RAPID COMMUNICATIONS PHYSICAL REVIEW A 91, 021402(R) (2015) Transient charge dynamics in argon-cluster nanoplasmas created by intense extreme-ultraviolet free-electron-laser irradiation H. Iwayama, 1, 2, 3 , * J. R. Harries, 3, 4 and E. Shigemasa 1, 2, 3 1 UVSOR Facility, Institute for Molecular Science, Nishigo-Naka 38, Myodaiji, Okazaki 444-8585, Japan 2 SOKENDAI, Nishigo-Naka 38, Myodaiji, Okazaki, Aichi 444-8585, Japan 3 RIKEN/SPring-8, Kouto 1-1-1, Sayo, Hyogo 679-5148, Japan 4 JAEA/SPring-8, Kouto 1-1-1, Sayo, Hyogo 679-5148, Japan (Received 20 June 2014; revised manuscript received 19 September 2014; published 27 February 2015) We present extreme-ultraviolet (EUV) fluorescence spectra of Ar clusters irradiated by intense EUV free- electron-laser (FEL) pulses focused to intensities of up to 3 × 10 13 W/cm 2 at a wavelength of 51 nm. The spectra reveal fluorescence at wavelengths shorter than that of the incident radiation, which can be assigned to EUV fluorescence lines from excited multiply charged ions Ar z+∗ with z as high as 6. This demonstrates that charge states as high as 7+ are produced by the FEL irradiation. The dependence of the spectra on cluster size shows that the highly charged ions are generated at the cluster surface, indicating inhomogeneous charging. The FEL power dependencies of the spectral features suggest that the inhomogeneous distribution of charge within the clusters reduces ionization thresholds at the cluster surface. DOI: 10.1103/PhysRevA.91.021402 PACS number(s): 36.40.−c, 32.50.+d, 41.60.Cr, 52.20.Hv Gas-phase clusters are fascinating objects for investigating laser-matter interactions since energy dissipation into the surrounding media is absent. The response of rare-gas clusters to intense infrared (IR) laser fields has been widely studied both experimentally [1–3] and theoretically [4,5]. From the kinetic energy distributions of emitted ions and electrons, it has been found that irradiated clusters absorb large amounts of en- ergy, form hot nanoplasmas, and dissociate through Coulomb explosion into many energetic fragments. Theoretical studies have revealed that the strong optical electric field of the IR laser pulses causes tunneling ionization and results in strong plasma heating of the cluster. Laser-cluster interactions at shorter wavelengths have also been investigated with the aid of free-electron lasers (FEL), which can now provide high-intensity laser pulses in the extreme-ultraviolet (EUV) [6], soft x-ray [7] and hard x-ray [8,9] regimes. Several experimental studies on rare-gas clusters have been performed using short wavelength FELs [10–16]. It has been demonstrated that the ionization, relaxation, and fragmentation dynamics are quite different from those in the IR regime, and depend strongly on the FEL wavelength [17]. When a cluster is irradiated by intense EUV laser pulses, it becomes highly ionized as a result of a multistep sequence of single-photon ionizations [13]. As photoionization progresses, the kinetic energies of emitted photoelectrons gradually decrease due to the increasing Coulomb potential of the in- creasingly positively charged cluster. Eventually, ionization is terminated at the point where the Coulomb potential produces an energy downshift larger than the atomic excess energy of direct photoelectrons. This frustration of photoelectron emission results in the formation of a nanoplasma [13,14]. The creation of a nanoplasma is thus common to irradiation by both intense EUV laser pulses and IR laser pulses. However, plasma heating under EUV irradiation is much weaker, since * Corresponding author: iwayama@ims.ac.jp the ponderomotive energy is proportional to the square of the wavelength and is thus much smaller at EUV wavelengths. Previous experimental studies have reported that the kinetic energies of the plasma electrons following EUV irradiation are only several eV [11,13], in contrast to the several keV reported for IR laser irradiation [2]. For EUV laser pulses, the nanoplasma created is in a strongly coupled regime, since the low kinetic energies of the electrons means that their dynamics is dominated by electron- ion interactions. The resulting charge dynamics is thus very different from that of the hot, weakly coupled nanoplasma produced by IR laser pulses [18,19]. This results in, for example, the inhomogeneous charge distribution reported for nanoplasmas produced by EUV laser pulses [12,15]. In the strongly coupled regime, the plasma electrons are pulled toward the center of the cluster by the ionic Coulomb potential, leaving the ions in the outermost shell essentially stripped [18]. As a result, two mechanisms drive the dissociation of irradiated clusters [20]. These are (i) Coulomb explosion of the ionic shell on a subpicosecond time scale, and (ii) the hydrodynamic expansion of the quasineutral cluster core on a time scale of several picoseconds. In the IR region, the charge redistribution effect is washed out due to the cluster-sized quivering amplitude of the hot nanoplasma. Since isolated atoms are highly ionized by intense EUV- FEL pulses, the nanoplasma created by EUV-FEL irradiation can also be expected to contain highly charged atomic ions. However, ion time-of-flight mass spectra have shown that the charge states of fragment ions emitted from clusters are much lower than those observed for isolated atoms. For example, whereas charge states of up to 26+ have been observed for Xe atoms exposed to intense x-ray FEL pulses, the production of singly charged monomer fragments dominates the pho- toionization of Xe clusters [16]. Since multistep sequences of single-photon, single ionizations can well reproduce the experimentally observed electron spectra [13], there has been no clear evidence for multiple ionization of atoms within the nanoplasma. 1050-2947/2015/91(2)/021402(5) 021402-1 ©2015 American Physical Society