatransferoftwoormorevirtualphotons<br />
fromtheionizedclusterunitstoaneutralone. UsingourFano-ADCtheoryfordoublyionizedstates,<br />
wehavedeterminedthedecayrateoftwoKr 4svacanciesinKr2Arforavarietyofclustergeometries<br />
asanexample. Thevirtualtwo-photontransitionis<br />
ordersofmagnitudefasterthantheradiativedecayof(4s −1 )Kr + andthecollectivedecaycancompete<br />
successfullywiththedissociativenucleardynamicsofthedoublyionizedsystem[2].<br />
AtomicAugerdecayinthefieldofapositivecharge Thesimplestpossiblemodelforthestudyof<br />
theeffectofaneighboringchargeonanAugertransitionisthatoftheatom(ormolecule)bearinga<br />
corevacancyinthefieldofapointcharge.WehavestudiedthedecaywidthsofaseriesofatomicAuger<br />
transitionsasfunctionsofatom-protondistance,namely (2s −1 )Mg + Coster-Kronigdecayaswellas<br />
(2p −1 )Mg + and (2p −1 )Ar + Augerdecay.Inallcases,wehavefoundanappreciableeffectoftheproton<br />
onthedecaywidth,rangingfromabout20%inthecasesof (2s −1 )Mg + and (2p −1 )Ar + tomorethanan<br />
orderofmagnitudeinthecaseof (2p −1 )Mg + [11].Weinterpretethiseffectintermsofthedistortionof<br />
avalenceorbitalbythestrongelectricfieldofthepointcharge.Whenasingledecaychanneldominates<br />
theAugertransition,amoredetailedunderstandingcanbegainedviamultipoleexpansionanalysisofthe<br />
correspondingAugermatrixelement. Calculationsofthesizesandthedipolemomentsofthevalence<br />
orbitalsasfunctionsoftheatom-protondistancehaveshownthat,justastheappreciablechangesin<br />
thedecaywidths,suchdistortionsoccuratthe”chemical”distancesofabout 1 − 2˚A,whileatdistances<br />
typicalofvanderWaalsclusters(∼ 3˚A),theneighboringchargeeffectcanbeneglected.Thesefindings<br />
arerelevantforthesimulationofclusterandmoleculardisintegrationinanXFELbeam.Suchsimulations<br />
usuallyapproximatetheratesofAugertransitionsinmultiplyionizedpolyatomicsystemsbytheAuger<br />
ratesofisolatedatoms. Ourworkshowsthatthisassumptionisjustifiedforraregasclustersbutvery<br />
questionableformoleculeswhichhasconsequencesforthevalidityofresultsonradiationdamageof<br />
moleculesintheXFELfield[12].<br />
PKS-Fellowship 103
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