Structural Order in Metal ClustersJoel H. ParksRowland Institute at HarvardCambridge, MassachusettsBrand Symposium on Size Selected Clusters 2005Research SupportXiaopeng XingRyan DanellMichael BurnsMartine Blom (Karlsruhe)Theory CollaborationI.L. Garzon, K. Michaelian (UNAM)Department of EnergyNational Science FoundationRowland InstituteOutline • Trapped Ion Electron DiffractionMethods and Review• Metal Cluster DiffractionAgn+ n=36-55Aun+, Aun-n=20, 32, 55Diffraction InstrumentDiffraction Beamline→● trapped ion number ● e-beam / ion cloud overlap● mass isolation ● number of isomers Diffraction Tradeoffs● space charge interactions● trapping potential● collisional relaxationrf heatingV-V, V-TTrapped Ion PhysicsSingle Mass SelectionTrapped Ion MSBroad Band ExcitationNarrow BandExcitationExperimental Data Cycle~ 500 trap cycles ~ 6 h~2x106 clustersAnnealing via Collision Induced HeatingAg38+Resonant ExcitationEquilibriate (Ar)Load IonsEquilibriate (He)EvacuateEvacuateDiffraction ExposureItotDiffraction Analysisatomsbackground gasesinelastic scatteringinterference poly(s) = s Ibkg/ f 2fit structuremodel(Imol)exp(Imol)theoItot-(Imol)theos Itot/ f 2s Ibkg/ f 2s (Imol)exp/ f 2Background Variation Agn+Diffraction AnalysisIndependent Scattering Background ~ Σ f 2s (Å‐1)‹Background›n/ f 2Molecular Difraction: Cluster Size DependenceCluster Temperature• Initially thermalized by He collisions ~105collisions during loading• No evidence of e-beam heating inelastic channel: multiple ionizationdata independent of exposure time • Annealing shows evidence of lower isomers• Patterns consistent with low temperature structures• No direct identification of vibrational temperatureMixture of Structural IsomersNaCl StructurefccCsCl StructurebccIModel=fNINaCl+fCICsCl(CsCl)nCs+Diffraction: CsCl Isomer Contribution(CsCl)32Cs+Silver Clusters: Total Scattering Intensitys (Ǻ)Agn+CCD ImageMolecular Diffraction vs Cluster SizeAgn+ Diffraction Exhibits Icosahedral Patternss1s2sshs (Å)Ag55+Ihs2/s1 ssh/s1Agn+cluster sizeIh55150 K150 K300 K43300 K 150 K39150 K300 K38150 K37150 K36Short Range Order ParametersSteinhardt, Nelson and RonchettiPhys Rev B 28, 784 (1983)Calculated Ag38+StrucuresI.L.Garzon and K.Michaelian (unpublished)0.148 eV0.064 eV0.0 eVOh0.045 eVAgn+Diffraction Fits to Calculated StructurescalculationdatauncertaintyGS36s (Å‐1)38.148 eV3739.49 eV.22 eV4355GSGSs (Å‐1)Ag38+Annealing Ne: 3x10-3T120 K0.148 eV0.064 eV42 %58 %s (Å‐1)s (Å‐1)Au20-Diffraction Fit to Calculations (Å‐1)Td GS.33 eVTd 65%150 K, annealedAu20+Diffraction Fit to Calculations (Å‐1)Td GS.49 eV150 K, annealedTd 40%Adsorption of O2on Aun-1921Dependence of Adsorption on Structure ?Au21-Td < 5 foldweak O2 adsorptionAu20-Td 0.64 5 fold 0.36strong O2 adsorptionAu19-Td > 5 foldweak O2 adsorptions (Å‐1)Au32Ion Diffraction Fit to CalculationAg32+ a9 isomera, a3, a9~0.05 eVD2hGSs (Å‐1)M. Johansson (unpubished)Ag32-s (Å‐1)Au55Ion Diffraction Fit to Calculation.638 eV80 %.09 eV20 %s (Å‐1)Au55-Binding Energy eVHannu Hakkinen, Michael Moseler, Oleg Kostko, Nina Morgner, Margarita Astruc Hoffmann, Bernd v. Issendorff PRL 93, 93401 (2004)Au55+s (Å‐1)Summary• Ag clusters short range order: local 5-point orderAg55+exhibits Ihorder parameters correlate with diffraction• Au clusters presence of Td n = 20, 195-point order n = 32, 55 • Annealing required for lowest energy isomers• nextstrucural change on adsorptionbimetallic