Licentiate seminar: Studying extreme states of matter induced by high-intensity femtosecond x-rays

Abstract:

We can use x-rays in a wide photon energy range to study the electronic and atomic structure of materials. We use x-rays because their wavelength at high photon energy approach the length of atomic bonds and, unlike electrons which scatter easily and neutrons that scatter rarely, strike a nice balance and offer elemental contrast. To achieve a strong signal for structural determination, the target must be arranged in a periodic structure. The development of free electron laser sources capable of high intensity and femtosecond-short x-ray pulses opens an opportunity to move away from crystallization and instead image single particles with atomic resolution. The high brilliance also brings some challenges. X-rays mainly interact with the deepest energetically available shell, rearranging in sub-femtosecond timescales the electronic structure, depositing large quantities of energy, and driving the material into a plasma state. This thesis deals with the interactions of x-rays with matter at photon energies and intensities available at free electron lasers using a combination of theoretical tools such as collisional-radiative and diffraction simulations. In one investigation, we study non-linear effects in a copper foil due to changes in the electronic level environment initialized by the photoionization of 1s electrons. We find transient changes in the opacity shorten the duration of transmitted and fluorescing x-ray pulses. These shortened pulses could be advantageous in other imaging techniques. In another project, we study the use of resonances to change the atomic structure factors of copper nano-sized clusters with the motivation to enhance the scattering signal in single particle experiments. In the last project, we explore a plasma-like state in ice during femtosecond x-ray exposure and find the ice melts in an anisotropic manner. We suggest the anisotropic melting could be experimentally detected with x-rays from a free electron laser facility.