Structure Determination of Noble Metal Clusters by Trapped Ion Electron Diffraction

The structures of noble metal cluster ions have been studied by the recently developed technique of trapped ion electron diffraction (TIED)$^{1}$. In brief, cluster ions are generated by a magnetron sputter source and injected into a cooled (95~K) quadrupole ion trap. After mass selection and thermalization, the trapped ions are irradiated with a 40~keV electron beam. The resulting diffraction pattern is integrated with a CCD detector. The assignment of the structural motif is done via a comparison of the experimental and simulated scattering function, calculated from density functional theory structure calculations. \newline The structures of mass selected silver cluster cations Ag$_{19}^{+}$, Ag$_{38}^{+}$, Ag$_{55}^{+}$, Ag$_{59}^{+}$, Ag$_{75}^{+}$ and Ag$_{79}^{+}$ have been investigated$^{2}$. The resulting experimental data are best described by structures based on the icosahedral motif, while closed packed structures could be ruled out. Additionally, we present a comparison of the structures of Cu$_{20}^{+/-}$, Ag$_{20}^{+/-}$ and Au$_{20}^{+/-}$. Our findings show unambiguously that the structure of Au$_{20}^{-}$ is predominantly given by a tetrahedron in agreement with the results of L.S. Wang et al.$^{3}$ In contrast, structures of Ag$_{20}^{-}$ and Cu$_{20}^{-}$ based on the icosahedral motif agree best with the experimental data. Small structural differences between the charge states are observed. The possibilities and limitations of the TIED method are discussed. \newline (1) M. Maier-Borst, D. B. Cameron, M. Rokni, and J. H. Parks, Physical Review A \textbf{59} (5), R3162 (1999); S.~Kr\"{u}ckeberg, D. Schooss, M. Maier-Borst, and J. H. Parks, Physical Review Letters \textbf{85} (21), 4494 (2000). \newline (2) D. Schooss, M.N. Blom, B. v. Issendorff, J. H. Parks, and M.M. Kappes, Nano Letters \textbf{5} (10), 1972 (2005). \newline (3) J. Li, X. Li, H. J. Zhai, and L. S. Wang, Science \textbf{299}, 864 (2003)