Polarizability of electronically excited molecular oxygen: theory and experiment

Multireference ab initio calculations (by using the extended multiconfiguration quasi-degenerate second-order perturbation theory method XMCQDPT2) have been employed to determine the static polarizabilities for the ground triplet ( X 3 Σ g − ) and two excited singlet ( a 1 Δ g and b 1 Σ g + ) electronic states of molecular oxygen. The corrections to polarizabilities for the nuclear zero point motion are also reported. It was revealed that the excitation of an O 2 molecule into the lowest singlet electronic states leads to a decrease in its polarizability: the polarizabilities of O 2 ( a 1 Δ g ) and O 2 ( b 1 Σ g + ) are smaller (by factors of 0.967 and 0.962, respectively) than that of O 2 ( X 3 Σ g − ). The theoretical results for the b 1 Σ g + state were quantitatively confirmed experimentally employing the diffraction at laser-induced gratings.