In ultra-violet photoelectron spectroscopy UV rays are used to probe surfaces. The technique is similar to XPS (X-ray Photoelectron Spectroscopy) but the energy of UV rays being much less than that of x-rays this technique cannot probe electrons that are as strongly attached to the nuclei as XPS does. On the other hand the technique is very sensitive and useful to probe the surface of samples and in particular, the valence electrons involved in bonding. The following figure shows how the range of electron bindingenergies over which XPS and UPS are useful differs.

UPS is useful to probe conduction band and valence band electrons and XPS is better suited to study core level electrons. Tutorials on UPS can be found here on a manufacturer's web site.