arXiv subjects

H. W. van der Hart

Publications and source records attributed to H. W. van der Hart.

At least 19 recordsLinked to original sources

arXiv2020

Electron correlation and short-range dynamics in attosecond angular streaking

We employ the R-matrix with time-dependence method to study attosecond angular streaking of F$^-$. Using this negative ion, free of long-range Coulomb interactions, we elucidate the role of short-range electron correlation effects in an attoclock scheme. Through solution of the multielectron time-dependent Schrodinger equation, we aim to bridge the gap between experiments using multielectron targets, and one-electron theoretical approaches. We observe significant negative offset angles in the photoelectron momentum distributions, despite the short-range nature of the binding potential. We show that the offset angle is sensitive to the atomic structure description of the residual F atom. We also investigate the response of co- and counter-rotating electrons, and observe an angular separation in their emission.

physics.atom-ph↗

arXiv2019

Modeling tomographic measurements of photoelectron vortices in counter-rotating circularly polarized laser pulses

Recent experiments [D. Pengel, S. Kerbstadt, L. Englert, T. Bayer, and M. Wollenhaupt, \href{https://journals.aps.org/pra/abstract/10.1103/PhysRevA.96.043426}{{\PRA} {\bf 96} 043426 (2017)}] have measured the photoelectron momentum distribution for three-photon ionization of potassium by counter-rotating circularly polarized 790-nm laser pulses. The distribution displays spiral vortices, arising from the interference of ionizing wavepackets with different magnetic quantum numbers. The high level of multidimensional detail observed in the distribution makes this an ideal case in which to demonstrate the accuracy of emerging theoretical techniques applicable to such problems. We use the $R$-matrix with time dependence approach to investigate this process. We calculate the full-dimensional photoelectron momentum distribution, and compare against a set of planar projections of this distribution previously measured in experiment.

physics.atom-ph↗

arXiv2019

Electron rotational asymmetry in strong-field photodetachment from F$^-$ by circularly polarized laser pulses

We use the $R$-matrix with time-dependence method to study detachment from F$^-$ in circularly-polarized laser fields of infrared wavelength. By decomposing the photoelectron momentum distribution into separate contributions from detached $2p_1$ and $2p_{-1}$ electrons, we demonstrate that the detachment yield is distributed asymmetrically with respect to these initial orbitals. We observe the well-known preference for strong-field detachment of electrons that are initially counter-rotating relative to the field, and calculate the variation in this preference as a function of photoelectron energy. The wavelengths used in this work provide natural grounds for comparison between our calculations and the predictions of analytical approaches tailored for the strong-field regime. In particular, we compare the ratio of counter-rotating electrons to corotating electrons as a function of photoelectron energy. We carry out this comparison at two wavelengths, and observe good qualitative agreement between the analytical predictions and our numerical results.

physics.atom-ph↗

arXiv2018

$R$-matrix with Time-dependence Theory for Ultrafast Atomic Processes in Arbitrary Light Fields

We describe an ab initio and non-perturbative $R$-matrix with time-dependence theory for ultrafast atomic processes in light fields of arbitrary polarization. The theory is applicable to complex, multielectron atoms and atomic ions subject to ultrashort (particularly few-femtosecond and attosecond) laser pulses with any given ellipticity, and generalizes previous time-dependent $R$-matrix techniques restricted to linearly polarized fields. We discuss both the fundamental equations, required to propagate the multielectron wavefunction in time, as well as the computational developments necessary for their efficient numerical solution. To verify the accuracy of our approach, we investigate the two-photon ionization of He, irradiated by a pair of time-delayed, circularly polarized, femtosecond laser pulses, and compare photoelectron momentum distributions, in the polarization plane, with those obtained from recent time-dependent close-coupling calculations. The predictive capabilities of our approach are further demonstrated through a study of single-photon detachment from F$^{-}$ in a circularly polarized, femtosecond laser pulse, where the relative contribution of the co- and counter-rotating $2p$ electrons is quantified.

physics.atom-ph↗

arXiv2018

On the Cooper Minimum in Singly ionized and Neutral Argon

We present an analysis of the appearance of the Cooper Minimum in singly ionized argon in both the photoionization cross-section (PICS) and high-harmonic generation (HHG) spectrum. We employ two computational approaches based on the same R-matrix technique to provide a coherent description of the atomic structure of the Ar+ system, finding that the PICS and HHG spectrum are affected differently by the inclusion of additional residual ion states and the improved description of correlation effects. Both the PICS and HHG spectrum possess a clear minimum for all atomic structure models used, with the centre of the minimum at 55 eV in the PICS and 60 eV in the HHG spectrum for the most complete description employed. The HHG minimum is systematically shifted to higher energies with respect to the PICS minimum. We also find that the initial magnetic alignment (magnetic quantum number) of the Ar+ system does not affect substantially the position and shape of the HHG minimum (given a sufficiently detailed atomic structure description), but the harmonic yield is enhanced by two-orders of magnitude for M_L = 1 over M_L = 0. We also perform similar calculations for neutral argon, finding that this system is more sensitive to enhancements in the atomic structure description.

