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Relativistic time-dependent configuration-interaction singles method
2022. Felipe Zapata (et al.). Physical Review A: covering atomic, molecular, and optical physics and quantum information 105 (1)Artikel
In this work, a derivation and implementation of the relativistic time-dependent configuration-interaction singles (RTDCIS) method is presented. Various observables for krypton and xenon atoms obtained by RTDCIS are compared with experimental data and alternative relativistic calculations. This includes energies of occupied orbitals in the Dirac-Fock ground state, Rydberg state energies, Fano resonances, and photoionization cross sections. Diagrammatic many-body perturbation theory, based on the relativistic random phase approximation, is used as a benchmark with excellent agreement between RTDCIS reported at the Tamm-Dancoff level. Results from RTDCIS are computed in the length gauge, where the negative energy states can be omitted with acceptable loss of accuracy. A complex absorbing potential, that is used to remove photoelectrons far from the ion, is implemented as a scalar potential and validated for RTDCIS. The RTDCIS methodology presented here opens for future studies of strong-field processes, such as attosecond transient absorption and high-order harmonic generation, with electron and hole spin dynamics and other relativistic effects described by first principles via the Dirac equation.
Angular anisotropy parameters for photoionization delays
2021. Soumyajit Saha (et al.). Physical Review A: covering atomic, molecular, and optical physics and quantum information 104 (3)Artikel
Anisotropy parameters describing the angular dependence of the photoionization delay are defined. The formalism is applied to results obtained with the relativistic random phase approximation with exchange for photoionization delay from the outermost s-orbitals in selected rare-gas atoms. Any angular dependence in the Wigner delay is induced here by relativistic effects, while the measurable atomic delay exhibits such a dependence also in the nonrelativistic limit. The contributions to the anisotropy from the different sources are disentangled and discussed. For the heavier rare gases, it is shown that measurements of the delay for electrons ejected in specific angles, relative to, e.g., those ejected along the laser polarization, are directly related here to the Wigner delay. For a considerable range of angles, the contributions from the second photon largely get canceled when the results in different angles are compared, and this angle-relative atomic delay is then close to the corresponding Wigner delay.
Attosecond photoionization dynamics in the vicinity of the Cooper minima in argon
2021. C. Alexandridi (et al.). Physical Review Research 3 (1)Artikel
Using a spectrally resolved electron interferometry technique, we measure photoionization time delays between the 3s and 3p subshells of argon over a large 34-eV energy range covering the Cooper minima in both subshells. The observed strong variations of the 3s−3p delay difference, including a sign change, are well reproduced by theoretical calculations using the two-photon two-color random-phase approximation with exchange. Strong shake-up channels lead to photoelectrons spectrally overlapping with those emitted from the 3s subshell. These channels need to be included in our analysis to reproduce the experimental data. Our measurements provide a benchmark for multielectronic theoretical models aiming at an accurate description of interchannel correlation.
Autoionizing Polaritons in Attosecond Atomic Ionization
2021. N. Harkema (et al.). Physical Review Letters 127 (2)Artikel
Light-induced states and Autler-Townes splitting of laser-coupled states are common features in the photoionization spectra of laser-dressed atoms. The entangled light-matter character of metastable Autler-Townes multiplets, which makes them autoionizing polaritons, however, is still largely unexplored. We employ attosecond transient-absorption spectroscopy in argon to study the formation of polariton multiplets between the 3s−14p and several light-induced states. We measure a controllable stabilization of the polaritons against ionization, in excellent agreement with ab initio theory. Using an extension of the Jaynes-Cummings model to autoionizing states, we show that this stabilization is due to the destructive interference between the Auger decay and the radiative ionization of the polaritonic components. These results give new insights into the optical control of electronic structure in the continuum and unlock the door to applications of radiative stabilization in metastable polyelectronic systems.
