[Show abstract][Hide abstract] ABSTRACT: This paper gives an account of our progress towards performing femtosecond
time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe
setup combining optical lasers and an X-ray Free-Electron Laser. We present
results of two experiments aimed at measuring photoelectron angular
distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and
dissociating, laseraligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss
them in the larger context of photoelectron diffraction on gas-phase molecules.
We also show how the strong nanosecond laser pulse used for adiabatically
laser-aligning the molecules influences the measured electron and ion spectra
and angular distributions, and discuss how this may affect the outcome of
future time-resolved photoelectron diffraction experiments.
[Show abstract][Hide abstract] ABSTRACT: X-ray free-electron lasers provide unique opportunities for exploring ultrafast dynamics and for imaging the structures of complex systems. Understanding the response of individual atoms to intense X-rays is essential for most free-electron laser applications. First experiments have shown that, for light atoms, the dominant interaction mechanism is ionization by sequential electron ejection, where the highest charge state produced is defined by the last ionic state that can be ionized with one photon. Here, we report an unprecedentedly high degree of ionization of xenon atoms by 1.5 keV free-electron laser pulses to charge states with ionization energies far exceeding the photon energy. Comparing ion charge-state distributions and fluorescence spectra with state-of-the-art calculations, we find that these surprisingly high charge states are created via excitation of transient resonances in highly charged ions, and predict resonance enhanced absorption to be a general phenomenon in the interaction of intense X-rays with systems containing high-Z constituents.
[Show abstract][Hide abstract] ABSTRACT: With the first X-ray free-electron laser (FEL), the Linac Coherent Light
Source (LCLS), multiphoton ionization has been pushed to a new regime,
where atoms and molecules are not just ionized by a series of valence
ionizations but "from the inside out". At unprecedented high intensities
and short pulse durations in the soft X-ray regime, a series of
inner-shell photoionizations followed by cascades of Auger decays was
observed to lead to highly charged final states in rare gases such as
Ne, Ar, Kr, and Xe. Ion time-of-flight and fluorescence spectra were
recorded for different FEL pulse energies and pulse lengths and compared
to theoretical models to explain the underlying processes that lead to
unexpectedly high charge states in Xe.
Journal of Physics Conference Series 11/2012; 388(2):2022-.
[Show abstract][Hide abstract] ABSTRACT: 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
Journal of Physics Conference Series 11/2012; 388(3):2011-.
[Show abstract][Hide abstract] ABSTRACT: Using a split-mirror setup attached to a Reaction Microscope at the Free
electron LASer in Hamburg (FLASH) we traced as function of time the
migration of a hydrogen atom in C2H4+
from one end of the molecule to the other by coincident CH+ +
CH3+ fragment detection. In addition, the observed
H3++C2H+ channel provides
for the first time evidence for an isomerization-induced formation
mechanism of H3+ molecules.
Journal of Physics Conference Series 11/2012; 388(3):2014-.
[Show abstract][Hide abstract] ABSTRACT: The multiple ionization of Ar-core–Ne-shell clusters in intense extreme-ultraviolet laser pulses (λ∼62 nm) from the free-electron laser in Japan was investigated utilizing a momentum imaging technique. The Ar composition dependence of the kinetic energies and the yields of the fragment ions give evidence for charge transfer from the Ar core to the Ne shell. We have extended the uniformly charged sphere model originally applied to pristine clusters [ Islam et al. Phys. Rev. A 73 041201 (2006)] to the core-shell heterogeneous clusters to estimate the amounts of charge and energy transfers.
Physical Review A 09/2012; 86(3). · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We traced the femtosecond nuclear-wave-packet dynamics in ionic states of oxygen and nitrogen diatomic molecules employing 38-eV XUV-pump–XUV-probe experiments at the free-electron laser in Hamburg (FLASH). The nuclear dynamics is monitored via the detection of coincident ionic fragments using a reaction microscope and a split-mirror setup to generate the pump and probe pulses. By comparing measured kinetic-energy-release spectra with classical and quantum-mechanical simulations, we identified electronic states of the molecular ions that are populated by ionization of the neutral molecule. For specific fragment charge states, this comparison allows us to assess the relevance of specific dissociation paths.
