The visible spectrum of highly charged ions:: A window to fundamental physics

The observation of forbidden transitions in the visible range was the first experimental sign of the existence of highly charged ions in nature. Such lines can nowadays be excited in electron beam ion traps with excellent control of the ionic charge state, allowing their identification and precise determination of wavelengths and transition probabilities. The accuracy achieved in such experiments has been central to the study of nuclear size effects and quantum electrodynamic contributions, which are drastically magnified in highly charged ions, thus enabling the study of the underlying fundamental interactions in few-electron systems. At the same time, forbidden lines can be used to probe conditions in high temperature plasmas present in both astrophysical and laboratory environments.