Similar to the optical diffraction of light passing through a material grating, the Kapitza-Dirac effect occurs when an electron is diffracted by a standing light wave. In its original description the effect is time-independent. In the present work, we extend the Kapitza-Dirac concept to the time domain. By tracking the spatiotemporal evolution of a pulsed electron wave packet diffracted by a femtosecond (10 15 second) standing wave pulse in a pump-probe scheme, we observe so far unseen time-dependent diffraction patterns. The fringe spacing in the observed pattern differs from that generated by the conventional Kapitza-Dirac effect, moreover it decreases as the pump-probe delay time increases. By exploiting this time-resolved diffraction scheme, we gather access to the time evolution of the previously inaccessible phase properties of a free electron.

Atoms can form a molecule by sharing their electrons in binding orbitals. These electrons are entangled. Is there a way to break a molecular bond and obtain atoms in their ground state that are spatially separated and still entangled? Here, we show that it is possible to prepare these spatially separated, entangled atoms on femtosecond time scales from single oxygen molecules. The two neutral atoms are entangled in the magnetic quantum number of their valence electrons. In a time-delayed probe step, we use nonadiabatic tunneling, which is a magnetic quantum number–sensitive ionization mechanism. We find a fingerprint of entanglement in the measured ionization probability as a function of the angle between the light’s quantization axis and the molecular axis. This establishes a platform for further experiments that harness the time resolution of strong-field experiments to investigate spatially separated, entangled atoms on femtosecond time scales.

The photoelectric effect describes the ejection of an electron upon absorption of one or several photons. The kinetic energy of this electron is determined by the photon energy reduced by the binding energy of the electron and, if strong laser fields are involved, by the ponderomotive potential in addition. It has therefore been widely taken for granted that for atoms and molecules, the photoelectron energy does not depend on the electron's emission direction, but theoretical studies have questioned this since 1990. Here, we provide experimental evidence that the energies of photoelectrons emitted against the light propagation direction are shifted toward higher values, while those electrons that are emitted along the light propagation direction are shifted to lower values. We attribute the energy shift to a nondipole contribution to the ponderomotive potential that is due to the interaction of the moving electrons with the incident photons.

X-Ray as well as electron diffraction are powerful tools for structure determination of molecules. Studies on randomly oriented molecules in the gas phase address cases in which molecular crystals cannot be generated or the interaction-free molecular structure is to be addressed. Such studies usually yield partial geometrical information, such as interatomic distances. Here, we present a complementary approach, which allows obtaining insight into the structure, handedness, and even detailed geometrical features of molecules in the gas phase. Our approach combines Coulomb explosion imaging, the information that is encoded in the molecular-frame diffraction pattern of core-shell photoelectrons and ab initio computations. Using a loop-like analysis scheme, we are able to deduce specific molecular coordinates with sensitivity even to the handedness of chiral molecules and the positions of individual atoms, e.g., protons.

The influence of the magnetic component of the driving electromagnetic field is often neglected when investigating light-matter interaction. We show that the magnetic component of the light field plays an important role in nonsequential double ionization, which serves as a powerful tool to investigate electron correlation. We investigate the magnetic-field effects in double ionization of xenon atoms driven by near-infrared ultrashort femtosecond laser pulses and find that the mean forward shift of the electron momentum distribution in light-propagation direction agrees well with the classical prediction, where no under-barrier or recollisional nondipole enhancement is observed. By extending classical trajectory Monte Carlo simulations beyond the dipole approximation, we reveal that double ionization proceeds via recollision-induced doubly excited states, followed by subsequent sequential over-barrier field ionization of the two electrons. In agreement with this model, the binding energies do not lead to an additional nondipole forward shift of the electrons. Our findings provide a new method to study electron correlation by exploiting the effect of the magnetic component of the electromagnetic field.

We experimentally and theoretically investigate the influence of the magnetic component of an electromagnetic field on high-order above-threshold ionization of xenon atoms driven by ultrashort femtosecond laser pulses. The nondipole shift of the electron momentum distribution along the light-propagation direction for high energy electrons beyond the 2U_{p} classical cutoff is found to be vastly different from that below this cutoff, where U_{p} is the ponderomotive potential of the driving laser field. A local minimum structure in the momentum dependence of the nondipole shift above the cutoff is identified for the first time. With the help of classical and quantum-orbit analysis, we show that large-angle rescattering of the electrons strongly alters the partitioning of the photon momentum between electron and ion. The sensitivity of the observed nondipole shift to the electronic structure of the target atom is confirmed by three-dimensional time-dependent Schrödinger equation simulations for different model potentials. Our work paves the way toward understanding the physics of extreme light-matter interactions at long wavelengths and high electron kinetic energies.

How long does it take to emit an electron from an atom? This question has intrigued scientists for decades. As such emission times are in the attosecond regime, the advent of attosecond metrology using ultrashort and intense lasers has re-triggered strong interest on the topic from an experimental standpoint. Here, we present an approach to measure such emission delays, which does not require attosecond light pulses, and works without the presence of superimposed infrared laser fields. We instead extract the emission delay from the interference pattern generated as the emitted photoelectron is diffracted by the parent ion’s potential. Targeting core electrons in CO, we measured a 2d map of photoelectron emission delays in the molecular frame over a wide range of electron energies. The emission times depend drastically on the photoelectrons’ emission directions in the molecular frame and exhibit characteristic changes along the shape resonance of the molecule.

