Steering chemical branching in photodissociation of molecules in a high vibrational overtone by combining laser and synchrotron radiation

如果从电子激发态可以获得两种或两种以上不同的解离途径，则分子中的初始振动激发可能影响光解离的化学分支。Crim的工作备受关注，他可以完成重水HOD的选择性破键。我们计划利用同步辐射（SR）在真空紫外区促进振动激发分子到电子激发态。主要目的是通过对其广泛的势能表面进行采样来阐明解离态的特性，例如确定产物最终状态分布的动力学，解离的非绝热跃迁，以及由俄歇衰变产生的未知多重激发态的分配和表征。本文研制了一种激光sr双光子电离实验系统，其核心是超高分辨率连续钛蓝宝石激光器和两束光子严格重叠的光束对准装置。该装置连接到筑波光子工厂的光束线上。利用锁相调制技术，通过观察激光激发水的光声信号，精确地确定了激光的波数。光子束对准装置位于中央光电离室的上游和下游。每个装置由两个安装在二维运动馈线上的SUS板组成。这些板可以在不破坏系统真空的情况下穿过光子束轴插入，这使我们能够很容易地获得两束光束的良好空间重叠。用飞行时间质谱法测定了H2O*(4νO-H)+hν→OH+（Χ3Σ-）+H(n=1)+e-离解产生的OH+离子，其离解阈值为18.05 eV，相对于水的中性基态。

Initial vibrational excitation in molecules may influence the chemical branching in photodissociation, if two or more different dissociation pathways are accessible from an electronically excited state. Much attention has been focused on the work of Crim, who could accomplish the selective bond-breaking of heavy water, HOD. We are planning to use synchrotron radiation (SR) to promote vibrationally excited molecules to electronically excited states in the vacuum UV region. The main objective is to elucidate the properties of dissociative states by sampling a wide range of their potential energy surfaces, such as dynamics determining the final-state distributions of the products, nonadiabatic transitions on dissociation, and assignments and characterization of unknown multiply-excited states produced by Auger decay. We have developed an experimental system for laser-SR two photon ionization the crux of which are an ultrahigh resolution continuous titanium-sapphire laser and beam alignment devices for strict overlapping of the two photon beams. The apparatus was connected to a beam line of the Photon Factory in Tsukuba. The wavenumber of the laser was determined precisely by observing photoacoustic signal from laser excited H2O measured with a lock-in modulation technique. The photon beam alignment devices are situated upstream and downstream of a central photoionization chamber. Each device is comprised of two SUS plates fitted on two-dimensional motion feedthroughs. These plates can be inserted across the photon beam axis without breaking the system vacuum, which permits us to easily attain good spatial overlap of the two beams. The OH+ ions produced via dissociation of H2O*(4νO-H)+hν→OH+(Χ3Σ-)+H(n=1)+e- were detected by time-of-flight mass spectrometry at SR photon energies near the dissociation threshold of 18.05 eV with respect to the neutral ground state of water.