Correlations between particles within a quantum system have serious consequences for its dynamical evolution. The quantum path chosen by a set of particles as a response to a light-stimulus will sensitively depend on the initial state, as well as on the specific properties -frequency, polarization, phase, temporal profile – of the electromagnetic wave. Its time-dependent observation and modelling represent major challenges to experiment and theory, but holds the promise of providing the most comprehensive description of light-matter interaction and facilitate the development of new mechanisms for dynamically controlling quantum states of matter.
We aim at studying and controlling the detailed evolution of photo induced processes in small quantum systems that are governed by correlation effects. The desired answers are expected from a suite of explicitly time-resolving experimental and theoretical methods, which are capable of revealing electronic dynamics on the relevant femtosecond to picosecond time scale. The consortium of investigators in research area A will make ample use of the close vicinity to the novel accelerator-based sources of ultra short X-ray pulses in Hamburg. Those greatly extend the capabilities for studying dynamics due to a unique combination of short wavelength, short duration and high intensity. The collaboration will benefit considerably from the mutual utilization of new time-resolving techniques and devices, recently developed by the participating groups.
Within the scientific programme of this SFB initiative, research area A takes up the position of considering correlated dynamics in comparably small quantum systems which are optically driven far above their ground state. The considered systems – atoms, molecules and clusters – can easily be isolated from any environment that could take over energy. This is in contrast with the solid-state systems studied in research area B, but on the other hand the isolation capability is also nicely available for the quantum gas systems studied in research area C. The degree of isolation can be adjusted to a great extent by e.g. embedding the studied atom into a molecular scaffold with a selectable size and type / strength of the bonds.
The processes studied in research area A are extremely fast – correspondingly their observation pushes the technical capabilities of the utilized techniques to their very limits. Also in research area B some dynamics evolves on a femtosecond scale; the findings of research area A will thus deliver input on the relevant times scales of the electronic relaxation. Ultracold matter in research area C is observed in the millisecond or even second range. Still, with some apology all the processes considered in the three research areas have to be considered as being “fast”. In the context of this SFB initiative, we use the term “fast” not as an absolute measure for speed, but as an indicator, how far a process has evolved – and was observed – prior to thermalisation, i.e. the coupling to an environmental bath of un-resolved and un-measured degrees of freedom. Even for a seemingly simple system like a small molecule as studied in research area A, this loss of energy and phase into non-accessible channels of electronic and nuclear motion will lead to a loss of information. In this sense, “fast” implies to finish a measurement before decoherence obscures the desired information about correlated dynamics.
Fastness in this sense is an ingredient of all research areas for beating decoherence and for exerting control over a system. For research area A this control implies the sculpturing of ultra short optical wave packets with tailored temporal intensity and frequency distributions. While this has technically been solved for the visible range, it remains a significant methodological challenge for the XUV range considered in most projects of research area A. Once successfully established we expect also considerable impact for the possibilities to steer strongly correlated systems in research area B. Owing to the different systems under study within this SFB, the “product states” over which control is to be achieved, are quite different. For the case of research area A these include the branching ratio for the population of certain final ionic states or molecular fragments (A1, A2, A3), the electronic dephasing and spin-orbit coupling in molecular spin transitions systems (A4) or the angular distribution and spin polarization of emerging electron waves (A2). As the spin degree of freedom is a major component of research area C, we expect significant exchange between the areas A and C on this issue.