The surface of the sample is illuminated homogeneously by UV-light thereby generating photo-electrons if the energy of the photons exceeds the local work function of the surface. These electrons are imaged by electron lenses onto the channel plate followed by a phosphor screen. The intensity on the screen is proportional to the local workfunction on the surface and can be recorded with a digital video camera through a glass window outside the reaction chamber. Since the work function of metals is affected by adsorbed molecules, reactions at the surface can be monitored.
This setup is only applicable at pressures below 10-3 mbar because the mean free path of electrons is limited by the pressure.
For more information, please see publications on Surface Imaging.