arXiv subjects

K. Bartschat

Publications and source records attributed to K. Bartschat.

At least 19 recordsLinked to original sources

arXiv2025

Extracting RABBITT-like phase information from time-dependent transient absorption spectra

We explore how the spectral phase of atto\-second pulse trains influences the optical cross section in transient absorption (TA) spectroscopy. The interaction of extreme ultraviolet (XUV) and time-delayed near-infrared (NIR) fields with an atomic or molecular system governs the dynamics. As already shown in RABBITT experiments, the spectral phase of the XUV pulses can be extracted from the photoionization spectrum as a function of the time delay. Similarly, this XUV phase imprints itself on delay-dependent optical cross-section oscillations. With a perturbative analytical approach and by simulating the quantum dynamics both in a few-level model and via solving the time-dependent Schr\"odinger equation for atomic hydrogen, we reveal the similarity between the spectral phase in RABBITT and TA spectroscopy.

physics.atom-ph↗

arXiv2024

Role of intermediate resonances in attosecond photoelectron interferometry in neon

Attosecond photoelectron interferometry based on the combination of an attosecond pulse train and a synchronized infrared field is a fundamental technique for the temporal characterization of attosecond waveforms and for the investigation of electron dynamics in the photoionization process. In this approach, the comb of extreme ultraviolet harmonics typically lies above the ionization threshold of the target under investigation, thus releasing a photoelectron by single-photon absorption. The interaction of the outgoing photoelectron with the infrared pulse results in the absorption or emission of infrared photons, thereby creating additional peaks in the photoelectron spectrum, referred to as sidebands. While, in the absence of resonances in the first ionization step, the phases imparted on the photoionization process evolve smoothly with the photon energy, the presence of intermediate resonances imprints a large additional phase on the outgoing photoelectron wave packet. In this work, using a comb of harmonics below and above the ionization threshold of neon, we investigate the effect of intermediate bound excited states on attosecond photoelectron interferometry. We show that the phase of the oscillations of the sidebands and their angular distributions are strongly affected by such resonances. By slightly tuning the photon energies of the extreme ultraviolet harmonics, we show how the contributions of selected resonances can be enhanced or suppressed.

physics.atom-ph↗

arXiv2024

R-matrix with time-dependence calculations for three-sideband RABBITT in helium

Following up on a recent paper [Bharti et al., Phys. Rev. A 109 (2024) 023110], we compare the predictions from severalR-matrix with time-dependence calculations for a modified three-sideband version of the "reconstruction of attosecond beating by interference of two-photon transitions" (RABBITT) configuration applied to helium. Except for the special case of the threshold sideband, which appears to be very sensitive to the details of coupling to the bound Rydberg states, increasing the number of coupled states in the close-coupling expansion used to describe the ejected-electron--residual-ion interaction hardly changes the results. Consequently, the remaining discrepancies between the experimental data and the theoretical predictions are likely due to uncertainties in the experimental parameters, particularly the detailed knowledge of the laser pulse.

physics.atom-ph↗

arXiv2023

High-Order Harmonic Generation in Helium: A Comparison Study

We report a detailed study of high-order harmonic generation (HHG) in helium. When comparing predictions from a single-active-electron model with those from all-electron simulations, such as ATTOMESA and R-matrix with time-dependence, which can include different numbers of states in the close-coupling expansion, it seems imperative to generate absolute numbers for the HHG spectrum in a well-defined framework. While qualitative agreement in the overall frequency dependence of the spectrum, including the cut-off frequency predicted by a semi-classical model, can be achieved by many models in arbitrary units, only absolute numbers can be used for benchmark comparisons between different approaches.

physics.atom-ph↗

arXiv2023

Multi-sideband interference structures by high-order photon-induced continuum-continuum transitions in helium

Following up on a previous paper on two-color photoionization of Ar(3p) [Bharti et al., Phys. Rev. A 103 (2021) 022834], we present measurements and calculations for a modified three-sideband (3-SB) version of the "reconstruction of attosecond beating by interference of two-photon transitions" (RABBITT) configuration applied to He(1s). The 3-SB RABBITT approach allows us to explore interference effects between pathways involving different orders of transitions within the continuum. The relative differences in the retrieved oscillation phases of the three sidebands provide insights into the continuum-continuum transitions. The ground state of helium has zero orbital angular momentum, which simplifies the analysis of oscillation phases and their angle-dependence within the three sidebands. We find qualitative agreement between our experimental results and the theoretical predictions for many cases but also observe some significant quantitative discrepancies.

