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We investigate the harmonic response of neon atoms to mid-IR laser fields (2000-3000nm) using a single-active electron (SAE) model and the fully ab initio all-electron R-Matrix with Time-dependence (RMT) method. The laser peak intensity and wavelength are varied to find suitable parameters for high-harmonic imaging in the water window. Comparison of the SAE and RMT results shows qualitative agreement between them as well as parameters such as the cutoff frequency predicted by the classical three-step model. However, there are significant differences in the details, particularly in the predicted conversion efficiency. These details indicate the possible importance of multi-electron effects, as well as a strong sensitivity of quantitative predictions on specific aspects of the numerical model.

Within the framework of the improved quantitative rescattering (QRS) model, we simulate the correlated two-electron momentum distributions (CMDs) for nonsequential double ionization (NSDI) of Ar by elliptically polarized laser pulses with a wavelength of 788 nm at an intensity of 0.7 × 1014 W/cm2 for the ellipticities ranging from 0 to 0.3. Only the CMDs for recollision excitation with subsequent ionization (RESI) are calculated and the contribution from recollision direct ionization is neglected. According to the QRS model, the CMD for RESI can be factorized as a product of the parallel momentum distribution (PMD) for the first released electron after recollision and the PMD for the second electron ionized from an excited state of the parent ion. The PMD for the first electron is obtained from the laser-free differential cross sections for electron impact excitation of Ar+ calculated using state-of-the-art many-electron R-matrix theory while that for the second electron is evaluated by solving the time-dependent Schrödinger equation. The results show that the CMDs for all the ellipticities considered here exhibit distinct anticorrelated back-to-back emission of the electrons along the major polarization direction, and the anticorrelation is more pronounced with increasing ellipticity. It is found that anticorrelation is attributed to the pattern of the PMD for the second electron ionized from the excited state that, in turn, is caused by the delayed recollision time with respect to the instant of the external field crossing. Our work shows that both the ionization potential of the excited parent ion and the laser intensity play important roles in the process.

This project concerns the development and subsequent distribution of a suite of computer codes that can accurately describe the interaction of charged particles (mostly electrons) and light (mostly lasers and synchrotrons) with atoms and ions. The results are of importance for the understanding of fundamental collision dynamics, and they also fulfil the urgent practical need for accurate atomic data to model the physics of stars, plasmas, lasers, and planetary atmospheres. With the rapid advances currently seen in computational resources, such studies can now be conducted for realistic systems. In particular, it has become possible to describe very complex targets, such as transition metals and other open-shell systems. The source code resulting from this project will be made publicly available. A website devoted to user-developer interaction will be developed and maintained together with the necessary code documentation and training materials.

Since its initial development in the 1970’s by Phil Burke and his collaborators, the R-matrix theory and associated computer codes have become the de facto approach for the calculation of accurate data for general electron-atom/ion/molecule collision and photoionization processes. The use of a non-orthonormal set of orbitals based on B-splines, now called the B-spline R-matrix (BSR) approach, was pioneered by Zatsarinny. It has considerably extended the flexibility of the approach and improved particularly the treatment of complex many-electron atomic and ionic targets, for which accurate data are needed in many modelling applications for processes involving low-temperature plasmas. Both the original R-matrix approach and the BSR method have been extended to the interaction of short, intense electromagnetic (EM) radiation with atoms and molecules. Here we provide an overview of the theoretical tools that were required to facilitate the extension of the theory to the time domain. As an example of a practical application, we show results for two-photon ionization of argon by intense short-pulse extreme ultraviolet radiation

The Quantemol database (QDB) provides cross sections and rates of processes important for plasma models; heavy particle collisions (chemical reactions) and electron collision processes are considered. The current version of QDB has data on 28 917 processes between 2485 distinct species plus data for surface processes. These data are available via a web interface or can be delivered directly to plasma models using an application program interface; data are available in formats suitable for direct input into a variety of popular plasma modeling codes including HPEM, COMSOL, ChemKIN, CFD-ACE+, and VisGlow. QDB provides ready assembled plasma chemistries plus the ability to build bespoke chemistries. The database also provides a Boltzmann solver for electron dynamics and a zero-dimensional model. Thesedevelopments, use cases involving O2, Ar/NF3, Ar/NF3/O2, and He/H2O/O2 chemistries, and plans for the future are presented.

