Directly imaging excited state-resolved transient structures of water induced by valence and inner-shell ionisation.

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 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.

Time-Resolved Resonant Auger Scattering Clocks Distortion of a Molecule.

Resonant Auger scattering (RAS) provides information on the core-valence electronic transition and impresses a rich fingerprint of the electronic structure and nuclear configuration at the time-initiating RAS process. Here, we suggest using a femtosecond X-ray pulse to trigger RAS in a distorted molecule, which is generated from the nuclear evolution on a valence excited state pumped by a femtosecond ultraviolet pulse. With the time delay varied, the amount of molecular distortion can be controlled and the RAS measurements imprint both their electronic structures and changing geometries. This strategy is showcased in H2O prepared in an O-H dissociative valence state, where molecular and fragment lines appear in RAS spectra as signatures of ultrafast dissociation. Given the generality of this approach for a broad class of molecules, this work opens a new alternative pump-probe technique for mapping the core and valence dynamics with ultrashort X-ray probe pulses.

Fragmentation of SO2 q+ (q = 2-4) induced by 1 keV electron collision.

We report an investigation on the fragmentation dynamics of SO2 q+ (q = 2-4) induced by 1 keV electron collision utilizing an ion momentum imaging spectrometer. Six complete Coulomb explosion channels were observed using the time-of-flight correlation map. The kinetic energy release distributions for these channels were obtained and compared with those available in the literature. The fragmentation mechanisms of the three-body dissociation channels were analyzed by the Dalitz plots and Newton diagrams. Both concerted breakup and sequential fragmentation pathways were identified in the channel SO2 3+ → O+ + O+ + S+, whereas only the concerted breakup mechanism was confirmed for the channels SO2 4+ → O+ + O+ + S2+ and SO2 4+ → O2+ + O+ + S+. Using the Coulomb explosion model, we determined the molecular geometry from the concerted fragmentation channels, and the obtained bond lengths and angles from the higher kinetic energy release peaks are close to that of the neutral SO2 obtained by high-level quantum chemical calculation. The present results indicate that the electron impact experiment is a potential tool for the Coulomb explosion imaging of small molecules.

Experimental and Theoretical Study on Electron Momentum Spectroscopy of SF6: Distorted-Wave and Vibrational Motion.

The vibrational and distorted-wave effects are usually invoked to explain the measured electron momentum profiles for molecular orbitals. The vibrational effect can be accounted for quantitatively by a harmonic analytical quantum mechanical approach within the plane-wave impulse approximation (PWIA). On the other hand, quantitative calculation considering the distorted-wave effect was available only recently by a multicenter-three-distorted-wave (MCTDW) method (Phys. Rev. A2022, 105, 042805). Here, we report a joint experimental and theoretical investigation on electron momentum spectroscopy of SF6. The experiments were performed using a high-sensitivity (e, 2e) spectrometer employing non-coplanar symmetric geometry with incident electron energy equal to 1200 eV + binding energy. The experimental electron momentum profiles are compared with theoretical calculations by the MCTDW method at equilibrium geometry and by the PWIA method both at equilibrium geometry and considering vibrational motions. For all the measured orbitals, large discrepancies were observed between the experiments and the PWIA calculations at equilibrium geometry. For the highest occupied molecular orbital 1t1g, the vibrational effect can partly explain the high intensity of the experimental momentum profile at low momenta. For the other orbitals, the influence of the vibrational effect is negligible. On the other hand, the MCTDW calculations improve the agreement with the experiments for all the observed orbitals, indicating that the distorted-wave effect plays an important role in reproducing the measured momentum profiles of SF6.

Fragmentation dynamics of nitrogen trifluoride induced by electron collision.

The fragmentation dynamics of nitrogen trifluoride (NF3) in collisions with a 500 eV electron is studied by using a momentum imaging spectrometer. The kinetic energy releases of two-body, three-body, and four-body fragmentation channels of NF3 q+ (q = 2, 3) are investigated. The fragmentation dynamics of three-body, as well as four-body, dissociation channels is analyzed by the Dalitz plot and the Newton diagram. It is found that for all of the dissociation channels, the fragment including N atom (ion) always shares significant momenta, regardless of whether it is charged. For F atom, however, it is always emitted with negligible momenta.

