92 research outputs found
Sensitivity of nonlinear photoionization to resonance substructure in collective excitation
Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body system with intense XUV radiation can be used as an effective probe for characterizing otherwise unresolved features of its collective response. Resonant photoionization of atomic xenon was chosen as a case study. The excellent agreement between experiment and theory strongly supports the prediction that two distinct poles underlie the giant dipole resonance. Our results pave the way towards a deeper understanding of collective behaviour in atoms, molecules and solid-state systems using nonlinear spectroscopic techniques enabled by modern short-wavelength light sources
Evidence for anisotropic final state interactions in the two-photon ionization of rare gases
Synopsis Anisotropic final state interactions are exhibited most clearly in the angular momentum dependent position of Cooper minima in the photoionization partial cross section and angular distribution asymmetry parameter. We show first indication of this effect in two-photon ionization of rare gases
The Variable Polarization XUV Beamline P04 at PETRA III Optics, mechanics and their performance
The layout of the Variable Polarization XUV Beamline P04 at PETRAIII is described with emphasison selected examples of optics, mirrors and gratings. A precise characterization of the optics, their performance inside the holder and of the surrounding mechanics is presented. This also includes a detailed characterization of the different beamline mechanics as a whole gratingunit, exit slit unit, re focusing unit including the environmen
Two-electron processes in multiple ionization under strong soft-x-ray radiation
In a combined experimental and theoretical study we have investigated the ionization of atomic argon upon irradiation with intense soft-x-ray pulses of 105 eV photon energy from the free-electron laser FLASH. The measured ion yields show charge states up to Ar7+. The comparison with the theoretical study of the underlying photoionization dynamics highlights the importance of excited states in general and of processes governed by electron correlation in particular, namely, ionization with excitation and shake-off, processes usually inaccessible by measurements of ionic yields only. The Ar7+ yield shows a clear deviation from the predictions of the commonly used model of sequential ionization via single-electron processes and the observed signal can only be explained by taking into account the full multiplet structure of the involved configurations and by inclusion of two-electron processes. The competing process of two-photon ionization from the ground state of Ar6+ is calculated to be orders of magnitude smaller
Emitter-site selective photoelectron circular dichroism of trifluoromethyloxirane
The angle-resolved inner-shell photoionization of R-trifluoromethyloxirane,
C3H3F3O, is studied experimentally and theoretically. Thereby, we investigate
the photoelectron circular dichroism (PECD) for nearly-symmetric O 1s and F 1s
electronic orbitals, which are localized on different molecular sites. The
respective dichroic and angular distribution parameters
are measured at the photoelectron kinetic energies from 1 to 16 eV by using
variably polarized synchrotron radiation and velocity map imaging spectroscopy.
The present experimental results are in good agreement with the outcome of ab
initio electronic structure calculations. We report a sizable chiral asymmetry
of up to about 9% for the K-shell photoionization of oxygen atom.
For the individual fluorine atoms, the present calculations predict asymmetries
of similar size. However, being averaged over all fluorine atoms, it drops down
to about 2%, as also observed in the present experiment. Our study demonstrates
a strong emitter- and site-sensitivity of PECD in the one-photon inner-shell
ionization of this chiral molecule
Angle resolved photoelectron spectroscopy of two-color XUV-NIR ionization with polarization control
Electron emission caused by extreme ultraviolet (XUV) radiation in the presence of a strong near infrared (NIR) field leads to multiphoton interactions that depend on several parameters. Here, a comprehensive study of the influence of the angle between the polarization directions of the NIR and XUV fields on the two-color angle-resolved photoelectron spectra of He and Ne is presented. The resulting photoelectron angular distribution strongly depends on the orientation of the NIR polarization plane with respect to that of the XUV field. The prevailing influence of the intense NIR field over the angular emission characteristics for He(1s) and Ne(2p) ionization lines is shown. The underlying processes are modeled in the frame of the strong field approximation (SFA) which shows very consistent agreement with the experiment reaffirming the power of the SFA for multicolor-multiphoton ionization in this regime
Multiple-core-hole resonance spectroscopy with ultraintense X-ray pulses
Understanding the interaction of intense, femtosecond X-ray pulses with heavy
atoms is crucial for gaining insights into the structure and dynamics of
matter. One key aspect of nonlinear light-matter interaction was, so far, not
studied systematically at free-electron lasers -- its dependence on the photon
energy. Using resonant ion spectroscopy, we map out the transient electronic
structures occurring during the complex charge-up pathways. Massively hollow
atoms featuring up to six simultaneous core holes determine the spectra at
specific photon energies and charge states. We also illustrate how the
influence of different X-ray pulse parameters that are usually intertwined can
be partially disentangled. The extraction of resonance spectra is facilitated
by the fact that the ion yields become independent of the peak fluence beyond a
saturation point. Our study lays the groundwork for novel spectroscopies of
transient atomic species in exotic, multiple-core-hole states that have not
been explored previously.Comment: Supplementary information is include
Femtosecond profiling of shaped x-ray pulses
Arbitrary manipulation of the temporal and spectral properties of x-ray pulses at free-electron lasers would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the free-electron laser driving electron bunch can be tuned to emit a pair of x-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the x-ray pulse cannot be predicted with sufficient precision. Furthermore, shot-to-shot instabilities that distort the pulse shape unpredictably cannot be fully suppressed. Therefore, the ability to directly characterize the x-rays is essential to ensure precise and consistent control. In this work, we have generated x-ray pulse pairs via electron bunch shaping and characterized them on a single-shot basis with femtosecond resolution through time-resolved photoelectron streaking spectroscopy. This achievement completes an important step toward future x-ray pulse shaping techniques
Controlling Fragmentation of the Acetylene Cation in the Vacuum Ultraviolet via Transient Molecular Alignment.
An open-loop control scheme of molecular fragmentation based on transient molecular alignment combined with single-photon ionization induced by a short-wavelength free electron laser (FEL) is demonstrated for the acetylene cation. Photoelectron spectra are recorded, complementing the ion yield measurements, to demonstrate that such control is the consequence of changes in the electronic response with molecular orientation relative to the ionizing field. We show that stable C2H2+ cations are mainly produced when the molecules are parallel or nearly parallel to the FEL polarization, while the hydrogen fragmentation channel (C2H2+ → C2H+ + H) predominates when the molecule is perpendicular to that direction, thus allowing one to distinguish between the two photochemical processes. The experimental findings are supported by state-of-the art theoretical calculations
Photoelectron circular dichroism of O 1-photoelectrons of uniaxially oriented trifluoromethyloxirane: Energy dependence and sensitivity to molecular configuration
The photoelectron circular dichroism (PECD) of the O 1s-photoelectrons of
trifluoromethyloxirane(TFMOx) is studied experimentally and theoretically for
different photoelectron kinetic energies. The experiments were performed
employing circularly polarized synchrotron radiation and coincidentelectron and
fragment ion detection using Cold Target Recoil Ion Momentum Spectroscopy. The
corresponding calculations were performed by means of the Single Center method
within the relaxed-core Hartree-Fock approximation. We concentrate on the
energy dependence of the differential PECD of uniaxially oriented TFMOx
molecules, which is accessible through the employed coincident detection. We
also compare results for differential PECD of TFMOx to those obtained for the
equivalent fragmentation channel and similar photoelectron kinetic energy of
methyloxirane (MOx), studied in our previous work. Thereby, we investigate the
influence of the substitution of the methyl-group by the trifluoromethyl-group
at the chiral center on the molecular chiral response. Finally, the presently
obtained angular distribution parameters are compared to those available in
literature.Comment: 6 fig
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