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Structural Order in Metal Clusters



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Structural Order in Metal Clusters Joel H Parks Rowland Institute at Harvard Cambridge Massachusetts Brand Symposium on Size Selected Clusters 2005 Research Support Xiaopeng Xing Ryan Danell Michael Burns Martine Blom Karlsruhe Theory Collaboration I L Garzon K Michaelian UNAM Department of Energy National Science Foundation Rowland Institute Outline Trapped Ion Electron Diffraction Methods and Review Metal Cluster Diffraction Agn n 36 55 Aun Aun n 20 32 55 Diffraction Instrument Diffraction Beamline Diffraction Tradeoffs Trapped Ion Physics trapped ion number space charge interactions e beam ion cloud overlap trapping potential mass isolation collisional relaxation number of isomers rf heating V V V T Single Mass Selection Trapped Ion MS Broad Band Excitation Narrow Band Excitation Experimental Data Cycle 500 trap cycles 6 h 2x106 clusters Annealing via Collision Induced Heating Ag38 Load Ions Equilibriate He Evacuate Resonant Excitation Equilibriate Ar Evacuate Diffraction Exposure Diffraction Analysis Itot atoms background gases inelastic scattering poly s s Ibkg f 2 Imol exp fit structure model Itot Imol theo s Itot f 2 s Imol exp f 2 s Ibkg f 2 interference Imol theo Background Variation Agn Diffraction Analysis Independent Scattering Background f 2 Background n f2 s 1 Molecular Difraction Cluster Size Dependence Cluster Temperature Initially thermalized by He collisions 105 collisions during loading No evidence of e beam heating inelastic channel multiple ionization data independent of exposure time Annealing shows evidence of lower isomers Patterns consistent with low temperature structures No direct identification of vibrational temperature Mixture of Structural Isomers NaCl Structure CsCl Structure fcc bcc IModel fN INaCl f C ICsCl CsCl nCs Diffraction CsCl Isomer Contribution CsCl 32Cs Silver Clusters Total Scattering Intensity Agn CCD Image s Molecular Diffraction vs Cluster Size Agn Diffraction Exhibits Icosahedral Patterns s1 s2 Ag55 Ih ssh s ssh s1 s2



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