Coherence control in helium-ion ensembles
2021. Saad Mehmood, Eva Lindroth, Luca Argenti. Physical Review Research 3 (2)Artikel
Attosecond pulses can ionize atoms in a coherent process. Since the emerging fragments are entangled, however, each preserves only a fraction of the initial coherence, thus limiting the chance of guiding the ion subsequent evolution. In this work, we use ab initio simulations of pump-probe ionization of helium above the 2s/2p threshold to demonstrate how this loss of coherence can be controlled. Thanks to the participation of 2ℓnℓ′ states, coherence between the ionic 2s and 2p states, which are degenerate in the nonrelativistic limit, results in a stationary, delay-dependent electric dipole. From the picosecond real-time beating of the dipole, caused by the fine-structure splitting of the n=2 manifold, it is possible to reconstruct all original ion coherences, including between antiparallel-spin states, which are a sensitive probe of relativistic effects in attosecond photoemission.
Implementation and validation of the relativistic transient absorption theory within the dipole approximation
2021. Felipe Zapata (et al.). Electronic structure 3 (1)Artikel
A relativistic transient absorption theory is derived, implemented and validated within the dipole approximation based on the time-dependent Dirac equation. In the non-relativistic limit, it is found that the absorption agrees with the well established non-relativistic theory based on the time-dependent Schrodringer equation. Time-dependent simulations have been performed using the Dirac equation and the Schrodinger equation for the hydrogen atom in two different attosecond transient absorption scenarios. These simulations validate the present relativistic theory. The presented work can be seen as a first step in the development of a more general relativistic attosecond transient absorption spectroscopy method for studying heavy atoms, but it also suggests the possibility of studying relativistic effects, such as Zitterbewegung, in the time domain.
Attosecond electron-spin dynamics in Xe 4d photoionization
2020. Shiyang Zhong (et al.). Nature Communications 11 (1)Artikel
The photoionization of xenon atoms in the 70-100 eV range reveals several fascinating physical phenomena such as a giant resonance induced by the dynamic rearrangement of the electron cloud after photon absorption, an anomalous branching ratio between intermediate Xe+ states separated by the spin-orbit interaction and multiple Auger decay processes. These phenomena have been studied in the past, using in particular synchrotron radiation, but without access to real-time dynamics. Here, we study the dynamics of Xe 4d photoionization on its natural time scale combining attosecond interferometry and coincidence spectroscopy. A time-frequency analysis of the involved transitions allows us to identify two interfering ionization mechanisms: the broad giant dipole resonance with a fast decay time less than 50 as, and a narrow resonance at threshold induced by spin-flip transitions, with much longer decay times of several hundred as. Our results provide insight into the complex electron-spin dynamics of photo-induced phenomena.
Electron-impact recombination and excitation rates for charge-state-selected highly charged Si ions
2020. Eva Lindroth (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 101 (6)Artikel
Charge-state selective recombination rate coefficients were measured by time of flight (TOF) analyzed highly charged Si ions extracted from an electron-beam ion trap. Additionally, the combination of simultaneous TOF and x-ray measurements and a separation of the dielectronic recombination contribution in the x-ray spectra is used for extracting electron-impact excitation rate coefficients for several overlaying charge states. Experimentally derived dielectronic recombination spectra for XIII and XIV Si are compared and found in excellent agreement with the results of relativistic many-body perturbation theory calculations.
Fano's Propensity Rule in Angle-Resolved Attosecond Pump-Probe Photoionization
2019. David Busto (et al.). Physical Review Letters 123 (13)Artikel
In a seminal article, Fano predicts that absorption of light occurs preferably with increase of angular momentum. We generalize Fano's propensity rule to laser-assisted photoionization, consisting of absorption of an extreme-ultraviolet photon followed by absorption or emission of an infrared photon. The predicted asymmetry between absorption and emission leads to incomplete quantum interference in attosecond photoelectron interferometry. It explains both the angular dependence of the photoionization time delays and the delay dependence of the photoelectron angular distributions. Our theory is verified by experimental results in Ar in the 20-40 eV range.
