Ultrafast Coulomb explosion of a diiodomethane molecule induced by an X-ray free-electron laser pulse
Ultrafast Coulomb explosion of a diiodomethane molecule induced by an X-ray free-electron laser pulse
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X 射线自由电子激光脉冲诱发二碘甲烷分子的超快库仑爆炸
DOI：
10.1039/c7cp01669g
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
Kiyoshi Ueda
中科院分区：
文献类型：
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作者：
Tsukasa Takanashi;Kosuke Nakamura;Edwin Kukk;Koji Motomura;Hironobu Fukuzawa;Kiyonobu Nagaya;(他28名);Hirohiko Kono;Kiyoshi Ueda
用多离子符合测量和基于自洽电荷密度泛函的紧束缚(SCC-DFTB)模拟研究了日本SACLA超短强X射线脉冲辐照下二碘甲烷CH2I2分子的库仑爆炸。二碘甲烷分子含有两个重原子的X射线吸收位，与前面研究的只有一个重原子的碘甲烷CH3I相比，它表现出相当不同的电荷产生和核运动动力学。与CH3I相比，我们重点研究了CH2I2中电荷的产生和分布。通过SCC-DFTB模拟与实验的比较，研究了动能在原子离子碎片中的释放。与以前的模拟相比，有几个关键的增强，例如引入了键轴反冲模型，在该模型中，在电荷产生过程中产生的振动能量只会引起键的拉伸或收缩。我们还提出了一个解析的库仑能量分配模型，在动量守恒的约束下，通过将每对库仑相互作用能划分为对中的两个离子，从计算和实验测量的碎片原子离子的动能中提取分子库仑爆炸的本质机制。然后根据离子的平均动能估算出库仑爆炸关键时刻分配给单个碎片离子的有效核间距离。我们演示了重电荷态碘碎片及其相互作用是如何定义CH2I2的库仑爆炸的特征的，这与实验和SCC-DFTB模拟的结果吻合得很好。本研究还证实了早先关于超短X射线脉冲持续时间中键伸长幅度的发现，表明除了C-H键之外，所有键的结构损伤在∼10fs的脉冲长度中都没有发展到明显的程度。
Coulomb explosion of diiodomethane CH2I2 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 CH3I with only a single heavy atom, as studied earlier. We focus on charge creation and distribution in CH2I2 in comparison to CH3I. 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 CH2I2. 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.