K. Fehre

Goethe University Frankfurt

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Trends

Author Order

Document Type

Co-Authors

R. Dörner

Goethe University Frankfurt

M. S. Schöffler

Goethe University Frankfurt

T. Jahnke

European X-Ray Free-Electron Laser

D. Trabert

Goethe University Frankfurt

J. Rist

Goethe University Frankfurt

S. Eckart

Goethe University Frankfurt

Florian Trinter

Fritz Haber Institute of the Max Planck Society

M. Kunitski

Goethe University Frankfurt

Alexander Hartung

Goethe University Frankfurt

Sven Grundmann

Goethe University Frankfurt

Cooperative Institutions

Goethe University Frankfurt

European X-Ray Free-Electron Laser

Max Planck Institute for Nuclear Physics

University of Kassel

Deutsches Elektronen-Synchrotron DESY

Center for Free-Electron Laser Science

Universität Hamburg

Centre National de la Recherche Scientifique

Lawrence Berkeley National Laboratory

Kansas State University

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Field

High-energy molecular-frame photoelectron angular distributions: a molecular bond-length ruler

Physical Chemistry Chemical Physics (2023)

Isabel Vela-Peréz Fukiko Ota A. Mhamdi Yoshiaki Tamura J. Rist

We present a study on molecular-frame photoelectron angular distributions (MFPADs) of small molecules using circularly polarized synchrotron light. We find that the main forward-scattering peaks of the MFPADs are slightly tilted with respect to the molecular axis. This tilt angle is directly connected to the molecular bond length by a simple, universal formula. We apply the derived formula to several examples of MFPADs of C 1s and O 1s photoelectrons of CO, which have been measured experimentally or obtained by means of ab initio modeling. In addition, we discuss the influence of the back-scattering contribution that is superimposed over the analyzed forward-scattering peak in the case of homo-nuclear diatomic molecules such as N2.

Diatomic molecule

Photoelectric effect

Molecular geometry

10.1039/d2cp05942h

Cite

Citations (7)

Direct Experimental Access to the Nonadiabatic Initial Momentum Offset upon Tunnel Ionization

Physical Review Letters (2018)

S. Eckart K. Fehre Nicolas Eicke Alexander Hartung J. Rist

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.

Momentum (technical analysis)

10.1103/physrevlett.121.163202

Cite

Citations (66)

Revealing the two-electron cusp in the ground states of He and H2 via quasifree double photoionization

Physical Review Research (2020)

Sven Grundmann Vladislav V. Serov Florian Trinter K. Fehre N. Strenger

This work studies quasifree double photoionization using reaction microscopy and companion $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations. The quasifree mechanism is confirmed for H${}_{2}$ and introduced as a probe for the electron-electron cusp density in the ground state of a two-electron target.

10.1103/physrevresearch.2.033080

Cite

Citations (8)

Photoelectron energy peaks shift against the radiation pressure in strong-field ionization

Science Advances (2022)

Kang Lin S. Eckart Alexander Hartung D. Trabert K. Fehre

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.

10.1126/sciadv.abn7386

Cite

Citations (19)

Observation of Nondipole-Induced Asymmetry in the Angular Emission Distribution of Photoelectrons from Fixed-in-Space CO Molecules

Physical Review Letters (2022)

Dmitrii V Rezvan Kim Klyssek Sven Grundmann A. Pier Nikolay M. Novikovskiy

We investigate experimentally and theoretically the C and O 1s photoionization of fixed-in-space CO molecules at a photon energy of 905 eV. We find a significant dependence of the photoelectron angular distributions on the direction of propagation of the ionizing radiation. It results from an interplay of nondipole effects, on one hand, and molecular effects, on the other. The nondipole effects lead to an increase of the emission probability in the forward direction along the light propagation, and the photoelectron wave being scattered by the molecular potential gives rise to a strong peak in the direction of the atom neighboring the emitter site. These effects can either conspire or extenuate each other, depending on the photoelectron emission direction and molecular orientation in space.

