Transient charge dynamics in argon-cluster nanoplasmas created by intense extreme-ultraviolet free-electron-laser irradiation

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 $3\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ 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 $\mathrm{A}{\mathrm{r}}^{z+*}$ 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.

Figure 1

(a) EUV fluorescence spectra from Ar clusters at three different cluster sizes $(\langle N\rangle \sim 20,500$, and 5000) following irradiation with 51-nm-wavelength FEL pulses. The peak assignments indicated above the spectra are based on Ref. [29]. (b) Cluster size dependence of fluorescence yield compared to the surface atom ratio.

Figure 2

EUV fluorescence spectra from Ar clusters $(\langle N\rangle \sim 100)$ at four different FEL intensities $(I\sim 6\times {10}^{12},1\times {10}^{13},1.7\times {10}^{13}$, and $3\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2)$.

Figure 3

FEL intensity dependencies of the fluorescence yields from ${\mathrm{Ar}}^{3+*}$ $\left(\lambda =40\mathrm{nm}\right)$ and ${\mathrm{Ar}}^{4+*}$ $\left(\lambda =25\mathrm{nm}\right)$ from Ar clusters $(\langle N\rangle \sim 100)$.

Figure 4

Sketch showing the effect of a neighboring ${\mathrm{Ar}}^{+}$ ion on the Coulomb potential of an ${\mathrm{Ar}}^{z+}$ ion. The solid line indicates the full cluster potential, and the dashed lines represent the potentials for isolated ions. $E_{ip}$ and $E_{ip}^{\mathrm{inner}}$ represent the ionization energies of the isolated ${\mathrm{Ar}}^{z+}$ ion and inner ionization energies of the ${\mathrm{Ar}}^{z+}$ ion with the contribution from the ${\mathrm{Ar}}^{+}$ ion.