Rydberg Tagging Surface Scattering Machine
Zusammenfassung der Projektergebnisse
A new apparatus was set up to investigate the inelasticity in H-atom scattering from surfaces. The goal is to gain a deeper understanding of the energy conversion processes at surfaces. We study a simple model system, H-atoms collide with single crystalline surfaces, in great detail and compare our results to high level theory. The conversion of kinetic energy carried by the atoms to surface excitations is of central interest here. To achieve this goal an apparatus combining a photolysis source for a nearly mono-energetic H-atom beam, the detection schema of Rydberg Atom Tagging (RAT) and an ultra-high-vacuum (UHV) surface scattering machine was constructed. In the photolysis source Hydrogen Iodide is dissociated with a UV/VUV photon. The received H-atom pulse has an energy spread of only a few wavenumbers. The H-atom pulse passes two differential pumping stages and is pointed onto a single crystalline surface mounted on a 6-axis manipulator in the UHV chamber. The scattered atoms are detected kinetic energy and angular resolved using RAT. In RAT, the H-atoms are excited into a high Rydberg state. The relatively long lifetime of the Rydberg state as well as the possibility to easily field ionize them allows Time-of-Flight analysis of the Rydberg atoms. The detection schema gives very high energy and angular resolution as well as high sensitivity. For the characterization of the surfaces an Auger-electron spectrometer and a LEED instrument are used. The instrument allows us to precisely control the experimental conditions as well as characterizing the scattered atoms in great detail. We recieve us a very detailed picture of the scattering event making it an ideal benchmark for testing new theoretical models. First experiments were carried out using a Au(111) crystal at room temperature. HI is dissociated with 212.5 nm light, resulting in a kinetic energy of 2.72 eV. The incident polar angle is 45° and the incident azimuthal direction is along [10-1]. The scattered H- atoms show a broad kinetic energy distribution with an average energy loss of 1 eV. Because of the large mass mismatch between Au and H, coupling to phonons cannot be responsible for this huge energy loss. The same experiment was carried out for a Xe covered Au(111) surface. In that case, a narrow kinetic energy distribution and minor energy loss are observed. The comparison of both systems clearly shows that in case of the metallic surface an additional dissipation channel beside phonon excitation must be present. Previous experiment using Schottky-diodes revealed that electron-hole-pair excitations occur in such systems. Our experiment suggests that electron-hole-pair excitations are actually the dominate relaxation channel. Comparison to high level theory leads to the same conclusion: only if electronic-hole-pair excitations are considered, theory can reproduce the experiment. This results show that in this case the Born-Oppenheimer approximation is not valid and coupling of nuclear motion to electronic excitations have to be considered. Currently, the angular and kinetic energy distributions of H-atoms scattered from Au(111) are studied in dependence of incident energy, incident angles and surface temperature. Additionally, the mass effect is studied by replacing H with its heavier isotope D. The aim is to generate an ideal dataset to validate up-todate theoretical models.
