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Alkali trimers, Ak3, located on the surface of He nanodroplets are triply ionized following multiphoton absorption from an intense femtosecond laser pulse, leading to fragmentation into three correlated Ak+ ions. Combining the information from threefold covariance analysis of the emission direction of the fragment ions and their kinetic energy distributions P(Ekin), we find that Na3, K3, and Rb3 have an equilateral triangular structure, corresponding to that of the lowest lying quartet state A 2 ′ 4 , and determine the equilibrium bond distance Req(Na3) = 4.65 ± 0.15 Å, Req(K3) = 5.03 ± 0.18 Å, and Req(Rb3) = 5.45 ± 0.22 Å. For K3 and Rb3, these values agree well with existing theoretical calculations, while for Na3, the value is 0.2–0.3 Å larger than the existing theoretical results. The discrepancy is ascribed to a minor internuclear motion of Na3 during the ionization process. In addition, we determine the distribution of internuclear distances P(R) under the assumption of fixed bond angles. The results are compared to the square of the internuclear wave function |Ψ(R)|2. [ABSTRACT FROM AUTHOR]

Site-selective probing of iodine 4d orbitals at 13.1 nm was used to characterize the photolysis of CH2I2 and CH2BrI initiated at 202.5 nm. Time-dependent fragment ion momenta were recorded using Coulomb explosion imaging mass spectrometry and used to determine the structural dynamics of the dissociating molecules. Correlations between these fragment momenta, as well as the onset times of electron transfer reactions between them, indicate that each molecule can undergo neutral three-body photolysis. For CH2I2, the structural evolution of the neutral molecule was simultaneously characterized along the C–I and I–C–I coordinates, demonstrating the sensitivity of these measurements to nuclear motion along multiple degrees of freedom. [ABSTRACT FROM AUTHOR]

ABSTRACT The present work deals with the Argon (Ar) Thermal plasma‐induced surface modification of Cu and its correlation with the electrical and field emission (FE) properties. Polycrystalline Cu targets were treated with Ar thermal plasma under atmospheric pressure at different treatment times ranging from 5 min to 30 min. XRD patterns revealed the absence of new phase in treated samples. However, significant variation in peak intensities and shifting is observed, which is explained on the basis of thermal plasma ions induced defect generation and annihilation processes. The electrical conductivity of processed Cu targets measured by four‐probe method ranges from 1.9 MS/m to 70 MS/m and is well correlated with the crystallite size variation. Surface modification induced work function alternations investigated by employing Scanning Kelvin Probe (SKP) technique are in the range of 4.69 eV 4.97 eV. The irradiated morphology explored by optical and scanning electron microscopy analyses revealed the formation of localized melt pools, grains, hillocks, spheroids, and sputtered patches, which are explainable on the basis of Coulomb's explosion, thermal spike model, plasma‐induced sputtering, and re‐deposition. FE properties of thermal plasma‐treated Cu are measured in diode configuration by measuring I‐V characteristics of target under ultra‐high vacuum condition. The improved FE parameters such as turn‐on field (Eo), field enhancement factor (β), and maximum current density (Jmax) come out to be in the range of 3–7.5 V/μm, 1715–3223, and 284–872 nA/cm2, respectively and their correlation with plasma‐induced surface structural, morphological and work function modifications is discussed. [ABSTRACT FROM AUTHOR]

We experimentally studied the three-body fragmentation dynamics of a noble gas cluster (ArKr2) upon its multiple ionization by an intense femtosecond laser pulse. The three-dimensional momentum vectors of correlated fragmental ions were measured in coincidence for each fragmentation event. A novel comet-like structure was observed in the Newton diagram of the quadruple-ionization-induced breakup channel of ArKr 2 4 + → Ar+ + Kr+ + Kr2+. The concentrated head part of the structure mainly originates from the direct Coulomb explosion process, while the broader tail part of the structure stems from a three-body fragmentation process involving electron transfer between the distant Kr+ and Kr2+ ion fragments. Due to the field-driven electron transfer, the Coulomb repulsive force of the Kr2+ and Kr+ ions with respect to the Ar+ ion undergoes exchange, leading to changes in the ion emission geometry in the Newton plot. An energy sharing among the separating Kr2+ and Kr+ entities was observed. Our study indicates a promising approach for investigating the strong-field-driven intersystem electron transfer dynamics by using the Coulomb explosion imaging of an isosceles triangle van der Waals cluster system. [ABSTRACT FROM AUTHOR]