Many-body calculations of two-photon, two-color matrix elements for attosecond delays
2019. Jimmy Vinbladh, Jan Marcus Dahlström, Eva Lindroth. Physical Review A: covering atomic, molecular, and optical physics and quantum information 100 (4)Artikel
We present calculations for attosecond atomic delays in the photoionization of noble-gas atoms based on the full two-color two-photon random-phase approximation with exchange in both the length and velocity gauge. Gauge-invariant atomic delays are demonstrated for the complete set of diagrams. The results are used to investigate the validity of the common assumption that the measured atomic delays can be interpreted as a one-photon Wigner delay and a universal continuum-continuum contribution that depends only on the kinetic energy of the photoelectron, the laser frequency, and the charge of the remaining ion, but not on the specific atom or the orbital from which the electron is ionized. Here, we find that although effects beyond the universal IR-photoelectron continuum-continuum transitions are rare, they do occur in special cases such as around the 3s Cooper minimum in argon. We conclude also that in general the convergence in terms of many-body diagrams is considerably faster in the length gauge than in the velocity gauge.
Anisotropic photoemission time delays close to a Fano resonance
2018. Claudio Cirelli (et al.). Nature Communications 9Artikel
Electron correlation and multielectron effects are fundamental interactions that govern many physical and chemical processes in atomic, molecular and solid state systems. The process of autoionization, induced by resonant excitation of electrons into discrete states present in the spectral continuum of atomic and molecular targets, is mediated by electron correlation. Here we investigate the attosecond photoemission dynamics in argon in the 20-40 eV spectral range, in the vicinity of the 3s(-1)np autoionizing resonances. We present measurements of the differential photoionization cross section and extract energy and angle-dependent atomic time delays with an attosecond interferometric method. With the support of a theoretical model, we are able to attribute a large part of the measured time delay anisotropy to the presence of autoionizing resonances, which not only distort the phase of the emitted photoelectron wave packet but also introduce an angular dependence.
Attosecond transient absorption spectrum of argon at the L-2,L-3 edge
2018. Andrew Chew (et al.). Physical Review A: covering atomic, molecular, and optical physics and quantum information 97 (3)Artikel
Progress in high-harmonic generation has led to high-energy attosecond pulses with cutoff above the carbon 1s edge (283.8 eV). These pulses are essential to extend time-resolved spectroscopies to the water window in order to control electron dynamics in solvated organic species. Here we report a step towards this goal: the measurement, with subcycle time resolution, of the attosecond transient absorption spectrum of argon at the 2p(-1) L-2,L-3 edge (similar to 250 eV) in the presence of a short-wave infrared control pulse. The measurements, supported by theoretical simulations, demonstrate the concurrent role of Auger decay and tunnel ionization in the driven evolution of inner-valence holes of polyelectronic atoms.
Photoionization of metastable heliumlike C4+ (1s2s S-3(1)) ions
2018. A. Müller (et al.). Physical Review A: covering atomic, molecular, and optical physics and quantum information 98 (3)Artikel
In a joint experimental and theoretical endeavor, photoionization of metastable C4+ (1s2s S-3(1)) ions via intermediate levels with hollow, double-K-vacancy configurations 2s2p, 2s3p, 2p3s, 2p3d, 2s4p, 2p4s, and 2p4d has been investigated. High-resolution photon-ion merged-beams measurements were carried out with the resolving power reaching up to 25 800 which is sufficient to separate the leading fine-structure components of the 2s2p P-3 term. Many-body perturbation theory was employed to determine level-to-level cross sections for K-shell excitation with subsequent autoionization. The resonance energies were calculated with inclusion of electron correlation and radiative contributions. Their uncertainties are estimated to be below +/- 1 meV. Detailed balance confirms the present photoionization cross-section results by comparison with previous dielectronic-recombination measurements. The high accuracy of the theoretical transition energies together with the present experimental results qualify photoabsorption resonances in heliumlike ions as new, greatly improved energy-reference standards at synchrotron radiation facilities.
