arXiv subjects

Armin Scrinzi

Publications and source records attributed to Armin Scrinzi.

At least 19 recordsLinked to original sources

arXiv2025

A general approximator for strong-field ionization rates

We address the long-standing problem of determining accurate, time-resolved ionization rates for atoms in strong laser fields, a quantity that is fundamental to attosecond science. We show that it is possible to retrieve sub-optical-cycle dynamics of strong-field ionization from ionization probabilities obtained for a set of few-cycle laser pulses that covers a sufficiently broad parameter space. To this end, we introduce the General Approximator for Strong-Field Ionization Rates (GASFIR), a retrieval tool that uses a few adjustable parameters to accurately reconstruct ab initio data. By imposing only essential physical constraints, our model provides a versatile framework for time-domain investigations of strong-field ionization and the role of ionization dynamics in attosecond metrology and lightwave electronics.

physics.atom-ph↗

arXiv2024

Single-photon and multi-photon Fano lines for helium and neon using tRecX-haCC

Single-photon and multi-photon ionization of helium and neon atoms by ultrashort extreme ultraviolet radiation is studied. The wavelength of radiation is chosen to excite a set of doubly excited states. We analyse the lineshapes arising from the presence of doubly excited states and numerically demonstrate the sensitivity of the Fano $q$ parameter to the order of multi-photon process. The computations were performed using the tRecX-haCC package developed by the authors. The tRecX-haCC package solves the time-dependent Schr\"odinger equation in the presence of a laser pulse and computes the photoelectron spectra including the narrow resonance peaks using the tSurff and iSurf techniques.

physics.atom-ph↗

arXiv2024

The hybrid anti-symmetrized coupled channels method (haCC) for the tRecX code

We present a new implementation of the hybrid antisymmetrized Coupled Channels (haCC) method in the framework of the tRecX [A. Scrinzi, Comp. Phys. Comm., 270:108146, 2022.]. The method represents atomic and molecular multi-electron functions by combining CI functions, Gaussian molecular orbitals, and a numerical single-electron basis. It is suitable for describing high harmonic generation and the strong-field dynamics of ionization. Fully differential photo\-emission spectra are computed by the tSurff method. The theoretical background of haCC is outlined and key improvements compared to its original formulation are highlighted. We discuss control of over-completeness resulting from the joint use of the numerical basis and Gaussian molecular orbitals by pseudo-inverses based on the Woodbury formula. Further new features of this tRecX release are the iSurff method, new input features, and the AMOS gateway interface. The mapping of haCC into the tRecX framework for solving the time-dependent Schr\"odinger equation is shown. Use, performance, and accuracy of haCC are discussed on the examples of high-harmonic generation and strong-field photo-emission by short laser pulses impinging on the Helium atom and on the linear molecules $N_2$ and $CO$.

physics.comp-ph↗

arXiv2021

tRecX -- an environment for solving time-dependent Schrödinger-like problems

TRecX is a C++ code for solving generalized inhomogeneous time-dependent Schrödinger-type equations $idΨ/dt = H[t,Ψ] + Φ$ in arbitrary dimensions and in a variety of coordinate systems. The operator $H[t,Ψ]$ may have simple non-linearities, as in Gross-Pitaevskii and Hartree(-Fock) problems. Primary application of tRecX has been non-perturbative strong-field single and double photo-electron emission in atomic and molecular physics. The code is designed for large-scale {\it ab initio} calculations, for exploring models, and for advanced teaching in computational physics. Distinctive numerical methods are the time-dependent surface flux method for the computation of single and double emission spectra and exterior complex scaling for absorption. Wave functions and operators are handled by tree-structures with the systematic use of recursion on the coarse-grain level. Numerical, analytic, and grid-based discretizations can be combined and are treated on the same abstract level. Operators are specified in the input using a script language including symbolic algebra. User-friendly in- and output, error safety, and documentation are integrated by design.

physics.comp-ph↗

arXiv2021

Potentials, exchange, and correlation in attosecond photoemission delays

We investigate attosecond time delays in the emission of photoelectrons using a hierarchy of models of the $CO_2$ molecule including the strong field approximation, Coulomb-scattering, short-range parts of the molecular potential, Hartree and Hartree-Fock descriptions. In addition, we present an {\it ab initio} calculation based on quantum-chemical structure in combination with strong-field techniques, which fully includes multi-electron exchange and correlation. Every single of these model constituents is found to modify delays on the scale of 10 as or more, with exchange and correlation having the most pronounced effect.