The ion impact multiple ionization and subsequent dissociation of CCl4 is studied using a beam of Ar7+ ion having the energy of about 1 MeV in a linear time- of-flight mass spectrometer, coupled with a position-sensitive detector. The complete, as well as incomplete Coulomb explosion pathways, for CCl 4 2 + and CCl 4 3 + ions are identified and studied. The kinetic energy release distributions of channels, kinetic energies, and momentum distributions of fragmented ions, as well as neutrals, are also calculated. Possible modes of fragmentation pathways, i.e., concerted and/or sequential, for all the identified channels are studied using Newton diagrams, Dalitz plots, and kinetic energy distributions. The dynamical information and fragmentation pathways were analyzed with the Dalitz plot and Newton diagram for the three-body dissociation channel. The nature of the fragmentation process is further investigated with simulated Dalitz plots and Newton diagrams using the simple classical mechanical model. [ABSTRACT FROM AUTHOR]

We report an investigation on the fragmentation dynamics of SO2q+ (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 SO23+ → O+ + O+ + S+, whereas only the concerted breakup mechanism was confirmed for the channels SO24+ → O+ + O+ + S2+ and SO24+ → 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. [ABSTRACT FROM AUTHOR]

Multi-ionization and subsequent Coulomb explosion (CE) of the N2O molecule irradiated by a linearly polarized 800 nm laser field is investigated by a reaction microscope, where a number of CE channels of N2Oq+ with q ≤ 5 for two-body fragmentation and q ≤ 8 for three-body fragmentation were observed. For two-body CE, by analyzing the internuclear separations extracted from kinetic energy releases (KERs), dissociation branching fractions, and laser intensity dependence, interestingly, we found that fragmentation N2O5+ → N3+ + NO2+ is produced directly from dissociating N2O3+ via non-sequential stairstep ionization, whereas most of the others result from the sequential stairstep ionization. For three-body CE, 25 fragmentation channels of N2Oq+ (q = 3–8) are distinguished in the present charge-encoded multi-photoion coincidence plot, and the concerted fragmentation mechanism is nominated in a typical Dalitz plot. With the help of the numerical computation with the measured KERs and momentum correlation angles, the geometric structures of molecular ions prior to fragmentation are reconstructed, which display the bending motion and simultaneous two-bond stretching before the CE. Increasing of the bond length for high charged N2Oq+ indicates the dominating stairstep ionization in the three-body fragmentation. [ABSTRACT FROM AUTHOR]

We study the competing mechanisms involved in the Coulomb explosion of 2-propanol C H 3 2 C H O H 2 + dication, formed by an ultrafast extreme ultraviolet pulse. Over 20 product channels are identified and characterized using 3D coincidence imaging of the ionic fragments. The momentum correlations in the three-body fragmentation channels provide evidence for a dominant sequential mechanism, starting with the cleavage of a C–C bond, ejecting C H 3 + and CH3CHOH+ cations, followed by a secondary fragmentation of the hydroxyethyl cation that can be delayed for up to a microsecond after ionization. The C–O bond dissociation channels are less frequent, involving proton transfer and double proton transfer, forming H2O+ and H3O+ products, respectively, and exhibiting mixed sequential and concerted character. These results can be explained by the high potential barrier for the C–O bond dissociation seen in our ab initio quantum chemical calculations. We also observe coincident COH+ + C2Hn+ ions, suggesting exotic structural rearrangements, starting from the Frank–Condon geometry of the neutral 2-propanol system. Remarkably, the relative yield of the H 3 + product is suppressed compared with methanol and alkene dications. Ab initio potentials and ground state molecular dynamics simulations show that a rapid and direct C–C bond cleavage dominates the Coulomb explosion process, leaving no time for H2 roaming, which is a necessary precursor to the H 3 + formation. [ABSTRACT FROM AUTHOR]

