Tracing transient charges in expanding clusters

Abstract

We study transient charges formed in methane clusters following ionization by intense nearinfrared laser pulses. Cluster ionization by 400 fs (I = 1 × 10 14 W/cm 2 ) pulses is highly efficient, resulting in the observation of a dominant C 3+ ion contribution. The C 4+ ion yield is very small, but is strongly enhanced by applying a time-delayed weak near-infrared pulse. We conclude that most of the valence electrons are removed from their atoms during the laser-cluster interaction, and that electrons from the nanoplasma recombine with ions and populate Rydberg states when the cluster expands, leading to a decrease of the average charge state of individual ions. Furthermore, we find clear bound-state signatures in the electron kinetic energy spectrum, which we attribute to Auger decay taking place in expanding clusters. Such nonradiative processes lead to an increase of the final average ion charge state that is measured in experiments. Our results suggest that it is crucial to include both recombination and nonradiative decay processes for the understanding of recorded ion charge spectra.The ionization of clusters by intense laser pulses induces highly complex dynamics that take place on attosecond to nanosecond timescales and that involve a large number of interacting particles. Intense lasercluster interactions commonly involve an ionization stage, the establishment of a nanoplasma and the subsequent expansion and break-up of this nanoplasma. Disentangling the different mechanisms is a very challenging task that requires sophisticated theoretical and experimental approaches. In spite of the large efforts that have been devoted to the study of laser-cluster interactions in the past 20 years (see e.g. [1-4]), the understanding of both the ionization and relaxation dynamics taking place during the cluster expansion are still far from being complete. For instance, up to now, experiments have only provided limited information about the relative importance of direct laser-induced (multi-photon and tunneling) ionization and electron-impact ionization by laserdriven electron-atom/ion collisions The development and application of pump-probe techniques promises novel insights into the relevant processes. For instance, the generation of seed electrons in a cluster by an extreme-ultraviolet (XUV) pulse allows the time-resolved investigation of strong-field processes induced by near-infrared (NIR) pulses [5]. Amongst the various processes that take place during the cluster expansion, electron-ion recombination processes are known to play an important role [1,3,[6][7][8][9]. Recently, recombination resulting in the production of a large number of excited atoms and ions was temporally resolved using pump-probe photoion and -electron spectroscopy [10][11][12], showing similar dynamics for clusters ionized by intense XUV and intense NIR pulses, respectively. These results suggest that the charge distributions that can be measured after the nanoplasma has expanded may differ significantly from the transient charge distributions ...

Low-Energy Electron Emission in the Strong-Field Ionization of Rare Gas Clusters

“…The shoulder was previously attributed to correlated electronic decay processes [36] and cascades of intra-Rydberg interatomic Coulombic decay transitions [37,38] producing singly charged ions and electrons with kinetic energies up to the ionization potential of the cluster atoms, i.e., up to about 15.8 eV for Ar. Further examples of recently discovered correlation-driven cluster ionization processes include collective autoionization [39], autoionization of spin-orbit-excited states [40], as well as the relaxation of multiply charged ions by Auger decay [41] and autoionization [42].…”

“…Atomic ionization events due to tunneling and electron impact are modeled via Monte Carlo sampling using effective quantum-mechanical rates, including local-field induced ionization potential depression [26]. Note that atomic autoionization [40,41] and many-body recombination processes in the nanoplasma, which dominantly couple to highly excited states [44], are only qualitatively included in the correlated classical treatment. The MD results obtained for Ar 5083 using parameters similar to the experiment reproduce two of the most prominent features of the measured electron spectra and show two exponential contributions with strongly differing decay constants; see Fig.…”

Evolution and ion kinetics of a XUV-induced nanoplasma in ammonia clusters

“…Many interesting properties of nanoplasmas come from the enhancement of recombination processes due to the large number of confined electrons and positive ions [13,14]. The decay of a nanoplasma is governed by a complex interplay of Coulomb explosion and hydrodynamic forces leading to shock shells in the outer Debye layer [15][16][17][18]. Single-shot x-ray diffraction imaging of the nanoplasma evolution in rare-gas clusters revealed sub-picosecond dynamics [19], and a core-shell structure [20].…”