arXiv subjects

Jakub Benda

Publications and source records attributed to Jakub Benda.

16 recordsLinked to original sources

arXiv2026

On the measurability of Wigner time delays at shape resonances in photodetachment of polyatomic anions

The energy dependence of the complex phases of electron continuum wave functions carries information about electron dynamics. Streaking and attosecond interference experiments (called RABBIT) seek to measure this energy dependence, and therefore, the time delays of ionization. The long-range Coulomb interaction dominates in those experiments, and can obscure the low-energy features of the Wigner time delays that are the object of the measurement. Photodetachment of electrons from negative ions has no long-range Coulomb interaction, and RABBIT and streaking measurements of photodetachment delays have the potential to reveal time delays of up to one femtosecond in low-energy features. We predict the results of such experiments on a particularly interesting polyatomic example, the nitrate anion (NO$_3^-$), for both valence and core electron detachment. We simulate the experiments in these cases and analyze the underlying physics of measurements on polyatomic anions where many electron partial waves contribute and find that the angular dependence of the measured delays generally differs from the Wigner delays. However, we demonstrate that measurements performed for ejection directions close to the polarization of the light sources can directly access the Wigner delays that give a time-dependent window on electron-molecule interactions. A promising experiment involving core photodetachment of NO$_3^-$ with X-rays is proposed.

physics.atom-ph↗

arXiv2026

Disentangling the Effect of Ionic Coupling and Multiple Interfering Terms in Attosecond Molecular Interferometry

Attosecond interferometry in a two-color field is central to attosecond metrology and spectroscopy. In this technique, a photoelectron wave packet is released when a single photon from an extreme ultraviolet comb is absorbed. The wave packet then either emits or absorbs one or more near-infrared photons, leading to the formation of sidebands of the main photoelectron peaks. This picture applies well to atoms and assumes that the near-infrared laser pulse only acts on the photoelectron leaving the parent ion. The effect of the near-infrared pulse on the electronic structure of the cation is not considered, since the field usually cannot induce transitions between its electronic levels. Here, we demonstrate how dynamics induced by the near-infrared field in the cation can significantly impact the amplitude and phases of the sideband signal of the photoelectrons associated with specific dissociative channels of CO$_2$ molecules. This coupling of the near-infrared field with the molecular cation opens a third quantum pathway contributing to the signal measured in attosecond interferometry. Through angle- and energy-resolved characterization of the sideband oscillations, we observe reduction of interference amplitude over specific energy range upon angle integration. By comparison with theoretical predictions, we can isolate the contributions of specific interfering pathways to the two-color multi-pathway photoionization process. The scheme investigated in our work is general, and our observations highlight the importance of the additional pathway for accurately interpreting attosecond interferometry experiments involving molecules and more complex quantum systems.

physics.atom-ph↗

arXiv2026

Roadmap on Attosecond Science

Twenty-five years have passed since the first experimental demonstration of attosecond pulses, marking the advent of our ability to resolve and control electron motion in real time. What began as a technological breakthrough - generating the shortest flashes ever produced - has evolved into a powerful approach for probing and steering electronic dynamics in atoms, molecules, and solids. This roadmap, authored by leading experts in the field, surveys the recent rapid progress in the generation and characterization of attosecond pulses, emerging attosecond measurement and control techniques, and their expanding range of applications. It reviews current and future developments in attosecond light sources, including novel laser technologies, waveform synthesizers, new schemes for high-order harmonic generation, attosecond pulse generation at free-electron lasers, and structured light. Advances in attosecond measurement methodologies are also discussed, encompassing all-attosecond pump-probe spectroscopy, attosecond four-wave mixing, attosecond microscopy, spectroscopy with light transients, and attosecond interferometry. Furthermore, the roadmap addresses applications of attosecond spectroscopy to reveal electron dynamics in molecules and condensed matter systems from both theoretical and experimental perspectives, and highlights emerging directions at the interface with quantum optics and quantum entanglement. Overall, this work aims to serve as a comprehensive resource for navigating the evolving landscape of attosecond science.

physics.optics↗

arXiv2025

Effective Core Potentials for calculations of continuum spectra of molecules using the molecular R-matrix method

Implementation of Effective Core Potentials (ECPs) into the molecular scattering suite UKRmol+ is presented together with a set of calculations for a range of targets relevant for plasma modeling. Continuum description in scattering and photoionization calculations for large targets or high-energy electrons often requires the use of numerical continuum functions and the associated molecular integrals. We derive expressions for ECP integrals over B-spline type orbitals using their momentum-space representation and describe their implementation. Sample calculations are presented for electron collision from bromine molecule (Br$_2$), silicon tetrabromide (SiBr$_4$) and tungsten hydride (WH) as well as photoionisation of methyl iodide (CH$_3$I).

