title: Direct experimental evidence for dissociative photoionization of oxygen molecule via 2Σ(u)(-) ionic "optical dark" state.
abstract: Direct experimental evidence for dissociative photoionization of oxygen molecule via the (2)Σ(u)(-) ionic optical dark state is presented by an investigation using the method of threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Besides vibrational progress of the B(2)Σ(g)(-) state, several weak vibrational bands of the (2)Σ(u)(-) ionic optical dark state are observed concomitantly in an excitation energy range of 20.2-21.1 eV. Only O(+) fragments are detected in the whole excitation energy range; therefore, all vibrational bands are completely predissociative. TPEPICO three-dimensional time-sliced velocity images of O(+) fragments dissociated from vibrational state-selected O(2)(+)((2)Σ(u)(-),v(+)) ions are recorded. For the (2)Σ(u)(-)(v(+)=0-3) vibrational states, only the lowest dissociation channel of O(+)((4)S) + O((3)P) is observed. Once the photon energy is slightly increased to the (2)Σ(u)(-)(v(+)=4) level, a new concentric doughnut appears in the image, indicating that the second dissociation channel of O(+)((4)S) + O((1)D) is identified indeed. With the aid of potential energy curves, the dissociative mechanism of O(2)(+) in the (2)Σ(u)(-)(v(+)) state is proposed.
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Xiaofeng
authors.initials: X
authors.lastname: Tang
authors.forename: Xiaoguo
authors.initials: X
authors.lastname: Zhou
authors.forename: Manman
authors.initials: M
authors.lastname: Wu
authors.forename: Yu
authors.initials: Y
authors.lastname: Cai
authors.forename: Shilin
authors.initials: S
authors.lastname: Liu
authors.forename: Liusi
authors.initials: L
authors.lastname: Sheng
mesh_terms: D007477:Ions / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry*; D055668:Photochemical Processes; D014732:Vibration
chemical_list: D007477:Ions; D010100:Oxygen