The specimen is excited with the primary X-radiation. In the process electrons from the inner electron shells are knocked. Electrons from outer electron shells fill the resultant voids emitting a fluorescence radiation that is characteristic in its energy distribution for a particular material. This fluorescence radiation is evaluated by the detector.
The generation of the X-ray fluorescence radiation is shown simplified in the figure above. One electron from the K shell is knocked. The resultant void is filled by either an electron from the L shell or an electron from the M shell. In the process the Kα and Kβ radiation is generated, which is characteristic for the particular material.