Alexander Hartung

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.

Electron spin polarization in strong-field ionization of xenon atoms

Electron spin polarization is experimentally detected and investigated via strong-field ionization of xenon atoms.

Observation of Enhanced Chiral Asymmetries in the Inner-Shell Photoionization of Uniaxially Oriented Methyloxirane Enantiomers

Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.

Ultrafast preparation and detection of ring currents in single atoms

Quantum particles can penetrate potential barriers by tunnelling1. If that barrier is rotating, the tunnelling process is modified2,3. This is typical for electrons in atoms, molecules or solids exposed to strong circularly polarized laser pulses4–6. Here we measure how the transmission probability through a rotating tunnel depends on the sign of the magnetic quantum number m of the electron and thus on the initial direction of rotation of its quantum phase. We further show that our findings agree with a semiclassical picture, in which the electron keeps part of that rotary motion on its way through the tunnel by measuring m-dependent modification of the electron emission pattern. These findings are relevant for attosecond metrology as well as for interpretation of strong-field electron emission from atoms and molecules7–14 and directly demonstrate the creation of ring currents in bound states of ions with attosecond precision. In solids, this could open a way to inducing and controlling ring-current-related topological phenomena15.When an electron with specific orbit — either clockwise or anticlockwise — in a rare gas atom is selectively ionized, the remaining ion will possess a stationary ring current, which can be probed in a time-delayed second ionization step.

Electron spin polarization in strong-field ionization and the effect of spin in attoclock measurements

Spin plays a fundamental role in the electronic structure of matter, from single atoms and molecules through to condensed matter. However, spin dynamics and the spin response of electrons to ultra-short, strong laser pulses has remained largely unexplored until today. In 2011 and 2013, pioneering theoretical work [1, 2] extended the fundamental PPT theory [3] to treat ionization of p+ and p” electrons by circular fields. This work predicted the sensitivity of ionization probabilities in strong field ionization using circular fields to the magnetic quantum number, showing that counter rotating electrons can tunnel ionize easier. It also predicted the possibility to obtain spin polarized electrons from strong, circularly polarized fields with a high degree of efficiency. The preference of ionization was later confirmed experimentally by [4], however, the experiment in [4] did not provide a confirmation of the spin polarization predictions.

Designing a COLTRIMS Reaction-Microscope for multi-hit coincident measurements with the SQS instrument at European XFEL

Using intense photon beams from the XFEL free electron laser, the COLTRIMS Reaction Microscope will be a versatile tool for examination of the dynamics in small quantum systems.

Measurement of the spin polarization of electrons in a strong laser field

We present first experimental data on the theoretically predicted effect [1] that photoelectrons created by ionization of noble gas atoms in an intense laser field are spin polarized.

A molecular movie of Interatomic Coulombic Decay in NeKr

Interatomic Coulombic Decay in mixed NeKr dimers has been measured time-resolved and the nuclear dynamics of the decay have been investigated.