text: In addition to causing secondary emission, positive ions can strike the cathode with sufficient force to eject particles of the material from which the cathode is made.  This process is called sputtering and it gradually ablates the cathode.  Sputtering is useful when using spectroscopy to analyze the composition of the cathode, as is done in Glow-discharge optical emission spectroscopy.
  However, sputtering is not desirable when glow discharge is used for lighting, because it shortens the life of the lamp.  For example, neon signs have hollow cathodes designed to minimize sputtering, and contain charcoal to continuously remove undesired ions and atoms.