Semirelativistic Schrodinger Equation for Relativistic Laser-Matter Interactions
2018. Tor Kjellsson Lindblom (et al.). Physical Review Letters 121 (25)Artikel
A semirelativistic formulation of light-matter interaction is derived using the so called propagation gauge and the relativistic mass shift. We show that relativistic effects induced by a superintense laser field can, to a surprisingly large extent, be accounted for by the Schrodinger equation, provided that we replace the rest mass in the propagation gauge Hamiltonian by the corresponding time-dependent field-dressed mass. The validity of the semirelativistic approach is tested numerically on a hydrogen atom exposed to an intense extreme ultraviolet laser pulse strong enough to accelerate the electron towards relativistic velocities. It is found that while the results obtained from the ordinary (nonrelativistic) Schrodinger equation generally differ from those of the Dirac equation, demonstrating that relativistic effects are significant, the semirelativistic formulation provides results in quantitative agreement with a fully relativistic treatment.
Alternative gauge for the description of the light-matter interaction in a relativistic framework
2017. Tor Kjellsson (et al.). Physical Review A: covering atomic, molecular, and optical physics and quantum information 96 (2)Artikel
We present a generalized velocity gauge form of the relativistic laser-matter interaction. In comparison with the (equivalent) regular minimal coupling description, this form of light-matter interaction results in superior convergence properties for the numerical solution of the time-dependent Dirac equation. This applies both to the numerical treatment and, more importantly, to the multipole expansion of the laser field. The advantages of the alternative gauge is demonstrated in hydrogen by studies of the dynamics following the impact of superintense laser pulses of extreme ultraviolet wavelengths and subfemtosecond duration.
Attosecond delays in laser-assisted photodetachment from closed-shell negative ions
2017. Eva Lindroth, Jan Marcus Dahlström. Physical Review A: covering atomic, molecular, and optical physics and quantum information 96 (1)Artikel
We study laser-assisted photodetachment time delays by attosecond pulse trains from the closed-shell negative ions F- and Cl. We investigate the separability of the delay into two contributions: (i) the Wigner-like delay associated with one-photon ionization by the attosecond pulse train and (ii) the delay associated with the exchange of an additional laser photon in the presence of the potential of the remaining target. Based on the asymptotic form of the wave packet, the latter term is expected to be negligible because the ion is neutralized, leading to a vanishing laser-ion interaction with increasing electron-atom separation. While this asymptotic behavior is verified at high photoelectron energies, we also quantify sharp deviations at low photoelectron energies. Further, these low-energy delays are clearly different for the two studied anions, indicating a breakdown of the universality of laser-ion-induced delays. The fact that the short-range potential can induce a delay of as much as 50 as can have implications for the interpretation of delay measurements also in other systems that lack long-range potential.
Photoionization in the time and frequency domain
2017. M. Isinger (et al.). Science 358 (6365), 893-896Artikel
Ultrafast processes in matter, such as the electron emission after light absorption, can now be studied using ultrashort light pulses of attosecond duration (10-18 seconds) in the extreme ultraviolet spectral range. The lack of spectral resolution due to the use of short light pulses has raised issues in the interpretation of the experimental results and the comparison with theoretical calculations. We determine photoionization time delays in neon atoms over a 40-electron volt energy range with an interferometric technique combining high temporal and spectral resolution. We spectrally disentangle direct ionization from ionization with shake-up, in which a second electron is left in an excited state, and obtain excellent agreement with theoretical calculations, thereby solving a puzzle raised by 7-year-old measurements.
Relativistic ionization dynamics for a hydrogen atom exposed to superintense XUV laser pulses
2017. Tor Kjellsson, Sølve Selstø, Eva Lindroth. Physical Review A: covering atomic, molecular, and optical physics and quantum information 95 (4)Artikel
We present a theoretical study of the ionization dynamics of a hydrogen atom exposed to attosecond laser pulses in the extreme ultraviolet region at very high intensities. The pulses are such that the electron is expected to reach relativistic velocities, thus necessitating a fully relativistic treatment. We solve the time-dependent Dirac equation and compare its predictions with those of the corresponding nonrelativistic Schrodinger equation. We find that as the electron is expected to reach about 20% of the speed of light, relativistic corrections introduce a finite yet small decrease in the probability of ionizing the atom.