physics.atom-ph↗

arXiv2020

Nonlocal mechanisms of attosecond interferometry in three-dimensional systems

Attosecond interferometry (AI) is an experimental technique based on ionizing a system with an attosecond pulse train in the presence of an assisting laser. This assisting laser provides multiple pathways for the photoelectron wave packet to reach the same final state, and interference of these pathways can be used to probe properties of matter. The mechanism of AI is well-understood for isolated atoms and molecules in the gas phase, but not so much in the condensed phase, especially if the substrate under study is transparent. Then, additional pathways open up for the electron due to scattering from neighbouring atoms. We investigate to what extent these additional pathways influence the measured photoionization delay with the help of one- and three-dimensional model systems. In both cases, we find that the total delay can be expressed as the sum of a local (photoionization) delay and a non-local delay which contains the effect of electron scattering during transport. The 1D system shows that the non-local delay is an oscillatory function of the distance between the sites where ionization and scattering take place. A similar result is obtained in 3D, but the modulation depth of the non-local delay is found to strongly depend on the effective scattering cross section. We conclude that attosecond interferometry of disordered systems like liquids at low photon energies (20-30 eV) is mainly sensitive to the local delay, i.e., to changes of the photoionization dynamics induced by the immediate environment of the ionized entity, and less to electron scattering during transport through the medium.

physics.chem-ph↗

arXiv2020

A Science Gateway for Atomic and Molecular Physics

We describe the creation of a new Atomic and Molecular Physics science gateway (AMPGateway). The gateway is designed to bring together a subset of the AMP community to work collectively to make their codes available and easier to use by the partners as well as others. By necessity, a project such as this requires the developers to work on issues of portability, documentation, ease of input, as well as making sure the codes can run on a variety of architectures. Here we outline our efforts to build this AMP gateway and future directions.

physics.comp-ph↗

arXiv2019

Electron double-emission spectra for Helium atoms in intense 400 nm laser pulses

Double photoelectron emission from He atoms by intense laser pulses with a wave length of $394.5\,nm$ is computed for intensities $3.5 - 9.2\times 10^{14}W/cm^2$. Joint momentum distributions confirm the characteristics seen in classical trajectory calculations. The pronounced transition from back-to-back to side-by-side emission with increasing intensity, the $He^{++}/He^+$ ratios, and a modulation of joint energy spectra agree well with a recent experiment [Henrichs et al., PRA 98, 43405 (2018)], if one admits an increase of experimental intensities by a factor $\sim 2$. We find that Freeman resonances enhance anti-correlated emission, we identify the signature of electron repulsion in joint angular distributions, and we interpret the modulation of joint energy spectra as a signature of multiple recollsions.

physics.atom-ph↗

arXiv2017

Implementation of infinite-range exterior complex scaling to the time-dependent complete-active-space self-consistent-field method

We present a numerical implementation of the infinite-range exterior complex scaling (irECS) [Phys. Rev. A 81, 053845 (2010)] as an efficient absorbing boundary to the time-dependent complete-active-space self-consistent field (TD-CASSCF) method [Phys. Rev. A 94, 023405 (2016)] for multielectron atoms subject to an intense laser pulse. We introduce Gauss-Laguerre-Radau quadrature points to construct discrete variable representation basis functions in the last radial finite element extending to infinity. This implementation is applied to strong-field ionization and high-harmonic generation in He, Be, and Ne atoms. It efficiently prevents unphysical reflection of photoelectron wave packets at the simulation boundary, enabling accurate simulations with substantially reduced computational cost, even under significant (~ 50%) double ionization. For the case of a simulation of high-harmonic generation from Ne, for example, 80% cost reduction is achieved, compared to a mask-function absorption boundary.

physics.atom-ph↗

arXiv2017

Multielectron effects in strong field ionization of CO$_2$: impact on differential photoelectron spectra

We report fully differential photoelectron spectra from an {\it ab-inito} coupled channels treatment of CO$_2$. Photoionization by laser pulses centered at 400 nm and 800 nm wavelength are considered, with arbitrary molecular alignment and polarization (linear and elliptic). Calculations reveal significant excited state channel contributions that are, at certain molecular orientations, an order of magnitude larger than the ground state channel in the rescattering plateau. Partial wash out of the multiphoton structure in the ATI spectra and of the nodal features in angle resolved spectra is observed due to ionization to excited state channels. The qualitative nature of the spectra is determined by ionization thresholds, orbital symmetries and interchannel coupling in the order of precedence.

physics.atom-ph↗

arXiv2015

Double photo-electron momentum spectra of Helium at infrared wavelength

Double photo-electron momentum spectra of the Helium atom are calculated \textit{ab initio} at extreme ultra-violet and near infrared wavelengths. At short wavelengths two-photon double ionization yields, two-electron energy spectra, and triply differential cross sections agree with results from recent literature. At the near infrared wavelength of $780\,nm$ the experimental single-to-double ionization ratio is reproduced up to intensities of $4\times 10^{14}W/cm^2$, and two-electron energy spectra and joint angular distributions are presented. The time-dependent surface flux (tSurff) approach is extended to full 3+3 spatial dimensions and systematic error control is demonstrated. We analyze our differential spectra in terms of an experimentally accessible quantitative measure of correlation.

