Explosions of Xenon Clusters in Ultraintense Femtosecond X-Ray Pulses from the LCLS Free Electron Laser

Explosions of large Xe clusters ($⟨N⟩\sim 11000$) irradiated by femtosecond pulses of 850 eV x-ray photons focused to an intensity of up to ${10}^{17}\mathrm{W}/{\mathrm{cm}}^2$ from the Linac Coherent Light Source were investigated experimentally. Measurements of ion charge-state distributions and energy spectra exhibit strong evidence for the formation of a Xe nanoplasma in the intense x-ray pulse. This x-ray produced Xe nanoplasma is accompanied by a three-body recombination and hydrodynamic expansion. These experimental results appear to be consistent with a model in which a spherically exploding nanoplasma is formed inside the Xe cluster and where the plasma temperature is determined by photoionization heating.

Figure 1

Ion-TOF spectra for 150 fs pulse length and 850 eV photon energy. (a) Xe clusters $⟨N⟩=11000$, $\mathrm{\text{Intensity}}=5.6\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. (b) Xe clusters $⟨N⟩=11000$, $\mathrm{\text{Intensity}}=5.6\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. (c) Xe clusters $⟨N⟩=11000$, $\mathrm{\text{Intensity}}=4.5\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$. (d) Xe atom $\mathrm{\text{Intensity}}=5.6\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 2

Intensity-dependent ion-TOF spectra from Xe clusters with $⟨N⟩=10000$ for select charge states ${\mathrm{Xe}}^{+}$ to ${\mathrm{Xe}}^{6+}$. X-ray pulse intensities are $4.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (lower trace, blue), $4.5\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (upper trace, black), and $5.6\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$ (middle trace, red). The gray bars show the ion positions for atomic photoionization.

Figure 3

(a) Measured Xe ion-energy spectrum from exploding Xe clusters at intensity $5.6\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$ from the field-free drift measurement. A hydrodynamic simulation of the ion-energy spectrum assuming a mean electron temperature of $\sim 125\mathrm{eV}$ is shown for comparison with the experimental ion drift measurement. (b) Kinetic electron spectra from Xe clusters for two different pulse energies at a pulse length of 142 fs. The distribution function for a 125 eV Maxwellian function is shown as a dashed line.

Figure 4

Rate equation calculation of the charge distribution from the irradiation of 10 000 atom Xe clusters by 150 fs, 850 eV x-ray pulses at intensity $5\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$. The top shows when no recombination is included and the bottom shows the inclusion of recombination.