A focused ion beam (FIB) system with a liquid metal ion source (LMIS) has been used to irradiate monocrystalline silicon with Au clusters. Energies of 6 keV per gold atom ( = 1,…,5) and 10 keV per gold atom (n = 1, 2, 3) were applied and fluences ranged from 1010 to 1017 cm−2. Laser induced modulated reflectivity at normal incidence has been used for characterization of the optical properties of the sample surface and the defect density after irradiation. The removal of substrate material induced by the cluster beam was measured by atomic force microscopy (AFM). The number of produced surface defects detected by the optical measurement did not significantly depend on the cluster size n. A small nonlinear increase of the total sputtering yield per incident gold atom was observed as a function of cluster size.