physics.atom-ph↗

arXiv2022

Two-path interference in the resonance-enhanced few-photon ionization of atoms

We investigate the resonance-enhanced few-photon ionization of atomic lithium by linearly polarized light whose frequency is tuned near the 2s-2p transition. Considering the direction of light polarization orthogonal to the quantization axis, the process can be viewed as an atomic "double-slit experiment" where the 2p states with magnetic quantum numbers m_l=+-1 act as the slits. In our experiment, we can virtually close one of the two slits by preparing lithium in one of the two circularly polarized 2p states before subjecting it to the ionizing radiation. This allows us to extract the interference term between the two pathways and obtain complex phase information on the final state. The experimental results show very good agreement with numerical solutions of the time-dependent Schroedinger equation. The validity of the two-slit model is also analyzed theoretically using a time-dependent perturbative approach.

physics.atom-ph↗

arXiv2021

Magnetic Dichroism in Few-Photon Ionization of Polarized Atoms

We consider few-photon ionization of atomic lithium by linearly polarized femtosecond laser pulses, and demonstrate that asymmetries of the electron angular distribution can occur for initially polarized (2p, m=+1) target atoms. The dependence of the photoelectron emission angle relative to the electric field direction is investigated at different laser intensities and wavelengths. The experimental spectra show excellent agreement with numerical solutions of the time-dependent Schroedinger equation. In the perturbative picture, the angular shift is traced back to interferences between partial waves with mean magnetic quantum number $\ne$0. This observation allows us to obtain quantum mechanical information on the electronic final state.

physics.atom-ph↗

arXiv2021

Carrier-Envelope-Phase Dependent Strong-Field Excitation

We present a joint experimental-theoretical study on the effect of the carrier-envelope phase (CEP) of a few-cycle pulse on the atomic excitation process. We focus on the excitation rates of argon as a function of CEP in the intensity range from 50-300 TW/cm$^2$, which covers the transition between the multiphoton and tunneling regimes. Through numerical simulations based on solving the time-dependent Schr\"{o}dinger equation (TDSE), we show that the resulting bound-state population is highly sensitive to both the intensity and the CEP. Because the intensity varies over the interaction region, the CEP effect is considerably reduced in the experiment. Nevertheless, the data clearly agree with the theoretical prediction, and the results encourage the use of precisely tailored laser fields to coherently control the strong-field excitation process. We find a markedly different behavior for the CEP-dependent bound-state population at low and high intensities with a clear boundary, which we attribute to the transition from the multiphoton to the tunneling regime.

physics.atom-ph↗

arXiv2021

Circular Dichroism in Atomic Resonance-Enhanced Few-Photon Ionization

We investigate few-photon ionization of lithium atoms prepared in the polarized 2$p$($m_\ell=\!+1$) state when subjected to femtosecond light pulses with left- or right-handed circular polarization at wavelengths between 665 nm and 920 nm. We consider whether ionization proceeds more favorably for the electric field co- or counter-rotating with the initial electronic current density. Strong asymmetries are found and quantitatively analyzed in terms of "circular dichroism" ($CD$). While the intensity dependence of the measured $CD$ values is rather weak throughout the investigated regime, a very strong sensitivity on the center wavelength of the incoming radiation is observed. While the co-rotating situation overall prevails, the counter-rotating geometry is strongly favored around 800 nm due to the 2$p$-3$s$ resonant transition, which can only be driven by counter-rotating fields. The observed features provide insights into the helicity dependence of light-atom interactions, and on the possible control of electron emission in atomic few-photon ionization by polarization-selective resonance enhancement.

physics.atom-ph↗

arXiv2021

Using circular dichroism to control energy transfer in multi-photon ionization

Chirality causes symmetry breaks in a large variety of natural phenomena ranging from particle physics to biochemistry. We investigate one of the simplest conceivable chiral systems, a laser-excited, oriented, effective one-electron Li target. Prepared in a polarized p state with |m|=1 in an optical trap, the atoms are exposed to co- and counter-rotating circularly polarized femtosecond laser pulses. For a field frequency near the excitation energy of the oriented initial state, a strong circular dichroism is observed and the photoelectron energies are significantly affected by the helicity-dependent Autler-Townes splitting. Besides its fundamental relevance, this system is suited to create spin-polarized electron pulses with a reversible switch on a femtosecond timescale at an energy resolution of a few meV.

