Female students working at laptop computer in library

APS - Appearance Potential Spectroscopy

Here's a brief introduction to Appearance Potential Spectroscopy, how it is done, and what type of experimental signal we obtain in this spectroscopy.

The experiment is done in vacuum. There, thermally excited electrons are used as a source of excitation. The voltage to the sample is increased in steps. Following the excitation of a core level electron, the system will de-excite. This de-excitation will appear as an emission of Auger electrons or Soft X-rays. Depending on what is monitored, the appropriate name is given to that technique.

The simplicity of the APS technique comes from its non-dispersive nature. Signal is extracted from the background using potential modulation technique. The signal strength is proportional to the Unoccupied Density of States at the Fermi Level. APS reveals the Total DOS.

In an APS spectrum, the peaks provide a means of elemental identification. The threshold can be used to measure the Binding Energy. The chemical shift correlates to changes in the chemical bonding and the shape of the peak can be used to characterize the Unoccupied DOS.

In our lab, we have two versions of APS: Soft X-ray APS and Auger Electron APS.  SXAPS is useful for studying High Z Materials and the signal is recorded in the first differential mode (Z is the atomic number).

AEAPS is useful for studying Low Z Materials and the signal is recorded in the second differential mode. We are setting up a Low Temperature AEAPS at A&M-Commerce which will allow us to conduct temperature variation studies.

Picture of APS equipment