The state-resolved differential cross sections for both the $1s^2$ ${}^1{S}_0\to 1s2s$ ${}^1{S}_0$ monopolar transition and the $1s^2$ ${}^1{S}_0\to 1s2p$ ${}^1{P}_1$ dipolar transition of atomic helium had been measured over a large momentum transfer region by high-resolution inelastic x-ray scattering (IXS). The almost-perfect match of the present measurement with the theoretical calculations gives a stringent test of the theoretical method and the calculated wave functions. Our results demonstrate that high-resolution IXS is a powerful tool for studying the excitations in atoms and molecules.

We report nonsequential and sequential fragmentation dynamics of ${{\mathrm{CO}}_2}^{3+}$ investigated by electron collision at an impact energy of 500 eV. The dissociation mechanisms are clearly distinguished by combined use of the Dalitz plot together with momentum correlation spectra. The angular distributions and kinetic-energy releases (KERs) of different fragmentation processes are obtained. The dissociation channels of higher excited states of the ${{\mathrm{CO}}_2}^{3+}$ molecular ion are opened, which are quite different from the previous studies of heavy-ion collision [N. Neumann, D. Hant, L. Ph. H. Schmidt, J. Titze, T. Jahnke, A. Czasch, M. S. Schöffler, K. Kreidi, O. Jagutzki, H. Schmidt-Böcking, and R. Dörner, Phys. Rev. Lett. 104, 103201 (2010)] and intense laser field [C. Wu, C. Wu, D. Song, H. Su, Y. Yang, Z. Wu, X. Liu, H. Liu, M. Li, Y. Deng, Y. Liu, L.-Y. Peng, H. Jiang, and Q. Gong, Phys. Rev. Lett. 110, 103601 (2013)]. By analyzing KERs together with the help of potential-energy curves exploration at the multireference configuration interaction level, we conclude that the sequential fragmentation occurs in the ${}^2\mathrm{\Pi },{}^4\mathrm{\Pi }$, and ${}^2{\mathrm{\Sigma }}^{+}$ states of the ${{\mathrm{CO}}_2}^{3+}$ ion. The bond length and bond angle are also determined based on the linear fragmentation, indicating that electron impact fragmentation is a potential method to precisely reconstruct the geometry of neutral molecules.

The Coulomb explosion of tribromomethane (bromoform, CHBr₃) induced by 28 fs near-infrared laser pulses is investigated by three-dimensional coincidence ion momentum imaging. We focus on the fragmentation into three, four, and five ionic fragments measured in coincidence and present different ways of visualizing the three-dimensional momentum correlations. We show that the experimentally observed momentum correlations for 4- and 5-fold coincidences are well reproduced by classical Coulomb explosion simulations and contain information about the structure of the parent molecule that could be used to differentiate structural isomers formed, for example, in a pump-probe experiment. Our results thus provide a clear path toward visualizing structural dynamics in polyatomic molecules by strong-field-induced Coulomb explosion imaging.