Jens Viefhaus

Isotope-induced partial localization of core electrons in the homonuclear molecule N2

Because of inversion symmetry and particle exchange, all constituents of homonuclear diatomic molecules are in a quantum mechanically non-local coherent state; this includes the nuclei and deep-lying core electrons. Hence, the molecular photoemission can be regarded as a natural double-slit experiment: coherent electron emission originates from two identical sites, and should give rise to characteristic interference patterns. However, the quantum coherence is obscured if the two possible symmetry states of the electronic wavefunction (‘gerade’ and ‘ungerade’) are degenerate; the sum of the two exactly resembles the distinguishable, incoherent emission from two localized core sites. Here we observe the coherence of core electrons in N2 through a direct measurement of the interference exhibited in their emission. We also explore the gradual transition to a symmetry-broken system of localized electrons by comparing different isotope-substituted species—a phenomenon analogous to the acquisition of partial ‘which-way’ information in macroscopic double-slit experiments.

Competition between direct dissociation and resonant Auger decay: a quasi-classical model applied to the 2p−1 σ∗ states of HCl, DCl and Cl2

Resonant Auger spectra of HCl, DCl, and Cl2 following the (Cl 2p32 → σ∗ excitation were studied. Both ‘molecular’ and ‘atomic’ Auger transitions are observed and assigned. The branching ratio of ‘molecular’ versus ‘atomic’ Auger transitions increases with increasing reduced mass of the target molecules. Using a quasi-classical model, we are able to deduce the principal Auger lineshape in agreement with experiment. The simultaneous occurrence of ‘atomic’ and ‘molecular’ Auger transitions is shown to be the general case for repulsive intermediate states. The kinematic model successfully describes the mass and lifetime dependence of the competition between the Auger process and dissociation.

Auger cascades versus direct double Auger: relaxation processes following photoionization of the Kr 3d and Xe 4d, 3d inner shells

The relaxation processes after non-resonant inner-shell photoionization are studied experimentally using electron?electron time-of-flight coincidence spectroscopy. Results for krypton 3d and xenon 4d as well as 3d photoionization are presented. The experimental data make it possible to disentangle sequential from simultaneous processes using the different electron emission characteristics as the differentiating property. For the population of final states having charges higher than 2, the measurements show a strong preference for sequential Auger cascade decay. Clear evidence for direct double Auger processes is found in the case of Xe 3d photoionization only.

Spin resolved auger electron spectroscopy after photoexcitation with circularly polarized radiation from the BESSY crossed undulator

Abstract The new crossed undulator beamline U2-FSGM at BESSY enables spin resolved spectroscopy of electrons emitted after excitation of atoms, molecules and solids by circularly polarized radiation, even beyond the energy limit of normal-incidence monochromators. It is thus possible to perform spin resolved Auger spectroscopy of an enlarged number of elements. We present first results obtained at the U2-FSGM in the gas phase and from solids. For the gas phase we show spin resolved data for 5p- and 4d-photoionization of Xe and for the N4,5O2,3O2,3 Auger decay in Xe which agree within the uncertainties with theory. They are measured by means of a newly developed time-of-flight apparatus including spin polarization analysis. For solids we present spin resolved M3VV Auger spectra obtained at Cu(100) in a normal incidence, normal emission setup. On the low energy side of the Auger peak they show a change of the preferential spin direction not expected within an atomic model of the Cu M3VV Auger decay. In the center of the peak the preferential spin direction is antiparallel to the spin of the photons exciting the primary M3 hole states.

Absolute cross sections for photoionization of Xeq + ions (1 q 5) at the 3d ionization threshold

The photon-ion merged-beams technique has been employed at the new Photon-Ion spectrometer at PETRA III for measuring multiple photoionization of Xeq + (q = 1–5) ions. Total ionization cross sections have been obtained on an absolute scale for the dominant ionization reactions of the type hν + Xeq + → Xer + + (q − r)e− with product charge states q + 2 ≤ r ≤ q + 5. Prominent ionization features are observed in the photon-energy range 650–750 eV, which are associated with excitation or ionization of an inner-shell 3d electron. Single-configuration Dirac–Fock calculations agree quantitatively with the experimental cross sections for non-resonant photoabsorption, but fail to reproduce all details of the measured ionization resonance structures.

