We demonstrate the visualization of ultrafast hydrogen migration in deuterated acetylene dication (${\mathrm{C}}_2\mathrm{D}_2{}^{2+}$) by employing the pump-probe Coulomb explosion imaging with sub-10-fs intense laser pulses (9 fs, $0.13\mathrm{PW}/{\mathrm{cm}}^2$, 800 nm). It is shown, from the temporal evolution of the momenta of the fragment ions produced by the three-body explosion, ${\mathrm{C}}_2\mathrm{D}_2{}^{3+}\to {\mathrm{D}}^{+}+{\mathrm{C}}^{+}+{\mathrm{CD}}^{+}$, that the migration proceeds in a recurrent manner: The deuterium atom first shifts from one carbon site to the other in a short time scale ($\sim 90\mathrm{fs}$) and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation.

Roaming dynamics in real time: Roaming is distinct from conventional reaction channels because of the unusual geometries that chemical systems use to bypass the minimum energy pathway. It is a relatively new phenomenon that is usually determined in experiments through spectroscopic characterization of the roaming products. Using a combination of time-resolved Coulomb explosion imaging and quasiclassical trajectory analysis, Endo et al. report real-time observation of individual fragments of the prototypical reaction of deuterated formaldehyde (D ₂ CO) dissociation as they roam on ultrafast time scales. They show that roaming not only occurs several orders of magnitude earlier than previously expected but also that it can terminate in a radical (D + DCO) rather than the well-known molecular (D ₂ + CO) product channel. Science , this issue p. 1072

Solid parahydrogen, known as a quantum crystal, is a novel matrix for the study of physical and chemical processes of molecules at low temperatures by high-resolution infrared spectroscopy. Ro-vibrational motion of molecules embedded in solid parahydrogen is well quantized on account of the weak interaction in the crystal. In addition, molecules are almost free from the cage effect because of the softness of the quantum crystal, so that a variety of chemical reactions could be observed for molecules in solid parahydrogen at liquid He temperatures. In this article, we survey our recent studies on photodissociation of methyl radicals and subsequent reactions in solid parahydrogen, and discuss (1) the nuclear spin selection rule in chemical reactions and (2) pure tunnelling reactions obtained from the analysis of the present system.

The visualization of ultrafast isomerization of deuterated acetylene dication (C₂D₂ ²⁺) is demonstrated by time-resolved Coulomb explosion imaging with sub-10 fs intense laser pulses (9 fs, 0.13 PW cm⁻², 800 nm). The Coulomb explosion imaging monitoring the three-body explosion process, C₂D₂ ³⁺ → D⁺ + C⁺ + CD⁺, as a function of the delay between the pump and probe pulses revealed that the migration of a deuterium atom proceeds in a recurrent manner; One of the deuterium atoms first shifts from one carbon site to the other in a short timescale (∼90 fs), and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation. Correlated motion of the two deuterium atoms associated with the hydrogen migration and structural deformation to non-planar geometry are identified by the time-resolved four-body Coulomb explosion imaging, C₂D₂ ⁴⁺ → D⁺ + C⁺ + C⁺ + D⁺.