The singlet oxygen absorption to the upper state of the Schumann–Runge system: the B 3Σu-←a 1Δg and B 3Σu-←b 1Σg+ transitions intensity calculation

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Abstract
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, a1Δg and b1Σg +, 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, B3Σu -, and with the 13Πu dissociative state. Calculations of the transition dipole moments for the spin allowed bands, SR, n1Σu +â†�b1Σg + and n1Δuâ†�a1Δg, 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 b1Σg + and X3Σg - (MS=0) states provides their effective mixing at all internuclear distances (r) and produces a strong contribution to the parallel component of the B3Σu -â†�b1Σg + transition dipole moment (Dz) by intensity borrowing from the Schumann–Runge band in a wide range of the r values. The Dz(B–b) integral has an averaged value 0.08 ea0† in the most important range, r=1.18 to 1.45 Å, but exhibits some irregular behaviour at longer distances. The perpendicular component of the B–b transition is negligible. The B3Σu -â†�a1Δg transition has only perpendicular dipole moment which is relatively non-intense, D(B–a)∽0.0004 ea0, in order to compete with the absorption in the Herzberg I continuum (D∽0.001 ea0). 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 13Πuâ†�a1Δg and 13Πuâ†�b1Σg + 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.

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