Ionization and charge migration through strong internal fields in clusters exposed to intense x-ray pulses

A general scenario for electronic charge migration in finite samples illuminated by an intense laser pulse is given. Microscopic calculations for neon clusters under strong short pulses as produced by x-ray free-electron laser sources confirm this scenario and point to the prominent role of field ionization by strong internal fields. The latter leads to the fast formation of a core-shell system with an almost static core of screened ions while the outer shell explodes. Substituting the shell ions with a different material such as helium as a sacrificial layer leads to a substantial improvement of the diffraction image for the embedded cluster thus reducing the consequences of radiation damage for coherent diffractive imaging.

Figure 1

(Color online) Electron trapping and strong-field effects of homogeneously charged neon clusters with an average charge $q$ per ion and a cluster radius $R$. Trapping of electrons detached by 12 keV photons (region above and including green/light-gray shaded band) and the Auger decays (region above and including blue/dark-gray shaded band). The thick lower borders take into account that electrons are released from any position in the cluster. Red/solid lines: ionization of surface ions at various charge states $\left(q=1,\dots ,6\right)$ due to the internal radial field of the charged cluster estimated by the Bethe rule, see text.

Figure 2

(Color online) Radial electric field (upper panels) and radial charge density (lower panels) for a ${\text{Ne}}_{1500}$ cluster and $T=10\text{fs}$ pulse without the effect of field ionization (left) and with field ionization (right). White lines show the average radial coordinate of cluster surface shell (the 5% outermost ions) and of cluster-core surface shell (the 5% outermost ions of the inner half of the cluster).

Figure 3

Mean displacement of ions $⟨\Delta r⟩$ of neon clusters at peak intensity $\left(t=0\right)$ for a full calculation (solid lines) and reduced calculations (dashed lines) neglecting field ionization. Dependence on the pulse duration $T$ (left, for ${\text{Ne}}_{1500}$) and the cluster size $N$ (right, $T=10\text{fs}$), respectively. The average $⟨\Delta r⟩$ is shown for all cluster ions in the upper panels and for only the inner half of the cluster ions in the lower panels.

Figure 4

(a) Mean displacement and (b) $R$ factor of ions of ${\text{Ne}}_{1500}$ cluster embedded in ${\text{He}}_{15000}$ droplet (full line) and without helium droplet (dashed line) for varying pulse lengths. The gray arrow indicates a typical improvement in the damage tolerance, see text.