By using timeofflight spectroscopy, the ionization and explosion of large argon clusters (n—=3×103-3×106) in the intense femtosecond pulsed lase r field (60 fs，2×1016 W/cm2) has been studied, and the dependence of a verage energy of ions emitted from argon clusters on the gas backing pressure ha s been measured. By comparing the average ion energies obtained with two differe nt supersonic conical nozzles and considering the Hagena's scaling law of clust ers, we have found that the average ion energy is determined by the cluster size when the laser parameters are kept unchanged. The experimental results indicate d that when the cluster size is less than 3×105 atoms per cluster, the Coulom b repulsion force is the dominating factor in the expansion mechanism. Beyond th is size, for 3×105

Keywords:
• atomic clusters / 
• femtosecond intense laser / 
• Coulomb explosion / 
• hydro dynamic expansion