Konventionelle Puls-NMR an 129Xe auf Einkristalloberflächen

In dieser Arbeit wurde mit konventioneller Puls-NMR die chemische Verschiebung von kernspinpolarisiertem 129Xe auf verschiedenen Präparationen der (111)-Oberfläche eines Iridium-Einkristalls untersucht. Das Experiment war als Nachweis der prinzipiellen Machbarkeit (proof of principle...

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Bibliographic Details
Main Author: Gerhard, Peter
Contributors: Jänsch, Heinz (Prof.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2004
Online Access:PDF Full Text
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In this work conventional pulse NMR was used to study the chemical shift of nuclear polarized 129Xe on different preparations of the (111)-surface of an iridium single crystal. This experiment was planned as proof of principal for NMR of adsorbates on single crystal surfaces. Nuclear magnetic resonance spectroscopy (NMR) is one of the most successful physical methods. Since the early 1980s xenon has been used in the study of surfaces. The low sensitivity of NMR has restricted its use so far to large area surfaces like clusters or zeolithes. The use of nuclear polarized xenon enhanced the development of this field greatly. Applying this technique to single crystal surfaces allows a more detailed control of the system studied. Associated with this is a reduction in number of the adsorption sites available by several orders of magnitude. In recent years, the prerequisites were established for conventional pulse NMR on single crystals under UHV conditions to be carried out successfully along with the use of highly polarized xenon. For the first time the NMR signal of an atomic layer of 129Xe adsorbed onto the surface of an iridium single crystal was observed in this work. The effect of different preparations of the substrate on the chemical shift sigma was demonstrated. For Xe/CO/Ir(111) and Xe/CH3C/Ir(111) sigma=153 ppm and sigma=193 ppm, respectively. For Xe/Ir(111) the chemical shift is sigma=853 ppm. This is far beyond what is to be expected for physisorbed Xe. On CO a temperature dependence of the chemical shift of -3.3 ppm/K was measured. In contrast to particles, this system is highly ordered and therefore allows to study the anisotropy of the chemical shift. For Xe/CO/Ir(111) one obtains sigma_iso=165 ppm and delta sigma=43 ppm. The corresponding values for Xe on the metallic surface (sigma_iso=1032 ppm and delta sigma=437 ppm) are surprisingly large. Within a model for the chemical shift, the analysis of the angle and temperature dependent data leads to conclusions about the interaction with the substrate as well as the coverage. The results point at the growth of islands already known from Xe. Investigations of the adsorption desorption dynamics are also possible with NMR even though the system is in equilibrium. Assuming a hypothesis, the astonishing behavior of the residence time can be explained by a simple model. Besides, experiments on highly polarized xenon films are presented. Polarization induced effects as well as the influence of the magnetization of the film itself and the substrate were studied. Polarizations up to P_z=0.8 could be proven.