Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
J Phys Chem A. 2012 Sep 27;116(38):9459-65. doi: 10.1021/jp3034038. Epub 2012 Sep 18.
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.
通过阈光电离-光电离符合（TPEPICO）速度成像方法的研究，提供了氧分子通过（2）Σ（u）（-）离子光暗态离解的直接实验证据。除了 B（2）Σ（g）（-）态的振动进展外，在 20.2-21.1 eV 的激发能范围内还同时观察到（2）Σ（u）（-）离子光暗态的几个较弱的振动带。在整个激发能范围内仅检测到 O（+）碎片；因此，所有的振动带都是完全预解离的。记录了从振动态选择的 O（2）（+）（（2）Σ（u）（-），v（+））离子解离的 O（+）碎片的 TPEPICO 三维时间切片速度图像。对于（2）Σ（u）（-）（v（+）=0-3）振动态，仅观察到 O（+）（（4）S）+ O（（3）P）的最低解离通道。一旦光子能量稍微增加到（2）Σ（u）（-）（v（+）=4）能级，图像中就会出现一个新的同心面包圈，表明确实鉴定了 O（+）（（4）S）+ O（（1）D）的第二个解离通道。借助势能曲线，提出了 O（2）（+）在（2）Σ（u）（-）（v（+））态下的离解机制。
