The dynamics of atoms and molecules initiated by irradiation  intense laser irradiations are investigated in our Centre both through numerical and experimental approaches.

Numerically a strong area of expertise is in the use of R-matrix with time dependence methods, which is the world’s most advanced computational framework in atomic, molecular and optical (AMO) physics. While most theoretical treatments of strong field physics use an ad hoc, lightweight model approach, RMT provides a general and flexible simulator of detailed multielectron dynamics in atomic or molecular systems. RMT has been applied to describe cutting-edge experimental techniques such as attosecond transient absorption spectroscopy, RABBITT with skewed laser polarisation, and XUV-initiated HHG. Recent development has yielded a unified computational approach to treat atoms and molecules in arbitrarily polarised laser fields, including semi-relativistic effects in atoms. Although other approaches have been developed for the treatment of intense-field processes for multi-electron systems, the RMT codes have the unrivalled capability of describing the full atomic or molecular structure allowing for the detailed capture of correlated multi-electron dynamics. This capability is of particular importance when fine details of the structure become pronounced in experiment (e.g. in attosecond transient absorption spectroscopy) and has enabled success in leading theoretical understanding of photoionisation time delays, and XUV-assisted high harmonic generation (HHG) for inner shell electrons.

As with other R-matrix approaches, RMT is based on the idea of the division of space. In an inner region, close to the nucleus, multi-electron correlations including exchange are described in full detail. We assume that a single electron can be ionised, and thus in an outer region far from the nucleus, we describe this single electron moving in the long-range potential of the residual ion and neglect electron-exchange. This allows the time-dependent Schrödinger equation to be solved in a large region of space by confining the computationally intensive electron exchange calculations to a relatively small region. Thus we have an inner region which is small but computationally arduous, and an outer region which is relatively less intensive, but quite large.

The strength of RMT compared with other implementations of the time-dependent R-matrix theory is the use of numerical techniques most appropriate to each region. Thus in the inner region where correlation is most important, the wavefunction is expressed using B-spline basis set techniques. In the outer region, where the size of the configuration space is the limiting factor, an ultra-efficient grid-based technique is used. This numerical flexibility is carried into the implementation, which uses a complex, three-layered parallel scheme to deploy the code on massively-parallel supercomputing facilities. Current projects are concerned with:

• extending and exploiting the codes to account for strong-field processes in a range of molecular targets
• developing new capability to describe double-ionisation phenomena
• exploiting capability to understand the role of relativistic effects in strong-field processes (e.g. strong field ionisation, harmonic generation)
• refactoring the RMT software to make it FAIR (Findable, Accessible, Interoperable, Reusable)

Experimental activities are based  on the use of   femtosecond (10^-15s) and attosecond (10¹⁸ s) laser technology to initiate and observe some of the fastest processes in nature. By studying DNA, amino acids, and other organic molecules we can study how these key building blocks of life respond to light at the molecular level. Ultrafast structural changes or charge transfer in these systems initiate processes in vision, photosynthesis, DNA damage, and molecular motors.

One of the research areas involves pump-probe studies of a range of molecular species, where an initial femtosecond/attosecond pulse (pump) excites or ionises a molecule setting electrons and the nuclei in motion. At a controlled delay later, another short (probe) pulse is used to observe a property of the molecule at that instant in time. By repeating this experiment with different delays, a “movie” of the molecular motion can be retrieved. With this technique we have observed how DNA molecules dissipate ultraviolet radiation in DNA, how charge oscillates coherently following exposure to ionizing radiation, and de-excitation of the pigment molecules such as the green fluorescent protein.

An emerging research area for the group, is using these laser pulses to detect the handedness of chiral molecules by studying the angular emission of electrons emitted from irradiation of the molecule by a circularly polarised laser pulse. A molecule is chiral if its mirror image cannot be superimposed on the original. Amino acids and sugars (and hence proteins and DNA) in natural systems are chiral but possess only one of these forms. As a result chirality is a key property in the development of pharmaceutical drugs. We are currently developing an apparatus which can identify what form the chiral molecule takes, but also to use this chiral observable as a way of sensitively tracking ultrafast changes in these molecules. (For more information, go to www.ultrafastbelfast.com)

KEY RECENT PUBLICATIONS:

1. J. B. Greenwood, I. D. Williams, Investigation of photoelectron elliptical dichroism for chiral analysis, PCCP, 25, 16238 (2023)

2. W. Jiang, G. S. J. Armstrong, J. Tong, Y. Xu, Z. Zuo, J. Qiang, P. Lu, D. D. A. Clarke, J. Benda, A. Fleischer, H. Ni, K.Ueda, H. W. van der Hart, A. C. Brown, X. Gong, J. Wu, Atomic partial wave meter by attosecond coincidence metrology, Nature Comm., 13, 5072 (2022)

3. E. P. Mansson, S. Latini, F. Covita, V. Wanie, M. Galli, E. Perfetto, G. Stefanucci, H. Hübener, U. De Giovannini, M. C. Castrovilli, A. Trabattoni, F. Frassetto, L. Poletto, J. B. Greenwood, F. Légaré, M. Nisoli, A. Rubio, F. Calegari, Real-time observation of a correlation-driven sub 3 fs charge migration in ionised adenine, Commun. Chem., 4, 73 (2021)

4. A. C. Brown , G. S.J. Armstrong , J. Benda , D. D.A. Clarke , J. Wragg , K. R. Hamilton , Z. Mašín , J. D. Gorfinkiel , H. W. van der Hart , RMT: R-matrix with time-dependence. Solving the semi-relativistic, time-dependent Schrödinger equation for general, multielectron atoms and molecules in intense, ultrashort, arbitrarily polarized laser pulses, Comp. Phys. Comm., 250, 107062 (2020)

5. J. Wragg, D. D. A. Clarke, G. S. J. Armstrong, A. C. Brown, C. P. Ballance, H. W. van der Hart, Resolving Ultrafast Spin-Orbit Dynamics in Heavy Many-Electron Atoms, Phys. Rev. Lett., 123, 163001 (2019)

6. A. Comby, E. Bloch, C. M. M. Bond, D. Descamps, J. Miles, S. Petit, S. Rozen, J. B. Greenwood, V. Blanchet & Y. Mairesse, Real-time determination of enantiomeric and isomeric content using photoelectron elliptical dichroism, Nature Comm., 9, 5212 (2018)

CURRENT FUNDING: 

PARAMOR- Platform And Resource for Atomic, Molecular and Optical Research, EPSRC  EP/V05208X/1 (2021-26)

Ultrafast spectroscopy of relativistic processes, EPSRC (2020-24)