Oliver Herrwerth

Attosecond tracing of correlated electron-emission in non-sequential double ionization

Despite their broad implications for phenomena such as molecular bonding or chemical reactions, our knowledge of multi-electron dynamics is limited and their theoretical modelling remains a most difficult task. From the experimental side, it is highly desirable to study the dynamical evolution and interaction of the electrons over the relevant timescales, which extend into the attosecond regime. Here we use near-single-cycle laser pulses with well-defined electric field evolution to confine the double ionization of argon atoms to a single laser cycle. The measured two-electron momentum spectra, which substantially differ from spectra recorded in all previous experiments using longer pulses, allow us to trace the correlated emission of the two electrons on sub-femtosecond timescales. The experimental results, which are discussed in terms of a semiclassical model, provide strong constraints for the development of theories and lead us to revise common assumptions about the mechanism that governs double ionization.

Second-order autocorrelation of XUV FEL pulses via time resolved two-photon single ionization of He

Second-order autocorrelation spectra of XUV free-electron laser pulses from the Spring-8 Compact SASE Source (SCSS) have been recorded by time and momentum resolved detection of two-photon single ionization of He at 20.45 eV using a split-mirror delay-stage in combination with high-resolution recoil-ion momentum spectroscopy (COLTRIMS). From the autocorrelation trace we extract a coherence time of 8 ± 2 fs and a mean pulse duration of 28 ± 5 fs, much shorter than estimations based on electron bunch-length measurements. Simulations within the partial coherence model [Opt. Lett. 35, 3441 (2010)] are in agreement with experiment if a pulse-front tilt across the FEL beam diameter is taken into account that leads to a temporal shift of about 6 fs between both pulse replicas.

Ultrafast dynamics in acetylene clocked in a femtosecond XUV stopwatch

Few-photon induced ultrafast dynamics in acetylene (C2H2) leading to several dissociation channels—deprotonation (H++C2H+ and H++C2H2+), symmetric break-up (CH++CH+) and isomerization (C++CH2+)-–were investigated employing the (XUV; extreme ultra-violet)-pump–(XUV; extreme ultra-violet)-probe scheme at the free-electron laser in Hamburg, combined with multi-hit coincidence detection. The kinetic energy releases and fragment-ion momentum distributions for various decay channels are presented. The C++CH2+ and H++C2H2+ channels reveal clear signatures of ultrafast molecular mechanisms, demonstrating potential applications of our pump-probe technique to complex systems in order to study a large variety of ultrafast phenomena in the XUV regime.

Wavelength dependence of sub-laser-cycle few-electron dynamics in strong-field multiple ionization

Recoil-ion momentum distributions for double and triple ionization of Ne and Ar, as well as for double ionization of N2 molecule by intense (0.3–0.5 PW cm-2), short (~35–40 fs) laser pulses have been recorded in a so far unexplored long laser-wavelength regime at 1300 nm. Compared to earlier results at 800 nm, the direct (e, ne) ionization pathway during recollision is strongly enhanced manifesting itself in a pronounced double-hump structure in the longitudinal ion momentum spectra not only for Ne, but also surprisingly distinct for Ar and, found for the first time, for molecules. Observed wavelength dependence of the sub-laser-cycle correlated few-electron dynamics might be of paramount importance for possible future applications in attosecond science, in particular, for imaging of ultrafast molecular processes via recollision-induced fragmentation.

Attosecond imaging of XUV-induced atomic photoemission and Auger decay in strong laser fields

Velocity-map imaging has been employed to study the photoemission in Ne and N4,5OO Auger decay in Xe induced by an isolated 85 eV extreme ultraviolet (XUV) pulse in the presence of a strong few-cycle near-infrared (NIR) laser field. Full three-dimensional momentum information about the released electrons was obtained. The NIR and XUV pulse parameters were extracted from the measured Ne streaking traces using a FROG CRAB retrieval algorithm. The attosecond measurements of the Auger decay in Xe show pronounced broadening of the Auger lines corresponding to the formation of sidebands. The temporal evolution of the sideband signals and their asymmetry along the laser polarization axis exhibit oscillations similar to those known from attosecond streaking measurements. The experimental results are in good agreement with model calculations based on an analytical solution of the Schr?dinger equation within the strong field approximation.

Characterization of Extreme Ultra-Violet Free-Electron Laser Pulses by Autocorrelation

We present EUV autocorrelation measurements of free-electron laser (FEL) pulses at 28 eV photon energy exploiting multiple ionization of argon as a non-linear process. In this way, the average pulse duration is measured while in parallel insight is gained into the temporal structure of the pulses. We compare the obtained results with FEL pulse simulations using our partial-coherence method (T. Pfeifer et al., Opt. Lett. 35:3441 (2010)).

Investigating temporal structure of FEL pulses by XUV pump--probe autocorrelation measurements

Using a split focusing mirror, XUV pump–probe measurements have been performed at the free-electron laser at Hamburg (FLASH). The obtained non-linear autocorrelation signals can be used to characterize the ultra-short photon pulses generated by the machine at different control parameters.

Time-resolved few-photon induced molecular fragmentation at FLASH

Multiple ionization and fragmentation of N2 induced by absorption of few photons (44 and 46 eV) has been explored in pump-probe measurements at the free electron laser FLASH [1] at Hamburg. Photo ionization of aligned N2+ ions, produced by photon absorption in sequential steps, is explored and few-photon absorption pathways are traced by inspecting the kinetic energy releases (KER) and the fragment-ion angular distributions as function of the pump-probe delay time.