We apply the molecular convergent close-coupling method to the calculation of linear polarization fractions for Fulcher-band fluorescence following electron-impact excitation of the ${\mathrm{H}}_{2}$ $d\phantom{\rule{0.16em}{0ex}}{}^{3}{\mathrm{\ensuremath{\Pi}}}_{u}$ state. The results exhibit the opposite threshold behavior compared to the only previous calculations [Meneses, Brescansin, Lee, Michelin, Machado, and Csanak, Phys. Rev. A 52, 404 (1995)], but are in agreement with the most recent measurements for the $Q(1),$ $R(1),$ and $Q(3)$ transitions [Maseberg, Bartschat, and Gay, Phys. Rev. Lett. 111, 253201 (2013)].

Benchmark intensity ratio measurements of the energy loss lines of krypton for excitation of the 4p61S0→4p55s[3/2]2, 4p55s[3/2]1, 4p55s′[1/2]0, and 4p55s′[1/2]1 transitions are reported, these being the lowest electronic excitations for krypton. The importance of these ratios as stringent tests of theoretical electron scattering models for the noble gases is discussed, as well as the role of spin-exchange and direct processes regarding the angular dependence of these ratios. The experimental data are compared with predictions from fully-relativistic B-spline R-matrix (close-coupling) calculations.

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., Comment: 7 pages, 4 figures

Using an improved quantitative rescattering model, we calculate the correlated two-electron momentum distributions (CMDs) for nonsequential double ionization of Ar exposed to intense laser pulses with a wavelength of 790 nm at a peak intensity of $1.0\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. We analyze the drastic variations in the CMDs that were observed by K\"ubel et al. [New J. Phys. 16, 033008 (2014)] in the transition from near-single-cycle to multicycle driving laser pulses. Our model reproduces their experimental data well. We also find that the transition from near-single-cycle to multicycle driving laser pulses depends strongly on the details of the pulse envelope. Special attention is paid to the mechanisms responsible for the cross-shaped structure observed experimentally with 4 fs pulses. Our analysis reveals that the cross-shaped structure in the carrier-envelope phase-averaged CMD for near-single-cycle pulses can be attributed to strong backward scattering of the recolliding electron as well as the narrow momentum distributions of the tunnel-ionized electrons compared to those for long pulses. This also explains why the cross-shaped distributions collapse to a rather structureless distribution when the pulse duration is increased to 8 fs.

The European physical journal / D 75(7), 205 (2021). doi:10.1140/epjd/s10053-021-00218-0, Ionization of laser-dressed atomic helium is investigated with focus on photoelectron angular distributions stemming from two-color multi-photon excited states. The experiment combines extreme ultraviolet (XUV) with infrared (IR) radiation, while the relative polarization and the temporal delay between the pulses can be varied. By means of an XUV photon energy scan over several electronvolts, we get access to excited states in the dressed atom exhibiting various binding energies, angular momenta, and magnetic quantum numbers. Furthermore, varying the relative polarization is employed as a handle to switch on and off the population of certain states that are only accessible by two-photon excitation. In this way, photoemission can be suppressed for specific XUV photon energies. Additionally, we investigate the dependence of the photoelectron angular distributions on the IR laser intensity. At our higher IR intensities, we start leaving the simple multi-photon ionization regime. The interpretation of the experimental results is supported by numerically solving the time-dependent Schr��dinger equation in a single-active-electron approximation., Published by Springer, Heidelberg

Analytic fits to the recommended electron-impact excitation and ionization cross sections for Be I are presented. The lowest 19 terms of configurations 2 s n l ( n ≤ 4) and 2 p 2 terms below the first ionization limit are considered. The fits are based on the accurate calculations with the convergent close coupling (CCC) method as well as the B-spline R-matrix (BSR) approach. The fitted cross sections provide rate coefficients that are believed to approximate the original data within 10% with very few exceptions. The oscillator strengths for the dipole-allowed transitions between all the considered states are calculated with the relativistic multi-configuration Dirac–Hartree–Fock (MCDHF) approach and compared with the CCC and BSR results. This comparison shows a very good agreement except for a handful of cases with likely strong cancellations.

We investigate few-photon ionization of lithium atoms prepared in the polarized $2p\phantom{\rule{0.16em}{0ex}}({m}_{\ensuremath{\ell}}=+1)$ state when subjected to femtosecond light pulses with left- or right-handed circular polarization at wavelengths between 665 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 $2p\text{\ensuremath{-}}3s$ 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.

