Abstract :
A number of unknown transitions below the Herzberg I band, A3Σu+←X3Σg−, have been studied by ab inito methods in the range 190–300 nm (B3Σu−←b1Σg+, B3Σu−←a1Δg, 13Πu←b1Σg+ and 13Πu←a1Δg) and also in the visible (B3Σu−←A3Σu+, 13Πg←A3Σu+ and 13Πg←A′3Δu) and in the IR regions (A3Σu+←c1Σu−). The singlet-triplet and singlet-singlet transitions from the metastable states of molecular oxygen were calculated by quadratic and linear response multiconfiguration self-consisted field (MCSCF) method, respectively. The larger active space and basis sets are used in comparison with previous calculations. A large value of the electric dipole B3Σu,0−←b1Σg+ transition moment (about 0.01 au) is confirmed. Absorbance in the range 192–231 nm could be determined by this B←b transition, since its cross-section exceeds the relatively intensive 11,3Πu←b1Σg+ continuous absorptions, which start to grow at ∼200 nm. At longer wavelengths the 13Πu←b1Σg+ continuum is about 20 times weaker than the B−b transition, but it is still more intensive than the A3Σu,1+←b1Σg+ continuum in the range 280–360 nm and than the A′3Δu,3←a1Δg absorption band, 210–300 nm. Absorption from the a1Δg state is practically negligible. The magnetic dipole and quadrupole transition moments for the B3Σu−←A3Σu+ and A3Σu,1+←c1Σu− transitions have been recalculated with new wave functions. The B−A transition intensity is negligible. The A−c infra-red transition (2340 cm−1) has the same magnetic nature like the b−X transition and could be observed, in principle, (f=1.5×10−9). The allowed triplet-triplet transitions 13Πg←A3Σu+ and 13Πg←A′3Δu are shown to be important.
