Frustration of direct photoionizations of rare gas clusters in intense extreme ultraviolet free-electron laser pulses

Abstract

We performed a momentum imaging measurements of Ar clusters irradiated by extreme ultraviolet free electron laser pulses (61nm 1.3×1011W/cm2) for various cluster sizes. Dominant fragment ions were Ar+ and Ar2+ ions with significant kinetic energy (up to 40eV). This indicates that Ar clusters absorbed many photons, become highly ionized and finally dissociated into many fragments through Coulomb explosion. The size dependence of the ion kinetic energy distribution indicates that larger clusters significantly suffer from the frustration of direct photoelectron emissions and multiple ionizations.