Ion-pair dissociation dynamics of O2 in the range 17.2–17.5 eV studied by XUV laser and velocity map imaging method

Abstract

The ion-pair dissociation dynamics of O2, O2 + hv → O(+)((4)S) + O(-)((2)P(1/2, 3/2)), in the photon energy range 17.20-17.50 eV has been studied using extreme ultraviolet laser and velocity map imaging method. The ion-pair yield spectrum and the fine structure resolved photofragment O(-)((2)P(j)) velocity map images have been recorded. The branching ratios between the two spin-orbit components O(-)((2)P(3/2)) and O(-)((2)P(1/2)) and the corresponding anisotropy parameters describing their angular distributions have been determined. It is found that the fragments O(-)((2)P(1/2)) are all from parallel transitions, while the fragments O(-)((2)P(3/2)) are from both parallel and perpendicular transitions. The main products for most of the excitation photon energies are O(-)((2)P(1/2)). The dissociation dynamics has been discussed based on the ab initio potential energy curves of the ion-pairs. The major peaks in the ion-pair yield spectrum have been assigned based on the angular distribution of the photofragments. The experimental results suggest that the so-called strong and weak series of Rydberg states converging to O2(+)b(4)Σg(-) should have symmetries of (3)Σu(-) and (3)Πu, respectively. In addition to the Rydberg states converging to O2(+)b(4)Σg(-), the Rydberg states converging to O2(+)A(2)Πu should also play a role in the ion-pair dissociation of O2.