physics.atom-ph↗

arXiv2018

Extreme-ultraviolet-initiated High-harmonic Generation in Ar$^{+}$

We employ the R-matrix with time-dependence method to investigate extreme-ultraviolet-initiated high-harmonic generation (XIHHG) in Ar$^{+}$. Using a combination of extreme-ultraviolet (XUV, $92\textrm{ nm}$, $3\times 10^{12}\,\textrm{Wcm}^{-2}$) and time-delayed, infrared (IR, $800\textrm{ nm}$, $3\times 10^{14}\,\textrm{Wcm}^{-2}$) laser pulses, we demonstrate that control over both the mechanism, and timing, of ionization can afford significant enhancements in the yield of plateau, and sub-threshold, harmonics alike. The presence of the XUV pulse is also shown to alter the relative contribution of different electron emission pathways. Manifestation of the Ar$^{+}$ electronic structure is found in the appearance of a pronounced Cooper minimum. Interferences amongst the outer-valence $3p$, and inner-valence $3s$, electrons are found to incur only a minor suppression of the harmonic intensities, at least for the present combination of XUV and IR laser light. Additionally, the dependence of the XIHHG efficiency on time delay is discussed, and rationalized with the aid of classical trajectory simulations.

physics.atom-ph↗

arXiv2017

Pulse-shape control of two-color interference in high-order-harmonic generation

We report on calculations of harmonic generation by neon in a mixed (800-nm + time-delayed 400-nm) laser pulse scheme. In contrast with previous studies we employ a short (few-cycle) 400-nm pulse, finding that this affords control of the interference between electron trajectories contributing to the cutoff harmonics. The inclusion of the 400-nm pulse enhances the yield and cutoff energy, both of which exhibit a strong dependence on the time delay between the two pulses. Using a combination of time-dependent R-matrix theory and a classical trajectory model, we assess the mechanisms leading to these effects.

physics.atom-ph↗

arXiv2016

XUV-initiated high harmonic generation: driving inner valence electrons using below-threshold-energy XUV light

We propose a novel scheme for resolving the contribution of inner- and outer-valence electrons in XUV-initiated high-harmonic generation in neon. By probing the atom with a low energy (below the 2s ionisation threshold) ultrashort XUV pulse, the 2p electron is steered away from the core, while the 2s electron is enabled to describe recollision trajectories. By selectively suppressing the 2p recollision trajectories we can resolve the contribution of the 2s electron to the high-harmonic spectrum. We apply the classical trajectory model to account for the contribution of the 2s electron, which allows for an intuitive understanding of the process.

physics.atom-ph↗

arXiv2015

Multiphoton inner-shell ionization of the carbon atom

We apply time-dependent R-matrix theory to study inner-shell ionization of C atoms in ultra-short high-frequency light fields with a photon energy between 170 and 245 eV. At an intensity of 10$^{17}$ W/cm$^2$, ionization is dominated by single-photon emission of a $2\ell$ electron, with two-photon emission of a 1s electron accounting for about 2-3\% of all emission processes, and two-photon emission of $2\ell$ contributing about 0.5-1\%. Three-photon emission of a 1s electron is estimated to contribute about 0.01-0.03\%. Around a photon energy of 225 eV, two-photon emission of a 1s electron, leaving C$^+$ in either 1s2s2p$^3$ or 1s2p$^4$ is resonantly enhanced by intermediate 1s2s$^2$2p$^3$ states. The results demonstrate the capability of time-dependent R-matrix theory to describe inner-shell ionization processes including rearrangement of the outer electrons.

physics.atom-ph↗

arXiv2015

Double Ionisation in R-Matrix Theory Using a 2-electron Outer Region

We have developed a two-electron outer region for use within R-matrix theory to describe double ionisation processes. The capability of this method is demonstrated for single-photon double ionisation of He in the photon energy region between 80 eV to 180 eV. The cross sections are in agreement with established data. The extended RMT method also provides information on higher-order processes, as demonstrated by the identification of signatures for sequential double ionisation processes involving an intermediate He$^{+}$ state with $n=2$.

physics.atom-ph↗

arXiv2015

Electron rescattering in strong-field photodetachment of F$^-$

We present ab initio studies of photoelectron spectra for above threshold detachment (ATD) of F$^-$ anions in short, 1300 nm and 1800 nm laser pulses. We identify and assess the importance of electron rescattering in strong-field photodetachment of a negative ion through comparison with an analytic, Keldysh-type approach, demonstrating the capability of ab-initio computation in the challenging near-IR regime. We further assess the influence of the strong electron correlation on the photodetachment.

physics.atom-ph↗

arXiv2014

Harmonic generation of noble-gas atoms in the Near-IR regime using ab-initio time-dependent R-matrix theory

