Radiation Physics and Chemistry 75 (2006) 1582–1585 Atomic cluster calculation of the X-ray near-edge absorption of copper C. Witte, C.T. Chantler à , E.C. Cosgriff, C.Q. Tran School of Physics, University of Melbourne, Victoria 3010, Australia Accepted 9 July 2005 Abstract The finite difference method for near-edge structure is used to calculate X-ray absorption near-edge structure (XANES) spectra. We extend the range of calculation for copper above the K-shell threshold and compare the results with recent experimental data in the X-ray absorption fine structure (XAFS) region. Qualitatively the calculation predicts the location of the peaks but fails to accurately describe relative amplitudes. r 2006 Elsevier Ltd. All rights reserved. PACS: 78.70.Dm; 61.10.Ht; 78.20.Bh; 32.80.Fb Keywords: XAFS; XANES; Copper 1. Background X-ray absorption fine structure (XAFS) contains detailed information about the local structure, in particular about nearest neighbour distances, coordina- tion numbers, fluctuations in bond distances, electronic structure and vibrational structure, providing a unique thumb print of the material being investigated. An accurate theoretical model validated by precise experi- ments is needed to utilise this field to its full potential. Significant progress has been made (Chantler, 1994), but discrepancies exist between theoretical approaches of up to 200% for numerous elements in the 1–3 keV X-ray energies (Chantler, 2000), and until recently experimen- tal inaccuracies have made it impossible to differentiate between them. Recent experimental work using the X-ray extended range technique (XERT) has improved accuracies to a level of 0.27–0.4% (Tran et al., 2005) prompting renewed efforts to improve theory. Qualitatively XAFS can be explained by many-body effects such as shake-up and shake-down, inelastic scattering and multiple scattering but accurate ab initio calculations that include these effects have been missing or are severely limited. Cluster methods can reproduce solid state effects but most use the muffin-tin potential. In this approximation the potential in a sphere of radius r surrounding the atom is expanded in spherical harmonics. In the interstitial region between atoms the potential is assumed constant. The size of the muffin tins is then a free parameter varied to obtain the best match. Often overlapping spheres produce the best results. This is a questionable formal assumption of the methods and can hide structural or electronic information. The presence of a free parameter to match experimental data questions the significance and sensitivity of this assumption. Joly (2001) proposed using the finite difference method (FDM) to calculate the potential in the ARTICLE IN PRESS www.elsevier.com/locate/radphyschem 0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.07.018 à Corresponding author. E-mail address: chantler@physics.unimelb.edu.au (C.T. Chantler).