# Ionization of Xenon Clusters by a Hard X-ray Laser Pulse
###### Abstract
In the case of molecular samples, the multiply charged molecular ions resulting from the photoemission and Auger decay cycles explode violently due to the Coulomb repulsive force [17; 18; 19; 20; 21; 22; 23]. These fundamental processes at the atomic level are indeed likely to be responsible for the observed radiation damage, but the size of the molecules studied, consisting of about a dozen atoms or less, are still relatively small compared to macromolecules and nanostructured samples. To investigate XFEL-ignited dynamics beyond the above simple targets, atomic clusters are ideal systems since their sizes are easily altered from a single atom to a bulk-like macroscopic region by employing a well-established method. Consequently, atomic clusters have been investigated extensively [24; 25; 26; 27]. When an intense X-ray laser-pulse shines into an atomic cluster, numerous individual atoms are core-ionized, and many secondary electrons are emitted by the following Auger decays. The net charge of the ionized cluster increases due to the escape of electrons from the system, the Coulomb potentials of which electrically capture the other electrons after their kinetic energies are lost by inelastic scattering with atoms and ions in the system. A nanoplasma is thus formed [24; 26]. This is expected to be a general phenomenon in XFEL-ignited dynamics, as the nanoplasma formation may be induced on any kind of nanometer-sized particles containing heavy elements. As such, understanding the formation and disintegration processes of the XFEL-ignited nanoplasma is not only of fundamental interest, but also of critical importance for the use of XFEL pulses for structural determinations [28; 29]. Our recent study [30] demonstrated that the dedicated molecular dynamics simulation tool, XMDYN, captured essential physical processes steering the XFEL-ionization dynamics of argon clusters. When the clusters are irradiated with hard X-ray lasers of a moderate fluence, the ionization dynamics have been markedly affected by chemical processes, distributing the increasing net charge within the clusters and causing bond-reorganization, which induces the formation of oligomer fragments [30]. These dynamics induced by the moderate-fluence X-ray lasers are different from those by high-fluence ones where the system entirely decomposes into atomic species. Moderate-fluence XFEL pulses are expected to provide the ability to modify and transform the molecular structure of nanometer-sized objects by achieving control over the disintegration dynamics of XFEL-ionized clusters. The XMDYN reported that on krypton clusters, the deep Coulomb potential effectively trapped the ionized electrons when its cluster size was greater than 5000 atoms [31]. Thermal electron emissions from the transient nanoplasma are highly suppressed owing to its deep Coulomb potential during the long-term plasma expansion associated with electron-ion recombination events. The simulation results show that this characteristic is likely to be observed in a wide range of sizes for relatively large-sized clusters. In other words, to decrypt the nanoplasma formation and disintegration processes, it is worth investigating the XFEL-ignited dynamics on small-sized clusters. For a deeper insight into the dynamics of laser-ignited nanoplasmas, a number of time-resolved experimental studies have been conducted [32; 33; 34; 35; 36; 37] using near-infrared (NIR) probe pulses. A recent study [34] demonstrated one novel concept that enables us to completely decouple the generation of seed electrons from the ionization dynamics driven by the NIR laser, both temporally and spatially. The study reported that inverse bremsstrahlung (IBS) heating, subsequent efficient avalanching by NIR pulses, and resonant excitation remain possibly well below the threshold of tunnelling ionization and can be activated by just several electrons seeded by an ultrashort extreme-ultraviolet (XUV) pulses produced by high-harmonic generation.