MICROSCOPIC METHODS ENABLING TO LOOK UPON DIFFUSIVITY IN SOLIDS .1. COMPARISON BETWEEN RAYLEIGH-SCATTERING AND QNS

The fullest information about the diffusivity in solids at microscopic scale (atomic) can be provided by methods sensitive to both energy an d momentum transfer. Due to the fact that jumps are usually of short d istance and the motion is rather slow one requires a high energy resol ution at relatively significant and well-defined momentum transfers. H ence, only two methods have been applied successfully up to now, i.e. QNS (quasi-elastic, incoherent neutron scattering) and QMS (quasi-elas tic Mossbauer spectroscopy) - the latter being limited to a fixed abso lute value of the momentum transfer. It seems that two other methods c an be considered presently despite their limitations, i.e. Rayleigh sc attering of monochromatized synchrotron radiation and TDPAC (time diff erential perturbed angular correlations). The paper reviews merits and drawbacks of Rayleigh scattering in comparison with the established t echnique of QNS. A detailed comparison between TDPAC and QMS is to be presented in the following paper.