Search

Abstract Real-time imaging of transient structure of the electronic excited state is fundamentally critical to understand and control ultrafast molecular dynamics. The ejection of electrons from the inner-shell and valence level can lead to the population of different excited states, which trigger manifold ultrafast relaxation processes, however, the accurate imaging of such electronic state-dependent structural evolutions is still lacking. Here, 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 with sub-10 picometre and few-femtosecond precision. We visualise that θ DOD and ROD are significantly increased by around 50∘ and 10 pm, respectively, within approximately 8 fs after initial ionisation for the A state, and the ROD further extends 9 pm within 2 fs along the ground state of the dication in the present condition. Moreover, the ROD can stretch more than 50 pm within 5 fs along autoionisation state of dication. The accuracies of the results are limited by the simulations. These results provide comprehensive structural information for studying the fascinating molecular dynamics of water, and pave the way towards to make a movie of excited state-resolved ultrafast molecular dynamics and light-induced chemical reaction.

Higher-order aromatic clusters are prevalent in biochemical systems, but a full understanding of their structural and dynamical properties is lacking. Here, the authors demonstrate that inner-valence ionization can induce ultrafast relaxation and further fragmentation mechanisms in benzene trimers.

This work reports atomic alignment of silver after L3-subshell ionization in collisions with electrons in energy range 10–50 keV, i.e., (3∼15)EL3, where EL3(=3.351 keV) is the binding energy of electrons in the L3-subshell. The characteristic spectra are measured at different emission angles from -40∘ to 40∘ intervals of 10∘. The angular dependence of X-ray emission is studied for Lα(L3→M4,5) and Lβ2(L3→N5) lines respectively, using a thin target. The anisotropy parameter β is determined from two angular distribution measurements of differential X-ray intensity ratios dI(Lα,θ)/dI(Lβ1,θ) and dI(Lβ2,θ)/dI(Lβ1,θ). This served to reduce the experimental uncertainties. For the derivation of the alignment factor A20 we took into account the Coster–Kronig (CK) correction factor κ, which allows for, CK transition probabilities fij among L-subshells. We found that the sign of A20 is mostly negative, and the influence of CK transition is significant for large alignment parameters. In addition, the experimental results are compared to theoretical predictions within the framework of Plane Wave Born Approximation (PWBA) and reasonable agreement is found in the energy range studied in the present work. Simultaneously, good scaling property of atomic alignment is demonstrated when we consider also previous experimental results. Our measurement extends the scaled velocity V (=vp/vt) to a range of 1∼4 for medium-Zt targets, and the results of atomic alignment can provide a fundamental testing ground for ionization models.

Reactions induced by low-energy electrons in hydrated systems are central to radiation therapy, but a full understanding of their mechanism is lacking. Here the authors investigate the electron-impact induced ionization and subsequent dissociation of tetrahydrofuran, model for biochemically relevant systems, in a micro-solvated environment.

The fragmentation dynamics of dicationic dimers of acetylene molecules initiated upon strong-field laser ionization is studied. Time-resolved pump-probe experiments with femtosecond laser pulses, accompanied by ab initio dynamical calculations, allow us to evaluate the detailed behavior of molecular ions during the dissociation process. The dynamical properties of the intermediate C_{2}H_{2}^{+}⋯C_{2}H_{2}^{+} state created by the pump pulse are probed by a second pulse which causes further ionization. The time-dependent yield of a coincident C_{2}H_{2}^{+} + C_{2}H_{2}^{2+} ion pair exhibits an oscillation feature with a periodicity of 240 ± 30 fs. Our studies demonstrate that this is caused by an ultrafast rotation of C_{2}H_{2}^{+} cations driven by intermolecular Coulomb force together with the orientation-dependent ionization rate. We suggest that the present observation of ultrafast molecular rotation can be a general phenomenon occurring in a wide variety of systems.

Inter-particle reactions in weakly bound systems are often difficult to pinpoint by detecting exclusively the kinetic energy of the produced ions. Here the authors present a full-coincidence experiment, where inter-particle reaction channels are determined by the measurement of all outgoing particles.