physics.atom-ph↗

arXiv2020

Positive-energy spectra of atomic hydrogen in a magnetic field with an adiabatic-basis-expansion method

The problem of photoionization of atomic hydrogen in a white-dwarf-strength magnetic field is revisited to understand the existing discrepancies in the positive-energy spectra obtained by a variety of theoretical approaches reported in the literature. Oscillator strengths for photoionization are calculated with the adiabatic-basis-expansion method developed by Mota-Furtado and O'Mahony [Phys. Rev. A {\bf 76}, 053405 (2007)]. A comparative study is performed between the adiabatic-basis-expansion method and our previously developed coupled-channel theory [Phys. Rev. A {\bf 94}, 033422 (2016)]. A detailed analysis of the positive-energy spectra obtained here and those from other theoretical approaches shows that the adiabatic-basis-expansion method can produce more accurate positive-energy spectra than other reported approaches for low field strengths.

physics.atom-ph↗

arXiv2020

Ellipticity Dependence of Excitation and Ionization of Argon Atoms by Short-Pulse Infrared Radiation

When atoms or molecules are exposed to strong short-pulse infrared radiation, ionization as well as "frustrated tunneling ionization" (FTI) can occur, in which a portion of the almost ionized electrons recombine into the initial ground or an excited bound state. We analyze the ellipticity dependence of the relative signals that are predicted in a single-active electron approximation (SAE), the validity of which is checked against a parameter-free multi-electron \hbox{$R$-matrix} (close-coupling) with time dependence approach. We find good agreement between the results from both models, thereby providing confidence in the SAE model potential to treat the process of interest. Comparison of the relative excitation probabilities found in our numerical calculations with the predictions of Landsman {\it et al.}\ (New Journal of Physics {\bf 15} (2013) 013001) and Zhao {\it et al.}\ (Optics Express {\bf 27} (2019) 21689) reveals good agreement with the former for short pulses. For longer pulses, the ellipticity dependence becomes wider than that obtained from the Landsman {\it et al.} formula, but we do not obtain the increase compared to linearly polarized radiation predicted by Zhao {\it et al.}

physics.atom-ph↗

arXiv2017

Quantum coherent control of the photo\-electron angular distribution in bichromatic ionization of atomic neon

We investigate the coherent control of the photo\-electron angular distribution in bichromatic atomic ionization. Neon is selected as target since it is one of the most popular systems in current gas-phase experiments with free-electron lasers (FELSs). In particular, we tackle practical questions, such as the role of the fine-structure splitting, the pulse length, and the intensity. Time-dependent and stationary perturbation theory are employed, and we also solve the time-dependent Schrödinger equation in a single-active electron model. We consider neon ionized by a FEL pulse whose fundamental frequency is in resonance with either $2p-3s$ or $2p-4s$ excitation. The contribution of the non\-resonant two-photon process and its potential constructive or destructive role for quantum coherent control is investigated.

physics.atom-ph↗

arXiv2017

Inelastic e+Mg collision data and its impact on modelling stellar and supernova spectra

Results of calculations for inelastic e+Mg effective collision strengths for the lowest 25 physical states of Mg I (up to 3s6p 1P), and thus 300 transitions, from the convergent close-coupling (CCC) and the B-spline R-matrix (BSR) methods are presented. At temperatures of interest, ~5000 K, the results of the two calculations differ on average by only 4%, with a scatter of 27%. As the methods are independent, this suggests that the calculations provide datasets for e+Mg collisions accurate to this level. Comparison with the commonly used dataset compiled by Mauas et al. (1988), covering 25 transitions among 12 states, suggests the Mauas et al. data are on average ~57% too low, and with a very large scatter of a factor of ~6.5. In particular the collision strength for the transition corresponding to the Mg I intercombination line at 457 nm is significantly underestimated by Mauas et al., which has consequences for models that employ this dataset. In giant stars the new data leads to a stronger line compared to previous non-LTE calculations, and thus a reduction in the non-LTE abundance correction by ~0.1 dex (~25%). A non-LTE calculation in a supernova ejecta model shows this line becomes significantly stronger, by a factor of around two, alleviating the discrepancy where the 457 nm line in typical models with Mg/O ratios close to solar tended to be too weak compared to observations.

