Directly imaging excited state-resolved transient structures of water induced by valence and inner-shell ionisation

Capturing the accurate transient structure of the electronic excited states for water molecule is a critical step towards the direct visualization of water-related ultrafast chemical reactions, such as the formation of H3O+ and OH radical, which attracts considerable broad interests in ultrafast physics and femtochemistry. Here, the authors achieve the transient structure retrieval of different electronic states for cation and dication of heavy water with a ew-picometer and few-femtosecond accuracy. By developing the laser-induced electron recollision-assisted Coulomb explosion imaging approach and molecular dynamics simulations, snapshots of the vibrational wave-packets of the excited (A) and ground states (X) of D2O+ are captured simultaneously . They visualise that the bond angle θ and bond length are both significantly increased by around 50° and 10 pm, respectively, within approximately 8 fs in the A state, and the ROD further extends 9 pm within 2 fs along the ground state of the dication in the present condition. In contrast, the ROD can extend at approximately 70 pm along a higher exited state of dication, which is populated by the inner shell ionisation. A highly asymmetric O-D-D type transient structure has been captured, which originates from a higher excited state of the cation. This indicates the higher exited states may play a role in the formation of radical or the protonated water. Finally, the authors achieved the retrieval so far of the most accurate structure of the neutral water with the Coloumb explosion imaging technique. The current work including the development both in measurement and simulation enables the quantitative transient structure retrieval for specific electronic states of polyatomic molecules, which is an important step for shooting the molecular movie of the ultrafast photo-induced chemical reaction.

精准获取水分子电子激发态（electronic excited states）的瞬态结构，是直接可视化水相关超快化学反应（如H₃O⁺与OH自由基（OH radical）的生成）的关键一步，该研究方向在超快物理与飞秒化学（femtochemistry）领域广受关注。本研究中，作者以亚皮米（sub-picometer，原文疑似笔误为ew-picometer）与飞秒级精度，实现了重水（heavy water）阳离子（cation）与双阳离子（dication）不同电子态的瞬态结构反演。通过开发激光诱导电子再碰撞辅助库仑爆炸成像（laser-induced electron recollision-assisted Coulomb explosion imaging）技术与分子动力学模拟（molecular dynamics simulations）方法，研究团队同时捕获了重水阳离子D₂O⁺激发态(A)与基态(X)的振动波包（vibrational wave-packets）快照。研究显示，在激发态(A)中，键角θ与键长分别在约8飞秒内显著增大约50°与10皮米（pm）；在本实验条件下，双阳离子基态中的O-D径向距离（ROD）进一步在2飞秒内延伸9皮米。与之相比，在通过内壳层电离（inner shell ionisation）布居的双阳离子更高激发态中，ROD可延伸约70皮米。本研究捕获到一种高度不对称的O-D-D型瞬态结构，该结构源于阳离子的更高激发态。这表明更高激发态可能在自由基或质子化水（protonated water）的生成过程中发挥作用。最终，本研究借助库仑爆炸成像技术，实现了迄今为止对中性水分子最精准的结构反演。本研究在测量与模拟两方面均取得进展，可实现多原子分子（polyatomic molecules）特定电子态的定量瞬态结构反演，为拍摄超快光诱导化学反应的分子电影迈出了重要一步。