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Structure Determination of Silicon Clusters in the Gas Phase A Vibrational Spectroscopy and DFT Investigation Jonathan T. Lyon, Philipp Gruene, Gerard.

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Presentation on theme: "Structure Determination of Silicon Clusters in the Gas Phase A Vibrational Spectroscopy and DFT Investigation Jonathan T. Lyon, Philipp Gruene, Gerard."— Presentation transcript:

1 Structure Determination of Silicon Clusters in the Gas Phase A Vibrational Spectroscopy and DFT Investigation Jonathan T. Lyon, Philipp Gruene, Gerard Meijer, André Fielicke, Ewald Janssens, Pieterjan Claes, and Peter Lievens lyon@fhi-berlin.mpg.de June 17, 2008

2 Why Study Silicon Clusters?

3 The Importance of Si ►Silicon is important to the electronics industry ►Current trend toward miniaturization ►Predictions that silicon clusters do not have bulk-like structures *Ho, K.-M.; Shvartsburg, A. A.; Pan, B.; Lu, Z.-Y.; Wang, C.-Z.; Wacker, J. G.; Fye, J. L.; Jarrold, M. F. Nature 1998, 392, 582.

4 Silicon Nanowires ►Silicon nanotubes potentially an assembly of clusters *Marsen, B.; Sattler, K. Phys. Rev. B 1999, 60, 11593.

5 What is Known About Si n ? ►Vibrational spectroscopy in matrices 1 ►Ion-mobility experiments 2 ►Photoelectron spectroscopy 3 ►UV Photodissociation spectroscopy 4 1 Honea, E. C.; et al. J. Chem. Phys. 1999, 110, 12161. 2 Shvartsburg, A. A.; et al. J. Chem. Phys. 2000, 112, 4517. 3 Cheshnovsky, O.; et al. Chem. Phys. Lett. 1987, 138, 119. 4 Antonietti, J. M.; et al. Phys. Rev. B 2003, 68, 035420.

6 IR Action Spectroscopy IR multiple photon dissociation (IR-MPD) IR resonance enhanced multiple photon ionization (IR-REMPI) (i) Processes following IR multiple photon excitation (ii) Detecting photon absorptions via the messenger technique - Weak interacting messenger - BDE typically <1000 cm -1 - Can be coupled with mass selection

7 IR-MPD Spectroscopy ►Vibrationally excite a molecule until part of it fragments (or fragment a weekly bound ligand) ►Need an intense radiation source in the region or molecular vibrations h d h h E IVR ►resonant absorption ►fast intramolecular vibrational redistribution (IVR), t IVR << 1 ns ►absorption of the next photon ►etc.

8 Free Electron Lasers ►Wavelength (λ) depends on: ►kinetic energy of the electrons (E) ►undulator period (λ u ) ►magnetic field (K)

9 Free Electron Laser for Infrared eXperiments (FELIX) ►Tunable between 40 and 2000 cm -1 ►≤ 100 mJ per macropulse (10 10 W/cm 2 in a micropulse)

10 Experimental Setup resonant absorption fragmentation of the complex

11 Silicon Cluster Cations ►Si cluster cations studied by tagging with Xe (Ar will not bind to the clusters) ►Good IR spectra of Si n + clusters in the n=6-20 range

12 The Structure of Si 8 + ►Theory predicts new structure (edge- capped pentagonal bipyramid) for Si 8 + ►Lowest in energy ►IR spectrum fits the best 69 12 17 kJ/mol higher in energy

13 Structures Identified ►Change from pentagonal bipyramid to trigonal prism building block occurs between Si 9 + and Si 10 +

14 A Complete Story? ►Not all sizes have been identified, e.g., Si 12 + ►Have fun theoreticians

15 Conclusions ►IR-MPD is a suitable technique to obtain the vibration spectra of Si n + in the n=6-23 range ►Determination of cluster geometry is possible by comparing experimental spectra with theoretical predictions ►Structures identified for Si 6 -Si 11, Si 13 -Si 15, and Si 18 ►Si 8 + has a novel structure previously not considered ►Transition from pentagonal bipyramid to trigonal prism building blocks occur between Si 9 + and Si 10 +

16 Acknowledgments ►Research Group ►André Fielicke ►Philipp Gruene ►Gerard Meijer ►Collaborators ►Peter Lievens – K. U. Leuven ►Ewald Janssens ►Pieterjan Claes ►FELIX Staff ►Britta Redlich ►Lex van der Meer ►Funding and Equipment ►FELIX ►Alexander von Humboldt Foundation

17 Other FELIX Users ►Robert C. Dunbar MG01 ►Monday June 16 th @ 1:30 PM ►“AMINO ACID COMPLEXES OF METAL IONS WITH +2 CHARGE. DETERMINATION OF SUCCESSIVE LIGAND CONFORMATIONS BY IRMPD SPECTROSCOPY” ►Anouk M. Rijs RD08 ►Thursday June 19 th @ 10:54 AM ►“ROTAXANES: MOLECULAR MOTION IN THE GAS PHASE” ►J. M. Bakker RG05 ►Thursday June 19 th @ 2:51 PM ►“FIRST EXPERIMENTS AT FELICE, THE FREE-ELECTRON LASER FOR INTRA-CAVITY EXPERIMENTS”

18 Gas Phase Spectroscopy More sensitive and selective ►Mass spectrometric detection of absorption ►Changes of the charge state = ionization ►Changes of particle mass = dissociation via absorption of multiple IR photons (IR-MPD) ►Problem: particle density in gas phase ►Direct measurement of absorption ►Not sensitive enough (low particle density) ►Not species specific (broad cluster distribution)

19 IR-MPD of Vanadium Clusters time-of-flight mass spectrometer no FELIX 7,0 8,0 7,1 7,2 * * * V x Ar y + gas pulse (Xe/He) metal rod/ silicon plate Nd:YAG laser liq. N2 resonant absorption Fragmentation of the complex depletion spectrumabsorption spectrum FELIX at 32  m (313 cm ) FELIX beam

20 Free Electron Laser for Infrared eXperiments (FELIX) ►Tunable between 40-2000 cm -1 (≤ 4000 cm -1 on 3 rd harmonic) ►≤ 100 mJ per macropulse (10 10 W/cm 2 in a micropulse)

21 Theoretical Chemistry ►Density Functional Theories (B3LYP, BPW91, BP86) ►Predict stable isomers ►Compute infrared spectrum ►Compare to experiment for structural assignment experimental spectrum calculated IR spectra for different structures structure assignment

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