Dynamic microscopy fundamentals

This tutorial will not attempt to deal with aU these ion implantation phenomena, although Mossbauer spectroscopy has been used in aU these fields. We wUl give several illustrative examples but we will mainly focus on semiconductors and to rather low implantation fluences where the implanted atoms are still isolated from each other or just start to coalesce and to form precipitates. The phenomena at high fluences and the dynamics of compound layer formation are beyond the scope of this tutorial. The reason for this limitation is that emission Mossbauer spectroscopy on radioactive probe atoms is particularly powerful in this low concentration range and allows to study the more fundamental phenomena of lattice location and defect association at the individual probe level, which is hard to study with other techniques. On the other hand, experience has shown that one has to be extremely careful in drawing conclusions from Mossbauer spectroscopy results only, as the possible interpretation of a particular Mossbauer spectrum is often not unique. Complementary data, e.g. from electron microscopy. X-ray difiEraction, transport measurements, channelUng experiments, are often more than welcome or even crucial for the interpretation of the hyperfine interaction data. 

A fundamental limitation of almost all microscopy investigations of materials is that the images are static and taken when the specimen is at room temperature. In some cases, as in electron microscopy (EM), the specimen is also in a high or ultra-high vacuum and under intense radiation. In situ microscopy allows observing materials dynamically under more realistic conditions approaching those of normal service life. 

Nowadays, computer simulations are treated as the third fundamental discipline of interface research in addition to the two classieal ones, namely theory and experiment. Based direetly on a microscopie model of the system, eomputer simulations can, in principle at least, provide an exact solution of any physicochemical problem. By far the most common methods of studying adsorption systems by simulations are the Monte Carlo (MC) technique and the molecular dynamics (MD) method. In this ehapter, a description of simidation methods will be omitted because several textbooks and review artieles on the subject are available [274-277]. The present discussion will be restricted to elementary aspects of simulation methods. In the deterministic MD method, the moleeular trajectories are eomputed by solving Newton s equations, and a time-correlated sequenee of configurations is generated. The main advantage of this technique is that it permits the study of time-dependent processes. In MC simulation, a stochastic element is an essential part of the method the trajectories are generated by random walk in configuration space. Struetural and thermodynamic properties are accessible by both methods. 

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