New magnetic dipole transition of the oxygen molecule: B′ 3Πg←X 3Σg−(0,0)

Through the use of isotopically pure gas at a temperature of 77 K, a weak photoabsorption band of O216 is found near 1856 Å, underlying the stronger Schumann–Runge (SR) band B 3Σu−←X 3Σg−(8,0). The location, structure, and intensity of this new band are consistent with expectation for the magnetic dipole transition B′ 3Πg←X 3Σg−(0,0), where the designation B′ is chosen to represent the II 3Πg valence state. This electronic transition contributes to the “excess absorption” underlying the SR bands [B. R. Lewis, S. T. Gibson, and E. H. Roberts, J. Chem. Phys. 115, 245 (2001)].