Low energy electron diffraction (LEED)

Introduction

Since the discovery of electron diffraction by Davisson and Germer in 1927 and subsequent developments in the vacuum technology and solid state physics during the following decades, the low energy electron diffraction (LEED) has become a standard technique in surface science. It allows quantitative determination of the atomic structure of an ordered solid surface often down to the first five interlayer spacings with an accuracy of a few tenths of an angstrom. Since the late 1960s, when the technique itself and the complicated theory required in the interpretation of the electron diffraction data had become established, LEED has contributed a major part of currently known, ordered clean and adsorbate covered surface structures.

Principle of operation

As its name implies, LEED is based on the detection of diffracted electrons. As shown in the figure below, a collimated and monoenergetic beam of electrons is accelerated towards the surface. Some of the impinging electrons are elastically scattered back towards the electron source. The spatial distribution of the intensity maxima (LEED pattern) of the backscattered electron stream at the electron source is determined by the surface structure. As it turns out, the positions of the intensity maxima correspond to the lattice structure of the surface in reciprocal space. Thus, the mere observation of a LEED pattern yields information about the surface structure and its long range order. A schematic representation of a LEED measurement is shown below.
 

The figure below shows an experimental LEED pattern of a clean Pd{110} surface at 120 K measured using electrons with kinetic energy of 140 eV. Also shown is the reciprocal space unit cell of the surface as indicated by the white rectangle.

The quantitative structural analysis consists of the measurement of the LEED beam intensities at various primary beam energies and theoretical calculations are based on this experimental data.

Applicability

Low energy electron diffraction can only be applied to the study of conductive or semiconductive surfaces with a long range order.