Ultraviolet Photoelectron Spectroscopy

The ultraviolet photoelectron spectroscopy , short ultraviolet photoelectron spectroscopy (UPS) is a specific method of analysis of the photoelectron spectroscopy (PES). It is mainly used to determine the valence band structure of solids (often also referred to as valence band spectroscopy in the solid state), surfaces and adsorbates . In addition to adsorption mechanisms, the chemical structure of bonds and vibrational energies of various molecular gases can also be examined.

Determined density (English density of states , DOS). In addition, the stimulating energy of light (see photoelectron spectroscopy ) is in the ultraviolet spectral range and is therefore only capable of triggering valence electrons . These energies are of course, the X-ray photoelectron spectroscopy (XPS) is also accessible only can by a suitable choice of the light source (usually helium - gas discharge lamps ), the kinetic energy of photoelectrons so be measured with extremely high accuracy. UPS can also resolve minimal energy differences between molecular orbitals or the physical environment (e.g. adsorption on surfaces) of the spectroscoped molecule .

Radiation sources

For ultraviolet photoelectron spectroscopy, radiation sources for the energy range from 10 to 100 eV are required:

1. He gas discharge lamp : He gas discharge lamps are usually used as radiation sources. These do not emit a continuous spectrum, but discrete energies. One disadvantage is that the photon energy cannot be continuously tuned. The advantage is that the energy resolution is very good:
   • Energy of radiation or ${\displaystyle E_{\gamma }=21{,}2\,{\text{eV}}}$ ${\displaystyle E\!\,_{\gamma }=40{,}8\,{\text{eV}}}$
   • resolution ${\displaystyle \Delta E_{\gamma }\approx \pm 10^{-3}\,{\text{eV}}}$
2. Synchrotron radiation : Continuous radiation that arises when charged particles are accelerated at high speeds. Synchrotron radiation shows good stability and can be tuned with a monochromator, which only allows certain measurement methods. However, the energy resolution is not quite as good as with a He gas discharge lamp and depends on the monochromator used . Synchrotron radiation is of course not available to every laboratory. One example is the beamline UE112-PGM2 at BESSY II in Berlin with energies E _γ from 4 to 200 eV and a resolution Δ E _γ of around ± 1 meV.

Determination of the density of states

There are three different methods with which either the so-called JDOS ( Joint Density of States ), a combination of occupied and unoccupied states, the density of states of the occupied states or the density of states of the unoccupied states is determined. With He gas discharge lamps, only the JDOS can be measured, since the other methods require a variation of the photon energy.

1. Measurement of JDOS: This is the normal case, since synchrotron radiation is only rarely available. Only the kinetic energy of the electrons is varied. The photon energy is kept constant. This changes both the initial state and the final state, and the relative intensity depends on a combination of the density of states.
2. Measurement of the DOS of the occupied states: only the photon energy is varied. The kinetic energy of the electrons is kept constant. This only changes the initial state, the end state remains constant. The relative intensity depends only on the density of states of the initial state, i.e. the density of states of the occupied states.
3. Measurement of the DOS of the unoccupied states: The photon energy and the kinetic energy of the electrons are varied so that the initial state remains constant and the end state is varied. The relative intensity depends only on the density of states of the final state, i.e. the density of states of the unoccupied states.

literature

• JM Hollas: Modern methods in spectroscopy. Vieweg, Braunschweig / Wiesbaden 1995, ISBN 3540670084 .
• D. Briggs, MP Seah (Ed.): Practical Surface Analysis, Volume I - Auger and X-ray photoelectron spectroscopy. John Wiley & Sons, Chichester 1990, ISBN 0471920819 .
• M. Henzler, W. Göpel: Surface physics of the solid. Teubner, Stuttgart 1991, ISBN 3519130475 .
• Gerhard Ertl , J. Küppers: Low Energy Electrons and Surface Chemistry. VCH, Weinheim 1985, ISBN 0895730650 .
• Stefan Hüfner : Photoelectron spectroscopy, principles and applications. Springer Series in Solid-State Sciences 82.Springer , Berlin / Heidelberg / New York 1996, ISBN 3540418024 .

Web links

• NIST X-ray Photoelectron Spectroscopy Database

Footnotes and individual references

1. ↑ BESSY II beamlines. Helmholtz Zentrum Berlin, September 2, 2010, accessed on April 12, 2013 .