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Methods to Produce and Study Clusters

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Methods to Produce and Study Clusters http://fielicke.lmsu.tu-berlin.de/ Methods to produce and study clusters André Fielicke Institut für Optik und Atomare Physik Technische Universität Berlin, Germany Program 1. What are clusters and why to study them? 2. Making and characterizing free clusters 3. Probing the structures 1. What are clusters and why to study them? Clusters Oxford English Dictionary: 1. A collection of things of the same kind, as fruits or flowers, growing closely together; a bunch. a. Originally of grapes (in which sense bunch is now the usual term). b. Of other fruits, or of flowers; also of other natural growths, as the eggs of reptiles, the air-cells of the lungs, etc. Cluster compounds • Thermodynamically and kinetically stable • Chemical synthesis in bulk quantities • Characterization with “classical” spectroscopic methods (IR, NMR, XRD etc.) Co (CO) 2- 4 12 B12H12 Isolated clusters • Generation through aggregation of the (atomic or molecular) constituents • (Nearly) free choice of size (n), + B V8 16 z composition (n/m) and charge (z): MnLm PRL 93 (2004) 023401 JCP 137 (2012) 014317 • Most clusters are not stable towards aggregation • Experimental investigations are usually performed in the gas phase Molecular beam techniques + H6O13 (“Zundel” cation) Clusters of atoms and molecules • multiples of a simple subunit, e.g. Cn, Arn, or (H2O)n • The cluster size n can vary and determines the properties Clusters Nano-crystals 2 3 4 5 6 7 8 Number of atoms 11010 10 10 10 10 10 10 2 3 4 5 Surface atoms 10 10 10 10 10 2 radius [nm] 11?0 10 • small clusters have (nearly) all atoms on the surface + Nbn 567 8 10 11 12 13 9 Volume and surface of a cluster with n atoms Spherical cluster approximation 4 VR 3 3 R SR 4 2 2r Assignment 1.1 How many Krypton atoms are in a spherical cluster of a) 1 nm, b) 10 nm, c) 100 nm radius? Assume that a single Kr atom fills an effective volume with 0.2 nm diameter in this cluster. 1.2 What is the ratio for surface vs. volume atoms for these clusters? The surface atoms contribute to the cluster surface by only ¼ of their ‚atomic surface‘, ¾ point toward the inside of the cluster. Clusters: nano and smaller Effects at the (sub)nano-scale quantum confinement large surface/volume ratio structural changes Emergence of new properties e.g.: magnetic optical / luminescence chemical / catalytic Size dependence of properties: Each atom counts Ionization energy Stability Magnetism Reactivity Fen + H2 Motivations for the study of free metal clusters Fundamental aspects How are properties emerging when going from the atom to the bulk? Reference systems Test and further development of theoretical model methods New materials Inspiration from particularly stable clusters application Model systems in heterogeneous catalysis 2. Making and characterizing free clusters Bonding in clusters EB≈100 neV 1s 1s dispersion HeHe2 He induction - - + - + - increasing dipole/dipole +++ bond strength - - - EB≈1-5 eV + + and cluster ion/dipole stability Cn metallic C=C=C or C≡C–C covalent EB≈4-6 eV ionic + - (Na Cl )n EB≈7-8 eV Experimental techniques for Cluster studies Cluster production: Top-down vs. Bottom-up Sputtering or Aggregation of the constituents sputtering bulk clusters material vaporization condensation supersaturated cooling vapor Cluster production Supersonic expansion of a gas Adiabatic and isenthalpic expansion leads to strong cooling formation of a cold supersonic beam Cluster formation via 3-body collisions near the nozzle e.g. Ar + Ar + Ar Ar* + Ar2 (conservation of energy and momentum) Dimers are condensation nuclei for larger clusters Seeded molecular beam: cooling of the internal degrees of freedom Cluster Production Gas aggregation (thermal) evaporation into a cold gas (°C) Typical vapour Na 289 pressures of ~10-2 mbar need to be Al 1217 reached Ag 1027 Smoke source for the production Au 1397 of C60, C70 and larger carbon clusters Cluster Production Laser ablation heating of a small surface part of a solid target by a focused, intense short- pulse laser (typically Nd-YAG, 532 nm) formation of a plasma that contains ions and electrons cooling with rare gas induces aggregation formation of neutral and charged (anionic and cationic) clusters Converts practically any solid into clusters, very frequently used! Can be easily combined with reaction or thermalization channels, etc. see: M.A. Duncan, Rev. Sci. Instr. 