Using the all-optical molecular orientation technique with intense nonresonant two-color laser pulses, stronger molecular orientation |⟨cos θ2D⟩| ∼ 0.34 is achieved by employing the following two strategies: (1) carbonyl sulfide molecules lying in the lower rotational states are selected using a home-built molecular deflector and (2) the rising parts of the two wavelengths of the pump pulse are adjusted by introducing a Michelson-type delay line in the optical path. The achieved degree of molecular orientation is higher than that observed in the proof-of-principle experiment [Oda et al., Phys. Rev. Lett. 104, 213901 (2010)] by about an order of magnitude and the highest ever characterized directly by Coulomb explosion imaging with appropriate probe polarization. [ABSTRACT FROM AUTHOR]

We propose a new scheme of a few MeV range uni-directional neutral particle generation via charge exchange mechanism (Rajeev et al. in Nat Phys 9:185, 2013) of energetic ions driven by intense laser. A two-dimensional stack of gold nanorods, embedded inside a hydrocarbon foil (CH), are ionized by the action of pulse of intensities more than 10 16 Wcm - 2 . The pre-pulse is diverted toward a gaseous/clustered environment and excites atoms/clusters to Rydberg states, while the main part of the pulse exerts a transverse ponderomotive force on electrons causing its immediate expulsion. Under Coulomb explosion, carbon ions and protons exit quickly, while the gold ions remain practically immobile due to inertia. The potential structure created by the positively charged nano-rod assembly has complete transverse confinement and axial acceleration. Carbon ions and protons are axially accelerated and undergo collision with Rydberg state atoms/cluster causing charge exchange. This scheme addresses the issues of complete transverse confinement, directionality, and in-situ charge exchange environment. Typical 40 femtosecond pulses with normalized amplitude ( a 0 = 1 - 2 ) can generate energetic neutrals of ∼ 0.5 - 5 MeV range. In the present scheme, one also gets the flexibility of using the changed nanorod geometry or alignment or using external magnetic field to control the beam direction prior to charge exchange stage. [ABSTRACT FROM AUTHOR]

Structural imaging of transient excited-state species is a key goal of molecular physics, promising to unveil rich information about the dynamics underpinning photochemical transformations. However, separating the electronic and nuclear contributions to the spectroscopic observables is challenging, and typically requires the application of high-level theory. Here, we employ site-selective ionisation via ultrashort soft X-ray pulses and time-resolved Coulomb explosion imaging to interrogate structural dynamics of the ultraviolet photochemistry of carbon disulfide. This prototypical system exhibits the complex motifs of polyatomic photochemistry, including strong non-adiabatic couplings, vibrational mode couplings, and intersystem crossing. Immediately following photoexcitation, we observe Coulomb explosion signatures of highly bent and stretched excited-state geometries involved in the photodissociation. Aided by a model to interpret such changes, we build a comprehensive picture of the photoinduced nuclear dynamics that follows initial bending and stretching motions, as the reaction proceeds towards photodissociation. Coulomb Explosion imaging is a promising technique to study the ultrafast nuclear dynamics which underpin molecular photochemistry. By initiating Coulomb explosion through soft X-ray ionization, the authors are able to image ultrafast nuclear dynamics of a prototypical photoreaction. [ABSTRACT FROM AUTHOR]