We calculate the correlated two-electron momentum distributions for nonsequential double ionization of helium in a 400-nm laser pulse with a peak intensity of $6.5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ $\mathrm{W}/{\mathrm{cm}}^{2}$, which is below the recollision threshold, by using the quantitative rescattering model in which the lowering of the threshold due to the presence of the electric field at the instant of recollision is taken into account. While distinct correlated back-to-back emission of the electrons along the polarization direction is predicted in accord with other existing theoretical simulations, we suggest an alternative mechanism that is responsible for the anticorrelation. At intensities below the recollision threshold, recollision excitation can only take place when the barrier of the Coulomb potential is sufficiently suppressed by the electric field. The excited electron begins to ionize at a ``delayed'' recollision time after a crossing of the field and hence the probability of being tunnel-ionized after the subsequent peak of the field is increased. It is demonstrated that the ``delayed'' recollision time predominantly determines the parallel momentum distribution of the tunneling electron and plays a decisive role in forming anticorrelation.

20 pags., 7 figs., 6 tabs., We report, over an extended energy range, recommended angle-integrated cross sections for elastic scattering, discrete inelastic scattering processes, and the total ionization cross section for electron scattering from atomic indium. In addition, from those angle-integrated cross sections, a grand total cross section is subsequently derived. To construct those recommended cross-section databases, results from original B-spline R-matrix, relativistic convergent close-coupling, and relativistic optical-potential computations are also presented here. Electron transport coefficients are subsequently calculated, using our recommended database, for reduced electric fields ranging from 0.01 Td to 10 000 Td using a multiterm solution of Boltzmann's equation. To facilitate those simulations, a recommended elastic momentum transfer cross-section set is also constructed and presented here., The work of K.R.H., O.Z., and K.B. was supported by the United States National Science Foundation under Grant Nos. OAC-1834740 and PHY-1803844 and by the XSEDE supercomputer Allocation No. PHY-090031. The (D)BSR calculations were carried out on Stampede2 at the Texas Advanced Computing Center. The work of D.V.F. and I.B. was supported by the Australian Research Council and resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. F.B. and G.G. acknowledge partial financial support from the Spanish Ministry MICIU (Project Nos. FIS2016- 80440 and PID2019-104727-RB-C21) and CSIC (Project No. LINKA20085). This work was also financially supported, in part, by the Australian Research Council (Project No. DP180101655), the Ministry of Education, Science and Technological Development of the Republic of Serbia, and the Institute of Physics (Belgrade).

Reconstruction of Attosecond Beating By Interference of Two-photon Transitions (RABBITT) is a technique that can be used to determine the phases of atomic transition elements in photoionization processes. In the traditional RABBITT scheme, the so-called "asymptotic approximation" considers the measured phase as a sum of the Wigner phase linked to a single-photon ionization process and the continuum-continuum (cc) phase associated with further single-photon transitions in the continuum. In this paper, we explore the possibility of extending the asymptotic approximation to multi-sideband RABBITT schemes. The predictions from this approximation are then compared with results obtained by an {\it ab initio} calculation based on solving the time-dependent Schr\"odinger equation for atomic hydrogen.

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 76, 053405 (2007)]. A comparative study is performed between the adiabatic-basis-expansion method and our previously developed coupled-channel theory [L. B. Zhao, O. Zatsarinny, and K. Bartschat, Phys. Rev. A 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.

We discuss a number of aspects regarding the physics of [Formula: see text] and H(2). This includes low-energy electron scattering processes and the interaction of both weak (perturbative) and strong (ultrafast/intense) electromagnetic radiation with those systems.

We propose an extension of coherent control using laser-synthesized free-electron matter waves. In contrast to coherent control schemes exploiting optical coherences to steer the dynamics of matter waves, we analyze the opposite and investigate the control of quantum light emission driven by laser-sculpted coherent free-electron matter waves. We apply this concept to the control of entangled photon-pair emission in electron-atom collisions, in which the incident electron wave packet, colliding with a target atom $B$, is engineered by interferometric resonantly-enhanced multiphoton ionization of a parent atom $A$. Each ionization pathway leads to electron wave packets that coherently interfere during temporal evolution in the continuum. Their mutual coherence can be controlled by adjusting the relative phases or time delays of the frequency components of the ionizing field contributing to the interfering pathways. We report coherent control of entangled photon-pair generation in radiative photo-cascade emission upon decay of the target atom after inelastic excitation triggered by the collision with the synthesized electron wave packet., 12 pages, 4 figures

