Metal nano-particles (diameter less than 5 nm) show unique optical, electronic, chemical, and especially catalytic properties. Ligand-protected Au13 clusters, Au 13 [PPh 3 ] 4 [S(CH 2) 11 CH 3 ] 4 , were deposited as catalyst precursors onto an anatase TiO 2 (300 nm average diameter) support [1]. The cluster ligands were then removed using either thermal or ozone treatments for exposing catalytically active surface sites, where the ozone treatment led to significantly less agglomeration of the Au13 clusters as compared to annealing only [2]. Here we report remarkably different sizes and shapes of the Au clusters due to exposure to an atomic oxygen beam with 5 eV kinetic energy used to remove the ligands. Atomic oxygen was produced by the breakdown of O 2 by laser detonation within an ultra-high vacuum (UHV) chamber. The number of gold atoms per Au particle was determined by the quantification of the absolute intensity of the high-angle annular dark-field (HAADF) image acquired with a VG HB501 scanning transmission electron microscope (STEM), and the morphologies and structures of the Au particles were investigated by high-resolution transmission electron microscopy (HRTEM) with a JEM 2010 FEG TEM.