83 (2012) 041101. A molecular beam cluster experiment Experiments under collision-free conditions Mean free path length (identical particles) Vacuum range Pressure in mbar Molecules / cm3 mean free path Ambient pressure 1013 2.7*1019 68 nm Medium vacuum 1-10-3 1016-1013 0.1-100 mm High vacuum 10-3-10-7 1013-109 10 cm - 1 km Ultra high vacuum 10-7-10-12 109-104 1 km-105 km Metal cluster lab at the FHI-FEL in Berlin VUV laser Source chamber operators time-of-flight mass spectrometer small pump big pump fore vacuum pump infrared laser beam from FEL Mass spectrometric characterization Ionization techniques for neutral clusters Electron impact Efficient ionization at 60-100 eV Ionization potentials (IPs) are on the order of 5-15 eV excess energy leads to fragmentation and changes mass distribution Photoionization h EB kin E UV lasers (nm) E (eV) Nd-YAG, 3rd 355 3.5 Nd-YAG, 4th 266 4.7 KrF 249 5.0 ArF 193 6.4 F2 157 7.9 single photon resonant multi photon Nd-YAG, 9th 118 10.5 species and state selective Mass spectrometric characterization Time-of-flight mass spectrometry acceleration of charged particles (ions) in an electric field particles having the same charge but different mass are accelerated to the same kinetic energy mv2 zeEs 2 2zeEs v m m t sDD 2zeEs Measurement of the arrival time on the detector gives mass information typical experimental conditions: s=1 cm, D=10-300 cm, E=100-10000 kV/cm A single mass spectrum can be measured within 5-100 µs. Mass resolution up to 10 000 amu can be achieved Mass spectrometric characterization Example: Cobalt cluster cations produced by Laser ablation (arb. units) intensity time-of-flight (µs) time-of-flight (µs) t mass resolution: Rmax 340 2t 2/1 2. Dirty Terbium clusters, what is in there? Mass spectrometric characterization Other types of mass spectrometers I Magnetic sector field II Quadrupole moderate to high (104) resolution experiments on beams of mass selected ions (MS/MS) III Ion traps, FT-ICR (ion cyclotron resonance) very high resolution (106), long storage times simultaneous detection of all ions expensive zeB m I-II are often used as mass filters, measurement of a full mass spectrum requires scanning (of voltages) and is relatively time consuming. Experiments are often performed on pulsed molecular beams, usage of a ToF-MS allows rapid and full mass analysis of a single ion pulse. Mass spectrometers: mass analyzer mass filters ion traps Approaches for size-selectivity in cluster studies: a) Mass selection, accumulation, spectroscopy b) Size-specific detection ( Action spectroscopy) 3. Probing the structures Linus Carl Pauling (1901-1994) Nobel Prize in Chemistry 1954 We like to understand, and to explain, observed facts in terms of structure. “The place of Chemistry in the Integration of the Sciences”, Main Currents in Modern Thought, 1950, 7, 110 Experimental methods for structure determination of clusters Anion PES Trapped Ion Ion Mobility Electron Diffraction Theory CLUSTER STRUCTURE Raman Chemical probe Spectroscopy method Vibrational Infrared Multiple Photon spectroscopies Dissociation Spectroscopy The chemical probe method Ligand molecules are brought into reaction with a cluster Complexes of the cluster with one or more ligands are formed depending on PL and T via consecutive reactions X + L XL + L XL2 + L … XLsat saturation numbers The chemical probe method plot (average) saturation number as function of P, plateaus indicate stable complexes 8 6 5 E.K. Parks, et al., The structure of nickel-iron clusters probed by adsorption of molecular nitrogen. Chem. Phys. 262 (2000) 151. Ion chromatography • The collision cross section is a measure of size (number of atoms) and shape of a cluster rotationally averaged collision cross sections: spherical < oblate < prolate Ion mobility measurements source: bowers.chem.ucsb.edu/theory_analysis/ion-mobility Mass selected ions are pulled through a collision gas (He) by a weak electric field leading to a resulting drift velocity: vd = K·E Mobility K in the gas is related to L2 3q 2 1 K the collision cross section Ut 16N kB T More compact structures have higher mobilities Comparison with collision cross sections for various isomers from theory geometric structure IMS-MS: a commercial technique Waters Synapt-G2 HDMS Si cations and anions oblate? ‘more prolate spherical’ Several families of cluster structures Similar transition size from prolate to oblate structures R.R. Hudgins, M. Imai, M.F. Jarrold, P. Dugourd, J. Chem. Phys. 111 (1999) 7865. Electron diffraction of trapped cluster ions wave - particle duality de Broglie wavelength on electrons: 12 pm for Ekin=10 keV Electron diffraction of trapped cluster ions mass selection, trapping, thermalization ~107 ions per cm3 40 keV e-beam, ~µA current J.H. Parks, X. Xing in The Chemical Physics of Solid Surfaces, Vol. 12 Atomic Clusters. (2007) 377. Overview: TIED of anionic gold clusters Total scattering intensity shows little size specific features use of reduced molecular intensity Gold clusters, some example structures from TIED observation of 2D and 3D isomer for Au12-: size for 2D/3D transition for anionic Au clusters Anion photoelectron spectroscopy (Photoemission) Ekin=h-EB • Anions can be mass selected • Excitation energies are within the UV-vis range • Electron affinity: vertical EA > adiabatic EA Measurement of photoelectron spectra 1. Production of cluster anions 2.
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