physics.comp-ph↗

arXiv2015

Perfect absorption in Schr\"odinger-like problems using non-equidistant complex grids

Two non-equidistant grid implementations of infinite range exterior complex scaling are introduced that allow for perfect absorption in the time dependent Schr\"odinger equation. Finite element discrete variables grid discretizations provide as efficient absorption as the corresponding finite elements basis set discretizations. This finding is at variance with results reported in literature [L. Tao et al., Phys. Rev. A 48, 063419 (2009)]. For finite differences, a new class of generalized $Q$-point schemes for non-equidistant grids is derived. Convergence of absorption is exponential $\sim \Delta x^{Q-1}$ and numerically robust. Local relative errors $\sim10^{-9}$ are achieved in a standard problem of strong-field ionization.

physics.comp-ph↗

arXiv2015

Anomalous Fano Profiles in External Fields

We show that external control of Fano resonances in general leads to complex Fano $q$-parameters. Fano line shapes of photo-electron and transient absorption spectra in presence of an infrared control field are investigated. Computed transient absorption spectra are compatible with a recent experiment [C. Ott {\it et al.}, Science 340, 716 (2013)] but suggest a modification of the interpretation proposed there. Control mechanisms for photo-electron spectra are exposed: control pulses applied {\em during} excitation modify the line shapes by momentum boosts of the continuum electrons. Pulses arriving {\em after} excitation generate interference fringes due to infrared two-photon transitions.

physics.atom-ph↗

arXiv2015

Dynamic exchange in the strong field ionization of molecules

We show that dynamic exchange is a dominant effect in strong field ionization of molecules. In $CO_2$ it fixes the peak ionization yield at the experimentally observed angle of $45^\circ$ between polarization direction and the molecular axis. In $N_2$ it changes the alignment dependence of yields by up to a factor of 2. The effect appears on the Hartree-Fock level as well as in full {\it ab initio} solutions of the Schrödinger equation.

physics.atom-ph↗

arXiv2015

Interpreting Attoclock Measurements of Tunnelling Times

Resolving in time the dynamics of light absorption by atoms and molecules, and the electronic rearrangement this induces, is among the most challenging goals of attosecond spectroscopy. The attoclock is an elegant approach to this problem, which encodes ionization times in the strong-field regime. However, the accurate reconstruction of these times from experimental data presents a formidable theoretical challenge. Here, we solve this problem by combining analytical theory with ab-initio numerical simulations. We apply our theory to numerical attoclock experiments on the hydrogen atom to extract ionization time delays and analyse their nature. Strong field ionization is often viewed as optical tunnelling through the barrier created by the field and the core potential. We show that, in the hydrogen atom, optical tunnelling is instantaneous. By calibrating the attoclock using the hydrogen atom, our method opens the way to identify possible delays associated with multielectron dynamics during strong-field ionization.

physics.atom-ph↗

arXiv2014

Photoionization of few electron systems with a hybrid Coupled Channels approach

We present the hybrid anti-symmetrized coupled channels method for the calculation of fully differential photo-electron spectra of multi-electron atoms and small molecules interacting with strong laser fields. The method unites quantum chemical few-body electronic structure with strong-field dynamics by solving the time dependent Schrödinger equation in a fully anti-symmetrized basis composed of multi-electron states from quantum chemistry and a one-electron numerical basis. Photoelectron spectra are obtained via the time dependent surface flux (tSURFF) method. Performance and accuracy of the approach are demonstrated for spectra from the helium and berryllium atoms and the hydrogen molecule in linearly polarized laser fields at wavelength from 21 nm to 400 nm. At long wavelengths, helium and the hydrogen molecule at equilibrium inter-nuclear distance can be approximated as single channel systems whereas beryllium needs a multi-channel description.

physics.comp-ph↗

arXiv2014

Mixed gauge in strong laser-matter interaction

We show that the description of laser-matter interaction in length gauge at short short and in velocity gauge at longer distances allows for compact physical modeling in terms of field free states, rapidly convergent numerical approximation, and efficient absorption of outgoing flux. The mathematical and numerical framework for using mixed gauge in practice is introduced. We calculate photoelectron spectra generated by a laser field at wavelengths of 400$\sim$800 nm from single-electron systems and from the helium atom and hydrogen molecule. We assess the accuracy of coupled channels calculations by comparison to full two-electron solutions of the time-dependent Schrodinger equation and find substantial advantages of mixed over velocity and length gauges.

physics.comp-ph↗

arXiv2014

Attosecond Photoscopy of Plasmonic Excitations

We propose an experimental arrangement to image, with attosecond resolution, transient surface plasmonic excitations. The required modifications to state-of-the-art setups used for attosecond streaking experiments from solid surfaces only involve available technology. Buildup and life times of surface plasmon polaritons can be extracted and local modulations of the exciting optical pulse can be diagnosed {\it in situ}.

physics.optics↗