Theory

A short academic paper is in preparation which gives an overview of Galore’s applications. Some of that information is repeated here; this section of the user guide aims to provide essential information and refer to the academic literature for those seeking more depth and context.

Photoelectron spectroscopy

History

Photoelectron spectroscopy (PES) is a family of methods used to characterise the chemical nature and electronic structure of materials. PES is based on the photoelectric effect, which was discovered by Hertz. [1] It was explored extensively by Rutherford and colleagues [2] and within a few years researchers including de Broglie [3] and Robinson [4] were using the technique to measure electron binding energies through the relationship

\[E_\text{k} = h\nu - E_\text{B}.\]

Photons with energies \(h\nu\) ranging from 5 eV up to 12 keV eject electrons (referred to as “photoelectrons”) from the occupied orbitals of a sample. The kinetic energy \(E_\text{k}\) of each photoelectron therefore depends on its binding energy \(E_\text{B}\). The names of various PES methods refer to the photon energy range used:

  • ultraviolet photoelectron spectroscopy (UPS): 5–100 eV

  • X-ray photoelectron spectroscopy (XPS): 0.3–2 keV

  • hard X-ray photoelectron spectroscopy (HAXPES, HE-PES, HXPS, HX-PES): above 2 keV

Broadening

Major sources of broadening include:

  • Intrinsic lifetime broadening (Lorentzian)

    • While this can play a significant role, the lifetime broadening is energy-dependent and care should be taken when applying it across the full data set.

  • Franck–Condon phonon broadening (Gaussian)

    • This is caused by vibrations in the system which lead to a distribution of accessible photoionization energies.

    • In oxides this is associated with as much as 0.8 eV broadening

  • Instrumental broadening (Gaussian)

    • Typical values are in the range 0.2–0.3 eV.

Weighting

The Gelius model

The Gelius model was originally developed to describe molecular systems. [5][6][7]

Asymmetry corrections

References

1

H. Hertz. Ueber einen Einfluss des ultravioletten Lichtes auf die electrische Entladung. Ann. der Phys. und Chemie, 267(8):983–1000, 1887. URL: http://doi.wiley.com/10.1002/andp.18872670827, doi:10.1002/andp.18872670827.

2

E. Rutherford. The Connexion between the β and γ Ray Spectra. Phil. Mag., Sept 1914. There are a number of other papers from Rutherford’s group in this volume of Phil. Mag. related to the ejection of β-rays (electrons) from samples exposed to γ-rays (photons). doi:10.1080/14786440908635214.

3

Maurice de Broglie. Les phénomènes photo-électriques pour les rayons x et les spectres corpusculaires des éléments. J. Phys. Radium, 2(9):265–287, sept 1921.

4

H Robinson. The Secondary Corpuscular Rays Produced by Homogeneous X-Rays. Proc. R. Soc. A Math. Phys. Eng. Sci., 104(727):455–479, nov 1923. URL: http://rspa.royalsocietypublishing.org/cgi/doi/10.1098/rspa.1923.0121, doi:10.1098/rspa.1923.0121.

5

U. Gelius and K. Siegbahn. ESCA studies of molecular core and valence levels in the gas phase. Faraday Discuss. Chem. Soc., 54:257, 1972. URL: http://xlink.rsc.org/?DOI=dc9725400257, doi:10.1039/dc9725400257.

6

U. Gelius. Molecular Orbitals and Line Intensities in ESCA Spectra. In D. A. Shirley, editor, Electron Spectrosc., pages 311. North-Holland, Amsterdam, 1972.

7

U. Gelius. Recent progress in ESCA studies of gases. J. Electron Spectros. Relat. Phenomena, 5(1):985–1057, jan 1974. URL: http://linkinghub.elsevier.com/retrieve/pii/0368204874850644, doi:10.1016/0368-2048(74)85064-4.