We present a new technique for detecting chirality in the gas phase: Chiral molecules are spatially aligned in three dimensions by a moderately strong elliptically polarized laser field. The momentum distributions of the charged fragments, produced by laser-induced Coulomb explosion, show distinct three-dimensional orientation of the enantiomers when the laser polarization ellipse is rotated by a non-right angle with respect to the norm vector of the detector plane. The resulting velocity-map-image asymmetry is directly connected to the enantiomeric excess and to the absolute handedness of molecules. We demonstrated our scheme computationally for camphor (C10H16O), with its methyl groups as marker fragments, using quantum-mechanical simulations geared toward experimentally feasible conditions. Computed sensitivity to enantiomeric excess is comparable to other modern chiroptical approaches. The present method can be readily optimized for any chiral molecule with an anisotropic polarizability tensor by adjusting the polarization state and intensity profile of the laser field. [ABSTRACT FROM AUTHOR]

CO2 single-photon double photoionization, Coulomb explosion, and dissociative ionization are studied with ultrafast extreme-ultraviolet pump and time-delayed near-infrared probe pulses. Kinetic energy release and momentum correlations for the two-body CO+ + O+ and three-body O+ + C+ + O fragmentation products are determined by 3D coincidence fragment imaging. The transient enhancement of the ratio of two-body vs three-body Coulomb explosion events and the time dependence of low and high kinetic energy release dissociation events are discussed in terms of dissociative ionization and Coulomb explosion dynamics. [ABSTRACT FROM AUTHOR]

Coulomb explosion velocity-map imaging is a new and potentially universal probe for gas-phase chemical dynamics studies, capable of yielding direct information on (time-evolving) molecular structure. The approach relies on a detailed understanding of the mapping between the initial atomic positions within the molecular structure of interest and the final velocities of the fragments formed via Coulomb explosion. Comprehensive on-the-fly ab initio trajectory studies of the Coulomb explosion dynamics are presented for two prototypical small molecules, formyl chloride and cis-1,2-dichloroethene, in order to explore conditions under which reliable structural information can be extracted from fragment velocity-map images. It is shown that for low parent ion charge states, the mapping from initial atomic positions to final fragment velocities is complex and very sensitive to the parent ion charge state as well as many other experimental and simulation parameters. For high-charge states, however, the mapping is much more straightforward and dominated by Coulombic interactions (moderated, if appropriate, by the requirements of overall spin conservation). This study proposes minimum requirements for the high-charge regime, highlights the need to work in this regime in order to obtain robust structural information from fragment velocity-map images, and suggests how quantitative structural information may be extracted from experimental data. [ABSTRACT FROM AUTHOR]

The behavior of nitrosyl chloride (ClNO) exposed to ionizing radiation was studied by direct probing valence-shell electrons in temporal coincidence with ions originating from the fragmentation process of the transient ClNO2+. Such a molecular dication was produced by double photoionization with synchrotron radiation in the 24–70 eV photon energy range. The experiment has been conducted at the Elettra Synchrotron Facility of Basovizza (Trieste, Italy) using a light beam linearly polarized with the direction of the polarization vector parallel to the ClNO molecular beam axis. ClNO molecules crossing the photon beam at right angles in the scattering region are generated by effusive expansion and randomly oriented. The threshold energy for the double ionization of ClNO (30.1 ± 0.1 eV) and six dissociation channels producing NO+/Cl+, N+/Cl+, N+/O+, O+/Cl+, ClN+/O+, NO+/Cl2+ ion pairs, with their relative abundance and threshold energies, have been measured. [ABSTRACT FROM AUTHOR]

Tpx3Cam is a newly developed time-stamped camera. With the ability to record the arrival time and position of each event simultaneously, it becomes a powerful tool in atomic and molecular research. As an inherent weakness, the time-walk effect resulting in a deviation in the arrival time is a major obstacle in improving the experimental resolution and reconstruction of momentum along the time-of-flight. We developed an analytic expression to describe and correct the time-walk effect, which is independent of different fragments once the parameters of the spectrometer, the microchannel plate (MCP), the phosphor screen, and the Tpx3Cam, such as voltages on the spectrometer and MCP, are fixed. With the time-walk correction, 3D momentum distributions of N+ and N2+ from the N2 molecule's Coulomb explosion were well extracted, paving a way for filming molecular dynamics in three dimensions with time-stamped velocity map imaging. Simultaneously, a denoising method based on data filtering is presented for Tpx3Cam. [ABSTRACT FROM AUTHOR]

