Abstract
The ionization dynamics of helium droplets irradiated by intense, femtosecond extreme ultraviolet (XUV) pulses is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited to atomic-like 2p states with a photon energy of 21.5 eV and autoionize by interatomic Coulombic decay (ICD). A complex evolution of the electron spectra as a function of droplet size (250 to 106 He atoms per droplet) and XUV intensity (109–1012 W cm−2) is observed, ranging from narrow atomic-like peaks that are due to binary autoionization, to an unstructured feature characteristic of electron emission from a nanoplasma. The experimental results are analyzed and interpreted with the help of a numerical simulation based on rate equations taking into account all relevant processes—multi-step ionization, electronic relaxation, ICD, secondary inelastic collisions, desorption of electronically excited atoms, and collective autoionization (CAI).
Highlights
The rapid development of short-wavelength free-electron lasers (FELs) [1,2,3] over recent decades has stimulated the investigation of the interaction between intense, high-energy light pulses and matter, and has become a very active field of research in atomic and molecular science [4,5,6]
We address the autoionization dynamics focusing on the transition from two-body interatomic Coulombic decay (ICD) to complex many-body autoionization, i.e. collective autoionization (CAI)
In our previous publications [26, 30], we showed that these autoionization processes are extremely efficient, which is a clear indication that the excitations are initially delocalized and during the lifetime of the excited states, the two excited atoms come into direct contact
Summary
The rapid development of short-wavelength free-electron lasers (FELs) [1,2,3] over recent decades has stimulated the investigation of the interaction between intense, high-energy light pulses and matter, and has become a very active field of research in atomic and molecular science [4,5,6]. A detailed understanding of these mechanisms is of fundamental interest and important for future studies using novel light sources such as x-ray free-electron lasers (FELs). Depending on the power density, samples can absorb a large number of photons and be transformed into highly excited, non-equilibrium systems within femtoseconds, which undergo complex relaxation. In this context, atomic clusters play an important role as well-controlled model systems
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