arXiv subjects

M. Kunitski

Publications and source records attributed to M. Kunitski.

At least 19 recordsLinked to original sources

arXiv2025

Chiral Electron Momentum Distribution upon Strong-Field Ionization of Atoms

We present a scheme to synthesize a three-dimensional laser field that produces a chiral electron momentum distribution upon strong-field ionization of atoms. Our approach employs two orthogonally propagating two-color laser beams. This results in a time-dependent three-dimensional electric field vector of the combined light field which varies for different positions within the focal volume. For each position, we conduct a simulation of the corresponding electron momentum distribution that includes non-adiabatic dynamics and Coulomb interaction after tunneling. For suitable laser parameters, only a small region of the focal volume contributes to the final momentum distribution. Thus, integrating over all position coordinates, a specific chiral laser field dominates. This leads to a volume-averaged electron momentum distribution, which is chiral, as well. This work will serve as a benchmark for future strong-field experiments aiming at the synthetization of well-defined, three-dimensional laser fields.

physics.atom-ph↗

arXiv2025

Modeling the laser-pulse induced helium trimer dynamics

Motivated by ongoing pump-probe spectroscopy experiments, this work develops a theoretical framework for describing the rovibrational wave packet dynamics that ensues when a single weakly-bound van der Waals trimer is exposed to a short, sub-picosecond linearly polarized pump laser pulse. The intensity I of the pump laser is chosen such that excitation and ionization of the electronic degrees of freedom are negligible while excitation of the wavepacket in the nuclear degrees of freedom is non-negligible. The numerical treatment, which takes advantage of the fact that the laser pulse is very short compared to typical molecular time scales, is based on a wave packet decomposition that utilizes hyperspherical coordinates. The framework is applied to the extremely floppy bosonic helium trimer. A convergence analysis of the partial wave decomposition is conducted. The kinetic energy release and orientation dynamics are presented. While the dynamics of more strongly-bound van der Waals trimers such as, e.g., the argon trimer display negligible coupling between vibrational and rotational degrees of freedom, rendering a description within a rigid-body picture appropriate, those of weakly-bound trimers display non-negligible coupling between vibrational and rotational degrees of freedom, rendering a description within a rigid-body picture inappropriate. It is shown that a model that constructs the helium trimer dynamics from the dynamics of the helium dimer captures a number of key characteristics of the alignment signal, including the interference between different angular momentum wave packet components.

quant-ph↗

arXiv2024

Ro-vibrational Dynamics of the Neon Dimer

Short intense laser pulses are routinely used to induce rotational wave packet dynamics of molecules. Ro-vibrational wave packet dynamics has been explored comparatively infrequently, focusing predominantly on extremely light and rigid molecules such as H$_2^+$, H$_2$, and D$_2$. This work presents quantum mechanical calculations that account for the rotational {\em{and}} the vibrational degrees of freedom for a heavier and rather floppy diatomic molecule, namely the neon dimer. For pumping by a strong and short non-resonant pump pulse, we identify several phenomena that depend critically on the vibrational (i.e., radial) degree of freedom. Our calculations show (i) fingerprints of the radial dynamics in the alignment signal; (ii) laser-kick induced dissociative dynamics on very short time scales (ejection of highly structured "jets"); and (iii) tunneling dynamics that signifies the existence of resonance states, which are supported by the effective potential curves for selected finite relative angular momenta. Our theory predictions can be explored by existing state-of-the-art experiments.

cond-mat.quant-gas↗

arXiv2024

Sub-cycle resolved strong field ionization of chiral molecules and the origin of chiral photoelectron asymmetries

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.

physics.atom-ph↗

arXiv2023

Ideal Two-Color Field Ratio for Holographic Angular Streaking of Electrons

We study strong field ionization of molecular hydrogen in highly intense co-rotating two-color (CoRTC) laser fields. The measured electron momentum distributions show alternating half-rings (AHR) that are characteristic for sub-cycle interference. We report on the role of the two-color field ratio for the visibility of this sub-cycle interference. The ratio of the peak electric field at 780 nm compared to the peak electric field at 390 nm $E_{780}/E_{390}$ 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 sub-cycle interference have to be similar, which is the case for low two-color field ratios $E_{780}/E_{390}$. 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 $E_{780}/E_{390}$. For typical experimental conditions, we find that two-color field ratios $E_{780}/E_{390}$ 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 (HASE).

physics.atom-ph↗

arXiv2023

Angular dependence of the Wigner time delay upon strong field ionization from an aligned p-orbital

We present experimental data on the strong-field ionization of the argon dimer in a co-rotating two-color (CoRTC) laser field. We observe a sub-cycle interference pattern in the photoelectron momentum distribution and infer the Wigner time delay using holographic angular streaking of electrons (HASE). 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 dimer-cation and is in good agreement with our theoretical model based on the ionization of an aligned atomic p-orbital.

physics.atom-ph↗

arXiv2022

Experimental fingerprint of the electron's longitudinal momentum at the tunnel exit in strong field ionization

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.

