Steplike Intensity Threshold Behavior of Extreme Ionization in Laser-Driven Xenon Clusters

Abstract

The generation of highly charged Xe(q+) ions up to q=24 is observed in Xe clusters embedded in helium nanodroplets and exposed to intense femtosecond laser pulses (λ=800 nm). Laser intensity resolved measurements show that the high-q ion generation starts at an unexpectedly low threshold intensity of about 10(14) W/cm2. Above threshold, the Xe ion charge spectrum saturates quickly and changes only weakly for higher laser intensities. Good agreement between these observations and a molecular dynamics analysis allows us to identify the mechanisms responsible for the highly charged ion production and the surprising intensity threshold behavior of the ionization process.

Charging dynamics of dopants in helium nanoplasmas

“…A series of experiments involving dopant clusters grown in He nanodroplets have led to similar conclusions in accordance with the generic nanoplasma dynamics of single component clusters [2,11] However, in certain cases the presence of a He shell surrounding the dopant cluster was found to distinctly influence the charging dynamics of the dopant atoms [12]. In a comparative study of lead (Pb) clusters formed in He droplets and as free clusters which were irradiated by two time-delayed pulses, the maximum yield of high charge states was reached at significantly earlier delay times for Pb clusters embedded in He droplets [13].…”

“…Nanoplasmas formed in the intense near-infrared irradiation of neutral nanoscale particles (1-100 nm) have continued to draw considerable attention due to the intriguing aspects of their dynamics ensuing in them such as the formation of an ionization avalanche on the few-femtosecond timescale [1][2][3][4][5]. For near-infrared (NIR) laser pulses with photon energies below atomic ionization potentials of the atomic constituents of these nanoclusters, ionization is initiated by nonlinear field photoionization of the atoms [6][7].…”

“…(2) For the given peak intensity, various higher dopant charge states must be accessible to EII. This condition is well fulfilled for Xe where the lowest ionization energies are rather closely spaced (12.1, 21.0, 31.1 eV), whereas it is less well fulfilled for K which has a low first ionization energy energy (4.3 eV) but the second ionization energy is much higher (31.6 eV).…”

Resonant charging of Xe clusters in helium nanodroplets under intense laser fields

“…Recent theoretical work on Xe clusters in He droplets suggests that (i) nanoplasma generation is triggered by ionization of the embedded cluster and (ii) subsequent avalanche ionization rapidly ionizes the He nanomatrix [25,26]. As the explosion of the He shell is fast, resonant conditions can be achieved at earlier times in the shell than in the embedded cluster.…”

“…Note that further details can be found elsewhere [26,33]. For the simulations we consider Xe 309 in relaxed icosahedral geometry, see Fig.…”