The laser energy per unit surface, necessary to trigger material removal, decreases with the pulse shortening, becoming pulse–time independent in the sub-picosecond range. These pulses are shorter than the electron-to-ion energy transfer time and electronic heat conduction time, minimising the energy losses. Electrons receiving an energy larger than the threshold drag the ions off the surface in the mode of electrostatic ablation. We show that a pulse shorter than the ion period (Shorter-the-Limit (StL)) ejects conduction electrons with an energy larger than the work function (from a metal), leaving the bare ions immobile in a few atomic layers. Electron emission is followed by the bare ion's explosion, ablation, and THz radiation from the expanding plasma. We compare this phenomenon to the classic photo effect and nanocluster Coulomb explosions, and show differences and consider possibilities for detecting new modes of ablation experimentally via emitted THz radiation. We also consider the applications of high-precision nano-machining with this low intensity irradiation. [ABSTRACT FROM AUTHOR]

Accelerator mass spectrometry (AMS) measurements of 10Be almost invariably utilize the BeO- molecular negative ion. Molecular ions experience the process of 'Coulomb explosion' when dissociated by a stripper in the high-voltage terminal of a tandem accelerator, resulting in energy and angular spread of the subsequently-accelerated positive ions. In this paper, the phenomenon is studied in detail for both foil and gas stripping of BeO, using the 14UD pelletron accelerator at the Australian National University operating at 8.5 MV. In a companion paper by Suter et al. [1], a model is developed to interpret, inter alia, these results. [ABSTRACT FROM AUTHOR]

Filming atomic motion within molecules is an active pursuit of molecular physics and quantum chemistry. A promising method is laser-induced Coulomb Explosion Imaging (CEI) where a laser pulse rapidly ionizes many electrons from a molecule, causing the remaining ions to undergo Coulomb repulsion. The ion momenta are used to reconstruct the molecular geometry which is tracked over time (i.e., filmed) by ionizing at an adjustable delay with respect to the start of interatomic motion. Results are distorted, however, by ultrafast motion during the ionizing pulse. We studied this effect in water and filmed the rapid "slingshot" motion that enhances ionization and distorts CEI results. Our investigation uncovered both the geometry and mechanism of the enhancement which may inform CEI experiments in many other polyatomic molecules. Laser-induced Coulomb explosion imaging allows the study of molecular geometries over time, but the results are often distorted by ultrafast motion during the ionizing laser pulse. Here, the authors film the rapid slingshot motion in D2O that induces this distortion and elucidate the underlying mechanism of enhanced ionization. [ABSTRACT FROM AUTHOR]

We performed reaction dynamics simulations to demonstrate that the vibrational dynamics of C60 induced by infrared (IR) pulses can be traced by triggering Coulomb explosion with intense femtosecond X-ray free electron laser (XFEL) probe pulses. The time series of the angular anisotropy β(t) of fast C+ and C2+ fragments of C6060+ produced by such an XFEL pulse reflects the instantaneous structure of C60 vibrationally excited by IR pulses. The phases and amplitudes of excited vibrational modes and the coupling between excited modes can be successfully extracted from the expansion of β(t) in terms of vibrational modes. This proof-of-principle simulation clearly demonstrates that various information of the structures and reaction dynamics of large clusters or biomolecules can be retrieved by decomposing the experimentally determined β(t) into vibrational modes. [ABSTRACT FROM AUTHOR]

