Dokument

Summary:
The essential core of this work is the study on xenon monolayers adsorbed on defined surfaces of single crystal metals and carbonic cover layers by means of Nuclear Magnetic Resonace (NMR). 129Xe is utilized as the probe nucleus because of its many excellent properties. First and foremost 129Xe can readily be polarized far above the thermodynamic equilibrium, which is also referred to as hyperpolarization. The rise in polarization is connected with an increase of the NMR signal strength up to five orders of magnitude. Only by these means the signal of a monolayer, containing not more than e15 atoms, is able to pass the detection threshold, corresponding to a signal from about e20 nuclei in a solid, of common NMR spectrometers. Furthermore xenon is a heavy noble gas that adsorbs on substrates at temperatures of about 80–90 K, without inducing chemical reactions. Nevertheless, the electron environment of the 129Xe is perturbed which causes a sensitive change in the magnetic field at the nuclear site. Especially in the immediate vicinity of metals the effect is so strong that the NMR signals, also called spectral lines, are frequency shifted up to the order of 1000 parts per million (ppm). Therefore, xenon NMR is not only a non-invasive chemical analysis method but also a local probe. Xenon NMR is performed on a Xe monolayer in contact with a copper single crystal with a surface oriented in the [100] direction. Two distinct and strong signals have been found. These lines are respectively shifted by 687 ppm and 772 ppm with respect to the xenon gas line. Hence, both shifts surpass clearly the typical physisorption range which means that 129 Xe is in contact with the metal electrons. Further the signal of the second layer is also made visible in the spectra showing a non metallic shift of 235 ppm. Another NMR experiment is performed on a xenon monolayer adsorbed on a CO buffer layer deposited on Cu(100). An NMR line is revealed at 163 ppm. 129Xe is also probed on graphene that has been created on an Ir(111) surface. Here, the presence of metallic electrons cannot be detected on basis of an extraordinary NMR line shift. In particular, the shift of the 129Xe signal in the monolayer on graphene has been determined to 173 ppm. For comparison xenon film surface layers are analyzed by NMR during their growth at substrate temperatures below 50 K. The results give information about the temporary configuration of the xenon atoms on the film surface and the corresponding line shifts. Two, thus far unknown, signals are found at 160 ppm and 240 ppm. Besides, the signal of the bulk at 320 ppm and the smooth film surface at 200 ppm are also visible in the spectra. In addition, an order phenomenon within a xenon film is demonstrated on the basis of the 129 Xe NMR linewidth. By strongly diluting the 129Xe with 132 Xe, all other solid effects on the linewidth are sufficiently diminished to reveal a heat induced structure change. The lowest temperature measured is 25 K. Starting from this temperature up to 45 K, the change is observed.

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