Markus Braune

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.

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.

Imaging the He2 quantum halo state using a free electron laser

Significance In bound matter on all length scales, from nuclei to molecules to macroscopic solid objects, most of the density of the bound particles is within the range of the interaction potential which holds the system together. Quantum halos on the contrary are a type of matter where the particle density is mostly outside the range of the interaction potential in the tunneling region of the potential. Few examples of these fascinating systems are known in nuclear and molecular physics. The conceptually simplest halo system is made of only two particles. Here we experimentally image the wavefunction of the He2 quantum halo. It shows the predicted exponential shape of a tunneling wavefunction. Quantum tunneling is a ubiquitous phenomenon in nature and crucial for many technological applications. It allows quantum particles to reach regions in space which are energetically not accessible according to classical mechanics. In this “tunneling region,” the particle density is known to decay exponentially. This behavior is universal across all energy scales from nuclear physics to chemistry and solid state systems. Although typically only a small fraction of a particle wavefunction extends into the tunneling region, we present here an extreme quantum system: a gigantic molecule consisting of two helium atoms, with an 80% probability that its two nuclei will be found in this classical forbidden region. This circumstance allows us to directly image the exponentially decaying density of a tunneling particle, which we achieved for over two orders of magnitude. Imaging a tunneling particle shows one of the few features of our world that is truly universal: the probability to find one of the constituents of bound matter far away is never zero but decreases exponentially. The results were obtained by Coulomb explosion imaging using a free electron laser and furthermore yielded He2’s binding energy of 151.9±13.3 neV, which is in agreement with most recent calculations.

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.

Electron angular distributions of noble gases in sequential two-photon double ionization

We present an angle resolved study of photoelectrons emitted from ions of the noble gases neon, argon and krypton by means of time-of-flight spectroscopy. The ionic targets are generated in a sequential two-photon process induced by the free-electron laser FLASH. Values of the anisotropy parameters and are derived from electron angular distribution measurements in the photon energy range from 38 to 91 eV and compared with recent theoretical calculations.

Soft X-ray multiphoton excitation of small iodine methane derivatives

The fragmentation pattern of the iodine-containing molecules and following a strong multiphoton excitation in the vicinity of the iodine 4d giant resonance regime is studied using soft X-ray free electron laser radiation. A strong difference of the charge distribution and the kinetic energy release (KER) for the two molecules is found. The effects can be attributed to charge rearrangement processes induced by the photoexcitation. The difference in the observed distribution for higher charge states of iodine and carbon fragments is consistent with an over-the-barrier model for the charge rearrangement in the dissociating molecules. The KER for singly ionised carbon fragments indicates an ultrafast charge rearrangement before the dissociation starts.

Dynamics of electron emission in double photoionization processes near the Krypton 3d threshold

Two-electron emission following photoabsorption near the Kr 3d threshold is investigated both experimentally and theoretically. On the experimental side, electron/electron coincidences using a magnetic bottle time-of-flight spectrometer allow us to observe the complete double photo ionization (DPI) continua of selected Kr2+ final states, and to see how these continua are affected by resonant processes in the vicinity of the Kr 3d threshold. The analysis is based on a quantum mechanical approach that takes into account the contribution of three different processes: (A) Auger decay of the inner 3d vacancy with the associated post-collision interaction (PCI) effects, (B) capture of slow photoelectrons into discrete states followed by valence multiplet decay (VMD) of the excited ionic states and (C) valence shell DPI. The dominant process for each Kr2+(4p−2) final state is the photoionization of the inner shell followed by Auger decay of the 3d vacancies. Moreover, for the 4p−2(3P) and 4p−2(1D) final ionic states an important contribution comes from the processes of slow photoelectron capture followed by VMD as well as from double ionization of the outer shell involving also VMD.

Appearance of Plasmons in Fullerenes

The valence electrons of fullerenes may be regarded as spherical distributions with a finite width of a jellium-like potential giving rise to collective motions of this orange peel electron cloud. They cause strong enhancement of the photoionization cross section, a resonant behavior phenomenon know as plasmon excitations. The number and characteristic features of these excitations will be discussed.

Evidence for anisotropic final state interactions in the two-photon ionization of rare gases

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.

Non-invasive online wavelength measurements at FLASH2 and present benchmark

The commissioning and the first year of operation of the online photoionization spectrometer OPIS at FLASH2 is reported.