Hollow-cathode lamp
If the voltage applied is large enough, the ions of the inert gas gather enough energy to remove some of the metal atoms of interest from the surface of the cathode by striking the surface. The removal of these atoms produces a cloud around the cathode. The process of producing this cloud is called sputtering. Some of these metal atoms in the cloud have moved to an excited energy state and as they return to their ground state, they emit a specific radiation which is characteristic to the metal of interest. The release of this radiation allows for data to be collected and information gathered pertaining to the metal of interest.  After a period of time, the atoms move to the glass walls of the lamp or back to the surface of the cathode.