The X-ray absorption spectrum (XAS) consists of XANES and EXAFS which are low and high energy parts respectively. EXAFS exhibits simple sine function like oscillation with energy. It is interpreted with a simple single scattering theory. XANES has more complex features in the spectra. It can be interpreted as an extension with higher multiple scatterings. Thus there is no gap between EXAFS and XANES in terms of the scattering theory.
The Multiple Scattering (MS) theory has been used widely for core electron spectroscopies, such as XAS, to construct the continuum states. This theory uses 1) numerical basis sets, which are solutions of Schrödinger equation at each scattering site, to expand the wave function or Green’s function and 2) scattering boundary condition, so that the continuum states are calculated precisely for wide energy range.
Conventionally, the MS calculation uses Muffin-tin (MT) approximation which approximates atomic potentials by spherically shaped and averaged potentials and sets a constant potential in interstitial region. It gives reasonable results for isotropic and closed packed systems. However it fails for anisotropic and open systems, especially for low dimensional materials. We have developed the Full Potential Multiple Scattering (FPMS) theory beyond the MT approximation, which utilizes space filling cells as scatterers without approximating the potentials inside the cells.
We implemented the FPMS code based on this theory. Our method avoids the expansion of the shaped potential by spherical harmonics which creates bad convergence problems. We applied this method to anisotropic systems, open molecules and graphene-related systems.
About the speakers
Prof. Keisuke Hatada obtained his Ph.D in 2001 from Chiba University, Japan. Post his Ph.D degree he won the Italian National Institute of Nuclear Physics (INFN) postdoctoral fellowship for foreigners and worked at the National Laboratory of Frascati, Italy for a period of 5 years. This was followed by work at various institutes including CSIC, Spain and CNRS, France, LMU, Munich, Germany. He was also won the Marie-Curie Individual Fellowships Action (IEF) during 2014-2016. Presently he is an Associate Professor at the Department of Physics, Univ. of Toyama, Japan.
Prof. Hatada has worked extensively in the development of theory, codes and analysis of core level spectroscopies (XANES, EXAFS, XRS, PHD, ELNES, XFEL), Multiple Scattering and Green’s function theory. He has developed his own scientific code known as FPMS apart from using other codes like MXAN, MsSpec, GNXAS, FELSAT.
Prof. Hatada was recipient of the Young Scientist Award (Dale Sayers Prize) in the year 2009 in the recognition of his contribution to the theoretical development of X-ray Absorption Near-Edge Structure given by the International X-ray Absorption Society (IXAS).