Charge transfer (CT) at avoided crossings of excited ionized states of argon dimers is observed using a two-color pump-probe experiment at the free-electron laser in Hamburg (FLASH). The process is initiated by the absorption of three 27-eV-photons from the pump pulse, which leads to the population of Ar2+*–Ar states. Due to nonadiabatic coupling between these one-site doubly ionized states and two-site doubly ionized states of the type Ar+*–Ar+, CT can take place leading to the population of the latter states. The onset of this process is probed by a delayed infrared (800 nm) laser pulse. The latter ionizes the dimers populating repulsive Ar2+ –Ar+ states, which then undergo a Coulomb explosion. From the delay-dependent yields of the obtained Ar2+ and Ar+ ions, the lifetime of the charge-transfer process is extracted. The obtained experimental value of (531 ± 136) fs agrees well with the theoretical value computed from Landau-Zener probabilities. [ABSTRACT FROM AUTHOR]

Structural and electronic changes induced by laser irradiation are currently of interest owing to the possibility to tune the mechanical, optical, and transport properties of the irradiated materials. In this work, we investigate the effects of laser irradiation on indium phosphide, InP, by modifying the electronic temperature, T e , of the system within the density functional theory framework and performing molecular dynamics simulations to prove that the laser irradiation also provokes a local thermalization effect. We found that the process can be described by a two-stage mechanism. First, at low T e values (0–1.0 eV), the laser energy induces electronic transitions, while the InP lattice remains undisturbed and cool. In the second stage (with T e in the range of 1.0–4.0 eV), both electron-electron scattering and electron-phonon coupling processes are triggered, increasing the energy of the lattice so as to provoke a Coulomb explosion, which changes some physical chemical properties of InP. The close agreement between the simulations helps explain the formation of metallic In as it is observed in the transmission electron microscopy images. [ABSTRACT FROM AUTHOR]

The long-time correlated decay dynamics of strong-field exposed helium nanodroplets is studied by means of photoelectron spectroscopy. As a result of the adiabatic expansion of the laser-produced, fully inner-ionized nanoplasma, delocalized electrons in the deep confining mean field potential are shifted towards the vacuum level. Meanwhile, part of the electrons localize in bound levels of the helium ions. The simple hydrogenlike electronic structure of He+ results in clear signatures in the experimentally observed photoelectron spectra, which can be traced back to bound-free and bound-bound transitions. Auger electron emission takes place as a result of the transfer of transition energy to weakly bound electrons in the quasifree electron band. Hence, the spatial and temporal development of the nanoplasma cloud is encoded in the experimental spectra, whereas the electronic properties of He+ help resolve the different contributions. [ABSTRACT FROM AUTHOR]

We have studied the fragmentation of the brominated cyclic hydrocarbons bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane upon Br(3d) and C(1s) inner-shell ionization using coincidence ion momentum imaging. We observe a substantial yield of CH3+ fragments, whose formation requires intramolecular hydrogen (or proton) migration, that increases with molecular size, which contrasts with prior observations of hydrogen migration in linear hydrocarbon molecules. Furthermore, by inspecting the fragment ion momentum correlations of three-body fragmentation channels, we conclude that CHx+ fragments (with x = 0, ..., 3) with an increasing number of hydrogens are more likely to be produced via sequential fragmentation pathways. Overall trends in the molecular-size-dependence of the experimentally observed kinetic energy releases and fragment kinetic energies are explained with the help of classical Coulomb explosion simulations. [ABSTRACT FROM AUTHOR]

We have studied the fragmentation of the brominated cyclic hydrocarbons bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane upon Br(3d) and C(1s) inner-shell ionization using coincidence ion momentum imaging. We observe a substantial yield of CH3+ fragments, whose formation requires intramolecular hydrogen (or proton) migration, that increases with molecular size, which contrasts with prior observations of hydrogen migration in linear hydrocarbon molecules. Furthermore, by inspecting the fragment ion momentum correlations of three-body fragmentation channels, we conclude that CHx+ fragments (with x = 0, ..., 3) with an increasing number of hydrogens are more likely to be produced via sequential fragmentation pathways. Overall trends in the molecular-size-dependence of the experimentally observed kinetic energy releases and fragment kinetic energies are explained with the help of classical Coulomb explosion simulations. [ABSTRACT FROM AUTHOR]

