Frustration of photoionization of Ar nanoplasma produced by extreme ultraviolet FEL pulses

We report on the laser power dependence of photoabsorptions of Ar clusters irradiated by extreme ultraviolet-free electron laser (EUV-FEL) at the wavelength of 62 nm. We measured kinetic energy distributions of fragment ions with our ion momentum spectrometer. From the measured kinetic energy distributions, we estimated charge states of irradiated Ar clusters using the classical uniformly charged sphere model. Our results show that the EUV-FEL irradiation of Ar clusters results in the frustration of direct photoionization leading to nanoplasma formation and subsequent strong electron-ion charge recombination.