Ultrafast Atomic Dynamics in Arbitrary Light Fields

Lead Research Organisation: Queen's University of Belfast
Department Name: Sch of Mathematics and Physics

Abstract

Throughout the last two decades, experimental developments in the field of attosecond optics have driven an unprecedented advancement in atomic and molecular physics. The attosecond represents the natural timescale for electron motion in atoms, and to capture the properties of this motion, we must achieve a temporal resolution even shorter. The feasibility of generating ultrashort light pulses, with durations as low as 70 attoseconds, has enabled the time-resolved investigation of a myriad of atomic excitation and ionisation processes, whose evolution is influenced profoundly by dynamic multi-electron correlations.

To complement the experimental insight acquired with attosecond light sources, we are obligated to develop sophisticated theoretical techniques, possessing the accuracy and flexibility needed to model ultrafast many-body dynamics in complex atomic systems. Over the last 10 years, at Queen's University Belfast (QUB), ab initio R-matrix with time-dependence (RMT) theory and the associated computer codes have been elaborated for precisely this purpose. The theory is unique in that it can be applied to a plethora of strong-field processes in general atoms, and is capable of yielding accurate data relevant for a broad range of experimentally accessible laser wavelengths, extending from the extreme ultraviolet (XUV) to the computationally demanding mid-infrared range.

To ensure that first-principles theory continues to play a role complementary to that of experiment, we must develop the existing R-matrix codes in a manner that reflects recent experimental trends. In particular, whilst many experiments to date have been performed with linearly polarised light, interest is increasingly shifting to laser fields with arbitrary polarisation. The latter could permit completely three-dimensional control over the ultrafast, laser-driven electron motion, and may thereby facilitate novel optical technologies with unparalleled precision and reliability. Recent examples include the production of isolated attosecond pulses, the possibility of measuring time with attosecond accuracy (the so-called attoclock), and the generation of circularly polarised harmonic radiation, with important implications for the future design of circularly polarised XUV laser sources.

The present computational implementation of RMT theory is confined to multi-electron dynamics driven by linearly polarised light fields. We propose the adaptation of the existing codes to the treatment of atoms in circularly polarised fields. Following this development, we aim to address a number of strong-field problems that have attracted recent experimental interest. We devote particular attention to the dynamics of negative halide ions subject to intense, circularly polarised light, as well as the novel attoclock scheme, for which detailed first-principles studies have, to date, been lacking. We anticipate that this work would yield both a more fundamental understanding of electron ejection pathways in such fields, as well as a rigorous assessment of the reliability of simplified models for these systems.

Ultimately, fulfilment of the objectives outlined above would represent an impactful and timely contribution to the field of attosecond science. Extending the predictive capacity of RMT theory, to include circularly polarised radiation fields, represents an essential first step towards investigating atomic dynamics in completely arbitrary light fields. More generally, the proposed endeavour bears important implications for other closely related, and strategically prioritised, domains of research, given the fundamental significance of the single-atom response. We expect progress here to support the evolution in laser technology, refinements in ultrafast spectroscopy and imaging for chemical reaction dynamics, in addition to multiscale methodological developments relevant for modelling the non-linear response of meso- and macroscopic media.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509541/1 01/10/2016 30/09/2021
1786236 Studentship EP/N509541/1 01/10/2016 30/09/2019 Daniel David Clarke
 
Description The primary outcomes of this research have been the development, implementation and application of an ab initio, R-matrix with time-dependence (RMT) methodology for ultrafast atomic processes in laser fields of arbitrary polarisation. The theory successfully extends the original RMT approach confined to linearly polarised laser light, and represents the first R-matrix formalism capable of modelling atomic photoionisation by circular, or more generally elliptical, femtosecond and attosecond laser pulses. To facilitate numerical computations, we have developed an elaborate suite of computer codes. They are especially suitable for deployment in high-performance computing environments, given the inherent parallel scalability conferred by the RMT approach. Exploiting these codes, in combination with national supercomputing resources, we have investigated the strong-field dynamics of atoms and atomic ions in circularly polarised laser fields, with photon energies extending from the extreme-ultraviolet to the computationally demanding, near-infrared regime. This research has been published in a number of recent, peer-reviewed journal articles.