Imaging molecular structure through femtosecond photoelectron diffraction on aligned and oriented gas-phase molecules.

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 (C(8)H(5)F) and dissociating, laser-aligned 1,4-dibromobenzene (C(6)H(4)Br(2)) 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.

Photon energy dependence of ionization-excitation in helium at medium energies

We have measured the photoionization-excitation-to-photoionization ratio for (n = 2 - 6) at several photon energies from 90 to 900 eV. By extrapolating these values we could determine the asymptotic high-energy ratios for (n = 2 - 5) which agree with theoretical predictions. We show that the satellite-to-main-line ratios are consistent with experimental double-to-single photoionization ratios and agree well with recent measurements.

Photoelectron spectroscopy as a non-invasive method to monitor SASE-FEL spectra

Since the summer of 2005, the vacuum ultra-violet Free-electron LASer in Hamburg (FLASH) has operated as a user facility at the Deutsches Elektronen-Synchrotron (DESY), delivering ultra-short laser pulses of tens of femtosecond duration with a high peak brilliance of up to 1028 photons/(s mm2 mrad2 0.1% bandwidth). Due to the statistics of the Self-Amplified Spontaneous Emission (SASE) process, each photon pulse differs from the previous one in the number of modes per pulse, the wavelength (0.5% fluctuations) and the intensity, making experiments more complicated. Thus, for certain experiments the detailed knowledge of the beam properties on a shot-to-shot basis is mandatory. In this paper we describe an online method to gain spectral information about the individual Free-Electron Laser (FEL) pulses that is based on rare-gas photoionization and photoelectron spectroscopy.

Field-free deterministic ultrafast creation of magnetic skyrmions by spin–orbit torques

Magnetic skyrmions are stabilized by a combination of external magnetic fields, stray field energies, higher-order exchange interactions and the Dzyaloshinskii-Moriya interaction (DMI). The last favours homochiral skyrmions, whose motion is driven by spin-orbit torques and is deterministic, which makes systems with a large DMI relevant for applications. Asymmetric multilayers of non-magnetic heavy metals with strong spin-orbit interactions and transition-metal ferromagnetic layers provide a large and tunable DMI. Also, the non-magnetic heavy metal layer can inject a vertical spin current with transverse spin polarization into the ferromagnetic layer via the spin Hall effect. This leads to torques that can be used to switch the magnetization completely in out-of-plane magnetized ferromagnetic elements, but the switching is deterministic only in the presence of a symmetry-breaking in-plane field. Although spin-orbit torques led to domain nucleation in continuous films and to stochastic nucleation of skyrmions in magnetic tracks, no practical means to create individual skyrmions controllably in an integrated device design at a selected position has been reported yet. Here we demonstrate that sub-nanosecond spin-orbit torque pulses can generate single skyrmions at custom-defined positions in a magnetic racetrack deterministically using the same current path as used for the shifting operation. The effect of the DMI implies that no external in-plane magnetic fields are needed for this aim. This implementation exploits a defect, such as a constriction in the magnetic track, that can serve as a skyrmion generator. The concept is applicable to any track geometry, including three-dimensional designs.

Cross sections and angular distributions of the photoelectron correlation satellites of the Xe atom

A theoretical and experimental study of the behaviour of the correlation satellites arising during 5s photoionization of xenon is presented. Many-body perturbation theory and configuration-interaction techniques have been applied to calculate the wavefunctions of the Xe II ionic states. An expression for the angular distribution parameter of the photoelectrons taking into account final-ionic-state configuration interaction is derived. Photoionization cross sections and angular distribution parameters were calculated for the 5s main line and the majority of the satellite lines and compared with our high-resolution measurements and earlier lower resolution measurements. The differences in the angular distribution parameter dependence on the photon energy for the 5s main line and satellites were analysed in terms of their origin. The most important mechanisms are: interference of several photoionization channels characterizing different orbital momenta of the photoelectron, the mixture of terms with different total orbital momenta in the final ionic state, and the dependence of the photoelectron wavefunctions on the total momentum of the photoelectrons.