Photoelectric effect

Photon energy

10.1103/physrevlett.129.253201

Cite

Citations (4)

Measuring the photoelectron emission delay in the molecular frame

Nature Communications (2021)

J. Rist Kim Klyssek Nikolay M. Novikovskiy M. Kircher Isabel Vela-Peréz

If matter absorbs a photon of sufficient energy it emits an electron. The question of the duration of the emission process has intrigued scientists for decades. With the advent of attosecond metrology, experiments addressing such ultrashort intervals became possible. While these types of studies require attosecond experimental precision, we present here a novel measurement approach that avoids those experimental difficulties. 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 measured emission times depend drastically on the emission direction and exhibit characteristic changes along the shape resonance of the molecule. Our approach can be routinely extended to other electron orbitals and more complex molecules.

Attosecond

Photon energy

10.1038/s41467-021-26994-2

Cite

Citations (35)

Chiral photoelectron angular distributions from ionization of achiral atomic and molecular species

Physical Review Research (2020)

A. Pier K. Fehre Sven Grundmann Isabel Vela-Peréz N. Strenger

We show that the combination of two achiral components - atomic or molecular target plus a circularly polarized photon - can yield chirally structured photoelectron angular distributions. For photoionization of CO, the angular distribution of carbon K-shell photoelectrons is chiral when the molecular axis is neither perpendicular nor (anti-)parallel to the light propagation axis. In photo-double-ionization of He, the distribution of one electron is chiral, if the other electron is oriented like the molecular axis in the former case and if the electrons are distinguishable by their energy. In both scenarios, the circularly polarized photon defines a plane with a sense of rotation and an additional axis is defined by the CO molecule or one electron. This is sufficient to establish an unambiguous coordinate frame of well-defined handedness. To produce a chirally structured electron angular distribution, such a coordinate frame is necessary, but not sufficient. We show that additional electron-electron interaction or scattering processes are needed to create the chiral angular distribution.

Photoelectric effect

Photon energy

10.1103/physrevresearch.2.033209

Cite

Citations (5)

A new route for enantio-sensitive structure determination by photoelectron scattering on molecules in the gas phase

Physical Chemistry Chemical Physics (2022)

K. Fehre Nikolay M. Novikovskiy Sven Grundmann Gregor Kastirke S. Eckart

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.

10.1039/d2cp03090j

Cite

Citations (5)

Fourfold Differential Photoelectron Circular Dichroism

Physical Review Letters (2021)

K. Fehre Nikolay M. Novikovskiy Sven Grundmann Gregor Kastirke S. Eckart

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.

Photoelectric effect

Dichroism

10.1103/physrevlett.127.103201

Cite

Citations (31)

Sidebands in Electron Energy upon Multi-Photon Ionization Depend on the Relative Helicity in ($\omega$, 2$\omega$) Fields

arXiv (Cornell University) (2020)

S. Eckart D. Trabert K. Fehre A. Geyer J. Rist

We experimentally and theoretically compare multi-photon ionization by counter-rotating two-color (CRTC) laser fields and co-rotating two-color (CoRTC) laser fields. The CRTC and the CoRTC fields are both a superposition of circularly polarized laser pulses at a central wavelength of 390 nm and 780 nm (intensitiy ratio $I_{390}/I_{780}\approx 250$). Unexpectedly, we find that the measured electron energy spectra strongly depend on the relative helicity in circularly polarized two-color laser fields. For the CRTC field, the measured electron energy spectrum is dominated by peaks that are spaced by $3.18$ eV (corresponds to light at 390 nm). For the CoRTC field, we observe additional energy peaks (sidebands). Comparison of the experimental results with our semi-classical, trajectory-based model shows excellent agreement. We conclude that the sideband visibility is modulated by a sub-cycle interference, which sensitively depends on the relative helicity in circularly polarized two-color fields.

Helicity

Photon energy

Source

Cite

Citations (0)