We demonstrate the capability of ab-initio time-dependent R-matrix theory to obtain accurate harmonic generation spectra of noble-gas atoms at Near-IR wavelengths between 1200 and 1800 nm and peak intensities up to 1.8 X 10(14) W/cm(2) . To accommodate the excursion length of the ejected electron, we use an angular-momentum expansion up to Lmax = 279. The harmonic spectra show evidence of atomic structure through the presence of a Cooper minimum in harmonic generation for Kr, and of multielectron interaction through the giant resonance for Xe. The theoretical spectra agree well with those obtained experimentally.

physics.atom-ph↗

arXiv2014

Angular distributions in two-colour two-photon ionization of He

We present R-Matrix with time dependence (RMT) calculations for the photoionization of helium irradiated by an EUV laser pulse and an overlapping IR pulse with an emphasis on the anisotropy parameters of the sidebands generated by the dressing laser field. We investigate how these parameters depend on the amount of atomic structure included in the theoretical model for two-photon ionization. To verify the accuracy of the RMT approach, our theoretical results are compared with experiment.

physics.atom-ph↗

arXiv2014

Electron dynamics in the carbon atom induced by spin-orbit interaction

We use R-Matrix theory with Time dependence (RMT) to investigate multiphoton ionization of ground-state atomic carbon with initial orbital magnetic quantum number $M_L$=0 and $M_L$=1 at a laser wavelength of 390 nm and peak intensity of 10$^{14}$ W cm$^{-2}$. Significant differences in ionization yield and ejected-electron momentum distribution are observed between the two values for $M_L$. We use our theoretical results to model how the spin-orbit interaction affects electron emission along the laser polarization axis. Under the assumption that an initial C atom is prepared at zero time delay with $M_L=0$, the dynamics with respect to time delay of an ionizing probe pulse modelled using RMT theory is found to be in good agreement with available experimental data.

physics.atom-ph↗

arXiv2014

Population trapping in bound states during IR-assisted ultra-fast photoionization of Ne$^+$

We have investigated photoionization of Ne$^+$ in the combined field of a short infra-red laser pulse and a delayed ultra-short pulse of the infra-red laser's 23$^r$$^d$ harmonic. We observe an ionization yield compatible with a picture in which one electron gets excited into Rydberg states by the harmonic laser field and is subsequently removed by the infra-red laser field. Modulations are seen in the ionization yield as a function of time delay. These modulations originate from the trapping of population in low members of the Rydberg series with different states being populated at different ranges of delay times. The calculations further demonstrate that single-threshold calculations cannot reproduce the Ne$^+$ photoionization yields obtained in multi-threshold calculations.

physics.atom-ph↗

arXiv2014

Time-dependent R-matrix theory applied to two-photon double ionization of He

We introduce a time-dependent R-matrix theory generalised to describe double ionization processes. The method is used to investigate two-photon double ionization of He by intense XUV laser radiation. We combine a detailed B-spline-based wavefunction description in a extended inner region with a single-electron outer region containing channels representing both single ionization and double ionization. A comparison of wavefunction densities for different box sizes demonstrates that the flow between the two regions is described with excellent accuracy. The obtained two-photon double ionization cross sections are in excellent agreement with other cross sections available. Compared to calculations fully contained within a finite inner region, the present calculations can be propagated over the time it takes the slowest electron to reach the boundary.

physics.atom-ph↗

arXiv2014

Multichannel Interference In High-Order Harmonic Generation From Ne(+) Driven By Ultrashort Intense Laser Pulse

We apply the time-dependent R-matrix method to investigate harmonic generation from Ne(+) at a wavelength of 390 nm and intensities up to 10(15) Wcm(-2). The 1s(2)2s(2)2p(4) (3)P(e), (1)D(e), and (1)S(e) states of Ne(2+) are included as residual-ion states to assess the influence of interference between photoionization channels associated with these thresholds. The harmonic spectrum is well approximated by calculations in which only the (3)P(e) and (1)D(e) thresholds are taken into account, but no satisfactory spectrum is obtained when a single threshold is taken into account. Within the harmonic plateau, extending to about 100 eV, individual harmonics can be suppressed at particular intensities when all Ne(2+) thresholds are taken into account. The suppression is not observed when only a single threshold is accounted for. Since the suppression is dependent on intensity, it may be difficult to observe experimentally.

physics.atom-ph↗

arXiv2013

The influence of multiple ionization thresholds on harmonic generation: Ar+

We apply time-dependent R-matrix theory to investigate harmonic generation from ground state Ar+ with M=0 at a wavelength of 390 nm. Contributions associated with the different 3s(2)3p(4) ionization thresholds are assessed, including the interference between these. The dominant contribution originates from the second ionization threshold, 3s(2)3p(4) (1)D. Changes to the harmonic yields arising from the higher 3s3p(5) thresholds are also assessed. We further confirm that Ar+ has a higher harmonic yield than He for the same laser pulse, despite having a higher ionization threshold.

physics.atom-ph↗