Oxygen absorption below and near the Herzberg I continuum. Ab initio calculation of the transitions probability from metastable states.

Abstract

A number of unknown transitions below the Herzberg I band, A 3Σ u +← X 3Σ g −, have been studied by ab inito methods in the range 190–300 nm ( B 3Σ u −← b 1Σ g +, B 3Σ u −← a 1Δ g , 1 3Π u ← b 1Σ g + and 1 3Π u ← a 1Δ g ) and also in the visible ( B 3Σ u −← A 3Σ u +, 1 3Π g ← A 3Σ u + and 1 3Π g ← A′ 3Δ u ) and in the IR regions ( A 3Σ u +← c 1Σ u −). 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 B 3Σ u,0 −← b 1Σ g + 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 1 1,3Π u ← b 1Σ g + continuous absorptions, which start to grow at ∼200 nm. At longer wavelengths the 1 3Π u ← b 1Σ g + continuum is about 20 times weaker than the B− b transition, but it is still more intensive than the A 3Σ u,1 +← b 1Σ g + continuum in the range 280–360 nm and than the A′ 3Δ u,3 ← a 1Δ g absorption band, 210–300 nm. Absorption from the a 1Δ g state is practically negligible. The magnetic dipole and quadrupole transition moments for the B 3Σ u −← A 3Σ u + and A 3Σ u,1 +← c 1Σ u − 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 1 3Π g ← A 3Σ u + and 1 3Π g ← A′ 3Δ u are shown to be important.