Controlling the interactions between atoms with external fields opened up new branches in physics ranging from strongly correlated atomic systems to ideal Bose1 and Fermi2 gases and Efimov physics3,4. Such control usually prepares samples that are stationary or evolve adiabatically in time. In contrast, in molecular physics, external ultrashort laser fields are used to create anisotropic potentials that launch ultrafast rotational wave packets and align molecules in free space5. Here we combine these two regimes of ultrafast times and low energies. We apply a short laser pulse to the helium dimer, a weakly bound and highly delocalized single bound state quantum system. The laser field locally tunes the interaction between two helium atoms, imparting an angular momentum of 2ℏ and evoking an initially confined dissociative wave packet. We record a video of the density and phase of this wave packet as it propagates from small to large internuclear distances. At large internuclear distances, where the interaction between atoms is negligible, the wave packet is essentially free. This work paves the way for future tomography of wave-packet dynamics and provides the technique for studying exotic and otherwise hardly accessible quantum systems, such as halo and Efimov states. Ultrashort laser fields applied to a helium dimer are able to tune the interactions between two helium atoms. A video of the dimer's response to this localized disturbance shows the effect of dissociation and alignment of the wave packets.

We report on kinematically complete measurements and ab initio nonperturbative calculations of double ionization of He and H_{2} by a single 800 eV circularly polarized photon. We confirm the quasifree mechanism of photoionization for H_{2} and show how it originates from the two-electron cusp in the ground state of a two-electron target. Our approach establishes a method for mapping electrons relative to each other and provides valuable insight into photoionization beyond the electric-dipole approximation.

Photoionization is one of the fundamental light-matter interaction processes in which the absorption of a photon launches the escape of an electron. The time scale of this process poses many open questions. Experiments have found time delays in the attosecond (10-18 seconds) domain between electron ejection from different orbitals, from different electronic bands, or in different directions. Here, we demonstrate that, across a molecular orbital, the electron is not launched at the same time. Rather, the birth time depends on the travel time of the photon across the molecule, which is 247 zeptoseconds (1 zeptosecond = 10-21 seconds) for the average bond length of molecular hydrogen. Using an electron interferometric technique, we resolve this birth time delay between electron emission from the two centers of the hydrogen molecule.

We experimentally investigate the effects of the linear photon momentum on the momentum distributions of photoions and photoelectrons generated in one-photon ionization in an energy range of 300  eV≤E_{γ}≤40  keV. Our results show that for each ionization event the photon momentum is imparted onto the photoion, which is essentially the system's center of mass. Nevertheless, the mean value of the ion momentum distribution along the light propagation direction is backward-directed by -3/5 times the photon momentum. These results experimentally confirm a 90-year-old prediction.

A central motivation for the development of x-ray free-electron lasers has been the prospect of time-resolved single-molecule imaging with atomic resolution. Here, we show that x-ray photoelectron diffraction—where a photoelectron emitted after x-ray absorption illuminates the molecular structure from within—can be used to image the increase of the internuclear distance during the x-ray-induced fragmentation of an O_{2} molecule. By measuring the molecular-frame photoelectron emission patterns for a two-photon sequential K-shell ionization in coincidence with the fragment ions, and by sorting the data as a function of the measured kinetic energy release, we can resolve the elongation of the molecular bond by approximately 1.2 a.u. within the duration of the x-ray pulse. The experiment paves the road toward time-resolved pump-probe photoelectron diffraction imaging at high-repetition-rate x-ray free-electron lasers.

The simplest molecular dimer, H-2-H-2, poses a challenge to both experiment and theory as a system with a multidimensional energy surface that supports only a single weakly bound quantum state. Here, we provide a direct experimental image of the structure of hydrogen dimers [(H-2)(2), H-2-D-2, and (D-2)(2)] obtained via femtosecond laser-induced Coulomb explosion imaging. Our results indicate that hydrogen dimers are not restricted to a particular geometry but rather occur as a mixture of all possible configurations. The measured intermolecular distance distributions were used to deduce the isotropic intermolecular potential as well as the binding energies of the dimers.

We present first results and plans for future neutron activation measurements at the Goethe University Frankfurt. The measurements were performed at the Van-de-Graaff accelerator employing the 7Li(p,n) reaction.

The investigation of the photoelectron circular dichroism (PECD) in the strong field regime (800 nm, 6.9 × 1013 W/cm2) on methyloxirane (MOX) reveals a flip of the sign of PECD between different fragmentation channels. This finding is of great importance for future experiments and applications in chemistry or pharmacy using PECD in the strong field regime as analysis method. We suggest that the observed sign change of PECD is not caused by ionization from different orbitals but by effectively selecting differently oriented nonisotropic subsamples of molecules via the fragmentation channel.

Wave-particle duality is an inherent peculiarity of the quantum world. The double-slit experiment has been frequently used for understanding different aspects of this fundamental concept. The occurrence of interference rests on the lack of which-way information and on the absence of decoherence mechanisms, which could scramble the wave fronts. Here, we report on the observation of two-center interference in the molecular-frame photoelectron momentum distribution upon ionization of the neon dimer by a strong laser field. Post-selection of ions, which are measured in coincidence with electrons, allows choosing the symmetry of the residual ion, leading to observation of both, gerade and ungerade, types of interference.

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.

Microchannel plates (MCP) convert one incoming particle (e.g. electrons, ions and higher energetic photons) into 106 to 107 secondary electrons. The leaving electron cloud’s centre is at the same spot as the impacting particle (in 2D). An anode measures the electron cloud and the particle’s original position can be estimated. Several types of anodes can be used (for a short summary see [1]). The voltage drop at the MCP is used for precise timing information. A resolution better than 150 ps FWHM is possible. State-of-the-art hexagonal delay-line anodes (hex-anode) with a time-to-digital-converter (TDC) as readout can obtain a spatial resolution about 40 μm FWHM [2]. If too many particles hit the detector in a short time window not all particles can be reconstructed with a simple TDC readout (see Figure 1).

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.