Observations on small anionic atomic clusters in an electrostatic ion beam trap [Elektronische Ressource] / vorgelegt von Markus Eritt

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Observations on Small Anionic AtomicClusters in an Electrostatic Ion Beam TrapI N A U G U R A L D I S S E R T A T I O NzurErlangungdesakademischenGradeseinesDoktorsderNaturwissenschaftenderMathematisch-NaturwissenschaftlichenFakultätderErnst-Moritz-Arndt-UniversitätGreifswaldvorgelegtvonMarkusErittgeborenam10.08.1977inJenaGreifswald, August2008Dekan: Prof. Dr. K.Fesser1. Gutachter: Prof. Dr. L.Schweikhard2. Prof. Dr. D.ZajfmanTagderPromotion: 2.10.2008Contents1. Introduction 12. Cluster properties 32.1. Shortintroductionintoclusters . . . . . . . . . . . . . . . . . . . . . . . . . 32.2. Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.1. Structurecalculations . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.2. Structureexperiments . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.3. Branchingratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.4. Polarisabilitycalculations . . . . . . . . . . . . . . . . . . . . . . . . 82.3. Aluminium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3.1. Structurecalculations . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3.2. Structureexperiments . . . . . . . . . . . . . . . . . . . . . . . . . . 92.3.3. Branchingratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.4. Silver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.4.1. Structurecalculations .
Publié le : mardi 1 janvier 2008
Lecture(s) : 40
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Source : UB-ED.UB.UNI-GREIFSWALD.DE/OPUS/VOLLTEXTE/2008/555/PDF/DISS_ERITT_MARKUS.PDF
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Observations on Small Anionic Atomic
Clusters in an Electrostatic Ion Beam Trap
I N A U G U R A L D I S S E R T A T I O N
zurErlangungdesakademischenGrades
einesDoktorsderNaturwissenschaftender
Mathematisch-Naturwissenschaftlichen
FakultätderErnst-Moritz-Arndt-Universität
Greifswald
vorgelegtvon
MarkusEritt
geborenam10.08.1977
inJena
Greifswald, August2008Dekan: Prof. Dr. K.Fesser
1. Gutachter: Prof. Dr. L.Schweikhard
2. Prof. Dr. D.Zajfman
TagderPromotion: 2.10.2008Contents
1. Introduction 1
2. Cluster properties 3
2.1. Shortintroductionintoclusters . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Structurecalculations . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.2. Structureexperiments . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.3. Branchingratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.4. Polarisabilitycalculations . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3. Aluminium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.1. Structurecalculations . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.2. Structureexperiments . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.3. Branchingratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4. Silver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4.1. Structurecalculations . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4.2. Structureexperiments . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4.3. Branchingratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4.4. Polarisabilitycalculations . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4.5. Giantdipoleresonances . . . . . . . . . . . . . . . . . . . . . . . . . 16
3. Setup 19
3.1. Beamline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2. Clustersource . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3. Trapchamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.4. Pickupelectrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.5. Electrontarget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.6. Micro-channelplatedetector . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.7. Vacuumsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4. Evaporative cooling experiment 35
4.1. Theoreticalapproachesand1/t-decaylaw. . . . . . . . . . . . . . . . . . . 35
4.2. Methodandexperimentalrealisation . . . . . . . . . . . . . . . . . . . . . . 37
4.2.1. Experimentalsetupandmeasurementtiming . . . . . . . . . . . . 37
4.2.2. Dataconditioning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.3. Resultsanddiscussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
iiiContents
5. Electron collision experiment 45
5.1. Electroncollisiondetails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.1.1. Firstprinciplesfromhistory . . . . . . . . . . . . . . . . . . . . . . . 45
5.1.2. Detachmentthreshold . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.1.3. Scalinglaws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.1.4. Semi-empiricalapproaches . . . . . . . . . . . . . . . . . . . . . . . 51
5.1.5. Decaychannels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
