Ultrahigh-Energy-Resolution Inelastic X-Ray Scattering Spectroscopy

Abstract

Very-high-energy-resolution measurements using X-rays can be achieved by extreme backreflection (Bragg angle close to 90°) from perfect crystals. This technique allowed the development of the instrument INELAX for inelastic scattering experiments at the HARWI wiggler at DORIS, DESY Hamburg. Recently, a high energy resolution of 9 meV could be achieved, and the instrument proved to be an excellent tool to investigate collective excitations in condensed matter. Energy transfers from 10 meV to 12 eV and wave vectors up to about 10 Ä -1 are accessible.Key words: Inelastic X-ray scattering; Collective excitations; Phonons; Electronic excitations; Zoneboundary collective states.Since its discovery in 1912 X-ray diffraction has been an extremely successful tool to reveal the local arrangement of atoms and molecules in crystal lattices. In addition, inelastic X-ray scattering based on Compton's observations in 1922 has given information on the electron momentum density. The energy resolution of such inelastic experiments, however, was not sufficient to allow the direct observation of excitations like lattice vibrations. The situation changed with the high intensity of X-rays emitted by synchrotron radiation sources. Meanwhile, very high energy resolution can be obtained by extreme backreflection with Bragg scattering from perfect single crystals at Bragg angles 6 close to 90° (backscattering) [1]. This technique allowed the development of the instrument INELAX for inelastic-scattering experiments at the HARWI wiggler at DORIS (HASYLAB, Hamburg) [2,3] and the direct measurements of lattice vibrations [2-4] and of low-lying electronic excitations [3,5].In a Bragg-scattering experiment the energy resolution is limited by the divergence 06 of the beam and the reflection width <5t of the perfect crystal. The energy resolution is given by ÖE ök öz -= -= -+ cot e • de, E k t