Clusters in intense FLASH pulses: ultrafast ionization dynamics and electron emission studied with spectroscopic and scattering techniques

Abstract

FLASH, the first FEL operating at short wavelength, has paved the way for novel types of experiments in many different scientific disciplines. Key questions for the first experiments with this new type of light source are linked to light–matter interaction and ionization processes. This paper gives an overview of the ultrafast ionization dynamics and electron emission of pure and doped rare gas clusters illuminated with intense short-wavelength pulses by summarizing the findings of recent years' work at FLASH. Atomic clusters are ideal for investigating the light–matter interaction because their size can be tuned from the molecular to the bulk regime, thus allowing us to distinguish between intra and interatomic processes. The ionization processes turned out to be strongly wavelength dependent. Plasma absorption, while dominant at 13 eV, becomes insignificant at photon energies above 40 eV. The cluster ionization and disintegration proceed in several steps on a time scale from fs to ps. Insight into the involved processes can be obtained with ion and electron spectroscopy. The high intensity of FLASH pulses opens the door for a new imaging approach to study nanoparticles. Scattering patterns of single and few clusters can be recorded in a single shot. Initial results of scattering experiments and their comparison to Mie calculations show that two- and three-dimensional structural information of gas phase particles can be obtained this way.

Nanoplasma Dynamics of Single Large Xenon Clusters Irradiated with Superintense X-Ray Pulses from the Linac Coherent Light Source Free-Electron Laser

“…This becomes particularly obvious for the highest power densities, where low charge states are entirely absent, a feature that has never been observed in nonlinear light-matter interaction experiments in other spectral regimes [10,11,24]. To shed more light on the transition from the low to the high power density spectra, the dominant charge state as well as the average charge state from single clusters with similar radii (30 AE 2 nm) are compared in Fig.…”

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

“…A large number of electrons (up to one electron per cluster atom) is released upon photoabsorption from individual atoms inside the cluster due to the dominant ionization of the 4d core level of Xe atoms at this photon energy, followed by a single or a cascade of two Auger decay processes [24]. Most of the activated electrons, however, remain quasi-free, i.e., trapped by the Coulomb potential of the cluster [12]. A temporally delayed, more intense probe pulse (4x10 13 W/cm 2 ) significantly increases the population of the nanoplasma electrons.…”

Heterogeneous clusters as a model system for the study of ionization dynamics within tampered samples

“…Their physical properties put them on the border between solid state and gas phase. Clusters are excellent objects to test the dynamics of samples irradiated with shortwavelength radiation from free-electron lasers (FELs) [1][2][3][4][5][6][7][8][9][10] and from high-harmonic-generation sources [11]. By studying the dependence of the cluster response on the cluster size, the influence of the atomic and condensed-matter effects on the ionization dynamics can be explored.…”

“…In this study we consider heterogeneous clusters composed of the noble gas atoms Xe and Ar, which have been extensively studied experimentally [1][2][3][4][6][7][8][9][10]. Using a fully nonequilibrium kinetic equation code (i.e., the Boltzmann solver described in [26,30,31]), we will demonstrate the significant effect of cluster composition on the ionization dynamics.…”