Ulrike Frühling

Single-shot timing measurement of extreme-ultraviolet free-electron laser pulses

Arrival time fluctuations of extreme-ultraviolet (EUV) pulses from the free-electron laser in Hamburg (FLASH) are measured single-pulse resolved at the experimental end-station. To this end, they are non-collinearly superimposed in space and time with visible femtosecond laser pulses on a GaAs substrate. The EUV irradiation induces changes of the reflectivity for the visible pulse. The temporal delay between the two light pulses is directly encoded in the spatial position of the reflectivity change which is captured with a CCD camera. For each single shot, the relative EUV/visible arrival-time can be measured with about 40 fs rms accuracy. The method constitutes a novel route for an improvement of future pump–probe experiments at short-wavelength free-electron lasers (FELs) by a pulse-wise correction with simultaneously measured arrival times of individual EUV pulses.

Time-resolved ion spectrometry on xenon with the jitter-compensated soft x-ray pulses of a free-electron laser

Atomic inner-shell relaxation dynamics were measured at the free-electron laser in Hamburg, FLASH, delivering 92 eV pulses. The decay of 4d core holes created in xenon was followed by detection of ion charge states after illumination with delayed 400 nm laser pulses. A timing jitter of the order of several hundred femtoseconds between laser- and accelerator-pulses was compensated for by a simultaneous delay measurement in a single-shot x-ray/laser cross-correlator. After sorting of the tagged spectra according to the measured delays, a temporal resolution equivalent to the pulse duration of the optical laser could be established. While results on ion charge states up to Xe4+ are compatible with a previous study using a high-harmonic soft x-ray source, a new relaxation pathway is opened by the nonlinear excitation of xenon atoms in the intense free-electron laser light field, leading to the formation of Xe5+.

Velocity map imaging of ion–molecule reactive scattering: The Ar++ N2 charge transfer reaction

We present a velocity map imaging spectrometer for the study of crossed-beam reactive collisions between ions and neutrals at (sub-)electronvolt collision energies. The charge transfer reaction of Ar(+) with N(2) is studied at 0.6, 0.8 and 2.5 eV relative collision energies. Energy and angular distributions are measured for the reaction product N. The differential cross section, as analyzed with a Monte Carlo reconstruction algorithm, shows significant large angle scattering for lower collision energies in qualitative agreement with previous experiments. Significant vibrational excitation of N(+)(2) is also observed. This theoretically still unexplained feature indicates the presence of a low energy scattering resonance.

Electron wave packet sampling with laser-generated extreme ultraviolet and terahertz fields

We report on transferring the concept of light-field streaking with intense terahertz fields from free-electron lasers to the laboratory scale. Utilizing a commercial laser system, synchronized 300 μm terahertz and 13 nm extreme ultraviolet pulses are generated by optical rectification and high harmonic generation, respectively. The terahertz fields are sufficiently strong to support electron wave packet sampling with a few fs resolution. The capability of this approach is demonstrated by measuring the duration of electron pulses formed by direct photoemission from a neon gas target.

Time-to-space mapping in a gas medium for the temporal characterization of vacuum-ultraviolet pulses

The authors introduce a method for cross correlating vacuum-ultraviolet with near-infrared femtosecond light pulses in a perpendicular geometry. Photoelectrons generated in an atomic gas by laser-assisted photoionization are used to create a two-dimensional image of the cross-correlation volume, thereby mapping time onto a space coordinate. Thus, information about pulse duration and relative timing between the pulses can be obtained without the need to scan an optical delay line. First tests using vacuum-ultraviolet pulses from the free-electron laser at the Deutsches Elektronen Synchrotron set an upper limit for their temporal jitter with respect to external optical laser pulses.

Evaporation of buffer gas-thermalized anions out of a multipole rf ion trap

We identify plain evaporation of ions as the fundamental loss mechanism out of a multipole ion trap. Using thermalized negative Cl- ions we find that the evaporative loss rate is proportional to a Boltzmann factor. This thermodynamic description allows us to extract the effective depth of the ion trap. As a function of the rf amplitude we find two distinct regimes related to the stability of motion of the trapped ions. For low amplitudes the entire trap allows for stable motion and the trap depth increases with the rf field. For larger rf amplitudes rapid energy transfer from the field to the ion motion can occur at large trap radii, which leads to a reduction of the effective trapping volume. In this regime the trap depth decreases again with increasing rf amplitude. We give an analytical parametrization of the trap depth for various multipole traps that allows predictions of the most favorable trapping conditions.

Light-field streaking for FELs

Free-electron lasers provide intense femtosecond radiation pulses with wavelengths in the extreme ultraviolet to x-ray range. Due to the stochastic nature of the light generation scheme, based on self-amplified spontaneous emission, the duration and temporal profile fluctuate from shot to shot. The temporal profile can be measured in a single-shot experiment by light-field streaking, where an infrared light field is used to accelerate photoelectrons generated by the x-ray pulse. The photoelectron energy change depends on the phase of the infrared field at the time of ionization. This technique is extensively used in attosecond metrology where near-infrared streaking fields are employed for the temporal characterization of attosecond XUV pulses. Here, it is adapted for the analysis of pulse durations in the femtosecond range by choosing far-infrared (terahertz) streaking fields.

Time-diagnostics for improved dynamics experiments at XUV FELs

Significantly structured and fluctuating temporal profiles of pulses from self-amplified spontaneous emission free electron lasers as well as their unstable timing require time diagnostics on a single-shot basis. The duration and structure of extreme-ultraviolet (XUV) pulses from the Free Electron Laser (FEL) in Hamburg (FLASH) are becoming accessible using a variation of the streak camera principle, where photoemitted electrons are energetically streaked in the electric field component of a terahertz electromagnetic wave. The timing with respect to an independently generated laser pulse can be measured in an XUV/laser cross-correlator, based on a non-collinear superposition of both pulses on a solid state surface and detection of XUV-induced modulations of its reflectivity for visible light. Sorting of data according to the measured timing dramatically improves the temporal resolution of an experiment sampling the relaxation of transient electronic states in xenon after linear- as well as nonlinear excitation with intense XUV pulses from FLASH.

Correlated electronic decay in expanding clusters triggered by intense XUV pulses from a Free-Electron-Laser

Irradiation of nanoscale clusters and large molecules with intense laser pulses transforms them into highly-excited non- equilibrium states. The dynamics of intense laser-cluster interaction is encoded in electron kinetic energy spectra, which contain signatures of direct photoelectron emission as well as emission of thermalized nanoplasma electrons. In this work we report on a so far not observed spectrally narrow bound state signature in the electron kinetic energy spectra from mixed Xe core - Ar shell clusters ionized by intense extreme-ultraviolet (XUV) pulses from a free-electron-laser. This signature is attributed to the correlated electronic decay (CED) process, in which an excited atom relaxes and the excess energy is used to ionize the same or another excited atom or a nanoplasma electron. By applying the terahertz field streaking principle we demonstrate that CED-electrons are emitted at least a few picoseconds after the ionizing XUV pulse has ended. Following the recent finding of CED in clusters ionized by intense near-infrared laser pulses, our observation of CED in the XUV range suggests that this process is of general relevance for the relaxation dynamics in laser produced nanoplasmas.