5.1.6. Polarisability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
5.1.7. Giantresonances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.2. Methodandexperimentalrealisation . . . . . . . . . . . . . . . . . . . . . . 59
5.2.1. Generalcrossedbeamapproach . . . . . . . . . . . . . . . . . . . . 59
5.2.2. Experimentalsetupandmeasurementtiming . . . . . . . . . . . . 61
5.2.3. Backgroundcollisionnormalisation . . . . . . . . . . . . . . . . . . 65
5.2.4. Dataconditioning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.2.5. Measurementuncertainties . . . . . . . . . . . . . . . . . . . . . . . 75
5.2.6. Crosssectioncomparison . . . . . . . . . . . . . . . . . . . . . . . . 76
5.3. Resultsanddiscussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.3.1. Giantresonances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.3.2. Carboncrosssectionresults . . . . . . . . . . . . . . . . . . . . . . . 82
5.3.3. Aluminiumcrosssectionresults . . . . . . . . . . . . . . . . . . . . 84
5.3.4. Silvercrosssectionresults . . . . . . . . . . . . . . . . . . . . . . . . 86
5.3.5. Comparisontodifferentapproaches . . . . . . . . . . . . . . . . . . 88
6. Summary 97
6.1. Evaporativecoolingexperiment . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.2. Electron-inducedelectron-detachmentexperiment . . . . . . . . . . . . . . 97
7. Outlook 99
Bibliography I
A. Appendix XVII
A.1. Thermionicevaporationresults . . . . . . . . . . . . . . . . . . . . . . . . . XVII
A.2. Crosssectionresults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XXI
Acknowledgements XXIII
ivList of Figures
2.1. Sodiumshellstructureand“Rayleigh-jets” . . . . . . . . . . . . . . . . . . 3
2.2. Carbonphotoelectronspectraandelectronaffinities . . . . . . . . . . . . . 6
!2.3. BranchingratioC ................................ 72
2.4. Polarisabilitycalculationsforneutralandanioniccarbonclusters . . . . . 8
2.5. Aluminiumvalencebandsandphoto-electronspectroscopydata . . . . . 10
2.6. Structureandgroundstateenergiesofdifferentsilverisotopes . . . . . . . 11
2.7. Silverphotoelectronspectra . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.8.branchingratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.9. Silverphoto-dissociationratiosandpolarisability . . . . . . . . . . . . . . 16
2.10. Giantdipoleresonances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1. Beamline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2. Clustersource . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.3. Carbonandaluminiummassspectra . . . . . . . . . . . . . . . . . . . . . . 23
3.4. Silvermassspectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.5. Trapchamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.6. Mirrorpotentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.7. Pickupsignal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.8. Simulationelectrontarget . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.9. Photoelectrontarget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.10. Cathodeandcollectorcurrent . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.11. Mass-dependentmcp-efficiency . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1. 1/t-law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.2. Setuplifetimeexperiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
!4.3. LifetimeAl insemi-logarithmicplot . . . . . . . . . . . . . . . . . . . . . 382!4.4. InjectionAl inplot . . . . . . . . . . . . . . . . . . . . . 409! !4.5. LifetimeAl andAg indouble-logarithmicplot . . . . . . . . . . . . . . 425 5
5.1. Gryzinskiscrosssectionshape . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.2. Electron-atomimpact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.3. Esaulovsscalinglaw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.4. Decaychannels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
5.5. Potentialwellsandplasmoncreation . . . . . . . . . . . . . . . . . . . . . . 56
5.6. Crossedbeamcollisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.7. Electroncollisionexperimentsetup . . . . . . . . . . . . . . . . . . . . . . . 62
5.8.ontargettiming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.9. Electroncollisiontiming . . . . . . . . . . . . . . . . . . . . . . 64
vListofFigures
5.10. Electroncollisionmeasurementscheme . . . . . . . . . . . . . . . . . . . . 65
5.11.onnormalisationtiming. . . . . . . . . . . . . . . . . . . . . 66
5.12. Mcppatternofabunchedbeam . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.13. Firstharmonicfft-peakofabunchedbeam . . . . . . . . . . . . . . . . . . 68
5.14. Frequencyspectraofabunchedbeam . . . . . . . . . . . . . . . . . . . . . 70
5.15. Backgroundnormalisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
