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Surface Science Techniques Title

UPS - Ultraviolet Photoemission Spectroscopy


Ultraviolet light is shone at the sample using a Helium lamp emitting at 21.2 eV (He I radiation) or 40.8 eV (He II radiation), although synchrotron radiation can provide photon energies from approx. 10 eV up to the XPS region. The low photon energy in UPS means that deep core electron levels cannot be excited, and only photoelectrons emitted from the valence band or shallow core levels are accessible. Angle Resolved UPS (ARUPS) can be used to determine the band structure of the material under investigation. UPS can also be used to identify molecular species on surfaces by identifying characteristic electron energies associated with the bonds of the molecules.

By using a synchrotron, the relative change in photoemission cross-section for various electron states can be used to determine the partial Density of States (DOS). Certain electron states undergo a 'Cooper minimum' whereby the cross-section drops to a very low value at a certain photon energy. Hence the shape of the DOS can be determined without the contribution of the electron state at the Cooper minimum.

Alternatively, a 'resonance' may occur whereby the photoemission intensity for a particular state may be enhanced at certain photon energies. This is caused when a high binding energy core level has its electrons excited to states whose energy is just above the Fermi level. The atom can relax by emitting an Auger electron (see AES ), or a resonant enhancement of a valence band state can occur. A typical resonance occurs in Ti where a 3p electron jumps to an empty 3d state for photon energies in the region of 33 eV. (see also Spin-Polarised UPS, SPUPS)



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