Ultrafast isomerization of acetylene cations ($[\mathrm{HC}=\mathrm{CH}{]}^{+}$) in the low-lying excited $A{}^2\Sigma _g{}^{+}$ state, populated by the absorption of extreme ultraviolet (XUV) photons (38 eV), has been observed at the Free Electron Laser in Hamburg, (FLASH). Recording coincident fragments ${\mathrm{C}}^{+}+\mathrm{CH}_2{}^{+}$ as a function of time between XUV-pump and -probe pulses, generated by a split-mirror device, we find an isomerization time of $52\pm 15\mathrm{fs}$ in a kinetic energy release (KER) window of $5.8<\mathrm{KER}<8\mathrm{eV}$, providing clear evidence for the existence of a fast, nonradiative decay channel.

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of ${\mathrm{Ne}}^{2+}$ production reveals the dominance of nonsequential two-photon double ionization at intensities of $I<6\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ and significant contributions of three-photon ionization as $I$ increases. ${\mathrm{Ne}}^{2+}$ recoil-ion-momentum distributions suggest that two electrons absorbing “instantaneously” two photons are ejected most likely into opposite hemispheres with similar energies.

The lifetime of interatomic Coulombic decay (ICD) [L. S. Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997)] in ${\mathrm{Ne}}_2$ is determined via an extreme ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The pump pulse creates a $2s$ inner-shell vacancy in one of the two Ne atoms, whereupon the ionized dimer undergoes ICD resulting in a repulsive ${\mathrm{Ne}}^{+}(2p^{-1})-{\mathrm{Ne}}^{+}(2p^{-1})$ state, which is probed with a second pulse, removing a further electron. The yield of coincident ${\mathrm{Ne}}^{+}-{\mathrm{Ne}}^{2+}$ pairs is recorded as a function of the pump-probe delay, allowing us to deduce the ICD lifetime of the ${\mathrm{Ne}}_2^{+}(2s^{-1})$ state to be $(150\pm 50)\mathrm{fs}$, in agreement with quantum calculations.

Recoil-ion momentum distributions for two-photon double ionization of He and Ne ($\hslash \omega =44\mathrm{eV}$) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of $\sim 1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ exploring the dynamics of the two fundamental two-photon–two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models.

A general numerical approach is described that allows obtaining model sets of temporal pulse shapes of free- electron lasers (FELs) operating in the self-amplified spontaneous emission mode. Based on a random partial- coherence approach, sets of pulse shapes can be calculated that satisfy statistical criteria of FEL light predicted by established FEL theory. Importantly, the numerically retrieved sets of pulses reproduce the experimentally accessible FEL light characteristics as measured at the Free-electron LASer at Hamburg (FLASH), such as the average spectrum, single-shot spectral shape, and pulse duration. The high-precision agreement with the experimental average spectral shape, without further knowledge of FEL machine parameters, makes this approach a convenient tool for the analysis and theoretical modeling of nonlinear optical or pump–probe experiments with FEL light.

The advent of free electron laser (FEL) facilities capable of delivering high intensity pulses in the extreme-UV to X-ray spectral range has opened up a wide vista of opportunities to study and control light matter interactions in hitherto unexplored parameter regimes. In particular, current short wavelength FELs can uniquely drive non-linear processes mediated by inner shell electrons and in fields where the photon energy can be as high as 10 keV and so the corresponding optical period reaches below one attosecond. Combined with ultrafast optical lasers, or simply employing wavefront division, pump probe experiments can be performed with femtosecond time resolution. As single photon ionization of atoms and molecules is by now very well understood, they provide the ideal targets for early experiments by which not only FELs can be characterised and benchmarked but can also be the natural departure point in the hunt for non-linear behaviour of atomistic systems bathed in laser fields of ultrahigh photon energy. In this topical review we illustrate with specific examples the gamut of apposite experiments in atomic, molecular physics currently underway at the SCSS Test Accelerator (Japan), FLASH (Hamburg) and LCLS (Stanford).