High-resolution hard-x-ray photoelectron diffraction in a momentum microscope—the model case of graphite

O Fedchenko,S Babenkov,A Gloskovskii,B Schönhense,Yu Matveyev,K Medjanik,A Winkelmann,S Chernov,D Vasilyev,W Drube,H J Elmers,G Schönhense,C Schlueter

doi:10.1088/1367-2630/ab51fe

Abstract

Hard x-ray photoelectron diffraction (hXPD) patterns recorded with a momentum microscope with high k-resolution (0.025 Å−1 equivalent to an angular resolution of 0.034° at 7 keV) reveal unprecedented rich fine structure. We have studied hXPD of the C 1s core level in the prototypical low-Z material Graphite at 20 photon energies between 2.8 and 7.3 keV. Sharp bright and dark lines shift with energy; regions of Kikuchi band crossings near zone axis exhibit a filigree structure which varies rapidly with energy. Calculations based on the Bloch wave approach to electron diffraction from lattice planes show excellent agreement with the experimental results throughout the entire energy range. The main Kikuchi bands in the [001] zone axis appear fixed on the momentum scale with a width of the corresponding reciprocal lattice vector, allowing to reconstruct the size of the projected Brillouin zone. The newly developed high-energy k-microscope allows full-field imaging of (kx, ky)-distributions in large k-fields (up to >22 Å−1 dia.) and time-of-flight energy recording.

Highlights

Launched by the pioneering work of Siegbahn, Fadley and others in the early 1970s [1, 2], x-ray photoelectron diffraction (XPD) has developed into a powerful method to gain information about the geometrical structure of the photo-emitting atomic layers, surface reconstruction and relaxation, as well as adsorbate geometries
In the present work we studied the inner region of the hard x-ray photoelectron diffraction (hXPD) pattern, which looks practically structureless in most previous XPD studies, see, e.g. the patterns recorded with hard x-rays at comparable energies (5.4 keV) in [11]
The quantity relevant for the diffraction dynamics is the photoelectron wavelength λe inside of the material, related to the final-state momentum kfinal as λe = 2π/|kfinal|. When deriving this quantity from the photon energy set by the monochromator and the tabulated binding energy of C 1s in Graphite (EB = 284 eV), two additional empirical quantities have to be taken into consideration

Summary

14 November 2019

Commons Attribution 3.0 Keywords: photoemission, hard x-rays, photoelectron diffraction, graphite, time-of-flight (ToF) momentum microscopy licence. Hard x-ray photoelectron diffraction (hXPD) patterns recorded with a momentum microscope with high k-resolution (0.025 Å−1 equivalent to an angular resolution of 0.034° at 7 keV) reveal unprecedented rich fine structure. Sharp bright and dark lines shift with energy; regions of Kikuchi band crossings near zone axis exhibit a filigree structure which varies rapidly with energy. The main Kikuchi bands in the [001] zone axis appear fixed on the momentum scale with a width of the corresponding reciprocal lattice vector, allowing to reconstruct the size of the projected. The newly developed high-energy k-microscope allows full-field imaging of (kx, ky)-distributions in large k-fields (up to >22 Å−1 dia.) and time-of-flight energy recording

Introduction

Photoelectron diffraction in the Kikuchi model

Experimental and theoretical details

Results and discussion

Summary and conclusions