We study ion-induced secondary electron emission from solid surfaces. This work focuses on the dependence of the energy distribution of secondary electrons on the orientation of Si-dimers impinging on thin carbon foils. The orientation was determined by the foil-induced Coulomb explosion method. Furthermore, the electron energy was measured with a retarding field analyzer. The correlation between the electron energy distribution and the cluster orientation was obtained through simultaneous measurements. A comparison of the electron energy distribution for parallel and perpendicular orientations indicated that the electron yields at an energy below 20 eV are lower for parallel orientation. This reduction can be explained by the orientation dependence of the energy loss of Si 2 + in carbon. It was suggested that the orientation effect is due to the distant collision between the cluster and the target atom, in which low-energy electrons are produced. [ABSTRACT FROM AUTHOR]

The break-up of molecules in the stripper of an AMS facility induces an energy and angular spread in the beam. This can cause beam losses, thereby reducing the performance of an AMS facility. With an appropriate design, these effects can be minimised and the efficiency enhanced. As an example, the break-up of BeO molecules is discussed here. First the basic physics is described and corresponding models and computer codes are explained. Then experimental data are shown and compared with the models. For gas stripping, beam profiles are available for small and large facilities. All show tails which can be attributed to the dissociation of the molecules. The size of the effect depends on various parameters such as beam energy, selected charge state, stripping gas type and ion optics. Foil stripping leads to energy distributions which are much wider, and show typically a double peak. [ABSTRACT FROM AUTHOR]

Graphene quantum dots (GQDs) have displayed significant momentum in numerous fields due to their unique electronic properties. However, how to obtain uniformly monolayer quantum dots and explain the synthesis mechanism are still a key technical problem. Here, a rapid and controllable photoexfoliation rate of monolayer graphene quantum dots of up to 80 % was achieved by using temporally and spatially shaped femtosecond laser to ablate bulk highly oriented pyrolytic graphite targets in liquid. Theoretical calculations suggested that the temporally shaped laser can minimize the spatial range of electron excitation, maximize the Coulomb repulsion between the outer layers and Coulomb explosion within the topmost layer through multi-level electron excitation. The above multilevel photoexfoliations lead to the formation of monolayer GQDs. These findings demonstrate a perfect theoretical explanation of controllable and rapid preparation of monolayer quantum dots, accelerating its industrialization in energy storage devices. [Display omitted] • A new way to prepare monolayer graphene quantum dots was proposed through temporally and spatially shaped femtosecond laser. • Rapid, controllable photoexfoliation of monolayer graphene quantum dots rate up to 80 %, which is benefited from the spatial optical field improves the effective area. • The photoexfoliation mechanism revealed by simulation shows that temporally shaped laserminimizes electron excitation range, maximizes the Coulomb repulsive force via multi-level electron excitation. • The method for preparing monolayer quantum dots has material applicability, accelerating the quantum dots industrialization in energy storage devices. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of CH3I and CH2ClI at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815 nm probe pulse. Fragment ion momenta over a wide m/z range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CH2ClI to be investigated. Furthermore, delay-dependent relative fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Dimers and trimers of carbonyl sulfide (OCS) molecules embedded in helium nanodroplets are aligned by a linearly polarized 160 ps long moderately intense laser pulse and Coulomb exploded with an intense 40 fs long probe pulse in order to determine their structures. For the dimer, recording of 2D images of OCS+ and S+ ions and covariance analysis of the emission directions of the ions allow us to conclude that the structure is a slipped-parallel shape similar to the structure found for gas phase dimers. For the trimer, the OCS+ ion images and the corresponding covariance maps reveal the presence of a barrel-shaped structure (as in the gas phase) but also other structures not present in the gas phase, most notably a linear chain structure. [ABSTRACT FROM AUTHOR]

