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.