Using the improved quantitative rescattering (QRS) model, we simulate the correlated two-electron momentum distributions (CMD) for nonsequential double ionization (NSDI) of Ar by near-single-cycle laser pulses with a wavelength of 750 nm at an intensity of 2.8 × 1014 W/cm2. With the accurate cross sections obtained from fully quantum mechanical calculations for both electron impact excitation and electron impact ionization of Ar+, we unambiguously identify the contributions from recollision direct ionization (RDI) and recollision excitation with subsequent ionization (RESI). Our analysis reveals that RESI constitutes the main contribution to NSDI of Ar under the conditions considered here. The simulated results are directly compared with experimental measurements [Bergues et al., Nat. Commun. 3, 813 (2012)10.1038/ncomms1807] in which each NSDI event is tagged with the carrier-envelope phase (CEP). It is found that the overall pattern of both the CEP-resolved and the CEP-averaged CMDs measured in experiment are well reproduced by the QRS model, and the cross-shaped structure in the CEP-averaged CMD is attributed to the strong forward scattering of the recolliding electron as well as the depletion effect in tunneling ionization of the electron from an excited state of the parent ion.

We present angle-integrated and angle-differential cross sections for electron-impact excitation of the (5s25p)P1/22→(5s26s)S1/22 transition in atomic indium. Experimental data for six incident electron energies between 10 and 100 eV are compared with predictions from semirelativistic and fully relativistic B-spline R-matrix calculations, as well as a fully relativistic convergent close-coupling model. Agreement between our measured and calculated data is, with a few exceptions, found to be typically very good. Additionally, the agreement between the present theoretical predictions is generally excellent, with the remaining small deviations being associated with the slightly different, although still very accurate, descriptions of the target structure. Agreement between the present results and an earlier relativistic distorted-wave computation [T. Das, R. Srivastava, and A. D. Stauffer, Phys. Lett. A 375, 568 (2011)PYLAAG0375-960110.1016/j.physleta.2010.12.037] was, however, found to be marginal, particularly at 10 and 20 eV.

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.}, Comment: 7 pages, 4 figures

We document recent progress made in the development and deployment of a science gateway for atomic and molecular physics (AMP) [10]. The molecular scattering applications supported in the gateway and the early phase of the project were described in a preliminary report [33]. Here, we present recent advances in both the platform’s capabilities and in its adoption for additional software suites and new possibilities for further development. The past year has seen substantial progress, with the addition of two new software suites and additional authors. A very successful workshop, supported by the MOLSSI, NSF, and NIST, was held at NIST from Dec 11-13, 2019. The agenda contained discussions of the science as well as demonstrations of the codes both in production and learning modes. More than 30 scientists participated in the workshop. Over the past few months, the number of registered gateway users has grown to over 60. The applications being deployed provide users with a number of state-of-the-art computational techniques to treat electron scattering from atomic and molecular targets, as well as the interaction of radiation with such systems. One may view all of these approaches as generalized close-coupling methods, where the inclusion of electron correlation is accomplished via the addition of generalized pseudostates. A number of the methods can also be employed to compute high-quality bound-state wave functions by closing the channels and imposing exponentially decaying boundary conditions. The application software suites are deployed on a number of NSF and DoE supercomputing systems. These deployments are brought to the user community through the science gateway with user interfaces, post-processing, and visualization tools. Below we outline our efforts in deploying the Django web framework for the AMPGateway using the Apache Airavata gateway middleware, discuss the new advanced capabilities available, and provide an outlook for future directions for the gateway and the AMP community.

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 counterrotating 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.

Using the quantitative rescattering model, we simulate the correlated two-electron momentum distributions for nonsequential double ionization of helium by 800 nm laser pulses at intensities in the range of (2 − 15) × 1014 W/cm2. The experimentally observed V-shaped structure at high intensities [Phys. Rev. Lett. 99, 263003 (2007)10.1103/PhysRevLett.99.263003] is attributed to the strong forward scattering in laser-induced recollision excitation and the asymmetric momentum distribution of electrons that are tunneling-ionized from the excited states. The final-state electron repulsion also plays an important role in forming the V-shaped structure.

Fully differential cross sections for single ionization of helium by fast proton impact in different kinematical regimes in the scattering plane were recently measured in a high-precision experiment [O. Chuluunbaatar et al., Phys. Rev. A 99, 062711 (2019)] and calculated using the first Born approximation. We use the nonperturbative wave-packet convergent close-coupling approach to calculate this process more accurately in all the kinematical regimes considered in the experiment. The obtained results show that the coupling between channels and multiple-scattering effects, combined with a more accurate treatment of the target structure, significantly improves the agreement between theory and experiment, especially in the apparently most difficult regions away from the so-called Bethe ridge, where the deviation in the positions of the binary peak observed in the experiment and calculated using the first Born approximation is largest. We also present fully differential cross sections in the same kinematical regimes but for incident projectile energies of 500 keV and 2 MeV. Corresponding results for the so-called perpendicular and azimuthal planes are also exhibited.

