The singlet oxygen absorption to the upper state of the Schumann-Runge system:: the B3Σu-←a1Δg and B3Σu-←b1Σg+ transitions intensity calculation

The singlet-triplet transition dipole moments were calculated by quadratic response (QR) multi-configuration self-consistent field (MCSCF) method from the two metastable singlet states of molecular oxygen, a(1)Delta(g) and b(1)Sigma(g)(+), to the upper triplet excited states. The most intense transitions in the near UV region (200-250 nm) are connected with the upper state of the Schumann-Runge (SR) system, B(3)Sigma(u)(-), and with the 1(3)Pi(u) dissociative state. Calculations of the transition dipole moments for the spin allowed bands, SR, nl(1)Sigma(u)(+)<--b(1)Sigma(g)(+) and n(1)Delta(u)<--a(1)Delta(g), by linear response (LR) MCSCF method were compared with previous studies and found to be quite reliable. A similar analysis was performed for the spin-orbit coupling matrix elements, calculated with the full Breit-Pauli operator. Spin-orbit coupling between the b(1)Sigma(g)(+) and X(3)Sigma(g)(-) (M-S=0) states provides their effective mixing at all internuclear distances (r) and produces a strong contribution to the parallel component of the B(3)Sigma(u)(-)<--b(1)Sigma(g)(+) transition dipole moment (D-z) by intensity borrowing from the Schumann-Runge band in a wide range of the r values. The D-z(B-b) integral has an averaged value 0.08 ea(0)dagger in the most important range, r=1.18 to 1.45 Angstrom, but exhibits some irregular behaviour at longer distances. The perpendicular component of the B-b transition is negligible. The B(3)Sigma(u)(-)<--a(1)Delta(g) transition has only perpendicular dipole moment which is relatively non-intense, D(B-a)similar to 0.0004 ea(0), in order to compete with the absorption in the Herzberg I continuum (D similar to 0.001 ea(0)). The transition dipole moments as functions of r have some oscillations at very short and long distances, connected with level crossings and avoided crossings. The singlet-triplet transitions 1(3)Pi(u)<--a(1)Delta(g) and 1(3)Pi(u)<--b(1)Sigma(g)(+) are 30-20 times weaker than the B-b absorption. Static and dynamic electric dipole polarizabilities for the ground triplet and both singlet excited states are also calculated and briefly discussed.