Angular dependence of photoemission time delay in helium
2016. Sebastian Heuser (et al.). Physical Review A 94 (6)Artikel
Time delays of electrons emitted from an isotropic initial state with the absorption of a single photon and leaving behind an isotropic ion are angle independent. Using an interferometric method involving XUV attosecond pulse trains and an IR-probe field in combination with a detection scheme, which allows for full three-dimensional momentum resolution, we show that measured time delays between electrons liberated from the 1s(2) spherically symmetric ground state of helium depend on the emission direction of the electrons relative to the common linear polarization axis of the ionizing XUV light and the IR-probing field. Such time delay anisotropy, for which we measure values as large as 60 as, is caused by the interplay between final quantum states with different symmetry and arises naturally whenever the photoionization process involves the exchange of more than one photon. With the support of accurate theoretical models, the angular dependence of the time delay is attributed to small phase differences that are induced in the laser-driven continuum transitions to the final states. Since most measurement techniques tracing attosecond electron dynamics involve the exchange of at least two photons, this is a general and significant effect that must be taken into account in all measurements of time delays involving photoionization processes.
Spectral phase measurement of a Fano resonance using tunable attosecond pulses
2016. M. Kotur (et al.). Nature Communications 7Artikel
Electron dynamics induced by resonant absorption of light is of fundamental importance in nature and has been the subject of countless studies in many scientific areas. Above the ionization threshold of atomic or molecular systems, the presence of discrete states leads to autoionization, which is an interference between two quantum paths: direct ionization and excitation of the discrete state coupled to the continuum. Traditionally studied with synchrotron radiation, the probability for autoionization exhibits a universal Fano intensity profile as a function of excitation energy. However, without additional phase information, the full temporal dynamics cannot be recovered. Here we use tunable attosecond pulses combined with weak infrared radiation in an interferometric setup to measure not only the intensity but also the phase variation of the photoionization amplitude across an autoionization resonance in argon. The phase variation can be used as a fingerprint of the interactions between the discrete state and the ionization continua, indicating a new route towards monitoring electron correlations in time.
Resonance Effects in Photoemission Time Delays
2015. M. Sabbar (et al.). Physical Review Letters 115 (13)Artikel
We present measurements of single-photon ionization time delays between the outermost valence electrons of argon and neon using a coincidence detection technique that allows for the simultaneous measurement of both species under identical conditions. The analysis of the measured traces reveals energy-dependent time delays of a few tens of attoseconds with high energy resolution. In contrast to photoelectrons ejected through tunneling, single-photon ionization can be well described in the framework of Wigner time delays. Accordingly, the overall trend of our data is reproduced by recent Wigner time delay calculations. However, besides the general trend we observe resonance features occurring at specific photon energies. These features have been qualitatively reproduced and identified by a calculation using the multiconfigurational Hartree-Fock method, including the influence of doubly excited states and ionization thresholds.
Double ionization probed on the attosecond timescale
2014. Erik P. Månsson (et al.). Nature Physics 10 (3), 207-211Artikel
Double ionization following the absorption of a single photon is one of the most fundamental processes requiring interaction between electrons(1-3). Information about this interaction is usually obtained by detecting emitted particles without access to real-time dynamics. Here, attosecond light pulses(4,5), electron wave packet interferometry(6) and coincidence techniques(7) are combined to measure electron emission times in double ionization of xenon using single ionization as a clock, providing unique insight into the two-electron ejection mechanism. Access to many-particle dynamics in real time is of fundamental importance for understanding processes induced by electron correlation in atomic, molecular and more complex systems.
Study of attosecond delays using perturbation diagrams and exterior complex scaling
2014. J. Marcus Dahlström, Eva Lindroth. Journal of Physics B 47 (12), 124012Artikel
We describe in detail how attosecond delays in laser-assisted photoionization can be computed using perturbation theory based on two-photon matrix elements. Special emphasis is laid on above-threshold ionization, where the electron interacts with an infrared field after photoionization by an extreme ultraviolet field. Correlation effects are introduced using diagrammatic many-body theory to the level of the random-phase approximation with exchange. Our aim is to provide an ab initio route to correlated multi-photon processes that are required for an accurate description of experiments on the attosecond time scale. Here, our results are focused on photoionization of the M-shell of argon atoms, where experiments have been carried out using the so-called reconstruction of attosecond beating by the two-photon interference transitions technique. An influence of autoionizing resonances in attosecond delay measurements is observed. Further, it is shown that the delay depends on both detection angle of the photoelectron and energy of the probe photon.
