Search

Coulomb explosion imaging is a promising experimental tool to study individual molecules. In this work simulation data for the x-ray induced Coulomb explosion of 2-iodopyridine is analyzed and the involved fragmentation dynamics are described. It is found that particular final ion momenta show correlations that reflect a collision of two atoms during the explosion. Variations of the proton kinetic energies can be associated with variations in the charge build-up time. Covariances of the forces during the explosion can be utilized to simplify the description of the dynamics in reduced dimensionality using only four collective coordinates. [ABSTRACT FROM AUTHOR]

We have deduced the structure of the bromobenzeneâ€"I2 heterodimer and the (bromobenzene)2 homodimer inside helium droplets using a combination of laser-induced alignment, Coulomb explosion imaging (CEI), and three-dimensional ion imaging. The complexes were fixed in a variety of orientations in the laboratory frame, then in each case multiply ionized by an intense laser pulse. A three dimensional ion imaging detector, including a Tpx3Cam detector allowed us to measure the correlations between velocity vectors of different fragments and, in conjunction with classical simulations, work backward to the initial structure of the complex prior to explosion. For the heterodimer, we find that the I2 molecular axis intersects the phenyl ring of the bromobenzene approximately perpendicularly. The homodimer has a stacked parallel structure, with the two bromine atoms pointing in opposite directions. These results illustrate the ability of CEI to determine the structure of large complexes, and point the way toward real-time measurements of bimolecular reactions inside helium droplets. [ABSTRACT FROM AUTHOR]

We report the comprehensive measurement of the Coulomb explosion of the double and triple ionization of N2 in collisions with slow highly-charged ions. The experiments are carried out with ion beams of different charge states for a given velocity (1 a.u.) and at three different velocities for a given charge state (O6+). By analyzing the kinetic energy release distributions of and molecular ions, the effects of projectile velocity and charge state on the population of different molecular states are inferred. We explain such a systematic variation in the population of different molecular states considering the energy deposition to N2 by the projectile ions. [ABSTRACT FROM AUTHOR]

XUV multiphoton ionization of molecules is commonly used in free-electron laser experiments to study charge transfer dynamics. However, molecular dissociation and electron dynamics, such as multiple photon absorption, Auger decay, and charge transfer, often happen on competing time scales, and the contributions of individual processes can be difficult to unravel. We experimentally investigate the Coulomb explosion dynamics of methyl iodide upon core–hole ionization of the shallow inner-shell of iodine (4d) and classically simulate the fragmentation by phenomenologically introducing ionization dynamics and charge transfer. Under our experimental conditions with medium fluence and relatively long XUV pulses (∼75 fs), we find that fast Auger decay prior to charge transfer significantly contributes to the charging mechanism, leading to a yield enhancement of higher carbon charge states upon molecular dissociation. Furthermore, we argue for the existence of another charging mechanism for the weak fragmentation channels leading to triply charged carbon atoms. This study shows that classical simulations can be a useful tool to guide the quantum mechanical description of the femtosecond dynamics upon multiphoton absorption in molecular systems. [ABSTRACT FROM AUTHOR]

With an intense femtosecond two-color pulse optimized for the generation of even-order harmonics from CO molecules, we directly measure the actual degrees of orientation by utilizing the Coulomb explosion imaging technique with appropriate probe polarization. We find that the macroscopic orientation of CO molecules is negligible even when significant even-order harmonics are observed. This finding shows that the generation of even-order harmonics cannot be ascribed to the macroscopic orientation of CO molecules. The rotational wave packet of CO molecules created with an intense femtosecond two-color pulse is thought to be in an uninvestigated quantum state, which cannot be explained by the theoretical model based on the Born–Oppenheimer approximation, without inversion symmetry at any of the three steps of high-order harmonic generation, leading to the generation of even-order harmonics. [ABSTRACT FROM AUTHOR]

Effects of the laser pulse wavelength and intensity on the high harmonic generation (HHG) production from the ionic and homolytic pre-ionization transient states of the two-electron H2 system exposed to ultrashort intense laser pulses are studied by solving the time-dependent Schrödinger equation. It is found that for the populated homolytic species in each half cycle of the laser pulse having enough strength, a pair of strong and weak HHG radiations are produced simultaneously. For the populated ionic species divided into two regions, in each half cycle, one of the ionic regions has a strong radiation and the other ionic region has a weak HHG radiation. The HHG spectra of the homolytic and ionic species are almost the same, except that in some limited parts of the HHG spectrum, one of them dominates the other. [ABSTRACT FROM AUTHOR]