Physical Review A 07/2012; 86(1). · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We traced the femtosecond nuclear wave-packet dynamics in ionic states
of oxygen and nitrogen diatomic molecules employing 38 eV XUV pump and
probe at the Free Electron Laser in Hamburg (FLASH).ootnotetextY.
H. Jiang et al., PRA 82, 041403(R) (2010). The nuclear dynamics is
monitored via the detection of coincident ionic fragments using a
reaction microscope and a split-mirror setup to generate the pump and
probe pulses. By comparing measured kinetic-energy-release (KER) spectra
with classical and quantum-mechanical simulations,ootnotetextI.
A. Bocharova et al., PRA 83, 013417 (2011) we identified electronic
states of the molecular ions that are populated by ionization of the
neutral molecule. The comparison of measured KER spectra for specific
fragment-charge states allows assessing the relevance of specific
[Show abstract][Hide abstract] ABSTRACT: The Radioactive Beam Experiment REX-ISOLDE [1–3] is a pilot experiment at ISOLDE (CERN) testing the new concept of post acceleration
of radioactive ion beams by using charge breeding of the ions in a high charge state ion source and the efficient acceleration
of the highly charged ions in a short LINAC using modern ion accelerator structures. In order to prepare the ions for the
experiments singly charged radioactive ions from the on-line mass separator ISOLDE will be cooled and bunched in a Penning
trap, charge bred in an electron beam ion source (EBIS) and finally accelerated in the LINAC. The LINAC consists of a radiofrequency
quadrupole (RFQ) accelerator, which accelerates the ions up to 0.3 MeV/u, an interdigital H-type (IH) structure with a final
energy between 1.1 and 1.2 MeV/u and three seven gap resonators, which allow the variation of the final energy. With an energy
of the radioactive beams between 0.8 MeV/u and 2.2 MeV/u a wide range of experiments in the field of nuclear spectroscopy,
astrophysics and solid state physics will be addressed by REX-ISOLDE.
[Show abstract][Hide abstract] ABSTRACT: 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)).
Multiphoton Processes and Attosecond Physics (ICOMP12). 01/2012;
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: A new and technologically challenging project, the electrostatic Cryogenic Storage Ring CSR, is presently
under construction at the Max-Planck-Institute for Nuclear Physics in Heidelberg. Applying liquid helium
cooling, the CSR, with 35 m circumference, will provide a low temperature environment of only a few
Kelvin and an extremely high vacuum of better than 10�13 mbar. To realize these conditions the mechanical
design has been completed and now the first quarter section is in the construction phase. For the
onion skin structure of the cryogenic system we have at the outer shell the cryostat chambers, realized
by welded rectangular stainless steel frames with aluminum plates. The next two shells are fabricated as
aluminum shields kept at 80 and 40 K. The inner vacuum chambers for the experimental vacuum consist
of stainless steel chambers cladded with external copper sheets connected to the LHe lines for optimized
thermal equilibration and cryopumping. Additional large surface 2 K units are installed for cryogenic
pumping of H2. The mechanical concepts and the realization will be presented in detail.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2011; 269:2871. · 1.27 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ultrafast isomerization of acetylene cations ([HC=CH](+)) in the low-lying excited A(2)Σ(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 C(+) + CH2(+) as a function of time between XUV-pump and -probe pulses, generated by a split-mirror device, we find an isomerization time of 52±15 fs in a kinetic energy release (KER) window of 5.8<KER<8 eV, providing clear evidence for the existence of a fast, nonradiative decay channel.
[Show abstract][Hide abstract] ABSTRACT: Using a split-mirror stage combined with a reaction microscope, nonlinear autocorrelation traces of XUV pulses from the Free-electron LASer at Hamburg were recorded for N2 multiphoton-induced fragmentation into noncoincident N2+ and coincident N2++N2+ channels. We find a pulse duration of 40±10 fs along with a sharp spike pointing to a coherence time of 4±1 fs, almost twice as short as in previous observations. Both are well reproduced by a simulation based on the partial-coherence model that includes the molecular dynamics leading to an ~12-fs substructure in the trace.
