Supervisors: Dr Arvo Kikas and dr Tanel Käämbre
Opponent: dr Franz Hennies (MaxLab, Rootsi)
The present thesis focuses on soft X-ray spectroscopy of insulators, especially resonant inelastic X-ray scattering (RIXS) experiments. RIXS is a relatively new technique for the study of electronic structure of matter and is especially useful for studying insulators. Unfortunately (soft X-ray) RIXS experiments are time-consuming and have to be carried out on a synchrotron. Both experimental and theoretical work on some beryllium and boron compounds including BeO, Be2SiO4, BeAl2O4, Be3Al2Si6O18, BN, BBO, LBO, La2Be2O5, Li6YB3O9, Li6GdB3O9, and Y2SiO5 has been carried out.
β-BaB2O4 (BBO) and LiB3O5 (LBO) materials are widely used in many applications of non-linear optics. The origin of their high values of non-linear susceptibilities is of great interest. RIXS studies show that a boron core exciton level exists in both LBO and BBO, similarly to BN and B2O3. The strong anisotropy of BBO and the small anisotropy of LBO were also evident in their X-ray spectra. Furthermore, it was shown that the measured BBO X-ray spectra can be interpreted remarkably well using a molecular picture of the solid.
The most important part of the work is the study of the phonon relaxation effect in solids. Namely, in some systems (BeO, BBO, Be2SiO4 etc.) a energy loss tail (ELT) to the elastic scattering line can be observed in the X-ray scattering spectra. A similar phenomenon in the optical region is known as hot luminescence. A simple model to account for the shape of the relevant emission band was developed. The results from that model are in agreement with experimental data and with previous theoretical work. Furthermore, a case was made that energy dissipation during the excited state lifetime may be important for the correct description of the shape of the ELT. The strength of the phonon relaxation effect was shown to be empirically well predicted by the electron-phonon interaction strength: the product between the estimated lattice relaxation energy and the energy of the LO (longitudinal optical) phonon.