physics.atom-ph↗

arXiv2021

Ultrafast preparation and strong-field ionization of an atomic Bell-like state

Molecules are many body systems with a substantial amount of entanglement between their electrons. Is there a way to break the molecular bond of a diatomic molecule and obtain two atoms in their ground state which are still entangled and form a Bell-like state? We present a scheme that allows for the preparation of such entangled atomic states from single oxygen molecules on femtosecond time scales. The two neutral oxygen atoms are entangled in the magnetic quantum number of their valence electrons. In a time-delayed probe step, we employ non-adiabatic tunnel ionization, which is a magnetic quantum number-sensitive mechanism. We then investigate correlations by comparing single and double ionization probabilities of the Bell-like state. The experimental results agree with the predictions for an entangled state.

physics.atom-ph↗

arXiv2021

Electric Nondipole Effect in Strong-Field Ionization

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.

physics.atom-ph↗

arXiv2020

Strong Differential Photoion Circular Dichroism in Strong-Field Ionization of Chiral Molecules

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. We observe a variation of the differential ionization probability in a range of several percent. Accordingly, we conclude that the helicity of light is a quantity that should be considered for the theoretical description of the strong field ionization rate of chiral molecules.

physics.atom-ph↗

arXiv2020

Sideband Modulation by Sub-Cycle Interference

We experimentally and theoretically show that the electron energy spectra strongly depend on the relative helicity in highly intense, circularly polarized two-color laser fields which is an unexpected finding. The employed counter-rotating two-color (CRTC) fields and the co-rotating two-color (CoRTC) fields are both a superposition of circularly polarized laser pulses at a central wavelength of 390 nm and 780 nm (intensitiy ratio $I_{390}/I_{780}\approx 250$). For the CRTC field, the measured electron energy spectrum is dominated by peaks that are spaced by 3.18 eV (corresponds to the photon energy of light at a wavelength of 390 nm). For the CoRTC field, we observe additional energy peaks (sidebands). Using our semi-classical, trajectory-based models, we conclude that the sideband intensity is modulated by a sub-cycle interference, which sensitively depends on the relative helicity in circularly polarized two-color fields.

physics.atom-ph↗

arXiv2019

Magnetic fields alter tunneling in strong-field ionization

When a strong laser pulse induces the ionization of an atom, momentum conservation dictates that the absorbed photons transfer their momentum $p_γ=E_γ/c$ to the electron and its parent ion. Even after 30 years of studying strong-field ionization, the sharing of the photon momentum between the two particles and its underlying mechanism are still under debate in theory. Corresponding experiments are very challenging due to the extremely small photon momentum ($~10^{-4}$ a.u.) and their precision has been too limited, so far, to ultimately resolve the debate. Here, by utilizing a novel experimental approach of two counter-propagating laser pulses, we present a detailed study on the effects of the photon momentum in strong-field ionization. The high precision and self-referencing of the method allows to unambiguously demonstrate the action of the light's magnetic field on the electron while it is under the tunnel barrier, confirming theoretical predictions, disproving others. Our results deepen the understanding of, for example, molecular imaging and time-resolved photoelectron holography.

physics.atom-ph↗

arXiv2018

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

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.

physics.atom-ph↗

arXiv2018

Multiphoton Double Ionization of Helium at 394nm - a Fully Differential Experiment

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 $3.5-5.7\times10^{14}\,W/cm^2$. 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 $2\times 10^{-4}$ to $1.5\times 10^{-3}$. We find the momentum distributions of the $\rm{He}^{2+}$ 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].

physics.atom-ph↗

arXiv2018

Timing Recollision in Nonsequential Double Ionization by Intense Elliptically Polarized Laser Pulses

We examine correlated electron and doubly charged ion momentum spectra from strong field double ionization of Neon employing intense elliptically polarized laser pulses. An ellipticity-dependent asymmetry of correlated electron and ion momentum distributions has been observed. Using a 3D semiclassical model, we demonstrate that our observations reflect the sub-cycle dynamics of the recollision process. Our work reveals a general physical picture for recollision-impact double ionization with elliptical polarization, and demonstrates the possibility of ultrafast control of the recollision dynamics.

physics.atom-ph↗

arXiv2018

Sub-Cycle Interference upon Tunnel-Ionization by Counterrotating Two-Color Fields

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.

physics.atom-ph↗

arXiv2018

Absolute ion detection efficiencies of microchannel plates and funnel microchannel plates for multi-coincidence detection

Modern momentum imaging techniques allow for the investigation of complex molecules in the gas phase by detection of several fragment ions in coincidence. For these studies, it is of great importance that the single-particle detection efficiency e is as high as possible, as the overall efficiency scales with e over n, i.e. the power of the number of detected particles. Here we present measured absolute detection efficiencies for protons of several micro-channel plates (MCPs), including efficiency enhanced "funnel MCPs". Furthermore, the relative detection efficiency for two-, three-, four-, and five-body fragmentation of CHBrClF has been examined. The "funnel" MCPs exhibit an efficiency of approx. 90 percent, gaining a factor of 24 (as compared to "normal" MCPs) in case of a five-fold ion coincidence detection.

physics.ins-det↗

arXiv2018

Determination of the He-He, Ne-Ne, Ar-Ar, and H$_{2}$ interaction potential by wave function imaging

We report on a direct method to measure the internuclear potential energy curve of diatomic systems. A COLTRIMS reaction microscope was used to measure the squares of the vibrational wave functions of H$_{2}$, He$_{2}$, Ne$_{2}$, and Ar$_{2}$. The Schrödinger equation relates the curvature of the wave function to the potential V(R) and therefore offers a simple but elegant way to extract the shape of the potential.

physics.atom-ph↗