Dajun Ding

This study investigates the ultrafast fragmentation of ammonia clusters,(NH 3) n (n= 2, 3, and 4), by using ion-ion multibody coincidence measurements after interaction with a strong femtosecond laser. Specifically, the Coulomb explosion and proton transfer channels in the dimer, as well as the Coulomb explosion of tri-and tetramers, are assigned. The kinetic energy release distributions and Newton plots of each channel are extracted and analyzed. To gain further insights into the dissociation process, an ab initio molecular dynamics simulation was performed to uncover the dissociation mechanism. In addition, the stretching of clusters during the Coulomb explosion was identified using classical trajectory calculation combined with the genetic algorithm. This combined experimental and theoretical study holds significant importance in improving the understanding of the ultrafast Coulomb explosion process in …

Phase-dependent photoelectron spectra in resonance-enhanced multiphoton ionization (REMPI) of Ar are obtained by scanning the relative phase of two-color femtosecond lasers. Five REMPI channels are resolved and the relative time delays in the attosecond-timescale are extracted. The channel-resolved time delays can be assigned as the contribution of the resonant phase shift of electrons trapped in the specific Rydberg states and also influenced by the phase shift during the continuum-continuum transition. Our observations highlight the important influences of Freeman resonances on photoelectron emission dynamics in strong laser fields and will enable a subfemtosecond photoelectron wavepacket reconstruction in quantum mechanics.

Ethylene, the simplest model of a carbon-carbon double bond system, is pivotal in numerous chemical and biological processes. By employing intense infrared (IR) laser pump-probe techniques alongside coincidence measurements, we investigated the ultrafast non-adiabatic dynamics involved in the breakage of carbon-carbon double bonds and hydrogen elimination in ethylene's dissociation. Our study entailed analyzing the dynamic Kinetic Energy Release (KER) spectra to assess three bond-breaking scenarios, the movements of nuclei, and the structural changes around the carbon atoms. This allowed us to evaluate the relaxation dynamics and characteristics of various dissociative states. Notably, we observed a significant rise in the yield of fragments resulting from CH bond breakage as the delay time extended, suggesting non-adiabatic coupling through conical intersections from CC bond breakage as a …

The study of molecular valence electron dynamics and their coupling with nuclear motion is one of the frontiers of ultrafast physics and ultrafast chemistry. With time-resolved strong-field ion momentum spectroscopy, we study electron valence and nucleus wavepacket evolution on a femtosecond timescale. Two orientation-dependent bond-breaks of N2O molecules from the same electronic state are studied, and the influence of orbital hybridization and polarization effect during molecular breaking is analyzed based on the measured time-resolved asymmetric Pzsum distributions, allowing a visual representation of electron localization during the dissociation of molecules into ions and atoms. Comparison of experimental and theoretical results on orientation-dependent dissociation dynamics allows us to understand how nuclear motions evolve during fragmentation and to control ultrafast molecular reactions.

The ring-breaking dynamics of C 6 H 6 in its three dimers [(C 6 H 6) 2, C 6 H 6 Ar, and C 6 H 6 Kr] are investigated by performing three-body coincidence measurements after interaction with a strong femtosecond laser. The measurements enabled the observation of three ring-breaking channels, resulting in products of C 1 H 3++ C 5 H 3+, C 2 H 3++ C 4 H 3+, and C 3 H 3++ C 3 H 3+ with the neighbor ions. Additionally, kinetic energies from intra-and intermolecular fragmentation are extracted. By using the neighbor ion as a reference, it is found that the lifetimes of corresponding dissociation states have a significant impact on the final angular distributions of the fragments. The vibration excitation of molecules induced by neighboring ions during the repelling process alters the relative ratio between the three observed ring-breaking channels. By examining the ring-breaking dynamics of aromatic molecules in different …

The initial momentum along laser propagation plays a significant role in the electron dynamics during tunneling ionization. Our study shows that the offset emission angle decreases with increasing lateral momentum. By using three-dimensional strong-field approximation calculations with initial transverse momentum and a Coulomb correction, we accurately reproduce the transverse momentum-dependent angular shifts of Ar, Kr, and Xe atoms. We find that these shifts are attributed to the influence of initial momentum, tunneling exits, and a long-range Coulomb potential during electron propagation. Additionally, we establish a formula to reconstruct the initial transverse momentum from the final transverse momentum. The result may provide another perspective on attosecond angular streaking, which is crucial for understanding the influence of the Coulomb potential on electron motion.

We present a high-resolution and delay-stabilized setup for attosecond spectroscopy based on high-order harmonic generation from the intense laser passing through a differential gas cell. It shows the ability to generate the tunable photons from VUV to Soft X-ray (20–180 eV) at low backing pressure of target gas. This atto-beamline consists of an HHG source, electron and XUV spectrometers, and an active stabilization system. The measurements show it can be stabilized to sub-20 as in hours, together with a home-built XUV spectrometer with an energy resolution better than E/Δ E≥ 1200 in the wavelength range of 5–25 nm. We demonstrated its applications for producing the narrowband XUV source, extending the cutoff energy of HHG, and performing RABBITT measurements, particularly for investigating the excitation and ionization dynamics of atoms and molecules in a broad energy range by measuring the …

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 θDOD 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 …