physics.chem-ph↗

arXiv2024

Angular momentum dependence in multiphoton ionization and attosecond time delays

Attosecond ionization time-delays at photoelectron energies above typically 10 eV are usually interpreted using the so called asymptotic approximation as a sum of the atomic or molecular delays with a universal laser-induced contribution. Here, we employ a two-harmonic RABITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transitions) configuration to isolate the multiphoton pathways and measure the ionization time delays as a function of the dressing field intensity. We show that the validity of the asymptotic theory can be extended to the threshold or to higher-order contributions by rigorously treating the angular-momentum dependence of the continuum-continuum transitions into universal and easily computable partial-wave-specific correction factors. Our asymptotic treatment is also valid for higher-order interfering amplitudes while significantly simplifying their evaluation and providing a transparent physical interpretation. The validity of the method for atomic and molecular targets in the vicinity of resonances, ionization thresholds, and for both the emission-integrated and angularly resolved signal is confirmed by comparison to ab initio calculations over a wide energy range.

physics.atom-ph↗

arXiv2024

Efficient configuration-interaction models for photoionization of molecular dimers

We present R-matrix calculations of photoionization of molecular monomers and dimers, focusing on ammonia (NH3) and formic acid (HCOOH), utilizing configuration-interaction models including the Occupation-Restricted Multiple Active Space (ORMAS) approach. We show that ORMAS is a highly efficient choice for calculating photoionization observables, yielding results that are in excellent agreement with those obtained using the much more demanding configuration-interaction method, Complete Active Space (CAS). We demonstrate that models incorporating single and double (SD) excitations with respect to the Hartree-Fock configuration provide good agreement with experimental data. The approach developed here can be readily applied to study photoionization in complex molecular systems.

physics.chem-ph↗

arXiv2024

Dipole-laser coupling delay in two-color (RABBITT) photoionization of polar molecules

We study theoretically the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) in strongly polar molecules. The time-dependent energy of a polar molecule in the infrared (IR) field gives rise to an additional dipole-laser coupling contribution to the sideband delay. In a time-independent picture this translates to the initial state becoming a linear combination of IR-dressed states. We extend the recently developed time-independent molecular R-matrix method to include the additional interfering ionization pathways arising from the IR-dressed initial state and obtain very good agreement with a reference non-perturbative time-dependent RABBITT simulation. We discuss the asymptotic behaviour of such ionization amplitudes and recover a known approximate asymptotic formula for the dipole-laser coupling delay derived earlier in the context of attosecond streaking. At low photon energies the dipole-laser coupling contributes significantly even in an unoriented molecular sample. Finally, we show that in photodetachment of polar singly charged negative ions the sideband delay is asymptotically proportional only to Wigner delay.

physics.atom-ph↗

arXiv2024

Anisotropy parameters for two-color photoionization phases in randomly oriented molecules: theory and experiment in methane and deuteromethane

We present a combined theoretical and experimental work investigating the angle-resolved phases of the photoionization process driven by a two-color field consisting of an attosecond pulse train and an infrared pulse in an ensemble of randomly oriented molecules. We derive a general form for the two-color photoelectron (and time-delay) angular distribution valid also in the case of chiral molecules and when relative polarizations of the photons contributing to the attosecond photoelectron interferometer differ. We show a comparison between the experimental data and the theoretical predictions in an ensemble of methane and deuteromethane molecules, discussing the effect of nuclear dynamics on the photoionization phases. Finally, we demonstrate that the oscillating component and the phase of the two-color signal can be fitted using complex asymmetry parameters, in perfect analogy with the atomic case.

physics.chem-ph↗

arXiv2023

UKRmol-scripts: a Perl-based system for the automated operation of the photoionization and electron/positron scattering suite UKRmol+

UKRmol-scripts is a set of Perl scripts to automatically run the UKRmol+ codes, a complex software suite based on the R-matrix method to calculate fixed-nuclei photoionization and electron- and positron-scattering for polyatomic molecules. Starting with several basic parameters, the scripts operatively produce all necessary input files and run all codes for electronic structure and scattering calculations as well as gather the more frequently required outputs. The scripts provide a simple way to run such calculations for many molecular geometries concurrently and collect the resulting data for easier post-processing and visualization. We describe the structure of the scripts and the input parameters as well as provide examples for photoionization and electron and positron collisions with molecules. The codes are freely available from Zenodo.

physics.comp-ph↗

arXiv2023

Isotopic effects in molecular attosecond photoelectron interferometry

Isotopic substitution in molecular systems can affect fundamental molecular properties including the energy position and spacing of electronic, vibrational and rotational levels, thus modifying the dynamics associated to their coherent superposition. In extreme ultraviolet spectroscopy, the photoelectron leaving the molecule after the absorption of a single photon can trigger an ultrafast nuclear motion in the cation, which can lead, eventually, to molecular fragmentation. This dynamics depends on the mass of the constituents of the cation, thus showing, in general, a significant isotopic dependence. In time-resolved attosecond photoelectron interferometry, the absorption of the extreme ultraviolet photon is accompanied by the exchange of an additional quantum of energy (typically in the infrared spectral range) with the photoelectron-photoion system, offering the opportunity to investigate in time the influence of isotopic substitution on the characteristics of the photoionisation dynamics. Here we show that attosecond photoelectron interferometry is sensitive to isotopic substitution by investigating the two-color photoionisation spectra measured in a mixture of methane (CH$_4$) and deuteromethane (CD$_4$). The isotopic dependence manifests itself in the modification of the amplitude and contrast of the oscillations of the photoelectron peaks generated in the two-color field with the two isotopologues. The observed effects are interpreted considering the differences in the time evolution of the nuclear autocorrelation functions in the two molecules.

