Table of Contents:
This thesis consists of two main parts. The first one reports about recent investigations of the electron dynamics on the Si(111) 7x7 surface employing time- and angle-resolved two-photon photoemission (2PPE). The second part describes the construction and demonstration of the capabilities of a new electron time-of-flight spectrometer.
Though nowadays Silicon is the basis of many technological applications and its bulk properties as well as the structure of its surfaces has been subject of intense research since a long time, this thesis represents, to my knowledge, the first systematic study of the electron dynamics of Si(111) 7x7 employing time- and angle-resolved 2PPE. It will be shown that the electron dynamics of this surface are governed by adatom and bulk states. The population of the adatom states is characterized by a hot electron distribution on short timescales after optical excitation. These hot electrons thermalize within a few hundred femtoseconds. On a similar timescale, a signature of the conduction band minimum evolves in the photoemission spectra. This feature is observable in normal emission, though the conduction band minimum of Silicon is located far off the Γ point. Variation of different experimental parameters leads to the suggestion that electrons scatter from the adatom states into the conduction band of Silicon. The latter shows a dependence of the electron dynamics on parallel momentum on short timescales, which probably is caused by intra-band processes and electron-phonon scattering. While thermalization of electrons within the adatom band results in an energy dependence, this state shows no dependence of the electron dynamics on parallel momentum. In conjunction with the observation of a vanishing dispersion this points to an electronic state of localized character. The localization in real space can be estimated from the distribution of the photoemission intensity in momentum space to be within one 7x7 unit cell. The electron population in the conduction band as well as those in the adatom band show a very long-living component. In addition to recombination through defect states, these electrons can undergo radiative recombination with holes in the valence band. Systematic studies of the electron dynamics as a function of temperature could help to unravel the role of phonons in the decay of the excited electrons.
The second part of this thesis reports about the design, construction and demonstration of the capabilities of a new electron time-of-flight spectrometer for applications in time- and angle-resolved 2PPE experiments. The new spectrometer is designed in a flexible manner to maximize either the energy resolution or the acceptance angle, respectively. By employing a position-sensitive electron detector it is possible for the first time to measure the energy as well as all components of the parallel momentum of the photoemitted electrons and thereby to fully characterize electrons from surface states. The energy- and position- and thus the momentum-resolution are governed predominantly by the electronic time-resolution. The time-resolution can be estimated from the width of a peak induced by photons scattered from the sample to be better than 150 ps. At the minimum of about 40 mm of the adjustable drift distance this leads to a energy resolution below 5 meV for electrons with kinetic energies of 1 eV. Thus, the achievable energy resolution of this spectrometer at typical kinetic energies of 2PPE experiments stays far below the spectral width of femtosecond laser pulses by far. Thereby, the parallel momentum resolution is below 5 mǺˉ¹ for parallel momentum values ≤1 Ǻˉ¹. At the same time, the maximum acceptance angle is about 25°.
The capabilities of the new spectrometer are demonstrated with the help of the well-characterized properties of image-potential states of the Cu(100) surface in a time- and angle-resolved 2PPE experiment. Through the possibility to capture the full vector of parallel momentum in a single measurement an anisotropy of the lifetime of electrons in the n=1 image state on the nominal flat surface can be detected. This anisotropy can be explained in accordance with experiments on vicinal Cu(100) surfaces. Hereafter, the full potential of the new spectrometer will be exploited particularly in the investigation of anisotropic electronic states.