Charge recombination in soft x-ray laser produced nanoplasmas

Abstract

The ionization and charge separation processes of nanoplasmas created by resonant excitation of atomic clusters in intense soft x-ray pulses have been investigated. Through irradiation with femtosecond pulses from the FLASH free electron laser (FEL) at λ = 13.7 nm and power densities exceeding 10 14 W cm −2 the clusters are highly ionized with transient atomic charge states up to 9+. Variation of the cluster composition from pristine to doped and core-shell systems allows tracking of the spatial origin and charge states of the fragments yielding insight into the nanoplasma dynamics. The data give evidence for efficient charge redistribution processes leading to a Coulomb explosion of the cluster outer part and recombination of the nanoplasma core. The experiments show qualitatively different processes for (soft) x-ray produced nanoplasmas from the optical (IR) strong-field regime where the clusters disintegrate completely in a Coulomb explosion.(Some figures in this article are in colour only in the electronic version)Understanding the interaction of light with matter has been a central theme of physics over the past century, starting with the concept of the photon and the inception of quantum theory. The invention of the laser and the continuing advance in laser technologies has made it possible to explore regimes of nonlinear light-matter interaction leading to novel laserbased concepts for particle accelerators, plasma formation and nuclear fusion [1]. Currently, we are witnessing the advent of intense lasers in the x-ray regime. One of the most exciting prospects of research with x-ray lasers is direct imaging of nonperiodic nanoscale objects, such as biomolecules, nanocrystals, living cells and viruses [2]. Even though it is crucial for the success of the imaging experiments, understanding the interaction of intense x-ray pulses with atomic systems and the underlying dynamics is still in its infancy. To date, virtually all studies about the ionization as well as nuclear dynamics of nanometer-sized structures in intense (soft) x-ray pulses are of theoretical nature [3][4][5][6] and no experimental data are available.For the experimental investigations of matter in intense light pulses atomic clusters are ideal because their size can be tuned from the molecular to the bulk-like regime and there is no energy dissipation into surrounding media. The ionization dynamics of clusters in intense laser pulses depend considerably on the radiation wavelength. In the infrared spectral regime the cluster is ionized by the optical field and the resulting transient nanoplasma is efficiently heated by the external laser field via inverse bremsstrahlung (IBS) and collective effects, leading to a Coulomb explosion of the cluster [7]. Because the ponderomotive energy scales with ω −2 (ω-laser frequency) and thus, the direct effect of the laser field on the electron movement is small, it was a big surprise when experiments in the vacuum-ultraviolet spectral regime at 100 nm and intensities up to 10 13 W cm −2 reported u...

Ionization dynamics in expanding clusters studied by XUV pump–probe spectroscopy

“…The charge states of cluster fragment ions are slightly higher for larger clusters (Figures 2 (a) and (b)) in agreement with previous work [21]. This is due to the charge transfer inside the cluster [19,21,23], which is especially effective in large clusters. Our findings are also in correspondence with previously reported results [33].…”

“…In particular, at the end of the XUV pulse the higher ionic charge states are preferably localized on the cluster outer shell while the inner core composed of nanoplasma electrons and ions remains quasi-neutral. The model has been confirmed experimentally by varying the cluster composition from pristine to doped and Xe core-Ar shell systems [23], by analysis of the kinetic energy distribution of fragment ions [21,22] as well as by theoretical calculations [18,19].…”

“…The lower ionic charge states produced by ionization of clusters compared to free atoms are mostly explained by the large Coulomb potential of the cluster, which prevents electrons to escape the cluster (outer ionization). Furthermore, efficient charge transfer and recombination can take place in clusters [23].…”

“…In these studies final distributions of photoelectrons or ionic fragments are detected long after cluster disintegration is completed [12,13]. Nevertheless, recent results [21,22,23] provide experimental evidence that a highly charged cluster surface explodes while in the inner part of the cluster an almost neutral plasma is formed.…”

Atomistic three-dimensional coherent x-ray imaging of nonbiological systems

“…Furthermore, there is intrinsic interest in the study of atomic clusters where the composition and structure can be controlled as a testing ground for new regimes of intense laser-matter interaction [55,56]. Rare gas clusters, bound by easy-to-model van der Waals forces, have traditionally served as testbeds as intense lasers have evolved from optical to x-ray wavelengths [10,38,[57][58][59][60][61][62][63][64]. FEL-induced transient dynamics in rare gas clusters have been observed in imaging experiments in the XUV [65] and, more recently, in the x-ray regime [66] providing evidence for femtosecond time scale electronic damage.…”