Elementary Diffusion Jump on Iron Surface

Diffusion in and on surfaces has been amply discussed in theoretical work, whereas experimental investigations because of considerable experimental difficulties are rare. That is why understanding of diffusion on surfaces is an issue which still is at its beginning. This is totally different from the situation in bulk (volume) material which has been extensively studied with a great variety of methods and, at least for simple materials as pure metals, ordered alloys, semiconductors, is in an advanced state. Even the atomistics of diffusion has been determined for selected systems. Here we propose to study surface diffusion by nuclear resonant scattering of synchrotron radiation (NRS). This should be done by studying the electric field gradient (EFG) relaxations resulting from random fluctuations caused by the movements of atoms on an iron monolayer surface.

The importance of diffusion for formation and stability of condensed matter is fundamental and there is no question that information on diffusion is a vital need. The bulk diffusion at least for metals, semiconductors and simple alloys is well understood, whereas for surfaces and thin films experimental investigations are still in their initial stages. This project was devoted to the studies of dynamics in low-dimensional structures. In particular we wanted to follow single atomic jumps on an Fe monolayer surface by studying the electric field gradient relaxations resulting from random fluctuations caused by the movements of atoms. For this purpose the method of nuclear resonant scattering has been exploited. This is the synchrotron variant of Mössbauer spectroscopy working in the time instead of the energy domain. It is an atomistic method, which means that information on the single atom behaviour is received. Investigations were divided into two parts. The first one concerned diffusion on Fe atoms in an Fe monolayer deposited on W(110), the second one was related to the diffusion of gas atoms such as CO, N2, O2 as well as Ag and Au atoms on the surface of an Fe monolayer on W(110)We registered formation and migration of defects (most probably vacancies) in an Fe monolayer on W(110). We succeeded also in the determination of the energies of migration EM=0.17(5)eV and formation EF=0.16(6)eV which are the components of the overall activation energy for diffusion of Fe atoms in an Fe monolayer. This is the first time that an atomistic nuclear method has been successfully applied for studying diffusion in a two-dimensional system. Diffusion studies of various adsorbates on the surface of an Fe monolayer turned out to be a more complicated task. We often observed a destabilization of a pseudomorphic Fe monolayer covered by adsorbates upon thermal treatment. However, these studies showed an Fe monolayer on W(110) to be a very interesting system from the point of view of adsorption and reactivity. Experiments on oxygen adsorption on an Fe monolayer resulted in the observation of a completely new ordered oxygen superstructure p(3x2) which for this system has been never registered before. By applying additional surface specific methods we derived a complete description of the initial stage of oxygen adsorption on an monolayer Fe/W(110) from the point of view of adsorption geometry and iron chemical state.