Energetic cluster ion bombardment shows unique and complicated interactions between constituents of the cluster and atoms of the target surface. Scanning tunneling microscopy (STM) observation is one of the most powerful methods to investigate the cluster ion-solid surface interaction. We have observed crater-shape traces on highly oriented pyrolitic graphite (HOPG) surface bombarded by energetic Ar cluster ions. However, STM images of HOPG surfaces are influenced by special electronic surface states. In this paper, Ar monomer and cluster ions are bombarded on Au( iii) surfaces on mica to observe ion traces by STM. In addition, the relationship between the parameters of cluster ion beams and the features of ion traces are discussed. The STM observation showed that Ar cluster ion bombardment had circular crater-like shapes, and their diameters were proportional to the cubic root of ion energy. These indicate that the kinetic energy of cluster ions is deposited on the surface isotropically. (C) 1998 Elsevier Science S.A. All rights reserved.