Attosecond and x-ray free-electron laser physics

Abstract

Currently, we are witnessing a revolution in photon science, driven by the vision to time-resolve ultra-fast electronic motion in atoms, molecules, and solids as well as by the quest for the characterization of time-dependent structural changes in large molecules and solids. Quantum jumps in the development of light sources are the key technologies for this emerging field of research. Thus, high harmonic radiation bursts now penetrate the attosecond (10-18 s) regime and free-electron lasers (FELs) deliver ultra-brilliant femtosecond, coherent VUV and x-ray pulses.This special issue presents a snapshot of this ongoing revolution and brings together, for the first time, pioneering results in both of these fields that are expected to evolve synergetically in the future. The volume is based on the spirit of the International Conference on Multi-Photon Processes, ICOMP08, which was held at the Max Planck Institute for Nuclear Physics in Heidelberg in summer 2008.The first contributions include articles that envision tracing electronic motion on an attosecond time scale and its relation to nuclear motion. After more technical papers on the generation of attosecond pulses via high harmonic generation (HHG), molecular and two-electron atomic dynamics in strong optical fields at a typical wavelength of 800 nm are presented pointing to sub-cycle, attosecond features. Making the transition to shorter wavelengths, nonlinear dynamics in atoms and molecules is explored via experimental and theoretical methods, where the present measurements are nearly exclusively performed at FEL sources. A substantial number of articles focus on the investigation of the most simple many- (few-) photon two-electron processes in double ionization of helium at optical and VUV wavelengths, with the goal of characterizing this fundamental reaction, not yet consistently solved theoretically, in spite of huge efforts.Finally, the behaviour of more complex nanoscaled systems, i.e. clusters, is investigated bridging the gap from atoms and molecules to solids introduced to intense FEL radiation. Beyond the basic interest in many-particle dynamics in finite systems, these studies are of enormous practical relevance for upcoming research at X-ray FELs. Here, realizing the dream of coherent imaging of the structure of single bio-molecules in the gas phase with atomic resolution is critically dependent on ultra-fast dynamics initiated by the pulse. In other words, it is reduced to the simple question of whether the molecule is first imaged and then destroyed or vice versa!During the preparation of this Editorial, the first lasing at the Stanford Linac Coherent Light Source (LCLS) was achieved at a photon energy of about 8 keV – a further milestone in this exciting revolution in the science related to light.