# Atomic Structure and Dynamics

## Attosecond physics

Attoscience is the study of electron dynamics in quantum systems, such as atoms, molecules and solids, on a time scale down to a few attoseconds (1 as = 10^-18 s). This high temporal resolution can be reached in experiments thanks to a nonlinear optical process called high-order harmonic generation (HHG). The HHG process can be understood as a 3-step process: 1) the electron tunnels from the atom due to the strong laser field, 2) the electron propagates in the continuum driven by the laser field and 3) the electron recombines with the atom and emits short bursts of high-energy photons. In this way, ultra-short laser pulse of low frequency can be converted to attosecond pulses of high frequency. Theoretically, the HHG process can be modelled using the strong-field approximation (SFA) or by solving the time-dependent SchrÃ¶dingar equation (TDSE). Quantum control of HHG is a vast subject with applications from the microscopic atomic scale, where electron correlation and molecular dynamics, to the macroscopic scale of optics. Recent attosecond experiments, based on laser-assisted photoionization (LAP), has allowed for direct measurements of photoionization delays, where diagrammatic many-body perturbation theory has been used to understand electron-electron correlation effects in photoionization.

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## Atomic Structure Theory

In spite of being the first testing ground of Quantum Mechanics, almost hundred years ago, Atomic Structure Theory is a vibrant and active area. It represents unique possibilities in front-line Science. The main challenges in general for many-body systems is to understand and represent the forces and to treat many-body effects efficiently, in the form of correlation. For atomic systems we have arguably a detailed understanding of the forces and we can therefore focus on the many-particle problem. Since Computational methods have reached a high degree of a accuracy, it is possible in atoms to test fundamental aspects of Physics, e.g. Quantum ElectroDynamic (QED) and parity violation. It is also possible to test the size and shape of the atomic nuclei, to complement results from collisions.

Most of the information we get from different plasma, in space and in the laboratory, reaches us via electromagnetic radiation - light - from atoms, ions and molecules. It is therefore important to understand in detail the structure and dynamics of these ions, to deduce the property of the plasma, e.g. temperature, density, chemical composition or presence of magnetic fields. In Lund we focus on Multiconfigurational method and the main part is concerned with solving the Dirac Equation for complex atoms and ions.

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