Research Centre for Surface and Materials Science
Ultraviolet Photoelectron Spectroscopy (UPS)
The measurement of kinetic energy spectra of photoelectrons emitted by molecules which have absorbed ultraviolet photons.
Similar to XPS but using vacuum UV (10-45 eV) radiation to examine valence levels.
In UPS the source of radiation is normally a noble gas discharge lamp; frequently a He-discharge lamp emitting He I or He II radiation of energy 21.2 eV or 40eV.
Such radiation is only capable of ionising electrons from the outermost levels of atoms - the valence levels. The advantage of using such UV radiation over X-rays is the very narrow line width of the radiation and the high flux of photons available from simple discharge sources. The main emphasis of work using UPS has been in studying:
- The electronic structure of solids - detailed angle resolved studies permit the complete band structure to be mapped out in k-space.
- The adsorption of relatively simple molecules on metals - by comparison of the molecular orbitals of the adsorbed species with those of both the isolated molecule and with calculations.
The surface of a sample often has a significantly different composition to the bulk of the material which can have profound implications for the behaviour of that material, especially in terms of corrosion resistance and adhesion, recent applications of XPS in RCSMS include analysis of:
- corrosion products.
- composition of coating materials.
- diamond like carbon films.
RCSMS has one instrument combining the techniques of XPS and UPS.
The Kratos Axis DLD is a state-of-the-art XPS system with full imaging capability and excellent energy resolution. The analysis area ranges from 700µm x 300µm to a spot of 15µm diameter.
Spatial resolution in imaging mode is <3µm. The axis is fitted with a He I and He II UV source for UPS.
Accessories include heating and cooling stages in both the analysis and sample preparation chambers and two ion guns for cleaning surfaces and depth profiling.