SFB 616: Energy Dissipation at Surfaces
Final Report Abstract
The CRC 616 "Energy dissipation at surfaces" had the goal to achieve far reaching scientific progress in our understanding of processes, in particular at solid state surfaces, which determine how a specific energy impact is redistributed over time into other degrees of freedom, so that by the end it results in unspecific warming. This specific energy disposal may result from bombardment with particles such as ions or from the irradiation with light. The important questions then are, what are the reservoirs receiving energy from the initial excitation and on what time scales does this happen. The answers to these general questions depend on the specific properties of the materials considered and the variable specifics of the initial excitation. An optical excitation, for instance, may lie within different regions of the optical spectrum. Furthermore, it may be resonant with various excitation processes at the surface. New instruments were developed within the framework of the CRC which allow us to thoroughly study the aforementioned questions. These instruments are so unique in their capabilities that these measurements are only possible in a few laboratories around the world. For example, one of those instrument allows to measure the extent of excitation of surface vibrations with a temporal resolution of 10-11 s. A spectrometer was set up in order to selectively stimulate an internal vibration of an adsorbed molecule and to follow in time the relaxation of this excitation. An apparatus providing a beam of highly charged ions, metal atoms, which have been stripped of 40 or more electrons, was built in order to direct these at a surface. A microscope was developed which allowed to observe the motion of single molecules at surfaces when these a tickled by electrons; an experiment which is only possible at temperatures a few degrees above absolute zero. Multilayer metal-semiconductor- and metal-insulator sensors were developed in order to uncover and quantify the conversion of chemical energy released during a reaction at a surface into excitations of electrons. Of the many experiments carried out by the CRC, only those which led to conclusions impossible to foresee will be mentioned here. It could be established that indeed a non-negligible fraction of the excess energy in a chemical reaction is transiently converted into electronic excitations. Also, a large fraction of the energy deposited by ion impact on a metal surface is first stored in the electronic system, in contrast to the findings for cluster impact. In both cases, these findings call into question common theoretical models that now may need to be revised as they are based on the assumption that the energy is directly dissipated into thermal motion. It could be established that the energy is not uniformly dissipated along the path of a heavy, fast ion through a substrate, such as SrTiO3, but rather at specific crystallographic locations. This results in nanometer sizes protrusion at the surface. Moreover, the vibrational energy after selective excitation of a molecule on a semi-conductor does often not directly dissipate into heat but flow first into bending motions. It thereby circumvents a bottleneck resulting in a relaxation rate several orders of magnitude faster than expected beforehand. Using electron and X-ray diffraction methods, the times could be determined which the atoms at a surface take to rearrange after a strong light pulse has delivered larger amounts of energy. Using a microscope, it was studied how voids are formed in an electric interconnect when a current passes through it, as the atoms start to move with or against the direction of electron flow depending on experimental detail. These experiments could only be such successful as they were supported by a close cooperation with theoretical physicists. New methods were developed that allow to address the upcoming questions. Extensive modelling calculations were carried out in order to reproduce the experimental results and to corroborate their interpretation.
