https://doi.org/10.1351/goldbook.A00502
Atomic and @[email protected] originate from specified electronic transitions between energy levels of atoms and ions, respectively. In the past it has been common usage to denote atomic lines as arc lines and ionic lines as spark lines. This usage is now considered to be incorrect. The correct way to indicate that lines are due to atomic or ionic transitions is: Element symbol I @[email protected] e.g. Cu I \(324.7\ \text{nm}\); and Element symbol II @[email protected] e.g. Cu II \(213.6\ \text{nm}\). Similarly for higher states of @[email protected], the type of line is represented by III, IV, etc.
See also:
ionic spectral lines