# Ionization of Xenon Clusters by a Hard X-ray Laser Pulse
## 4 Conclusions  
We investigated the ionization dynamics of xenon clusters irradiated with a hard X-ray laser pulse with the photon energy of 5.5 keV by using electron and ion spectroscopies. The kinetic energy spectra of the ion fragments ejected by xenon clusters with an average size of 5000 atoms have shown that multiply charged ions with high kinetic energies are ejected from the vicinity of the cluster's surface. Since the charges in the cluster are redistributed  
Figure 7: Total energy absorption of xenon clusters with an average size of 5000 atoms irradiated by the XFEL and NIR pulses as a function of the strength of the NIR-laser field. The peak fluence of XFEL pulse is \(\sim\)4.6 \(\upmu\)J/\(\upmu\)m\({}^{2}\) and the time delay is 2 ps. The electric field to the fifth power is shown as a dashed curve to guide the eye. to reduce the stored Coulomb energy, the XFEL-ignited nanoplasma is formed with a quasi-neutral core and a charged thin spherical shell, a finding which has been reported by both XUV [61] and soft X-ray experiments [62]. The dependence of ion yields on the XFEL fluence has evidenced the minimum charge required on the nanoplasma to produce multiply charged ions from the thin spherical shell of the clusters. The dependence of the yields of singly charged ions on the XFEL fluence indicates that the oligomer formation due to the bonding reorganization is disturbed by increasing the mean charge state of the ions within the XFEL-ignited nanoplasma. The percentages of oligomer ions to the total ion yields being a function of the cluster size suggests that increasing the cluster size enhances the bonding reorganization. Consequently, we have experimentally demonstrated the importance of XFEL-fluence and the object's size in achieving control over the bonding reorganization to create oligomer fragments. Using xenon clusters with an average size of 5000 atoms, we have conducted time-resolved spectroscopy experiments using delayed NIR laser pulses using a wavelength of 800 nm as a probe. The XFEL-ignited nanoplasma was heated up by the surface plasma resonance when the electron density of expanding nanoplasma met the resonance condition. The gradual decline in the yield of the multiply-charged ions after the peaks were reached might suggest that the quasi-free electrons are inhomogeneously distributed within the XFEL-ignited nanoplasma. [PERSON], [PERSON], and [PERSON] conceived the experiment; [PERSON] coordinated the experimental team; [PERSON], [PERSON], [PERSON] ([PERSON]), [PERSON]-i.W., [PERSON], [PERSON] ([PERSON]), [PERSON], [PERSON], [PERSON] ([PERSON]), [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] set up the experiment; [PERSON]. ([PERSON]), [PERSON], and [PERSON] prepared the NIR laser; [PERSON], [PERSON], [PERSON] ([PERSON]), [PERSON]-i.W., [PERSON], [PERSON] ([PERSON]), [PERSON], [PERSON], [PERSON] ([PERSON]), [PERSON], [PERSON], [PERSON], [PERSON] ([PERSON]), [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] conducted the experiment; Y.K., W.X., K.A., [PERSON], [PERSON] ([PERSON]), [PERSON] analyzed the data; Y.K., W.X., K.A., [PERSON], [PERSON], [PERSON], and K.U. interpreted the data and wrote the paper. All authors have read and agreed to the published version of the manuscript. This study was supported by the X-ray Free Electron Laser Utilization Research Project and the X-ray Free Electron Laser Priority Strategy Program of the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT); by the Proposal Program of SACLA Experimental Instruments of RIKEN; by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant No. JP21244042, No. JP23241033, No. JP15K17487, No. JP16K05016; by MEXT KAKENHI Grant No. JP22740264; by the IMRAM project; and by the National Nature Science Foundation of China (Grant No. 12174259). H.F. and K.U. acknowledge \"Dynamic Alliance for Open Innovation Bridging Human, Environment, and Materials\". S.W. and K.N.