astro-ph.IM↗

arXiv2017

Benchmark calculations for electron-impact excitation of Mg$^{4+}$

There are major discrepancies between recent B-spline R-matrix (BSR) and Dirac Atomic R-matrix Code (DARC) calculations regarding electron-impact excitation rates for transitions in Mg$^{4+}$, with claims that the DARC calculations are much more accurate. To identify possible reasons for these discrepancies and to estimate the accuracy of the various results, we carried out independent BSR calculations with the same 86 target states as in the previous calculations, but with a different and more accurate representation of the target structure. We find close agreement with the previous BSR results for the majority of transitions, thereby confirming their accuracy. At the same time the differences with the DARC results are much more pronounced. The discrepancies in the final results for the collision strengths are mainly due to differences in the structure description, specifically the inclusion of correlation effects, and due to the likely occurrence of pseudoresonances. To further check the convergence of the predicted collision rates, we carried out even more extensive calculations involving 316 states of Mg$^{4+}$. Extending the close-coupling expansion results in major corrections for transitions involving the higher-lying states and allows us to assess the likely uncertainties in the existing datasets.

physics.atom-ph↗

arXiv2017

Electron-impact excitation of N$^{3+}$ using the B-spline R-matrix method: Importance of the target structure description and the size of the close-coupling expansion

There are major discrepancies between recent ICFT (Intermediate Coupling Frame Transformation) and DARC (Dirac Atomic R-matrix Code) calculations (Fernández-Menchero et al. 2014, Astron. Astroph. 566 A104, Aggarwal et al. 2016 Mon. Not. R Astr. Soc. 461 3997) regarding electron-impact excitation rates for transitions in several Be-like ions, as well as claims that the DARC calculations are much more accurate and the ICFT results might even be wrong. To identify possible reasons for these discrepancies and to estimate the accuracy of the various results, we carried out independent B-Spline R-Matrix (BSR) calculations for electron-impact excitation of the Be-like ion N$^{3+}$. Our close-coupling expansions contain the same target states (238 levels overall) as the previous ICFT and DARC calculations, but the representation of the target wave functions is completely different. We find close agreement among all calculations for the strong transitions between low-lying states, whereas there remain serious discrepancies for the weak transitions as well as for transitions to highly excited states. The differences in the final results for the collision strengths are mainly due to differences in the structure description, specifically the inclusion of correlation effects, rather than the treatment of relativistic effects or problems with the validity of the three methods to describe the collision. Hence there is no indication that one approach is superior to another, until the convergence of both the target configuration and the close-coupling expansions have been fully established.

physics.atom-ph↗

arXiv2017

Coherent control with a short-wavelength Free Electron Laser

XUV and X-ray Free Electron Lasers (FELs) produce short wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilised for many experiments previously possible at long wavelengths only: multiphoton ionization, pumping an atomic laser, and four-wave mixing spectroscopy. However one important optical technique, coherent control, has not yet been demonstrated, because Self- Amplified Spontaneous Emission FELs have limited longitudinal coherence. Single-colour pulses from the FERMI seeded FEL are longitudinally coherent, and two-colour emission is predicted to be coherent. Here we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5 nm ionized neon, and the asymmetry of the photoelectron angular distribution was controlled by adjusting the phase, with temporal resolution 3 attoseconds. This opens the door to new shortwavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity.

physics.optics↗

arXiv2016

Spin entanglement in elastic electron scattering from lithium atoms

In two recent papers (Phys. Rev. Lett. {\bf 116} (2016) 033201; Phys. Rev. A {\bf 94} (2016) 032331), the possibility of continuously varying the degree of entanglement between an elastically scattered electron and the valence electron of an alkali target was discussed. In order to estimate how well such a scheme may work in practice, we present results for elastic electron scattering from lithium in the energy regime of 1$-$5~eV and the full range of scattering angles $0^\circ - 180^\circ$. The most promising regime for Bell-correlations in this particular collision system are energies between about 1.5 eV and 3.0 eV, in an angular range around $110^\circ \pm 10^\circ$. In addition to the relative exchange asymmetry parameter, we present the differential cross section that is important when estimating the count rate and hence the feasibility of experiments using this system.

physics.atom-ph↗