Direct observation of the momentum distribution and renormalization factor in lithium

We have measured the momentum distribution and renormalization factor Z(kF) in liquid and solid lithium by high-resolution Compton scattering. High-resolution data over a wide momentum range exhibit a clear feature of the renormalization and a sharp drop of momentum densities at the Fermi momentum k(F). These results are compared with those computed by quantum Monte Carlo simulation performed both on a disordered crystal and a liquid exhibiting very good agreement. Asymptotic behavior of the experimental and theoretical momentum distributions are examined to estimate Z(kF). The experimentally obtained Z(kF) = 0.43(-0.01)(+0.11) for liquid Li and 0.54(-0.02)(+0.11) for solid Li are in good agreement with theoretical results of 0.54 +/- 0.01 and 0.64 +/- 0.01, respectively.Peer reviewe

Coulomb implosion of tetrabromothiophene observed under multiphoton ionization by free-electron-laser soft-x-ray pulses

Soft-x-ray free-electron-laser pulses were used to create highly charged molecular tetrabromothiophene species by sequential multiphoton ionization from bromine 3d orbitals. The experiment was performed at the SACLA facility in Japan and the products of molecular dissociation were analyzed by means of multicoincidence momentum-resolved ion time-of-flight spectroscopy. Total charge states up to +13 atomic units were produced, creating a particular dissociation pattern for the carbon ions, a Coulomb implosion, due to the concerted forces by the surrounding heavy bromine ions. This behavior was explored both experimentally and by numerical molecular-dynamics simulations and the fingerprints of the Coulomb implosion were identified in both. In simulations, Coulomb implosion was predicted to be highly sensitive to the initial (thermal) motion of the atoms and, after including vibrational motion for several temperatures, good general agreement between the experiment and simulations was found. The agreement with the experiment was further improved by adding charge dynamics to the simulation, according to our point-charge dynamics model with empirical rate constants

Disentangling sequential and concerted fragmentations of molecular polycations with covariant native frame analysis

Using covariance analysis methods, we study the fragmentation dynamics of multiply ionized 1- and 2-iodopropane. Signatures of isomer-specific nuclear motion occurring during sequential fragmentation pathways are identified.</jats:p

Direct momentum imaging of charge transfer following site-selective ionization

We study ultrafast charge rearrangement in dissociating 2-iodopropane (2−C3H7I) using site-selective core ionization at the iodine atom. Clear signatures of electron transfer between the neutral propyl fragment and multiply charged iodine ions are observed in the recorded delay-dependent ion momentum distributions. The detected charge-transfer pathway is only favorable within a small (few angstroms), charge-state-dependent spatial window located at C-I distances longer than that of the neutral ground-state molecule. These results offer insights into the physics underpinning charge transfer in isolated molecules and pave the way for a different class of time-resolved studies

Characterizing crystalline defects in single Xe nanoparticles from angular correlations of single-shot diffracted X-rays

We performed a wide-angle X-ray scattering experiment of single Xe nanoparticles using an X-ray free electron laser. We developed a novel analysis method that focuses on the angular correlation between plural Bragg spots in single-shot diffraction patterns. The angular correlations of the Bragg spots encode rich structural information and offer an evidence of twinning and stacking faults in Xe nanoparticles.</p

Disentangling sequential and concerted fragmentations of molecular polycations with covariant native frame analysis

We present results from an experimental ion imaging study into the fragmentation dynamics of 1-iodopropane and 2-iodopropane following interaction with extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Using covariance imaging analysis, a range of observed fragmentation pathways of the resulting polycations can be isolated and interrogated in detail at relatively high ion count rates (∼12 ions shot−1). By incorporating the recently developed native frames analysis approach into the three-dimensional covariance imaging procedure, contributions from three-body concerted and sequential fragmentation mechanisms can be isolated. The angular distribution of the fragment ions is much more complex than in previously reported studies for triatomic polycations, and differs substantially between the two isomeric species. With support of simple simulations of the dissociation channels of interest, detailed physical insights into the fragmentation dynamics are obtained, including how the initial dissociation step in a sequential mechanism influences rovibrational dynamics in the metastable intermediate ion and how signatures of this nuclear motion manifest in the measured signals.</p

Dissociation and ionization dynamics of CF3I and CH3I molecules via pump-and-probe experiments using soft x-ray free-electron laser

The dissociation and ionization dynamics of CF3I and CH3I molecules were investigated using a pump-and-probe technique that employs a soft x-ray free-electron laser (SACLA) in Japan. First, time-resolved inner-shell photoelectron spectroscopy was employed to observe the ultrafast reaction of CF3I by monitoring iodine 4d electrons. The change in the I 4d state observed in the photoelectron spectra is found to occur with a rise time τ of approximately 40 fs after a pump laser pulse, which is faster than that observed when an ultrafast gas-phase electron diffraction technique is employed. This implies that the inner-shell photoelectron spectroscopy is more sensitive to the potential surface near the Franck–Condon region. Second, a strong laser intensity at 266 nm, corresponding to a power density of 1.9 × 1014 W cm−2, can easily ionize CH3I molecules via multiphoton ionization processes, and the time dependence of the valence photoelectron spectra clearly shows that at the picosecond timescale, this pump laser pulse causes spectral peaks to shift owing to space-charge effects in response to the large amount of ions generated. Thus, the SACLA can be a useful tool to investigate not only the dynamical process of molecular dissociation but also the ionization process through the shift in the peaks of photoelectron spectra

Formative period in the x-ray-induced photodissociation of organic molecules

Absorption of x-ray photons by atomic inner shells of light-element organics and biomolecules often leads to formation of dicationic electronic states and to molecular fragmentation. We investigated the x-ray-induced dissociation landscape of a representative medium-sized organic molecule, thiophene, by femtosecond x-ray pulses from the Super Photon Ring-8 GeV (SPring-8) Angstrom Compact Free-Electron Laser (SACLA). Holes, created in the sulfur 2p orbital by photoemission, were filled by the Auger process that created dicationic molecular states within a broad range of internal energies—a starting point particular to x-ray-induced dynamics. The evolution of the ionized molecules was monitored by a pump-probe experiment using a near-infrared (800 nm) laser pulse. Ion-ion coincidence and ion momentum analysis reveals enhanced yields of ionic fragments from multibody breakup of the ring, attributed to additional ionization of the highly excited fraction of the dicationic parent molecular states. The transient nature of the enhancement and its decay with about a 160-fs time constant indicate formation of an open-ring parent geometry and the statistical survival time of the parent species before the dissociation events. By probing specific Auger final states of transient, highly excited nature by near-infrared light, we demonstrate how pump-probe signatures can be related to the key features in dynamics during the early period of the x-ray-induced damage of organic molecules and biomolecules

The role of momentum partitioning in covariance ion imaging analysis

We present results from a covariance ion imaging study, which employs extensive filtering, on the relationship between fragment momenta to gain deeper insight into photofragmentation dynamics. A new data analysis approach is introduced that considers the momentum partitioning between the fragments of the breakup of a molecular polycation to disentangle concurrent fragmentation channels, which yield the same ion species. We exploit this approach to examine the momentum exchange relationship between the products, which provides direct insight into the dynamics of molecular fragmentation. We apply these techniques to extensively characterize the dissociation of 1-iodopropane and 2-iodopropane dications prepared by site-selective ionization of the iodine atom using extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Our assignments are supported by classical simulations, using parameters largely obtained directly from the experimental data