Takanashi Tsukasa, Nakamura Kosuke, Kukk Edwin, Motomura Koji, Fukuzawa Hironobu, Nagaya Kiyonobu, Wada Shin-Ichi, Kumagai Yoshiaki, Iablonskyi Denys, Ito Yuta, Sakakibara Yuta, You Daehyun, Nishiyama Toshiyuki, Asa Kazuki, Sato Yuhiro, Umemoto Takayuki, Kariyazono Kango, Ochiai Kohei, Kanno Manabu, Yamazaki Kaoru, Kooser Kuno, Nicolas Christophe, Miron Catalin, Asavei Theodor, Neagu Liviu, Schöffler Markus, Kastirke Gregor, Liu Xiao-Jing, Rudenko Artem, Owada Shigeki, Katayama Tetsuo, Togashi Tadashi, Tono Kensuke, Yabashi Makina, Kono Hirohiko, Ueda Kiyoshi
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan.
Phys Chem Chem Phys. 2017 Aug 2;19(30):19707-19721. doi: 10.1039/c7cp01669g.
Coulomb explosion of diiodomethane CHI molecules irradiated by ultrashort and intense X-ray pulses from SACLA, the Japanese X-ray free electron laser facility, was investigated by multi-ion coincidence measurements and self-consistent charge density-functional-based tight-binding (SCC-DFTB) simulations. The diiodomethane molecule, containing two heavy-atom X-ray absorbing sites, exhibits a rather different charge generation and nuclear motion dynamics compared to iodomethane CHI with only a single heavy atom, as studied earlier. We focus on charge creation and distribution in CHI in comparison to CHI. The release of kinetic energy into atomic ion fragments is also studied by comparing SCC-DFTB simulations with the experiment. Compared to earlier simulations, several key enhancements are made, such as the introduction of a bond axis recoil model, where vibrational energy generated during charge creation processes induces only bond stretching or shrinking. We also propose an analytical Coulomb energy partition model to extract the essential mechanism of Coulomb explosion of molecules from the computed and the experimentally measured kinetic energies of fragment atomic ions by partitioning each pair Coulomb interaction energy into two ions of the pair under the constraint of momentum conservation. Effective internuclear distances assigned to individual fragment ions at the critical moment of the Coulomb explosion are then estimated from the average kinetic energies of the ions. We demonstrate, with good agreement between the experiment and the SCC-DFTB simulation, how the more heavily charged iodine fragments and their interplay define the characteristic features of the Coulomb explosion of CHI. The present study also confirms earlier findings concerning the magnitude of bond elongation in the ultrashort X-ray pulse duration, showing that structural damage to all but C-H bonds does not develop to a noticeable degree in the pulse length of ∼10 fs.
利用多离子符合测量和基于自洽电荷密度泛函紧束缚（SCC-DFTB）的模拟，对由日本X射线自由电子激光设施SACLA产生的超短强X射线脉冲辐照的二碘甲烷CH₂I₂分子的库仑爆炸进行了研究。与之前研究的仅含有单个重原子的碘甲烷CH₃I相比，含有两个重原子X射线吸收位点的二碘甲烷分子表现出截然不同的电荷产生和核运动动力学。我们重点研究了CH₂I₂与CH₃I相比的电荷产生和分布情况。还通过将SCC-DFTB模拟与实验进行比较，研究了动能向原子离子碎片的释放。与早期模拟相比，进行了几项关键改进，例如引入了键轴反冲模型，其中电荷产生过程中产生的振动能量仅导致键的拉伸或收缩。我们还提出了一种解析库仑能量分配模型，通过在动量守恒的约束下将每对库仑相互作用能量分配给该对中的两个离子，从计算得到的和实验测量的碎片原子离子动能中提取分子库仑爆炸的基本机制。然后根据离子的平均动能估计库仑爆炸关键时刻分配给各个碎片离子的有效核间距。实验与SCC-DFTB模拟之间具有良好的一致性，我们展示了电荷更多的碘碎片及其相互作用如何定义CH₂I₂库仑爆炸的特征。本研究还证实了早期关于超短X射线脉冲持续时间内键伸长幅度的发现，表明在约10飞秒的脉冲长度内，除C-H键外的所有键的结构损伤都不会发展到明显程度。