Multiconfigurational Hartree-Fock close-coupling ansatz
2013. Thomas Carette (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 87 (2), 023420Artikel
We present a robust, ab initio method for addressing atom-light interactions and apply it to photoionization of argon. We use a close-coupling ansatz constructed on a multiconfigurational Hartree-Fock description of localized states and B-spline expansions of the electron radial wave functions. In this implementation, the general many-electron problem can be tackled thanks to the use of the ATSP2K libraries [C. Froese Fischer et al., Comput. Phys. Commun. 176, 559 (2007)]. In the present contribution, we combine this method with exterior complex scaling, thereby allowing for the computation of the complex partial amplitudes that encode the whole dynamics of the photoionization process. The method is validated on the 3s3p(6)np series of resonances converging to the 3s extraction. Then, it is used for computing the energy dependent differential atomic delay between 3p and 3s photoemission, and agreement is found with the measurements of Guenot et al. [Phys. Rev. A 85, 053424 (2012)]. The effect of the presence of resonances in the one-photon spectrum on photoionization delay measurements is studied. DOI: 10.1103/PhysRevA.87.023420
Photoionization of helium by attosecond pulses
2013. Luca Argenti (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 87 (5), 053405Artikel
We investigate the photoionization spectrum of helium by attosecond XUV pulses both in the spectral region of doubly excited resonances as well as above the double ionization threshold. In order to probe for convergence, we compare three techniques to extract photoelectron spectra from the wave packet resulting from the integration of the time-dependent Schrodinger equation in a finite-element discrete variable representation basis. These techniques are projection on products of hydrogenic bound and continuum states, projection onto multichannel scattering states computed in a B-spline close-coupling basis, and a technique based on exterior complex scaling implemented in the same basis used for the time propagation. These methods allow one to monitor the population of continuum states in wave packets created with ultrashort pulses in different regimes. Applications include photo cross sections and anisotropy parameters in the spectral region of doubly excited resonances, time-resolved photoexcitation of autoionizing resonances in an attosecond pump-probe setting, and the energy and angular distribution of correlated wave packets for two-photon double ionization.
Diagrammatic approach to attosecond delays in photoionization
2012. J. Marcus Dahlström, Thomas Carette, Eva Lindroth. Physical Review A. Atomic, Molecular, and Optical Physics 86 (6), 061402Artikel
We study laser-assisted photoionization by attosecond pulses using a time-independent formalism based on diagrammatic many-body perturbation theory. Our aim is to provide an ab initio route to the delays for this above-threshold ionization process, which is essential for a quantitative understanding of attosecond metrology. We present correction curves for characterization schemes of attosecond pulses, such as streaking, that account for the delayed atomic response in ionization from neon and argon. We also verify that photoelectron delays from many-electron atoms can be measured using similar schemes if, instead, the so-called continuum-continuum delay is subtracted. Our method is general and it can be extended also to more complex systems and additional correlation effects can be introduced systematically. DOI: 10.1103/PhysRevA.86.061402
Photoemission time delay measurements and calculations close to the 3s ionization cross section minimum in ar
2012. D. Guenot (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 85 (5), 053424Artikel
We present experimental measurements and theoretical calculations of photoionization time delays from the 3s and 3p shells in Ar in the photon energy range of 32-42 eV. The experimental measurements are performed by interferometry using attosecond pulse trains and the infrared laser used for their generation. The theoretical approach includes intershell correlation effects between the 3s and 3p shells within the framework of the random-phase approximation with exchange. The connection between single-photon ionization and the two-color two-photon ionization process used in the measurement is established using the recently developed asymptotic approximation for the complex transition amplitudes of laser-assisted photoionization. We compare and discuss the theoretical and experimental results, especially in the region where strong intershell correlations in the 3s -> kp channel lead to an induced Cooper minimum in the 3s ionization cross section.
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