Frustration of direct photoionization of Ar clusters in intense extreme ultraviolet pulses from a free electron laser

We have measured the kinetic energies of fragment ions from Ar clusters (average cluster size < N > similar to 10-600) exposed to intense extreme ultraviolet free electron laser pulses (lambda similar to 61 nm, I similar to 1.3 x 10(11) W cm(-2)). For small clusters (< N > less than or similar to 200), the average kinetic energy of ions strongly increases with increasing the cluster size, indicating a promotion of the multiple ionization, whereas the average kinetic energy is observed to be saturated for < N > greater than or similar to 200. Considering how many photoelectrons can escape from the cluster, it was found that the size dependence of the ion kinetic energy exhibited the frustration of direct photoionization, which resulted from the strong Coulomb potential of the highly ionized cluster.