[Show abstract][Hide abstract] ABSTRACT: Two-photon double ionization of He is studied at the Free Electron Laser in Hamburg (FLASH) by inspecting He 2+ momentum ( ) distributions at 52 eV photon energy. We demonstrate that recoil ion momentum distributions can be used to infer information about highly correlated electron dynamics and find the first experimental evidence for 'virtual sequential ionization'. The experimental data are compared with the results of two calculations, both solving the time-dependent Schrödinger equation. We find good overall agreement between experiment and theory, with significant differences for cuts along the polarization direction that cannot be explained by the experimental resolution alone.
[Show abstract][Hide abstract] ABSTRACT: We have investigated multiple ionization of N2 and O2 molecules by 52 nm extreme-ultraviolet light pulses at the free-electron laser facility SCSS in Japan. Coulomb break-up of parent ions with charge states up to 5+ is found by the ion-ion coincidence technique. The charge-state dependence of kinetic energy release distributions suggests that the electrons are emitted sequentially in competition with the elongation of the bond length.
The Journal of Chemical Physics 05/2010; 132(20):204305-204305-5. · 3.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We used a split-mirror setup attached to a reaction microscope at the free-electron laser in Hamburg (FLASH) to perform an XUV-pump–XUV-probe experiment by tracing the ultrafast nuclear wave-packet motion in the D2+(1sσg) with <10 fs time resolution. Comparison with time-dependent calculations shows excellent agreement with the measured vibrational period of 22±4 fs in D2+, points to the importance of accurately knowing the internuclear distance-dependent ionization probability, and paves the way to control sequential and nonsequential two-photon double-ionization contributions.
[Show abstract][Hide abstract] ABSTRACT: Fourth generation accelerator-based light sources, such as VUV and X-ray Free Electron Lasers (FEL), deliver ultra-brilliant (∼1012–1013 photons per bunch) coherent radiation in femtosecond (∼10–100fs) pulses and, thus, require novel focal plane instrumentation in order to fully exploit their unique capabilities. As an additional challenge for detection devices, existing (FLASH, Hamburg) and future FELs (LCLS, Menlo Park; SCSS, Hyogo and the European XFEL, Hamburg) cover a broad range of photon energies from the EUV to the X-ray regime with significantly different bandwidths and pulse structures reaching up to MHz micro-bunch repetition rates. Moreover, hundreds up to trillions of fragment particles, ions, electrons or scattered photons can emerge when a single light flash impinges on matter with intensities up to 1022W/cm2.In order to meet these challenges, the Max Planck Advanced Study Group (ASG) within the Center for Free Electron Laser Science (CFEL) has designed the CFEL-ASG MultiPurpose (CAMP) chamber. It is equipped with specially developed photon and charged particle detection devices dedicated to cover large solid-angles. A variety of different targets are supported, such as atomic, (aligned) molecular and cluster jets, particle injectors for bio-samples or fixed target arrangements. CAMP houses 4π solid-angle ion and electron momentum imaging spectrometers (“reaction microscope”, REMI, or “velocity map imaging”, VMI) in a unique combination with novel, large-area, broadband (50eV–25keV), high-dynamic-range, single-photon-counting and imaging X-ray detectors based on the pnCCDs.This instrumentation allows a new class of coherent diffraction experiments in which both electron and ion emission from the target may be simultaneously monitored. This permits the investigation of dynamic processes in this new regime of ultra-intense, high-energy radiation—matter interaction. After an introduction into the salient features of the CAMP chamber and the properties of the redesigned REMI/VMI spectrometers, the new 1024×1024 pixel format pnCCD imaging detector system will be described in detail. Results of tests of four smaller format (256×512) devices of identical performance, conducted at FLASH and BESSY, will be presented and the concept as well as the anticipated properties of the full, large-scale system will be elucidated. The data obtained at both radiation sources illustrate the unprecedented performance of the X-ray detectors, which have a voxel size of 75×75×450μm3 and a typical read-out noise of 2.5 electrons (rms) at an operating temperature of −50°C.
Nuclear Instruments and Methods in Physics Research Section A, v.614, 483-496 (2010). 01/2010;