Chemists and physicists have always collaborated in my research group, a combination that has turned out to be very successful. It has enabled us to cover a wide range of research activities, from synthesizing new material, pursuing physical properties using non-routine techniques down to very low temperatures as well as application of external pressure and magnetic field, developing software for special data reduction, interpreting experimental results in the frame of theoretical models, optimizing instrumentation and developing new measuring techniques. A few examples are briefly described in the following:
- In the frame of our research work on physical and chemical after-effects of nuclear transformation in solid coordination compounds we have designed and constructed the only ever successfully operating Mössbauer coincidence spectrometer for lifetime measurements on excited electronic states following nuclear decay in a time window of 5-500 ns and in a temperature regime down to 4 K /43, 106, 116, 144/. This technique has enabled us to identify the excited ligand field states generated by nuclear processes to be of the same nature as those induced by light irradiation by the so-called LIESST effect. The results have been recognized to be of fundamental importance in ”hot atom chemistryâ
- The influence of external pressure on the magnetic properties of molecule-based magnets and spin crossover systems has attracted much interest in many research groups dealing with molecular magnetism. We have designed and built hydrostatic pressure cells for magnetic susceptibility as well as Mössbauer measurements for pressures up to 14 kbar (1.4 GP) and temperatures down to 4.2 K. The cells have been employed in many pressure effect studies /378, 389, 400, 419, 439/. A particular highlight is the pressure-induced electron transfer study in ferrimagnetic Prussian Blue analogue compounds /369/
- Mössbauer spectroscopy as a function of temperature renders this tool particularly valuable in many electronic structure problems. The regime from room temperature down to ca. 1.8 K (pumping on liquid helium) is relatively easily accessible as the relevant cryogenic equipment is commercially available. High temperature studies well above room temperature, however, are more difficult to perform, particularly in the case of in situ-studies of solid state reactions. In a project within the DFG Priority program ”Reactivity of Solidsâ we have constructed a high temperature oven for Mössbauer effect measurements up to a. 1000 K and employed it for the in situ-analysis of the formation of nitrides and nitridoferrates on steel surfaces at high temperatures /358/. The results have enabled us to draw conclusions about the mechanism of nitrification on steel surfaces
- Most spectacular has turned out to be the Miniaturization of a Mössbauer Spectrometer (MIMOS), which began under E. Kankeleit and G. Klingelhöfer at the Technical University Darmstadt in the nineties, and continued after 1997 under my responsibility and active involvement of G. Klingelhöfer at the University of Mainz /344, 354, 372/. MIMOS has been scaled down by a factor of ca. 100 as compared to a laboratory spectrometer, has the dimensions of 9x5x5 cm3 and weighs only 400 g. MIMOS measures the Mössbauer effect in scattering mode and yields spectra of comparable quality as with standard laboratory equipment. After many successful terrestrial applications, NASA and the European Space Agency ESA decided in 2000 that MIMOS would participate in the next Mars missions. It so happened in 2003, when the two NASA missions Spirit and Opportunity, both furnished with MIMOS together with other equipment, were launched. They landed successfully on Mars in January 2004 and ever since are have been recording and communicating Mössbauer spectra of fantastic quality down to Earth. Two Science papers /382, 392/ from our group have reported already in 2004 on the first results measured by Spirit and Opportunity. An extended report followed shortly later /408/. Despite the excessively harsh weather and environment conditions the missions on the whole and MIMOS in particular have been operating extremely well up to now. About a dozen iron containing minerals have been identified, among typical ones which only could have developed in wet surroundings, like Goethite and Jarosite. This is indirect evidence that water must have been present on Mars or even still is. In the meantime, direct evidence for the presence of water on Mars has been detected in various ways. Mössbauer spectroscopy has played such an important role in the current NASA missions that it has already been decided that it will be operational again in future missions to Mars and other extraterrestrial missions.