Clusters in intense XUV pulses: Effects of cluster size on expansion dynamics and ionization

Abstract

We examine the effect of cluster size on the interaction of Ar 55 -Ar 2057 with intense extreme ultraviolet (XUV) pulses, using a model we developed earlier that includes ionization via collisional excitation as an intermediate step. We find that the dynamics of these irradiated clusters is dominated by collisions. Larger clusters are more highly collisional, produce higher charge states, and do so more rapidly than smaller clusters. Higher charge states produced via collisions are found to reduce the overall photon absorption, since charge states of Ar 2+ and higher are no longer photoaccessible. We call this mechanism collisionally reduced photoabsorption, and it decreases the effective cluster photoabsorption cross section by more than 30% for Ar 55 and 45% for Ar 2057 . The time evolution of the electron kinetic energy distribution begins as a (mostly) Maxwellian distribution. Further, the electron velocity distribution of large clusters quickly become isotropic while smaller clusters retain the inherent anisotropy created by photoionization. Last, the total electron kinetic-energy distribution is integrated over the spatial profile of the laser and the log-normal distribution of cluster size for comparison with a recent experiment [C. Bostedt et al., Phys. Rev. Lett. 100, 133401 (2008)], and good agreement is found.

Recombination effects in soft-x-ray cluster interactions at the xenon giant resonance

“…We find that the highest charge states all originate from atoms in the outermost shell of the cluster (shell 3) [4]. While the physical center of the cluster produces higher charge states during the laser-cluster interaction [26], in the soft-x-ray regime, these recombine to much lower charge states (including neutral) prior to detection. Most of the neutrals from the outer shell are atoms that are simply never ionized due to being in the low intensity region of the laser's spatial profile.…”

“…This simulation is run using the shortest time step, and includes all microscopic interactions. Eventually (after about 2 ps for small clusters) ionization becomes infrequent as the cluster disintegrates [26]. Once this happens, phase two is initiated.…”

Induced transparency in the XUV: a pump-probe test of laser-cluster interactions

“…Thus, during the pulse there are no more targets to further photoionize. This is the high intensity limit to the previously observed collisionally reduced photoabsorption where clusters absorb less photons due to fast collisional ionization depleting ionizable targets [29]. The result is the same saturated AICS, both with and without the probe-pulse.…”

“…These plasma mechanisms attempt to distort the atomic potentials and allow ionizations to occur which would otherwise be energetically impossible for photoionization. Work which only included augmented collisional ionization using the local ionization threshold model [20,28], which treats the cluster potential as a constant potential perturbation, has successfully reproduced the results of multiple experiments [29], including experiments where Auger ionization is dominant [30]. Thus, the question arises: are the plasma effects on ionization, such as ionization potential lowering, negligible in small clusters?…”

A validation of a simple model for the calculation of the ionization energies in X-ray laser-cluster interactions

“…Further, the implementation of LIT model is straight-forward and may easily include all cluster particles (including electrons) with results that agree well with experiments. 4,9,15,16 Atoms and ions in nanoplasmas will also have their ionization probabilities further altered due to electron and ion screening. 17,18 There have largely been two methodologies for dealing with these changes.…”