The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br2 sample was characterized by Coulomb explosion imaging (CEI) using a time-delayed extreme ultra-violet (XUV) FEL pulse, obtained from the Free electron LASer in Hamburg (FLASH). The momentum distribution of atomic fragments resulting from the 400 nm-induced dissociation was measured with a velocity map imaging spectrometer and used to monitor the internuclear distance as the molecule dissociated. By employing the simultaneously recorded in-house timing electro-optical sampling data, the time resolution of the final results could be improved to 300 fs, compared to the inherent 500 fs time-jitter of the FEL pulse. Before dissociation, the Br2 molecules were transiently 'fixed in space' using laser-induced alignment. In addition, similar alignment techniques were used on CO2 molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO2 molecules, together with our investigation of the dissociation dynamics of the Br2 molecules with CEI, show that information about the evolving molecular structure and electronic geometry can be retrieved from such experiments, therefore paving the way towards the study of complex non-adiabatic dynamics in molecules through XUV time-resolved photoion and photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]

Single-shot time-of-flight spectra for Coulomb explosion of N2 and I2 molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N4+-N5+ channel for nitrogen and up to the I8+-I9+ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N2 ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers. [ABSTRACT FROM AUTHOR]

To greatly enhance output of fusion-produced neutrons in a laser-initiated Coulomb explosion (CE) of D clusters, we propose to accelerate D+ ions by an electrical pulse to the energies where the collision cross-section is the highest. With ions bombarding a D-rich cathode, this solves a few problems simultaneously by (a) removing the electron cloud hindering the CE, (b) utilizing up to 100% of the ions to hit the high-density packed nuclei, and (c) reaching the highly increased cross-section of neutron production in a single collision, by using a multi-layered target. We show that neutron output can reach up to 1015 per shot, which provides a powerful and compact neutron source. We also consider the use of E-pulse acceleration for diagnostic purposes. [ABSTRACT FROM AUTHOR]

We studied the relative yields and dissociation dynamics for two- and three-body Coulomb explosion (CE) channels from highly charged carbonyl sulfide molecules in intense laser fields using the CE imaging technique. The electron recollision contributions are evaluated by comparing the relative yields for the multiple ionization process in linearly polarized and circularly polarized (LP and CP) laser fields. The nonsequential multiple ionization is only confirmed for the charge states of 2 to 4 because the energy for further ionization from the inner orbital is much larger than the maximum recollision energy, 3.2Up. The novel deviations of kinetic energy releases distributions between LP and CP pulses are observed for the charge states higher than 4. It can be attributed to the stronger molecular bending in highly charged states before three-body CE with CP light, in which the bending wave packet is initialed by the triple or quartic ionization and spread along their potential curves. Compared to LP light, CP light ionizes a larger fraction of bending molecules in the polarization plane. [ABSTRACT FROM AUTHOR]

This tutorial is based on a doctoral thesis that was shortlisted for the 2016 AMOP dissertation prize of the German Physical Society (DPG). The principal achievement of the thesis was to use Coulomb explosion imaging (CEI) to determine the microscopic handedness (‘chirality’) of molecular structures on a single-molecule level. It thus shows how a technique developed in atomic physics can address a long-standing problem in chemistry. Owing to these disparate backgrounds, the tutorial has two facets: on the one hand, the history of molecular chirality and recent developments are very briefly reviewed. On the other hand, an account is given of different experimental approaches to CEI, on the physical processes in light-induced Coulomb explosion and—most importantly—on the aspects that are relevant when designing and performing such an experiment. As structural chirality occurs only in polyatomic molecules, special attention will be given to multiple ionization and multi-coincidence measurements. A short discussion of the results presented in earlier papers is given, followed by an outlook on experiments that are under way or can realistically be performed within the next years. [ABSTRACT FROM AUTHOR]

