Resonant elastic X-ray scattering: where from? where to?

The International Conference on Anomalous Scattering held in Malente, Germany, in 1992 examined the broad issue of 'dispersion or resonant scattering' for X-rays. The significant aspects discussed at that time included: the evaluation of X-ray scattering factors, the role of local broken symmetries and the application to structure determinations, the impact on magnetic X-ray scattering, but also resonant Raman scattering and nuclear resonant scattering. However, many aspects of resonant elastic X-ray scattering (REXS) were still being revealed and soon it was realised that new productive paths of research would open up. In the last two decades, applications have developed thanks to concurrent developments in X-ray instrumentation and theoretical approaches. Focusing on REXS only, noteworthy fields are: the observation of multiple order parameters, the detection of multipolar moments, but also applications in chemistry and materials sciences, macromolecules and soft matter in the soft X-ray range. REXS has evolved from the exploitation of large resonant signals to a powerful experimental and characterisation method aimed at unraveling new phenomena. The recent technical and theoretical advances have paved the way to further REXS developments in many fields of science. (author)