Photo-stationary state of UV pumped matrix isolated O2

The 193 nm excitation of O2 in crystalline Ar and Kr leads to a photo-stationary state, whereby ≈ 50% of the population is transferred from O2(X3 Σg−) to a(1Δg). A kinetic analysis yields absorption cross sections of ≈ 10−21 cm2, from X and a states, and establishes the final states as A′(3Δu) and the repulsive 1Π surface, respectively. The A′ ← X transition is enhanced ≈ 3 orders of magnitude in the solid, by borrowing intensity from the 3Π ← X transition. Only molecules isolated at defect sites, undergo permanent dissociation. Substitutionally isolated O2 does not dissociate at excess energies ⩽ 2.5 eV.