Assignment of the excess absorption underlying the Schumann–Runge bands of molecular oxygen

A long-standing problem, pertaining to the origin of the excess absorption found experimentally to underlie the high-vibrational Schumann–Runge bands of molecular oxygen, is resolved. Through new calculations, with parameter sets based on recently obtained experimental information, it is shown that the excess absorption arises from transitions into the lowest valence states of Πu3 and Πug symmetry.