Description
Fundamental processes in photon-matter interactions:
In the first half of this block course, a basic introduction to the theory underlying x-ray processes will be provided. After general remarks on the practical advantages of using x-rays for probing matter, the use of the minimal-coupling Hamiltonian within nonrelativistic quantum electrodynamics will be outlined. Perturbation theory will be reviewed and applied to describe x-ray-induced processes. In connection with x-ray absorption, inner-shell binding energies and the photon energy dependence of the x-ray absorption cross section will be discussed. In the context of x-ray scattering, atomic and molecular scattering factors will be introduced and the complex index of refraction will be derived. This will be complemented by a discussion of x-ray fluorescence and Auger decay of inner-shell-excited systems.
The first half of this block course will end with an elementary introduction to accelerator-based x-ray sources such as storage-ring-based synchrotron radiation sources and x-ray free-electron lasers.
The second half of the block course applies the concepts introduced in the first half to examples from two categories. The first category is the high-intensity optical laser control of x-ray processes. Topics addressed in this category include the electronic alignment and dynamics in atomic ions produced in a strong laser field; laser-induced transparency in the x-ray regime; and laser-induced alignment of molecules.
The second category is the electronic-structure problem in high-intensity x-ray fields. Generally, the probability that a given atom in a material absorbs an x-ray photon in a single x-ray pulse is much less than unity for storage-ring-based x-ray sources, even for third-generation synchrotron radiation sources. This situation has changed dramatically with the arrival of x-ray free-electron lasers: In the micro-focus of an x-ray free-electron laser, saturation of x-ray photoabsorption is routinely achieved. The immediate consequence is that the overall behavior of matter under such extreme conditions is characterized by efficient multiphoton absorption via a sequence of single-photon absorption events combined with inner-shell decay cascades and collisional ionization processes. In this way, unusual, highly excited states of matter are formed, whose properties and dynamics are challenging to describe theoretically. The progress towards developing suitable electronic structure tools will be discussed. The performance of currently available electronic structure tools will be assessed by direct comparison with experimental data.