The research conducted under this project has also proven instrumental to a broader collaborative effort, embodied by the EPSRC-funded, Software Flagship project entitled 'R-matrix suites for multielectron attosecond dynamics in atoms and molecules irradiated by arbitrarily polarised light' (or R-MADAM for brevity). This work, conducted jointly among academics from Queen's University Belfast (Belfast, UK), The Open University (Milton Keynes, UK) and Charles University (Prague, Czech Republic) in particular, aims to establish a unified computational approach for modelling ultrafast phenomena in multielectron atomic and molecular systems, exposed to intense and ultrashort laser pulses of arbitrary polarisation. The code maintenance and development also benefits from support by CoSeC, the Computational Science Centre for Research Communities, through CCPQ. An important outcome of this effort has been the non-commercial release of a joint atomic/molecular RMT code to the wider academic community, of which the code developed under the present project forms a key component.
Exploitation Route The establishment of an RMT methodology for arbitrary light fields is conducive to a number of further developments. In a parallel initiative, we have modified the atomic RMT approach to incorporate one-body relativistic corrections to the electronic motion, most notably spin-orbit coupling. The dynamics of atomic spin-orbit wavepackets have been manifest in recent time-resolved, pump-probe experiments with atomic negative ions, employing laser pulses with different relative orientations of their linear polarisation axes. A semi-relativistic RMT approach, combined with the latest capability to handle arbitrary polarisation, would constitute a unique computational tool in support of such experiments, and may provide a deeper insight into the role of relativistic interactions in atomic strong-field processes. Another emerging vein of research pertains to the multiphoton double-ionisation of complex atomic systems. Presently, an RMT approach for the double-ionisation of entirely general atoms in linearly polarised pulses is under development. In the future, we expect to incorporate the arbitrary-polarisation capability within this framework, making feasible the investigation of correlated, multiple-electron emission in elliptically polarised fields, a subject of active experimental and theoretical interest. Finally, we mention that the progress achieved in this project provides important foundations for the development of multi-scale modelling approaches, with the aim of simulating the non-linear response of atomic clusters, nanostructures and even bulk solids to high-intensity, arbitrarily-polarised laser light.
Sectors Aerospace, Defence and Marine,Chemicals,Digital/Communication/Information Technologies (including Software),Electronics,Energy,Pharmaceuticals and Medical Biotechnology

 
Title R-matrix with time-dependence (RMT) suite 
Description RMT is a program which solves the time-dependent Schroedinger equation for the interaction of general, multielectron atoms, ions and molecules with laser light. As such, it can be used to model ionisation (single-photon, multiphoton and strong-field), recollision (high-harmonic generation, strong-field rescattering), and more generally, absorption or scattering processes with a full account of multielectron correlation effects in a time-dependent manner. Calculations can be performed for targets interacting with short, intense laser pulses of long-wavelength and arbitrary polarisation. Calculations for atoms can optionally include the Breit-Pauli correction terms for the description of relativistic (spin-orbit) effects. 
Type Of Technology Software 
Year Produced 2018 
Open Source License? Yes  
Impact The RMT suite of codes have been applied to investigate the two-photon ionisation of He, irradiated by a pair of time-delayed, circularly polarised, femtosecond laser pulses, with the numerical results obtained comparing favourably with recent time-dependent close-coupling calculations. The predictive capabilities afforded by the RMT codes have also been demonstrated through a study of single-photon detachment from the F- ion, exposed to a circularly polarised, extreme-ultraviolet, femtosecond laser pulse, where the relative contribution of the co- and counter-rotating 2p electrons was quantified. This latter work has recently been extended to the investigation of multiphoton detachment in infrared laser fields, representing some of the first ab initio calculations evidencing electron rotational asymmetry in the transient, few-photon regime. 
URL https://pure.qub.ac.uk/portal/en/persons/daniel-clarke(7ca62a4c-ca79-47eb-bb7d-1e4f545fdf48).html