Theoretical Atomic Attosecond Spectroscopy: Monitor and Control of Electron Correlation in Real Time

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This project is aimed at the development of new, non-standard computational tools for representing the interaction of poly-electronic atomic systems with ultra-short "attosecond" light pulses. These ultra-short pulses act like high speed cameras capable of imaging the motion of electrons within atoms and/or molecules. They accordingly provide a unique picture of the complex dynamics associated with electron motion, and may also provide an opportunity to control the outcome of ionization and other fragmentation processes, even within complex, highly-correlated materials. The severe computational challenge of representing the motion of electrons on such short time scales derives from the uncertainty principle, which indicates that atomic and/or molecular states spanning a broad energy range will need to be accurately described, including high energy states in which two or more electrons are simultaneously excited. The project should accordingly shed light on the role of multiply-excited states in fragmentation processes induced by the absorption of attosecond radiation pulses. The theoretical description of the dynamical regimes triggered in atoms by ultrashort laser pulses is challenging because it entails the representation of several electronic states, across a wide energy range, that decay by emitting photoelectrons. The electrons liberated to the continuum interact strongly with the parent ion they leave behind, as well as with the dressing laser pulses commonly employed in attosecond pump-probe experiments. To tackle these difficulties, this project will merge parallel simulation techniques for the time-dependent Schrodinger equation of general poly-electronic atoms in the presence of arbitrary external fields, with state-of-the-art numerical methods to compute stationary bound states, multichannel single-ionization scattering atomic functions, and Siegert states, represented in terms of a multi-configuration Hartree Fock B-spline close-coupling approach. The numerical tools developed in the project will be applied to study unexplored aspects of driven multi-electron attosecond dynamics in rare gas atoms, with a special focus on the role of autoionizing states, which constitute the best approximation of a persistent reference onto which building coherent-control schemes for photo-fragmentation processes. In particular, the project will explore the creation, imaging and tailoring of transiently bound wavepackets, by means of both optical and photoelectron attosecond interferometric pump-probe schemes. The project will study how transient coherences can be used to control branching ratios, angular distributions, and residual-ion coherence in photoionization events. Properties of laser-driven autoionizing states, such as their ac-Stark shift, their tunnelling rate, and the electric susceptibility of dressed atoms in the extreme ultraviolet (XUV) range, will be charted. The affect of electron correlation on these properties will be studied, and this information will be used to help control population transfer within the system as well as the spectrum and shape of the XUV light transmitted through the target.