physics.atom-ph↗

arXiv2022

Theory of nuclear motion in RABBITT spectra

Reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) is a powerful photoelectron spectroscopy, offering direct access to internal dynamics of the target. It is being increasingly applied to molecular systems, but a general, computationally tractable theory of RABBITT spectra in molecules has so far been lacking. We show that under quite general assumptions, RABBITT spectra in molecules can be expressed as a convolution of the vibronic cross-correlation functions and two-electron photoionization matrix elements. We specialize the general expressions to the commonly-encountered special cases. We expect our theory to enable accurate modeling and interpretation of molecular RABBITT spectra in most medium-sized molecules.

physics.chem-ph↗

arXiv2022

Multi-photon above threshold ionization of multi-electron atoms and molecules using the R-matrix approach

We formulate a computationally efficient time-independent method based on the multi-electron molecular R-matrix formalism. This method is used to calculate transition matrix elements for the multi-photon ionization of atoms and molecules under the influence of a perturbative field. The method relies on the partitioning of space which allows us to calculate the infinite-range free-free dipole integrals analytically in the outer region, beyond the range of the initial bound wave function. This approach is valid for an arbitrary order, that is, any number of photons absorbed both in the bound and the continuum part of the spectrum (below- and above-threshold ionization). We calculate generalized multi-photon cross sections and angular distributions of different systems (H, He, H2, CO2) and validate our approach by comparison with data from the literature.

physics.atom-ph↗

arXiv2022

Analysis of RABITT time delays using the stationary multi-photon molecular R-matrix approach

We employ the recently developed multi-photon R-matrix method for molecular above-threshold photoionization to obtain second-order ionization amplitudes that govern the interference in RABITT experiments. This allows us to extract RABITT time delays that are in better agreement with non-perturbative time-dependent simulations of this process than the typically used combination of first-order (Wigner) delays and asymptotic corrections. We calculate molecular-frame as well as orientation-averaged RABITT delays for H$_2$, N$_2$, CO$_2$, H$_2$O and N$_2$O and analyze the origin of various structures in the time delays including the effects of partial wave interference, shape resonances and orientation-averaging. Time-delays for B and C states of CO$_2^{+}$ are strongly affected by absorption of the second (IR) photon in the ion. This effect corresponds to an additional contribution, $τ_{\text{coupl}}$, to the asymptotic approximation for the RABITT delays $τ\approx τ_{\text{mol}} +τ_{cc} + τ_{\text{coupl}}$. Applicability of the asymptotic theory depends on the target and IR photon energy but typically starts at approximately 30 -- 35 eV of XUV photon energy.

physics.atom-ph↗

arXiv2021

Enhancing spin polarization using ultrafast angular streaking

Through solution of the multielectron, semi-relativistic, time-dependent Schrödinger equation, we show that angular streaking produces strongly spin-polarized electrons in a noble gas. The degree of spin polarization increases with the Keldysh parameter, so that angular streaking -- ordinarily applied to investigate tunneling -- may be repurposed to generate strongly spin-polarized electron bunches. Additionally, we explore modifications of the angular streaking scheme that also enhance spin polarization.

physics.atom-ph↗

arXiv2019

RMT: R-matrix with time-dependence. Solving the semi-relativistic, time-dependent Schrodinger equation for general, multi-electron atoms and molecules in intense, ultrashort, arbitrarily polarized laser pulses

RMT is a program which solves the time-dependent Schrodinger equation for general, multielectron atoms, ions and molecules interacting with laser light. As such it can be used to model ionization (single-photon, multi-photon and strong-field), recollision (high-harmonic generation, strong-field rescattering), and more generally absorption or scattering processes with a full account of the multielectron correlation effects in a time-dependent manner. Calculations can be performed for targets interacting with ultrashort, intense laser pulses of long-wavelength and arbitrary polarization. Calculations for atoms can optionally include the Breit-Pauli correction terms for the description of relativistic (in particular, spin-orbit) effects.

physics.comp-ph↗

arXiv2019

UKRmol+: a suite for modelling of electronic processes in molecules interacting with electrons, positrons and photons using the R-matrix method

UKRmol+ is a new implementation of the UK R-matrix electron-molecule scattering code. Key features of the implementation are the use of quantum chemistry codes such as Molpro to provide target molecular orbitals; the optional use of mixed Gaussian -- B-spline basis functions to represent the continuum and improved configuration and Hamiltonian generation. The code is described, and examples covering electron collisions from a range of targets, positron collisions and photionisation are presented. The codes are freely available as a tarball from Zenodo.

physics.comp-ph↗