The ionization of He by 10-45 keV proton impact was investigated employing recoil-ion momentum spectroscopy. For projectile velocities up to 1 a.u, the electron emission pattern in velocity space shows two intense "banana"-shaped areas similar to the emission pattern previously observed for transfer ionization in collision of He2+ with He. These structures extend from the target to the projectile velocities and exhibit a similar focusing towards either of the two nuclei. With increasing impact velocity, an accumulation of the electron velocity distribution at the nuclei becomes more pronounced. At 45 keV proton impact a target-cusp-like peak is observed. In addition to this peak the emission pattern shows signatures of the quasimolecular 2pZ promotion, which is characteristic for the process of saddle-point ionization.

Electronically excited atoms or molecules may deexcite by emission of a secondary electron through an Auger-Meitner decay. This deexcitation process is typically considered to be exponential in time. This is strictly speaking, however, only true for the case of an atom. Here, we present a study experimentally demonstrating the non-exponential time dependence of the decay of an inner-shell hole in a diatomic molecule. In addition, we provide an intuitive explanation for the origin of the observed variation of the molecular lifetimes and their dependence on the kinetic energy of the ionic fragments measured in coincidence with the photoelectrons.

We report a fully differential study of ionization of the Ne L-shell by Compton scattering of 20 keV photons. We find two physical mechanisms which modify the Compton-electron emission. Firstly, we observe scattering of the Compton electrons at their parent nucleus. Secondly, we find a distinct maximum in the electron momentum distribution close-to-zero momentum which we attribute to a focusing of the electrons by the Coulomb potential.

We studied strong -field multiphoton ionization of 1-iodo-2-methylbutane enantiomers with 395 nm circularly polarized laser pulses experimentally and theoretically. For randomly oriented molecules, we observe spin polarization up to about 15%, which is independent of the molecular enantiomer. Our experimental findings are explained theoretically as an intricate interplay between three contributions from HOMO, HOMO -1, and HOMO -2, which are formed of 5p -electrons of the iodine atom. For uniaxially oriented molecules, our theory demonstrates even larger spin polarization. Moreover, we predict a sizable enantiosensitive photoelectron circular dichroism of about 10%, which is different for different spin states of photoelectrons.

We experimentally study the influence of the binding energy on nondipole effects in K-shell single-photon ionization of atoms at high photon energies. We find that for each ionization event, as expected by momentum conservation, the photon momentum is transferred almost fully to the recoiling ion. The momentum distribution of the electrons becomes asymmetrically deformed along the photon propagation direction with a mean value of 8/(5c)(E_{γ}-I_{P}) confirming an almost 100 year old prediction by Sommerfeld and Schur [Ann. Phys. (N.Y.) 396, 409 (1930)10.1002/andp.19303960402]. The emission direction of the photoions results from competition between the forward-directed photon momentum and the backward-directed recoil imparted by the photoelectron. Which of the two counteracting effects prevails depends on the binding energy of the emitted electron. As an example, we show that at 20 keV photon energy, Ne^{+} and Ar^{+} photoions are pushed backward towards the radiation source, while Kr^{+} photoions are emitted forward along the light propagation direction.

We report on strong field ionization of S- and R-propylene oxide in circularly polarized two-color laser fields. We find that the relative helicity of the two single color laser fields affects the photoelectron circular dichroism (PECD). Further, we observe that PECD is modulated as a function of the sub-cycle release time of the electron. Our experimental observations are successfully described by a heuristic model based on electrons in chiral initial states, which are selectively liberated by the laser field and, after tunneling, interact with an achiral Coulomb potential.

We study strong-field ionization of molecular hydrogen in highly intense corotating two-color laser fields. The measured electron momentum distributions show alternating half rings (AHRs) that are characteristic of subcycle interference. We report on the role of the two-color field ratio for the visibility of this subcycle interference. The ratio of the peak electric field at 780 nm compared to the peak electric field at 390 nm E780/E390 is varied from 0.037 to 0.18. We find very good agreement with the results from our semiclassical simulation. We conclude that the AHR pattern is visible if two conditions are fulfilled. First, the amplitudes of the two pathways that lead to the subcycle interference have to be similar, which is the case for low two-color field ratios E780/E390. Second, the phase difference of the two pathways must be strong enough to allow for destructive interference, which is the case for high two-color field ratios E780/E390. For typical experimental conditions, we find that two-color field ratios E780/E390 in the range from 0.037 to 0.12 lead to good visibility of the AHR pattern. This guides future experiments to measure the Wigner time delay using holographic angular streaking of electrons.