We investigate AC Stark-shifted resonances in argon with ultrashort near-infrared pulses. Using 30 fs pulses we observe periodic enhancements of the excitation yield in the intensity regions corresponding to the absorption of 13 and 14 photons. By reducing the pulse duration to 6 fs with only a few optical cycles, we also demonstrate that the enhancements are significantly reduced beyond what is measurable in the experiment. Comparing these to numerical predictions, which are in quantitative agreement with experimental results, we find that even though the quantum-state distribution can be broad, the enhancements are largely due to efficient population of a select few AC Stark-shifted resonant states rather than the closing of an ionization channel. Because these resonances are dependent on the frequency and intensity of the laser field, the broad bandwidth of the 6 fs pulses means that the resonance condition is fulfilled across a large range of intensities. This is further exaggerated by volume-averaging effects, resulting in excitation of the $5g$ state at almost all intensities and reducing the apparent magnitude of the enhancements. For 30 fs pulses, volume averaging also broadens the quantum state distribution but the enhancements are still large enough to survive. In this case, selectivity of excitation to a single state is reduced below 25% of the relative population. However, an analysis of TDSE simulations indicates that excitation of up to 60% into a single state is possible if volume averaging can be eliminated and the intensity can be precisely controlled., 7 pages, 5 figures

Coherent control of the photoelectron angular distribution (PAD) in ionization by a bichromatic circularly polarized field is theoretically studied in neon in the photon energy range 16--20 eV. The breakdown of the PAD axial symmetry with respect to the light propagation direction is analyzed. The coherent control of the PAD manifests itself through: (1) a change from a one-lobe to a three-lobe shape, respectively, for co- and counter-rotating harmonics, (2) a variation of the polar asymmetry with the light frequency, and (3) a rotation of the PAD around the direction of the beam depending on the relative phase between the harmonics. Tuning the lowest harmonic at the $2{p}^{5}3s$ or $2{p}^{5}4s$ intermediate excited states strongly influences the PAD. In contrast to atomic hydrogen and the alkali atoms, the cases of equal and opposite helicity are not related in neon. Features of a conjugate polarization setup, when one harmonic is linearly polarized while the other one is circularly polarized, are also discussed.

The tunnelling of a particle through a potential barrier is a key feature of quantum mechanics that goes to the core of wave–particle duality. The phenomenon has no counterpart in classical physics, and there are no well constructed dynamical observables that could be used to determine ‘tunnelling times’. The resulting debate1–5 about whether a tunnelling quantum particle spends a finite and measurable time under a potential barrier was reignited in recent years by the advent of ultrafast lasers and attosecond metrology6. Particularly important is the attosecond angular streaking (‘attoclock’) technique7, which can time the release of electrons in strong-field ionization with a precision of a few attoseconds. Initial measurements7–10 confirmed the prevailing view that tunnelling is instantaneous, but later studies11,12 involving multi-electron atoms—which cannot be accurately modelled, complicating interpretation of the ionization dynamics—claimed evidence for finite tunnelling times. By contrast, the simplicity of the hydrogen atom enables precise experimental measurements and calculations13–15 and makes it a convenient benchmark. Here we report attoclock and momentum-space imaging16 experiments on atomic hydrogen and compare these results with accurate simulations based on the three-dimensional time-dependent Schrodinger equation and our experimental laser pulse parameters. We find excellent agreement between measured and simulated data, confirming the conclusions of an earlier theoretical study17 of the attoclock technique in atomic hydrogen that presented a compelling argument for instantaneous tunnelling. In addition, we identify the Coulomb potential as the sole cause of the measured angle between the directions of electron emission and peak electric field: this angle had been attributed11,12 to finite tunnelling times. We put an upper limit of 1.8 attoseconds on any tunnelling delay, in agreement with recent theoretical findings18 and ruling out the interpretation of all commonly used ‘tunnelling times’19 as ‘time spent by an electron under the potential barrier’20. Simulation and measurement of the photoionization of atomic hydrogen at attosecond resolution confirm that the tunnelling of the ejected electron is instantaneous.

