SXAPS - Soft X-ray Appearance Potential Spectroscopy SXAPS is a member of the Appearance Potential Spectroscopies (see APS). The experimental apparatus for SXAPS is very simple. A filament mounted near the sample emits electrons which are accelerated towards the sample. X-rays generated within the sample are detected via photoelectrons generated by the X-ray within the detector. Due to the long mean free path of X-rays in matter, it might be thought that SXAPS is not particularly surface sensitive, but as it is a threshold technique, the incident electrons will only travel a short distance before they are unable to excite the level of interest. As X-rays are very inefficiently generated, SXAPS suffers from poor signal. Indeed the bremsstrahlung background may be 100 times larger than the signal leading to very bad signal to noise ratios. Its main asset is its simplicity.