high­-resolution electron energy loss spectroscopy abbr., HREELS (rus. спектроскопия высокого разрешения характеристических потерь энергии электронами abbr., СХПЭЭ-ВР) — Spectroscopy of electrons' characteristic energy loss designed to analyse the loss of energy for excitation of atomic vibrations on a solid surface and adsorbates on it.

Description

Since the loss of energy for the excitation of surface and adsorbate atoms is usually measured in fractions of eV, respective energy peaks are placed very close to the elastic peak, and to observe them a very high energy resolution is required. This goal can be achieved through the use of monochromators when electrons' primary beam is formed and high resolution energy analysers to record the spectra of electrons scattered from the sample.

This method (SECEL) proved to be a powerful method to study the adsorption of atoms and especially molecules on solid surfaces. It makes it possible to identify the type of adsorbate and gives information about the geometry of its chemical bonds. Considerations are usually based on a comparison of vibrational modes measured by SECEL, at known vibrational spectra of molecules measured in the gas phase by infra-red spectroscopy or Raman spectroscopy.

Using the SECEL technique, the following problems are usually solved.

1. Identification of adsorbate type. Since each molecule is characterised by a set of certain vibrational modes, SECEL-derived spectra can be used to identify the adsorbate type. The example shown in the Figure demonstrates the ability to distinguish between adsorption of oxygen as O atoms and O2 molecules.

2. Identification of adsorption places. This can be done on the basis of vibrational modes corresponding to a specific tie between adsorbate and substrate atoms. For example, in the case of the adsorption of organic molecules on the GaAs surface the appearance of vibrational modes corresponding to the As-H bond indicates that it is the As atoms where adsorption takes place.

3. Determination of adsorbed molecules' orientation in the space. The use of different scattering geometries facilitates, in particular, determining whether certain chemical bonds are oriented parallel or perpendicular to the surface.

Illustrations

Investigation of oxygen adsorption on the silver surface us
Investigation of oxygen adsorption on the silver surface using the HR-EELS technique. In case of adsorption in normal room temperature conditions (medium spectrum), a typical peak is observed at 40.2 meV that corresponds to the vibrational mode O–Ag, which reflects dissociation of O2 molecules. In case of specimen's exposure to O2 at 100 K (upper spectrum), molecular adsorption of O2 is observed; typical peaks at 29.4 meV and 79.1 meV correspond to vibrational modes Ag-O2 and O-O, respectively. Ag(110) clean surface spectrum (lower curve) is provided for benchmarking [2].

Authors

  • Zotov Andrey V.
  • Saranin Alexander A.

Sources

  1. Oura K. et al. Surface Science: An Introduction // Springer, 2010 - 452 pp.
  2. Stietz F., Pantfoerder A., Schaefer J. A. et al. High-resolution study of dipole-active vibrations at the Ag(110)(nx1)O surface // Surf. Sci. 1994. V. 318. P. L1201–L1205.

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