Cross sections for electron scattering from atomic and molecular iodine are calculated based on the R-matrix (close-coupling) method. Elastic and electronic excitation cross sections are presented for both I and I2. The dissociative electron attachment and vibrational excitation cross sections of the iodine molecule are obtained using the local complex potential approximation. Ionization cross sections are also computed for I2 using the BEB model.

22 pags., 23 figs., We study the transport of electrons and propagation of the negative ionisation fronts in indium vapour. Electron swarm transport properties are calculated using a Monte Carlo simulation technique over a wide range of reduced electric fields E/N (where E is the electric field and N is the gas number density) and indium vapour temperatures in hydrodynamic conditions, and under non-hydrodynamic conditions in an idealised steady-state Townsend (SST) setup. As many indium atoms are in the first metastable state at vapour temperatures of a few thousand Kelvin, the initial Monte Carlo code was extended and generalized to consider the spatial relaxation and the transport of electrons in an idealised SST experiment, in the presence of thermal motion of the host-gas atoms and superelastic collisions. We observe a significant sensitivity of the spatial relaxation of the electrons on the indium vapour temperature and the initial conditions used to release electrons from the cathode into the space between the electrodes. The calculated electron transport coefficients are used as input for the classical fluid model, to investigate the inception and propagation of negative ionisation fronts in indium vapour at various E/N and vapour temperatures. We calculate the electron density, electric field, and velocity of ionisation fronts as a function of E/N and indium vapour temperature. The presence of indium atoms in the first metastable state significantly affects the characteristics of the negative ionisation fronts. The transition from an avalanche into a negative ionisation front occurs faster with increasing indium vapour temperature, due to enhanced ionisation and more efficient production of electrons at higher vapour temperatures. For lower values of E/N, the electron density behind the streamer front, where the electric field is screened, does not decay as one might expect for atomic gases, but it could be increased due to the accumulation of low-energy electrons that are capable of initiating ionisation in the streamer interior., The work of SD, JA, DB, MSR, DS, and BPM was sup- ˇported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, and the Institute of Physics (Belgrade). The work of KRH, OZ, and KB was supported by the United States National Science Foundation under Grant Nos. OAC-1834740, PHY-1803844, and PHY-2110023, and by the XSEDE supercomputer Allocation No. PHY-090031. The work of DVF and IB, was supported by the Australian Research Council and resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. FB and GG acknowledge partial financial support from the Spanish Ministry MICIU (Project Nos. FIS2016-80440 and PID2019-104727-RB-C21) and CSIC (Project No. LINKA20085). This work was also financially supported by the Australian Research Council (Project No. DP180101655).

Oleg Ivanovich Zatsarinny (Figure 1) was born in the city of Uzhgorod, Ukraine, on 4 November 1953 [...]

Two-color XUV plus near-IR multiphoton ionization of the helium ion by circularly polarized light is studied in the vicinity of the 3p resonance. Combining the analysis of results obtained by solving the time-dependent Schrodinger equation and that of the quasienergy spectrum of He+ reveals the physical mechanisms that determine the photoelectron spectra and the variation of the circular dichroism as a function of the near-IR intensity.

We revisit the excitation-tunneling process in nonsequential double ionization (NSDI) of helium in an 800-nm laser field. The correlated two-electron momentum distributions are calculated by using the improved quantitative rescattering (QRS) model, in which the lowering of the threshold energy due to the presence of an electric field at the instant of recollision is taken into account. In the framework of the QRS model, the correlated two-electron momentum distributions for excitation-tunneling in NSDI can be factorized as a product of the returning-electron wave packet (RWP) and the field-free differential cross section (DCS) for electron impact excitation of the parent ion multiplied by the tunneling ionization rate of electrons in the excited states. The RWPs, which describe the momentum distribution of the returning electrons, are obtained within the strong-field approximation for high-order above-threshold ionization. The DCSs for electron impact excitation of ${\mathrm{He}}^{+}$ are calculated using the state-of-the-art many-electron $R$-matrix theory, and the tunneling ionization rates for electrons in the excited states are evaluated by solving the time-dependent Schr\"odinger equation. The calculated correlated two-electron momentum distribution shows that the fourfold symmetry with regard to the parallel momentum components is broken. This is in contradiction to the prevalent view that the correlation pattern for excitation-tunneling in NSDI is symmetric with respect to the coordinate axes. By including the recollisional ($e,2e$) process, the predicted correlated two-electron momentum distributions are found to be in good qualitative agreement with experiment.