We present experimental data on the strong-field ionization of the argon dimer in a co-rotating two-color laser field. We observe a subcycle interference pattern in the photoelectron momentum distribution and infer the Wigner time delay using holographic angular streaking of electrons. We find that the Wigner time delay varies by more than 400 attoseconds as a function of the electron emission direction with respect to the molecular axis. The measured time delay is found to be independent of the parity of the dimercation and is in good agreement with our theoretical model based on the ionization of an aligned atomic p orbital.

We report on the strong field ionization of H2 by a corotating two-color laser field. We measure the electron momentum distribution in coincidence with the kinetic energy release (KER) of dissociating hydrogen molecules. In addition to a characteristic half-moon structure, we observe a low-energy structure in the electron momentum distribution at a KER of about 3.5 eV. We speculate that the outgoing electron interacts with the molecular ion, despite the absence of classical recollisions under these conditions. Time-dependent density functional theory simulations support our conclusions.

We present experimental data on the strong field tunnel ionization of argon in a counter-rotating two-color (CRTC) laser field. We find that the initial momentum component along the tunneling direction changes sign comparing the rising and the falling edge of the CRTC field. If the initial momentum at the tunnel exit points in the direction of the ion at the instant of tunneling, this manifests as an enhanced Coulomb interaction of the outgoing electron with its parent ion. Our conclusions are in accordance with predictions based on strong field approximation.

The center of the momentum sphere of O 1s electrons ionized from trifluoromethyloxirane varied between left-circularly and right-circularly polarized light for the particular energies of 3.2, 4.2, and 5.2 eV for unknown reasons.Due to the applied analysis methodology of subtracting the respective data sets, this has erroneously led to significant deviations regarding the determined photoelectron circular dichroism (PECD) at these energies as shown in Fig. 1.The revised PECD values are presented and discussed in light of a recent study on the same target [1].The findings regarding the emitter-site selectivity of inner-shell PECD as the main subject of this paper remain unaltered by this Erratum.We gratefully acknowledge K. Fehre from the University of

During the last decade, X-ray free-electron lasers (XFELs) have enabled the study of light-matter interaction under extreme conditions. Atoms which are subject to XFEL radiation are charged by a complex interplay of (several subsequent) photoionization events and electronic decay processes within a few femtoseconds. The interaction with molecules is even more intriguing, since intricate nuclear dynamics occur as the molecules start to dissociate during the charge-up process. Here, we demonstrate that by analyzing photoelectron angular emission distributions and kinetic energy release of charge states of ionic molecular fragments, we can obtain a detailed understanding of the charge-up and fragmentation dynamics. Our novel approach allows for gathering such information without the need of complex ab initio modeling. As an example, we provide a detailed view on the processes happening on a femtosecond time scale in oxygen molecules exposed to intense XFEL pulses.

We present the momentum distributions of the nucleus and of the electrons from double ionization of the helium atom by Compton scattering of photons with hν=40  keV. We find that the doubly charged ion momentum distribution is very close to the Compton profile of the nucleus in the ground state of the helium atom, and the momentum distribution of the singly charged ion to give a precise image of the electron Compton profile. To reproduce these results, nonrelativistic calculations require the use of highly correlated initial- and final-state wave functions.

We investigate the differential ionization probability of chiral molecules in the strong-field regime as a function of the helicity of the incident light. To this end, we analyze the fourfold ionization of bromochlorofluoromethane (CHBrClF) with subsequent fragmentation into four charged fragments and different dissociation channels of the singly ionized methyloxirane. By resolving for the molecular orientation, we show that the photoion circular dichroism signal strength is increased by 2 orders of magnitude.

