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

A number of unknown transitions below the Herzberg I band, A(3)Sigma(u)(+) <-- X(3)Sigma(g)(-), have been studied by ab inito methods in the range 190-300 nm (B(3)Sigma(u)(-) <-- b(1)Sigma(g)(+), B(3)Sigma(u)(-) <-- a(1)Delta(g), l(3)Pi(u) <-- b(1)Sigma(g)(+) and l(3)Pi(u) <-- a(1)Delta(g)) and also in the visible (B(3)Sigma(u)(-) <-- A(3)Sigma(u)(+), l(3)Pi(g) <-- A(3)Sigma(u)(+) and l(3)Pi(g) <-- A'(3)Delta(u)) and in the IR regions (A(3)Sigma(u)(+) <-- c(1)Sigma(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(3)Sigma(u,0)(-) <-- b(1)Sigma(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 l(1,3)Pi(u) <-- b(1)Sigma(g)(+) continuous absorptions, which start to grow at similar to 200 nm. At longer wavelengths the l(3)Pi(u) <-- b(1)Sigma(g)(+) continuum is about 20 times weaker than the B - b transition, but it is still more intensive than the A(3)Sigma(u,1)(+) <-- b(1)Sigma(g)(+) continuum in the range 280-360 nm and than the A'(3)Delta(u,3) <-- a(1)Delta(g) absorption band, 210-300 nm. Absorption from the a(1)Delta(g) state is practically negligible. The magnetic dipole and quadrupole transition moments for the B(3)Sigma(u)(-) <-- A(3)Sigma(u)(+) and A(3)Sigma(u,1)(+) <-- c(1)Sigma(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 x 10(-9)). The allowed triplet-triplet transitions l(3)Pi(g) <-- A(3)Sigma(u)(+) and l(3)Pi(g) <-- A'(3)Delta(u) are shown to be important. (C) 2000 Elsevier Science B.V. All rights reserved.