!5.16. C crosssectionandnormalisationtoabsolutemeasurement . . . . . . . . 762!5.17. C cross . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778
5.18. Crosssectioncomparisontodifferentexperiments . . . . . . . . . . . . . . 78
!5.19. Ag crosssection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797
5.20. Ordinaryvs. polarisationalbremsstrahlung . . . . . . . . . . . . . . . . . . 80
25.21. Carbonresultsvs. m/E ............................. 82b
25.22.rvs."!#·m/E ........................... 84b
25.23. Aluminiumresultsvs. m/E 85b
25.24. Silverresultsvs. m/E .............................. 87b
25.25.rvs."!#·m/E ............................ 89b
25.26. Ratiosbetweenmeasuredand! 1/E calculatedvalues . . . . . . . . . . . 90b
25.27.edand!"!#/Evalues . . . . . . . . . 91b
2 25.28. Ratiosbetweenmeasuredand! r /E calculated . . . . . . . . . . 92b
1/3 2 25.29.edand! ("r#!"!# ) /E calculatedcarbonvalues 93b
5.30. Electronimpactincludingpolarisability . . . . . . . . . . . . . . . . . . . . 94
1/3 2 25.31. Ratiosbetweenmeasuredand! ("r#!"!# ) /E calculatedsilvervalues 95b
7.1. Bent-trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
7.2. Isobaricseparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
! !A.1. EvaporativecoolingofAl andAl ......................XVII1 2! ! ! ! ! !A.2.ofAl ,Al ,Al ,Al ,Al andAl ..........XVIII3 4 5 6 7 8! !A.3.coolingofAl andAlXIX9 10! ! ! !A.4. EvaporativeofAg ,Ag ,Ag andAg ...............XIX1 2 3 4! ! ! ! ! !A.5.coolingofAg ,Ag ,Ag ,Ag ,Ag andAg ........XX5 6 7 8 9 10
vi1. Introduction
The primary content of this thesis deals with observations on small anionic clusters
which is a purely fundamental research. The term atomic cluster relates to small com-
pounds of several up to thousands of atoms. Anionic clusters are furthermore charac-
terisedbyanadditionalweaklyattachedelectronthatleadstoanoverallsinglynegative
charge. Especially at small cluster sizes simple approximations, as e.g. the liquid drop
model do only very limited describe the observed behaviours. The extended but not
infinitenumberofinvolvedinteractionsbetweenelectronsandatomiccoresofthesame
elementarethereforeagoodtestgroundforquantummechanicalcalculationmethods.
Systematicstudiesofclusterswithonlyfewconstituentsshowedcharacteristicandnot
smooth size dependent property changes like e.g. the shell structure expressed in the
129specificstabilities(section2.1),negativeheatcapacities orgiantresonancesthatwere
51for anionic metal clusters predicted to occur above detachment treshold. The advan-
tageinworkingwithgas-phaseparticlesisthattheinfluenceofexternalforcestointer-
nal processes can be neglected. In a trap it is furthermore possible to observe property
developments of individual particles over time. Conclusions can thereby be extracted
fromindirectobservationsase.g. thedetectionofaneutralparticleisonlypossibleafter
anelectron wasreleased. The electrostaticionbeamtrapwasoriginallydevelopedasa
tooltodosimplemeasurementsase.g. lifetimedeterminationsinatabletopapparatus
instead of requiring a huge storage ring. The big advantage towards the various still
used storage rings with magnetic benders is that in electrostatic setups only the kinetic
particle energy defines the confinement conditions and therefore no upper mass limit
exists. The mass limit is the reason why so far no measurements of electron detach-
ment and only a rare number of lifetime measurements for heavier particles exists. In
ionbeamtrapsneutralparticlescanbedirectlydetected,whichopensnewobservation
possibilitiescomparedtorf-andPenningtraps.
The thesis is arranged into following chapters: this introduction is succeeded by an
overview of the for this work most important known properties of the observed car-
bon, aluminium and silver clusters. Main aspects therein are theoretically calculated
structures, electron detachment spectroscopy, branching decay ratios and if available
polarisation calculations. The third chapter describes experimental setup and condi-
tions in detail. Chapter four deals with evaporative cooling of the hot clusters from
the sputter source just after injection into the trap. Chapter five contains the main ex-
11. Introduction
periment of this thesis where forthefirsttimesystematicallyelectroncollisioninduced
electron detachment of anionic clusters is observed. Therein a short introduction into
the history and first principles of electron detachment as well as in the predicted giant
resonances is given – followed by a description of the method. The main issues in the
discussion of the results section are the electron detachment measurements as well as
possible cross section scaling factors in the conclusion part. The summary in chapter
sixshortlysketchesthemostimportantpointsofthisthesiswhilechaptersevenbriefly
looksforwardtowardsrecentexperimentsatthesetupattheWeizmann-Instituteandto
themassseparatorIdesignedforthelastoneandahalfyears.
