The research focused on development of the experimental and theoretical methodology of Very-Low-Energy Electron Diffraction (VLEED) as a spectroscopic method to probe the dispersions and lifetimes of the photoemission final states. This information is of vital importance for reliable determination of three-dimensional electronic dispersions from the Angle-Resolved Photoelectron Spectroscopy (ARPES) experiment. The habilitation (senior doctorate) thesis was defended with the Institute of Analytic Instrumentation, St. Petersburg. This activity has resulted in pushing the soft-X-ray ARPES technique to previously unthinkable applications to electronic structure of buried nanostructure and impurity systems actual for electronic, spintronic and quantum devices. As of the end of 2023, more than 260 publications in refereed journals and monographs.