Neutral H2 formation via intramolecular hydrogen migration in hydrocarbon molecules plays a vital role in many chemical and biological processes. Here, employing cold target recoil ion momentum spectroscopy (COLTRIMS) and pump-probe technique, we find that the non-adiabatic coupling between the ground and excited ionic states of ethane through conical intersection leads to a significantly high yield of neutral H2 fragment. Based on the analysis of fingerprints that are sensitive to orbital symmetry and electronic state energies in the photoelectron momentum distributions, we tag the initial electronic population of both the ground and excited ionic states and determine the branching ratios of H2 formation channel from those two states. Incorporating theoretical simulation, we established the timescale of the H2 formation to be ~1300 fs. We provide a comprehensive characterization of H2 formation in ionic states of ethane mediated by conical intersection and reveals the significance of non-adiabatic coupling dynamics in the intramolecular hydrogen migration.

Real-time imaging of transient structure of the electronic excited state is fundamentally critical to understand and control ultrafast physics and chemistry. However, the coherent nuclear motion in a specific excited state is yet to be captured accurately. Here, snapshots of the vibrational wave-packets of the excited (A) and ground states (X) of D$\rm_{2}$O$^{+}$ were acquired simultaneously with sub-10 picometer and few-femtosecond spatiotemporal precision, using a novel approach based on electron recollision-assisted Coulomb explosion. We visualised large amplitude bending and stretching motions for the A state, which significantly increased $\rm \theta _{DOD}$ and R$\rm_{OD}$ by approximately 50$^\circ$ and 10 pm, respectively, within 8 fs after initial tunneling ionization. In contrast, for the X state, only the prominant stretching motion is initialised with a bond extension of 7 pm within 5 fs. Another higher excited state with an asymmetric folding structure is experimentally established, which can be efficiently populated via the excitation of electron recollision. We demonstrate the remarkable dynamical vibrational fingerprints of these excite states and pave the way towards to make a movie of excited state-resolved ultrafast molecular dynamics and light-induced chemical reaction.