High-energy Photoelectron Diffraction Research Articles

High-energy photoelectron diffraction: model calculations and future possibilities

We discuss the theoretical modeling of x-ray photoelectron diffraction (XPD) with hard x-ray excitation at up to 20 keV, using the dynamical theory of electron diffraction to illustrate the characteristic aspects of the diffraction patterns resulting from such localized emission sources in a multilayer crystal. We show via dynamical calculations for diamond, Si and Fe that the dynamical theory predicts well the available current data for lower energies around 1 keV, and that the patterns for energies above about 1 keV are dominated by Kikuchi bands, which are created by the dynamical scattering of electrons from lattice planes. The origin of the fine structure in such bands is discussed from the point of view of atomic positions in the unit cell. The profiles and positions of the element-specific photoelectron Kikuchi bands are found to be sensitive to lattice distortions (e.g. a 1% tetragonal distortion) and the position of impurities or dopants with respect to lattice sites. We also compare the dynamical calculations with results from a cluster model that is more often used to describe lower energy XPD. We conclude that hard XPD (HXPD) should be capable of providing unique bulk-sensitive structural information for a wide variety of complex materials in future experiments.

Simulation of high energy photoelectron diffraction using many-beam dynamical Kikuchi-band theory

We use the many-beam dynamical theory of electron diffraction for the calculation of x-ray photoelectron diffraction $(\mathrm{XPD})$ patterns of the substrate emission. The reciprocity principle is used to apply a Bloch wave model for the diffraction of an incoming plane wave by a three-dimensional crystal. In this way, many-beam dynamical simulations of $\mathrm{XPD}$ in the context of Kikuchi-band theory can be carried out. This extends the results of the two-beam theory used so far and leads to quantitative descriptions of $\mathrm{XPD}$ patterns in the picture of photoelectrons reflected by lattice planes. The effects of forward scattering directions, substrate polarity, circular structures due to onedimensional diffraction, and emitter specific extinction of Kikuchi lines can be reproduced by Kikuchi-band theory. The results are compared with single scattering cluster calculations. In this way, the equivalence of the cluster approach and the Kikuchi-band picture can be demonstrated completely in both directions

Atomic structure of the Sb-terminated Si(111) surface: A photoelectron diffraction study

The deposition under certain conditions of antimony on a $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface removes the $7\ifmmode\times\else\texttimes\fi{}7$ reconstruction, producing a passivated Si(111)-$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$-Sb (1 monolayer) surface. In this work, a quantitative determination of the atomic structure of this reconstruction using photoelectron diffraction is reported. In particular, high-energy photoelectron diffraction (forward-focusing regime) has been applied to investigate the stacking sequence of the atomic layers of the silicon substrate, and scanned-energy photoelectron diffraction (backscattering regime) has been used to determine quantitatively the atomic structure of the surface. Our results show that the formation of a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ phase produces a bulklike-terminated $\mathrm{Si}(111)1\ifmmode\times\else\texttimes\fi{}1$ substrate free of stacking faults. Regarding the atomic structure of the interface, this study strongly favors the $T4$-site milkstool model over the $H3$ one.

Splitting effects in high resolution and high energy photoelectron diffraction: the case of MgO(001)

In some cases, high energy photoelectron diffraction peaks corresponding to well defined crystallographic directions show a splitting that cannot be explained by the traditional interpretation. Such an effect occurs in the O 1s 0° peak in MgO(001). Using cluster calculations, we demonstrate that this effect is induced by close neighbours via an inter-chain process and explain why it does not occur for the Mg 2s peak.

Temperature dependent photoelectron diffraction of the Si(001) surface

A high energy photoelectron diffraction (XPD) study of Si(001) at different temperatures reveals that at about 1400 K a surface phase transition occurs. It has been identified as being an incomplete melting transition. The thickness of the liquid film is estimated to be about 2 Å (or two atomic layers) on the basis of single-scattering cluster calculations.

Comparative high-energy photoelectron diffraction study of Pt(111), Ni(111), and Pt50Ni

We report new experimental results concerning high-energy photoelectron diffraction from Ni core levels in pure Ni(111), from Pt core levels in pure Pt(111), and from Pt and Ni core levels in ${\mathrm{Pt}}_{50}$${\mathrm{Ni}}_{50}$(111). We show that, qualitatively, the observed diffraction patterns may be easily interpreted when proper account is taken of the strong internuclear axis enhancement. This enhancement, due to the predominance of forward single scattering over multiple scattering and backscattering, is a characteristic of the high-kinetic-energy region. More quantitatively, we reproduce the experimental results by performing theoretical calculations based on the quasidynamical method. Furthermore, an estimation of the degree of Pt segregation in ${\mathrm{Pt}}_{50}$${\mathrm{Ni}}_{50}$(111) is deduced from this comparison between Pt(111), Ni(111), and ${\mathrm{Pt}}_{50}$${\mathrm{Ni}}_{50}$(111) diffraction patterns.