No abstract

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.

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 report on the non-adiabatic offset of the initial electron momentum distribution in the plane of polarization upon single ionization of argon by strong field tunneling and show how to experimentally control the degree of non-adiabaticity. Two-color counter-and co-rotating fields (390 and 780 nm) are compared to show that the non-adiabatic offset strongly depends on the temporal evolution of the laser electric field. We introduce a simple method for the direct access to the non-adiabatic offset using two-color counter-and co-rotating fields. Further, for a single-color circularly polarized field at 780 nm we show that the radius of the experimentally observed donut-like distribution increases for increasing momentum in the light propagation direction. Our observed initial momentum offsets are well reproduced by the strong-field approximation (SFA). A mechanistic picture is introduced that links the measured non-adiabatic offset to the magnetic quantum number of virtually populated intermediate states.

A central motivation for the development of x-ray free-electron lasers has been the prospect of timeresolved 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 timeresolved pump-probe photoelectron diffraction imaging at high-repetition-rate x-ray free-electron lasers.

We report on three-dimensional (3D) electron momentum distributions from single ionization of helium by a laser pulse consisting of two counterrotating circularly polarized fields (390 nm and 780 nm). A pronounced 3D low energy structure and sub-cycle interferences are observed experimentally and reproduced numerically using a trajectory based semi-classical simulation. The orientation of the low energy structure in the polarization plane is verified by numerical simulations solving the time dependent Schrödinger equation.

We report on a kinematically complete experiment on strong field double ionization of helium using laser pulses with a wavelength of 394 nm and intensities of 3.5 − 5.7 × 10 14 W/cm 2 . Our experiment reaches the most complete level of detail which previously has only been reached for single photon double ionization. We give an overview over the observables on many levels of integration, starting from the ratio of double to single ionization, the individual electron and ion momentum distributions over joint momentum and energy distributions to fully differential cross sections showing the correlated angular momentum distributions. Within the studied intensity range the ratio of double to single ionization changes from 2 × 10 −4 to 1.5 × 10 −3 . We find the momentum distributions of the He 2+ ions and the correlated two electron momentum distributions to vary substantially. Only at the highest intensity both electrons are emitted to the same direction while at the lowest intensity back-to-back emission dominates. The joint energy distribution of the electrons shows discrete structures from the energy quantization of the photon field which allows us to count the number of absorbed photons and thus access the parity of the final state. We find the energy of the individual electron to show a peak structure indicating a quantized sharing of the overall energy absorbed from the field. The joint angular momentum distributions of the two electrons show a highly directed emission of both electrons along the polarization axis as well as clear imprints of electron repulsion. They strongly change with the energy sharing between the electrons. The aspect of selection rules in double ionization which are also visible in the presented dataset has been subject to a preceding publication [1]. arXiv:1808.03516v1 [physics.atom-ph]

The spin polarization of electrons from multiphoton ionization of Xe by 395 nm circularly polarized laser pulses at 6 · 10 13 W/cm 2 has been measured. At this photon energy of 3.14 eV the above threshold ionization peaks connected to Xe + ions in the ground state (J = 3/2, ionization potential Ip = 12.1 eV) and the first exicted state (J = 1/2, Ip = 13.4 eV) are clearly separated in the electron energy distribution. These two combs of ATI peaks show opposite spin polarizations. The magnitude of the spin polarization is a factor of two higher for the J = 1/2 than for the J = 3/2 final ionic state. In turn the data show that the ionization probability is strongly dependent on the sign of the magnetic quantum number.Light-driven ionization processes are sensitive to the spin of the electron. Surprisingly, this important and fundamental fact of light matter interaction is experimentally well validated only for the special cases of single photon and resonant enhanced two and three photon processes [1]. For strong-field ionization it rests on only one single experiment [2], which did not even resolve the quantum state from which the electron was ejected.The role of the spin in single photon ionization was adressed soon after the discovery of the electron spin [3]. Starting in the 1960s, it became clear that spin selectivity of single photon ionization of atoms and molecules is very general. Today it is well studied experimentally and theoretically (see [4] for a review). The generalization to the multiphoton regime was achieved first in pioneering theoretical work by Lambropoulus [5]. Recently Barth and Smirnova [6] predicted a high degree of spin polarization for strong-field ionization by circularly polarized femtosecond pulses. The proposed mechanism giving rise to spin sensitivity of strong-field ionization consists of two independent steps. The primary effect is that the nonadiabatic tunneling probability through a rotating barrier depends on the sign of the magnetic quantum number m l of the orbital. This finding was confirmed experimentally [7] and by solving the time-dependent Schrödinger equation [8] without invoking the concept of tunneling explicitly. Together with the strong binding energy dependence of strong-field ionization the m l dependence then leads to a spin selectivity. Because, due to the spinorbit interaction, the binding energy differs for parallel or antiparallel orientation of the spin with respect to the projection of the orbital angular momentum m l on the quantization axis.The purpose of the present paper is to experimentally show this theoretically suggested connection of spin, magnetic quantum number and binding energy in strongfield ionization. This relies on experimentally determining both: the ionization potential and the spin polarization of the electron. For xenon this is possible as illus-Schematics of multiphoton ionization in xenon by 395 nm (hν = 3.14 eV) laser pulses. The ground state ( 2 P , J = 3/2) and first excited state ( 2 P , J = 1/2) of Xe + differ by ∆Ip = 1.3 eV in i...