K. U. Kühnel

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.

Femtosecond x-ray photoelectron diffraction on gas-phase dibromobenzene molecules

We present time-resolved femtosecond photoelectron momentum images and angular distributions of dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules measured in a near-infrared pump, soft-x-ray probe experiment performed at an x-ray free-electron laser. The observed alignment dependence of the bromine 2p photoelectron angular distributions is compared to density functional theory calculations and interpreted in terms of photoelectron diffraction. While no clear time-dependent effects are observed in the angular distribution of the Br(2p) photoelectrons, other, low-energy electrons show a pronounced dependence on the time delay between the near-infrared laser and the x-ray pulse.

Unusual under-threshold ionization of neon clusters studied by ion spectroscopy

We carried out time-of-flight mass spectrometry for neon clusters that were exposed to intense free electron laser pulses with the wavelength of 62 nm, which induce optical transition from the ground state (2s 2 2p 6 ) to an excited state (2s 2 2p 5 nl ) in the Ne atoms. In contrast to Ne + ions produced by two-photon absorption from isolated Ne atoms, the Ne + ion yield from Ne clusters shows a linear dependence on the laser intensity (I). We discuss the ionization mechanisms which give the linear behaviour with respect to I and expected features in the electron emission spectrum. (Some figures may appear in colour only in the online journal)

EUV-photon-induced multiple ionization and fragmentation dynamics: from atoms to molecules

Multiple ionization (MI), induced by a few EUV photons at energies of 28.2 eV, 38 eV and 44 eV from FLASH (the free-electron laser at Hamburg), has been studied for atoms (He, Ne, Ar) and N2 molecules utilizing our multi-hit coincident technology?the reaction microscope. At comparably low intensities of I 1011?1013 W cm?2 we find the non-sequential (NS) MI mechanism dominating Ar3+ and Ar4+ production. Inspecting recoil ion and electron momentum distributions evidence is provided (i) for preferential back-to-back emission of electrons for NS double ionization of He and (ii) for angular entanglement between two outgoing electrons in sequential ionization (SI). In contrast to atoms, SI is observed to be most effective for MI of N2 molecules at an intensity of ~1013 W cm?2 leading, among others, to N2+2 ? N+ + N+, N3+2 ? N2+ + N+, N4+2 ? N2+ + N2+ Coulomb explosion channels. Fragment ion momentum distributions are investigated and are demonstrated to allow tracing SI pathways.

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)).

CAMP – A new endstation for simultaneous detection of photons and charged particles in free electron lasers experiments

We have designed a multi-purpose experimental chamber especially adapted to accommodate unique large-area, single-photon counting pnCCD detectors, developed by the Max Planck Institute Semiconductor Laboratory, together with advanced many-particle ion and electron imaging spectrometers (reaction microscope, REMI; velocity map imaging, VMI) for simultaneous detection of scattered and fluorescent photons and charged particles in experiments at Free Electron Lasers.

Multiple photoionization of rare-gas clusters by EUV-FEL at Spring-8

Multiple photoionization processes of giant rare-gas clusters are investigated in the wavelength region of 51 to 61nm at Spring8. We report here some latest results of our investigation; the frustration of photoionization for giant xenon clusters and the charge transfer in Ar-core Ne-shell clusters.

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.

Ion-ion coincidence studies on multiple ionizations of N2 and O2 molecules irradiated by EUV free-electron laser at SPring-8

We investigated multiple ionizations of N2 and O2 molecules by 51 nm EUV-FEL at the SPring-8 Compact SASE Source Test Accelerator in Japan using ion–ion coincidence spectroscopy. Coulomb break-up of parent ions with charge states up to four (N24+ → N2+–N2+) or five (O25+ → O3+–O2+) is found.

Multiple ionization of rare gas atoms by intense free electron laser radiation at 51 and 61 nm

We have studied multiple ionization of rare gas atoms (Ne, Ar, Kr, Xe) by intense 51 nm and 61 nm radiation delivered by the EUV-FEL at SPring-8 Compact SASE Source (SCSS) Test Accelerator. Charge states as high as Ar7+ and Kr8+ have been observed. We present intensity dependent yields of multiply charged ions providing benchmark data for theory, and discuss the role of direct, sequential and resonant ionization channels.