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In 1928 he introduced the Grotrian diagram in atomic spectroscopy, showing the allowed transitions between atomic energy levels
Grotrian studied the emission line from the solar corona in the green region of the spectrum; this emission line could not be attributed to any known chemical element and was thought to be a new element (which scientists named "coronium"). Grotrian and Bengt Edlén from Sweden demonstrated that the two observed emission lines arise from ironatoms that have lost about half their 26 electrons.