AbstractTo explore ultra-fast dynamics in quantum systems one needs detection schemes which allow time measurements in the attosecond regime. During the recent decades, the pump & probe two-pulse laser technique has provided milestone results on ultra-fast dynamics with femto- and attosecond time resolution. Today this technique is applied in many laboratories around the globe, since complete pump & probe systems are commercially available. It is, however, less known or even forgotten that ultra-fast dynamics has been investigated several decades earlier even with zeptosecond resolution in ion-atom collision processes. A few of such historic experiments, are presented here, where the particle motion (due to its very fast velocity) was used as chronometer to determine ultra-short time delays in quantum reaction processes. Finally, an outlook is given when in near future relativistic heavy ion beams are available which allow a novel kind of “pump & probe” experiments on molecular systems with a few zeptosecond resolution. However, such experiments are only feasible if the complete many-particle fragmentation process can be imaged with high momentum resolution by state-of-the-art multi-particle coincidence technique.

When a very strong light field is applied to a molecule an electron can be ejected by tunneling. In order to quantify the time-resolved dynamics of this ionization process, the concept of the Wigner time delay can be used. The properties of this process can depend on the tunneling direction relative to the molecular axis. Here, we show experimental and theoretical data on the Wigner time delay for tunnel ionization of H2 molecules and demonstrate its dependence on the emission direction of the electron with respect to the molecular axis. We find, that the observed changes in the Wigner time delay can be quantitatively explained by elongated/shortened travel paths of the emitted electrons, which occur due to spatial shifts of the electrons' birth positions after tunneling. Our work provides therefore an intuitive perspective towards the Wigner time delay in strong-field ionization.

K. Fehre, G. Nalin, N. M. Novikovskiy, S. Grundmann, G. Kastirke, S. Eckart, F. Trinter, J. Rist, A. Hartung, D. Trabert, Ch. Janke, M. Pitzer, S. Zeller, F. Wiegandt, M. Weller, M. Kircher, M. Hofmann, L. Ph. H. Schmidt, A. Knie, A. Hans, L. Ben Ltaief, A. Ehresmann, R. Berger, H. Fukuzawa, K. Ueda, H. Schmidt-Böcking, J. B. Williams, T. Jahnke, R. Dörner, Ph. V. Demekhin, and M. S. Schöffler Institut für Kernphysik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, 60438 Frankfurt am Main, Germany Institut für Physik und CINSaT, Universität Kassel, Heinrich-Plett-Straße 40, 34132 Kassel, Germany Institute of Physics, Southern Federal University, 344090 Rostov-on-Don, Russia 4 Molecular Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany Department of Chemistry, Philipps-Universität Marburg, Hans-Meerwein-Straße 4, 35032 Marburg, Germany Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Department of Physics, University of Nevada, Reno, Nevada 89557, United States

We report on a joint experimental and theoretical study of photoelectron circular dichroism (PECD) in methyloxirane. By detecting O 1s photoelectrons in coincidence with fragment ions, we deduce the molecule's orientation and photoelectron emission direction in the laboratory frame. Thereby, we retrieve a fourfold differential PECD clearly beyond 50%. This strong chiral asymmetry is reproduced by ab initio electronic structure calculations. Providing such a pronounced contrast makes PECD of fixed-in-space chiral molecules an even more sensitive tool for chiral recognition in the gas phase.

Strong-field ionization of atoms by circularly polarized femtosecond laser pulses produces a donut-shaped electron momentum distribution. Within the dipole approximation this distribution is symmetric with respect to the polarization plane. The magnetic component of the light field is known to shift this distribution forward. Here, we show that this magnetic non-dipole effect is not the only non-dipole effect in strong-field ionization. We find that an electric non-dipole effect arises that is due to the position dependence of the electric field and which can be understood in analogy to the Doppler effect. This electric non-dipole effect manifests as an increase of the radius of the donut-shaped photoelectron momentum distribution for forward-directed momenta and as a decrease of this radius for backwards-directed electrons. We present experimental data showing this fingerprint of the electric non-dipole effect and compare our findings with a classical model and quantum calculations.

We present experimental data on the nonadiabatic strong field ionization of atomic hydrogen using elliptically polarized femtosecond laser pulses at a central wavelength of 390 nm. Our measured results are in very good agreement with a numerical solution of the time-dependent Schrödinger equation (TDSE). Experiment and TDSE show four above-threshold ionization peaks in the electron's energy spectrum. The most probable emission angle (also known as "attoclock offset angle" or "streaking angle") is found to increase with energy, a trend that is opposite to standard predictions based on Coulomb interaction with the ion. We show that this increase of deflection angle can be explained by a model that includes nonadiabatic corrections of the initial momentum distribution at the tunnel exit and nonadiabatic corrections of the tunnel exit position itself.