Giant Dipole Resonances as Probes of Local Environment in Molecules, Clusters and Solidsl

Doktorsavhandling, 1995

In this thesis, I study effects of the local environment on the giant dipole resonances (GDR) in lanthanides and actinides. The effects of embedding in jellium, bonding in small molecules and incorporation into different fullerene systems are studied. All systems are described using density-functional theory and the dynamic response is treated either with a local-density based random phase approximation (LDRPA) or the time-dependent local-density approximation (TDLDA). In Paper I, the effects of a metallic environment on the GDR in Ba, La and Th is investigated using LDRPA. We show that the lowering of the continuum in the metal brings the shape of the GDR in Th to a form similar to that in Ba or La. Our results for the photo-yield in Th metal are in good agreement with experimental photo-yield measurements on Th metal in the region of the Th 5d-5f giant dipole resonance. In Paper II, the response of a O-Ba-O structure is treated within the framework of TDLDA to describe the combined local-screening and multiple-scattering effects. The Ba-O distance is chosen to be the same as the Ba-O distance in YBaCuO. The molecular potential is expanded in terms of spherical harmonics, and we also study the convergence properties of the finite expansion. We have calculated the x-ray absorption in the region of the Ba 4d-4f GDR and showed that this method will qualitatively describe the experimental data. Papers III and IV show how the x-ray absorption near-edge structure (XANES) can be used to provide a "finger-print" characteristic of the position of atoms trapped in various fullerene systems. We approximate the fullerene cages by closed two-dimensional jellium systems and study the effects of screening and molecular scattering within the TDLDA in a spherical-harmonics expansion. Specifically, we show how the GDR can be used to distinguish between center and off-center positions in C60, and we demonstrate the effect on the GDR of the non-spherically symmetric fullerene model system C90.