Nanoplasma Dynamics of Single Large Xenon Clusters Irradiated with Superintense X-Ray Pulses from the Linac Coherent Light Source Free-Electron Laser

The plasma dynamics of single mesoscopic Xe particles irradiated with intense femtosecond x-ray pulses exceeding ${10}^{16}\mathrm{W}/{\mathrm{cm}}^2$ from the Linac Coherent Light Source free-electron laser are investigated. Simultaneous recording of diffraction patterns and ion spectra allows eliminating the influence of the laser focal volume intensity and particle size distribution. The data show that for clusters illuminated with intense x-ray pulses, highly charged ionization fragments in a narrow distribution are created and that the nanoplasma recombination is efficiently suppressed.

Figure 1

Experimental setup (top panel) and single-shot scattering images from individual clusters (bottom panel). The cluster beam is crossed with the focused x-ray beam above the aperture of a time-of-flight spectrometer. The scattered photons are detected by pnCCDs covering scattering angles from 0.5° to 5°. The scattering signal exhibits a strong intensity variation due to the different positions (a)–(c) of the clusters in the focus as depicted in the inset.

Figure 2

Top panel: Single shot ion spectra recorded in coincidence with the scattering images (a)–(c) shown in Fig. 1. Bottom panel: Focal volume integrated ion yield spectra for cluster (top) and atomic (bottom) targets. The single-shot spectra strongly correlate with the position of the cluster in the FEL focus and are strikingly different from the integrated data.

Figure 3

Average (open circles) and the dominant charge state (filled diamonds) for increasing FEL intensities. The FEL intensities are deduced from the integrated scattering intensity of the similarly sized clusters and plotted on a relative scale. The dashed lines are guides to the eye and demonstrate the different behavior of a constantly growing average charge state versus the rapid saturation of the dominant charge state. The ion charge states are estimated without considering the initial ion kinetic energy release.

Figure 4

Comparison between FLYCHK simulations and experimental single-shot ion data (inset). Both the data and simulation show a strong suppression of recombination for the highly excited x-ray-induced nanoplasma (initial $q={26}^{+}$) in the center of the FEL focus and a significant recombination for lower excitation (initial $q=8^{+}$). For the initial $q={26}^{+}$ nanoplasma, the high charge states freeze out due to rapid expansion of the cluster and low three-body recombination rates for hot electron temperatures.