Fragmentation dynamics of CS2 in collisions with 1.0 keV electrons.

The dissociation dynamics of CS2 molecules in collisions with 1.0 keV electrons is studied. We observe a series of two- and three-body fragmentation channels which are identified from the correlation map between fragment ions. For all of the channels, the kinetic energy release (KER) distributions are obtained. The Dalitz plot and Newton diagram are adopted to analyze the fragmentation dynamics of the three-body dissociation channels. For C S 2 3 + and C S 2 4 + , both the concerted and sequential fragmentation mechanisms are observed where the concerted mechanism dominates. For C S 2 5 + , only the concerted mechanism is observed. Two types of Coulomb explosion models considering the molecular vibration are adopted to simulate the experimental KER distributions of the three-body channels. While obvious deviations are observed considering each ion during the whole dissociation process with an integer charge, good agreement can be achieved within deviation less than 5% if the charge state of the ions are adopted from ab initio calculations.

Theoretical study of (e, 2e) triple differential cross sections of tetrahydrofuran using multicenter distorted-wave method.

We present in this work a theoretical study of the triple differential cross sections for electron-impact single ionization of 12a' and 9b orbitals of a tetrahydrofuran molecule. The calculations are performed by the multicenter distorted-wave (MCDW) method in coplanar asymmetric kinematics with an incident electron energy of 250 eV, where previous (e, 2e) experiments and theories are available for comparison. The present calculations reproduce the experimental measurements satisfactorily, indicating that the MCDW method can be easily extended to study biomolecules with sufficiently high calculation efficiency.

Imaging molecular geometry with electron momentum spectroscopy.

Electron momentum spectroscopy is a unique tool for imaging orbital-specific electron density of molecule in momentum space. However, the molecular geometry information is usually veiled due to the single-centered character of momentum space wavefunction of molecular orbital (MO). Here we demonstrate the retrieval of interatomic distances from the multicenter interference effect revealed in the ratios of electron momentum profiles between two MOs with symmetric and anti-symmetric characters. A very sensitive dependence of the oscillation period on interatomic distance is observed, which is used to determine F-F distance in CF4 and O-O distance in CO2 with sub-Ångström precision. Thus, using one spectrometer, and in one measurement, the electron density distributions of MOs and the molecular geometry information can be obtained simultaneously. Our approach provides a new robust tool for imaging molecules with high precision and has potential to apply to ultrafast imaging of molecular dynamics if combined with ultrashort electron pulses in the future.

Fragmentation dynamics of carbonyl sulfide in collision with 500 eV electron.

The fragmentation dynamics of OCSq+ (q = 2, 3, 4) induced by electron collision at an impact energy of 500 eV is studied. By using the momentum imaging technique, the three dimensional momentum vectors of all the fragments are obtained, which enables us to analyse both the kinetic energy release and the momentum correlations for a certain fragmentation channel. Up to fifteen dissociation channels are analyzed including six, five, and four channels for two-body, and incomplete and complete three-body Coulomb fragmentations. For three-body dissociation, the fragmentation mechanisms are investigated with the help of Dalitz plot and Newton diagram. It is found that the sequential fragmentation involves in OCS2+→O+C++S+ with S+ emitted first and in OCS3+→O++C++S+ with O-C and C-S bonds breaking first. The remaining channels, however, always dissociate through a concerted mechanism. The relative intensities of the channels are also presented in this work.

Note: A high-efficiency multi-coincidence method designed for molecular fragmentation studies.

A high-efficiency multi-coincidence method is developed based on the hardware electronic multiple coincidence units. The multi-hit signals originating from one single detector can be selected and measured in coincidence. The performance of the method is tested by the electron impact three-body fragmentation of CO2(3+). Compared to the conventional method, the relative and absolute coincidence efficiencies of the triple-coincidence measurement are improved by about 200 and 3 times, respectively.