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In the experiments,.cationic vanadium clusters are formed in a laser vaporization cluster source by ablating a
vanadium target and quenching the plasma with a short pulse of a gas mixture of 0.5 % Ar in He.Complexes with Ar
atoms are formed after passing through a copper channel that is cooled to about 80 K. The molecular beam containing
these complexes is overlapped with the far-IR output of Free Electron Laser for Infrared eXperiments (FELIX). Resonant
absorption of one or multiple IR photons by the complexes can be followed by evaporation of single or more Ar atoms
from the complex leading to decreases in their intensities, and thus the IR spectrum can be constructed. The experiments
have been done by our collaborators in the department
of molecular phyiscs at the Fritz-Haber-Institute in Berlin, in particular
Andre Fielicke and Gert van
Helden.
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We have perforned DFT calculations using the computer code DMol3, which is an all-electron code that uses numerical
atomic orbitals as a basis set. It is an extremely efficient code for small clusters. We compared the energy of
a large number of different geometries for clusters of all sizes. The vibrational spectra were then calculated
by displacing all atoms in all directions, and subsequent diagonalization of the resulting force constant matrix.
By comparing the caclulated spectra with the ones measured in the IR experiments, we were able to identify the
atomic structure of most clusters.
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Here we show typical results for V8. We clearly see that the energetically preferred structure (top) is also the one that show best agreement between the vibrational spectrum calculated and the IR spectrum measured in the experiment.
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The method described above has been applied to Vanadium clusters of sizes 3 to 15. We are currently in the process
of doing similar calculation for neutral and cationic Niobium. We believe that this approach is applicable to all
kind of clusters, and more general, to any nano-particle.
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A. Fielicke, A. Kirilyuk, C. Ratsch, J. Behler, M. Scheffler, G. von Helden, and G. Meijer, Structure
determination of isolated meatal clusters via far-infrared spectroscopy,
Phys. Rev. Lett 93, 023401
(2004).