Ab Initio X-ray Physics

Group Leader: Prof. Dr. Robin Santra

The subgroup working in the area of Ab Initio X-ray Physics focuses on the microscopic, quantum-mechanical characterization of the interaction of x rays with atoms and molecules. “Ab Initio” means that the theoretical description employed is based on first principles, i.e., fundamental natural laws. The information obtained in our research is important for maximizing the utility of novel radiation sources such as x-ray free-electron lasers. Of central interest to us is the exploration of novel techniques made possible by the extremely short pulse duration, and the extremely high intensity, offered by X-ray free-electron lasers. We are exploring, for instance, whether such pulses can be employed to directly watch the behavior of molecules in strong optical laser fields. A goal of this research is to find out whether it is generally possible to use strong optical lasers as catalysts of chemical reactions. We are also exploring opportunities for directly observing and manipulating the motion of electrons in matter. Since electrons provide the glue that binds atoms together to form molecules, the ability to control electrons in matter could have revolutionary consequences for applications.

Probability densities of the photoelectron originating from the 4d₀ and 5s shells of xenon are shown for different times after the ionizing attosecond pulse. The pulse has a duration of 10 as, a photon energy of 136 eV, and a peak electric field strength of 25 GV/m.

 

A recent research highlight: Unexpected decoherence in ultrafast photoionization
Attosecond science holds the promise of controlling electron motion to manipulate physical processes at the atomic level. One way of inducing electron motion is photoionization using an attosecond laser pulse. The focus of our recent paper [Phys. Rev. Lett. 106, 053003 (2011)] was to answer an open fundamental question about electron control via attosecond photoionization: Can the nonstationary state of the parent ion be described by a Schrödinger wave function, i.e., is the state coherent? Pulses with sufficiently broad coherent bandwidth can now bridge the energy splitting between valence and inner-shell atomic orbitals. One might expect that by ionizing these orbitals using an attosecond pulse, a coherent superposition of the corresponding ionic eigenstates is formed. While the entire system---ion plus photoelectron---is described by a wave function, the ion alone however must be described by a density matrix. This opens up the possibility that the state of the ion is not perfectly coherent. We showed that the Coulomb interaction between the photoelectron and the parent ion triggers complex many-body effects, which unexpectedly enhance the entanglement between photoelectron and ion---leading subsequently to decoherence within the ion. We pointed out strategies for controlling the decoherence, offering new opportunities for x-ray free-electron lasers.