The effects of electron impact ionization (EII) in laser–metal-cluster interactions are investigated with two-dimensional particle-in-cell simulations. For large Cu clusters (R = 10 nm) heated by moderate laser pulses (peak intensities IM = 8.8 × 1015 W cm−2), the effects of EII depend on the atom/ion density of neutral cluster or cluster plasma. In high density neutral cluster (HDNC), EII is the dominant ionization mechanism and EII efficiency reaches 55%. However, in the case of low density cluster plasma (LDCP), EII plays a minor role that only increases the mean ion charges by 5%. Moreover, when EII is considered, the energy of ions with the same charges is reduced by 60% in the HDNC case but not in the LDCP case. This is due to the fact that ions in HDNC mainly gain energy through hydrodynamic expansion while ions in LDCP obtain energy through Coulomb explosion. More importantly, it is found that EII efficiency increases when the density of cluster plasma increases and is most pronounced in the neutral cluster. The density dependence of the EII efficiency provides a control mechanism for cluster ionization products with pump–probe technology. [ABSTRACT FROM AUTHOR]

X-ray single-photon ionization and fragmentation of the chiral molecule halothane from a racemic mixture have been investigated using the cold target recoil ion momentum spectroscopy technique. Two important facets related to the core ionization of this species are examined: Firstly, the distinction of enantiomers (mirror isomers) and the determination of absolute configuration on a single-molecule level by four-body Coulomb explosion; secondly, the interplay of site-selective excitation and fragmentation patterns. These results are easily transferable to other molecular species and show the wealth of features that can be investigated by coincidence spectroscopy of chiral molecules. [ABSTRACT FROM AUTHOR]

The article discusses the similarity of strength of all Coulomb interactions in the electron and hydrogen atom (e-H) collision, based on the hypothesis that the three-Coulomb (3C) wave function should be treated democratically. It references the article "On the Quantum Mechanics of Collision Processes," by Max Born in the 1926 issue of "Zeitschrift für Physik." The introduction of dynamically screened 3C (DS3C) wave functions is considered.

The recent availability of intense and ultrashort extreme ultraviolet sources opens up the possibility of investigating ultrafast electronic relaxation processes in matter in an unprecedented regime. In this work we report on the observation of two-photon excitation of interatomic Coulombic decay (ICD) in neon dimers using the tunable intense pulses delivered by the free electron laser FERMI. The unique characteristics of FERMI (narrow bandwidth, spectral stability, and tunability) allow one to resonantly excite specific ionization pathways and to observe a clear signature of the ICD mechanism in the ratio of the ion yield created by Coulomb explosion. The present experimental results are explained by ab initio electronic structure and nuclear dynamics calculations. [ABSTRACT FROM AUTHOR]

We demonstrate the ability to control the molecular dissociation rate using femtosecond pulses shaped with third-order dispersion (TOD). Explicitly, a significant 50% enhancement in the dissociation yield for the low lying vibrational levels (v ∼ 6) of an ion-beam target was measured as a function of TOD. The underlying mechanism responsible for this enhanced dissociation was theoretically identified as non-adiabatic alignment induced by the pre-pulses situated on the leading edge of pulses shaped with negative TOD. This control scheme is expected to work in other molecules as it does not rely on specific characteristics of our test-case molecule. [ABSTRACT FROM AUTHOR]

Using a new multi-electron multi-ion coincidence apparatus and soft x-ray synchrotron radiation we have determined branching ratios to final Xen+ states with 2 < n < 9 from the 4d−1, 4p−1, 4s−1, 3d−1 and 3p−1 Xe+ hole states. The coincident electron spectra give information on the Auger cascade pathways. We show that by judicious choice of coincident electrons, almost pure single charge states of the final ions can be selected. [ABSTRACT FROM AUTHOR]

Using the example of the molecular ion and its isotopes, we show how the Coulomb explosion triggered by ionization of the molecule can be used to reconstruct the emission times and identify the physical mechanism contributing to the emission of near- and below-threshold harmonics (BTH). The key idea is that recombination responsible for the harmonic emission halts Coulomb explosion at different times, depending on the final vibrational state of the molecule. As a consequence, interferences between different vibrational channels show up, enabling enhanced temporal resolution in the harmonic spectrum. The reconstruction of the emission times for BTH shows that they are associated with the so-called short trajectories and that the time–energy mapping for such harmonics is possible, similar to the well-known mapping for the plateau harmonics. [ABSTRACT FROM AUTHOR]