Cluster ion beam processes can produce special surface modification effects, such as surface smoothing, high rate sputtering and very shallow implantation. In order to realize high-speed surface processing, a high-energy gas cluster ion beam irradiation system was developed. Since cluster energy depends on chamber pressure, the energy distribution of the high-energy cluster ion beams was measured for various pressure regimes. These distributions show that both cluster size and energy decrease with increasing collisions with residual gas. The number of atoms scattered by one collision with residual gas is proportional to the velocity of the cluster ion. The results suggest that an analytical equation can be used to calculate irradiation energy from the acceleration energy, size of cluster, chamber pressure and transportation length. (c) 2007 Elsevier B.V. All rights reserved.