Generation of X-Rays

1. Ionization: A hole in an inner shell (here: K shell) is generated by an incident high-energy electron that loses the corresponding energy E transferred to the ejected electron*.
2. X-ray emission: The hole in the K shell is filled by an electron from an outer shell (here: L3). The superfluous energy is emitted as a characteristic X-ray quantum.

In a typical X-ray spectrum, there are many peaks caused by such a process. The X-ray energy corresponds to a certain difference in inner-shell energies. Thus, the detection of characteristic X-ray is specific for a element in the sample, and X-ray spectroscopy can be employed for qualitative analysis.

* Energy losses of the incident electrons are measured by EELS.