RELATIVISTIC EFFECTS IN THE X-RAY-ABSORPTION FINE-STRUCTURE

A relativistic model for the extended x-ray-absorption fine structure, in which the energy loss of the emitted photoelectron is accounted fo r by using a complex energy-dependent exchange-correlation potential, has been developed. Relativistic curved-wave single-, double-, and tri ple-scattering formulas for excitation from any core hole have been fo und. The dominant single-scattering signal has been computed for the f irst shell at the L2 and L3 edges of Th (Z = 90), Pt (Z = 78), and Eu (Z = 63) and at the K edge of Sr (Z = 38) using this model. Comparison s of the relativistic scattering amplitudes of the Dirac model with th e current standard Schrodinger curved-wave model reveal that the latte r deviates from the more exact Dirac model by 20% above 240 eV for Th, by 15% above 240 eV for Pt, and by 10% above 140 eV for Eu. For lower energies, the deviations are as much as two times larger. Differences between the Dirac model L2- and L3-edge amplitudes occur below 240 eV for Th and Pt with values of 2% and 6%, respectively. For Eu, this di fference diminishes to 4% and occurs only below 140 eV. In the case of the K edge of Sr, the Schrodinger single-scattering amplitude differs from the Dirac amplitude by 5% for energies above 95 eV. The spin dep endence of the generalized Ramsauer-Townsend effect is also exhibited.