22. Cluster properties
2.1. Short introduction into clusters
Theterm clusterintheframeworkofthisthesisreferstocompoundsthatconsistofsev-
eral atoms of the same element. In particular, this thesis deals with singly negatively
charged clusters in the gas-phase that means without interferences due to surrounding
forces. Asastartpointthatopenedthefieldofclusterphysicsonecanrefertothediscov-
82ery of Knight et al. in 1984. In their experiment they observed the abundance of neu-
tral sodium clusters that were produced in a supersonic expansion source and ionised
by broadband UV light. The spectrum obtained with a quadrupole mass analyser is
presented on the left in figure 2.1. The astonishing result was that the mass abundance
82Figure 2.1.: (left side) Mass abundance spectra of hot neutral sodium clusters Na . Clustersn
with closed shell configurations (n=8, 20, 40, 58) appear to be more stable. (right side) High-
34speedmicroscopyphotographofajetofelectronsejectedfromaethyleneglycolcluster.
changed in a way as it was already known from the building laws of the electron shell
modelinsingleatoms.
The probably simplest description of a cluster is the liquid-drop model which was
originally adopted from nuclear physics. It allows to calculate the stability of a cluster
and how much energy is required to detach an atom or an electron. Within this model
it is assumed that atoms arrange like a incompressible fluid so that the entire surface
minimises. Forastableclusterthecohesiveforcebetweenatomsneedstobeinbalance
32. Clusterproperties
with the surface tension and the Coulomb repulsion of the electrons. These properties
vary only smoothly with cluster size n but additionally terms for the shell and for the
odd-even energy are required whose averaged values are zero but show large value
fluctuations for specific cluster sizes. In order to observe an influence of these last two
parametersonthemonotonicbehaviourinternallyhotandthereforefastdecayingclus-
ters are necessary. At room temperature electron affinity and ionisation energy of most
clustersscalesmoothlywithclustersize n ormorepreciselywiththeinverseclusterra-$
3 65diusthatisproportionalto1/ n towardsthebulkvalues. Averyniceapplicationof
34theliquiddropmodelistheexperimentofDuftetal. whereachargedethyleneglycol
clusterwithµmradiuslevitatedoversecondsinarf-trap. Themaincoolingmechanism
wassteadyevaporationofneutralsresultinginadecreaseoftheclusterradiuswhereas
thechargeremainedconstant. Atleastuntilitreachedthesocalled“Rayleigh-limit”that
relatesthemaximumchargeadropletcanbeartoitssurfacetensionandradius. Exactly
137asRayleighpredicted thechargeddropletbecameunstableandejectedamicroscopic
jet of liquid from each end before returning to equilibrium. A high-speed microscopy
34photograph ofthese”Rayleigh-jets”ispresentedontherightsideoffigure2.1.
Another very successful approach is the so called jellium model. In this concept the
discreteness of the cores is eliminated by smearing out the positive charge into an uni-
formstructurlessbackground“jelly”thatisembeddedinahomogenousgasofvalence
23 1electrons. A sodium atom has a [Ne]3s structure that means the closed shell core
has the same size as neon and only the attached delocalised 3s valence electron defines
the binding properties. For a simple cluster like this the pseudo potential can be sim-
ulated by a spherical-symmetric square well potential. Solving the Schrödinger equa-
tion for the electron gas yields to discrete electronic energy levels characterised by the
82angular momentum quantum number L with degeneracy 2(2L+1) (including spin).
Analogue to noble gases in atomic systems it turned out that closed shell orbitals are
more stable and mark at the same time a step to a shell of less stable clusters like in
22figure2.2. Clemenger showedinhiscalculationsthatonlyclosedshellstructureswith
n = 2,8,20,40,58,92 valence electrons are really spherical. Open shell configurationse
lead to ellipsoidal distortions and thereby to sub-shell and electron energy level split-
tings.
The jellium model turned out to work very well for group I and II elements, where
undirected "-bonds support a spherical cluster structure. For small clusters or clusters
with directed #-bonds the situation is more complicated and geometric effects become
relevant. Thusreasonableresultscanbeonlyobtainedwithdetailedquantumchemical
structurecalculations.
The following sections will give a short review to the known properties of cluster
species and sizes which were observed in this thesis. The sections contain theoretical
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