Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.

The ability to fully characterize ultrashort, ultra-intense X-ray pulses at free-electron lasers (FELs) will be crucial in experiments ranging from single-molecule imaging to extreme-timescale X-ray science. This issue is especially important at current-generation FELs, which are primarily based on self-amplified spontaneous emission and radiate with parameters that fluctuate strongly from pulse to pulse. Using single-cycle terahertz pulses from an optical laser, we have extended the streaking techniques of attosecond metrology to measure the temporal profile of individual FEL pulses with 5 fs full-width at half-maximum accuracy, as well as their arrival on a time base synchronized to the external laser to within 6 fs r.m.s. Optical laser-driven terahertz streaking can be utilized at any X-ray photon energy and is non-invasive, allowing it to be incorporated into any pump–probe experiment, eventually characterizing pulses before and after interaction with most sample environments

A general expression for the angular correlation function of the two emitted photoelectrons in sequential two-photon double ionization of atoms is derived and discussed. The expression can be used in the analysis of angle-resolved coincidence experiments. The angular distributions of the emitted electrons as measured in non-coincidence experiments are also discussed. As an example, the cross sections, angular distributions of photoelectrons and angular correlation functions for the sequential two-photon double ionization of Ne and Ar atoms have been calculated within the MCHF and MCDF approaches. The results are compared with recent experiments performed at the free-electron laser (FLASH) facility.

Experimental set-up-angular streaking spectroscopy. Previous streaking measurements of XFEL pulses 37-39 used a linearly polarized streaking field to encode their temporal profile onto the kinetic energy of photoelectrons. Depending on the amplitude and phase of

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We present kinematically complete data on two-photon double ionization of Ne induced by short (~25 fs) intense (~5x10 13 W/cm 2) free-electron laser pulses at 44 eV. The observed electron energy spectrum points to the dominance of "sequential" ionization. We analyze state-selective angular distributions as well as the two-electron angular correlation function, and suggest a method to determine the time delay between both ionization steps. The measured angular asymmetry (β-) parameters strongly deviate from the results of an earlier non-coincident experiment providing benchmark data for theory.

Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions. For the studied case of WSe, the photoemission events are time ordered with rising initial-state angular momentum. Including intra-atomic electron-electron interaction and angular momentum of the initial localized state yields excellent agreement between theory and experiment. This has required a revision of existing models for solid-state photoemission, and thus, attosecond time-resolved photoemission from solids provides important benchmarks for improved future photoemission models.

The angular distribution of Auger electrons in the decay of resonantly excited atomic states is studied theoretically. The two-step model is used for B description of the Auger process with the excited electron as a spectator. Expressions for the angular anisotropy parameters in the jK and jj coupling schemes are obtained for excitation of a closed-shell system. A close relation between the angular anisotropies of the resonant Auger and normal Auger processes is found within the framework of the spectator model.Numerical calculations for resonant Auger transitions in Ar, Kt and Xe are presented and compared with recent experiments.