We consider the dust charging processes in protoplanetary disks. It is shown that in the atmosphere of a disk, grains can acquire anomalously high positive charges, which can lead to their destruction as a result of the Coulomb explosion. The charging is due to the combined effect of radiation from a protostar and of electron and ion currents from the surrounding plasma. For typical protoplanetary disk conditions, such a process turns out to be effective both for porous dust grains of micron sizes and for submicron crystalline grains. Critical charges that are necessary for the destruction of dust grains and the charges acquired by the particles are found for their different sizes and various parameters of the dusty plasmas. Critical grain sizes as functions of the distance to the host star are calculated. It is shown that the Coulomb explosion leads to the appearance of a cascade of crushing of large dust grains. The character of the dependence of crushing conditions on the dust particle radius can lead to the formation of a bimodal grain size distribution in some areas of protoplanetary disks. One fraction corresponds to large particles that are not subject to destruction and the other, to small particles of submicron size. [ABSTRACT FROM AUTHOR]

We investigated the ultrafast proton migration and the Coulomb explosion (CE) dynamics of methyl chloride (CH3Cl) in intense femtosecond laser fields at thewavelengths of 800 nm (5.5 x 1014 W/cm2) and 400 nm (4 x 1014W/cm2), respectively. Various fragment channels from molecular dication and trication were observed by coincidence momentum imaging through the measurement of their kinetic energy releases (KERs). The proton migration from different charged parent ions was analyzed from the obtained KER distributions. For the direct CE channel of CH3+ + Cl+ and CH3+ + Cl2+, the contribution of multiply excited electronic states and multicharged states is identified. In addition, the measurements of relative yields of the fragmentation channel at different laser wavelengths provide a selective control of proton migration for CH3Cl molecules in intense laser fields. [ABSTRACT FROM AUTHOR]

In this work we instigated the fragmentation of Au microparticles supported on a thin amorphous carbon film by irradiating them with a gradually convergent electron beam inside the Transmission Electron Microscope. This phenomenon has been generically labeled as "electron beam-induced fragmentation" or EBIF and its physical origin remains contested. On the one hand, EBIF has been primarily characterized as a consequence of beam-induced heating. On the other, EBIF has been attributed to beam-induced charging eventually leading to Coulomb explosion. To test the feasibility of the charging framework for EBIF, we instigated the fragmentation of Au particles under two different experimental conditions. First, with the magnetic objective lens of the microscope operating at full capacity, i.e. background magnetic field B = 2 T, and with the magnetic objective lens switched off (Lorenz mode), i.e. B = 0 T. We observe that the presence or absence of the magnetic field noticeably affects the critical current density at which EBIF occurs. This strongly suggests that magnetic field effects play a crucial role in instigating EBIF on the microparticles. The dependence of the value of the critical current density on the absence or presence of an ambient magnetic field cannot be accounted for by the beam-induced heating model. Consequently, this work presents robust experimental evidence suggesting that Coulomb explosion driven by electrostatic charging is the root cause of EBIF. [Display omitted] • Experimental study of the magnetic field effect on the e-beam induced fragmentation • Theoretical framework explaining the difference of critical current densities • The simulated Coulomb explosion occuring in pico-second order • Raman, XRD, SAED studies of the amorphous carbon films and 4-probe resistance measurements [ABSTRACT FROM AUTHOR]

Nanoparticle mediated laser induced breakdown can be used for Nanoparticle synthesis, cell nanosurgery and laser induced breakdown spectroscopy. To investigate the photo-thermal conversion of gold nanoparticles during pulsed laser irradiation, the electron-phonon two temperature model was established in this study. The impact of laser energy density and pulse width on the thermal conversion and morphology change of gold nanorods were investigated and compared with experimental observations. The results show that the melting threshold of gold nanorods under nanosecond laser irradiation is about twenty times that of femtosecond laser irradiation. The mechanisms of nanorod fragmentation are different between femto and nanosecond laser irradiation: particle melting is more likely to occur in nanosecond laser irradiation, while the Coulomb explosion is highly likely to occur in femtosecond laser irradiation. [ABSTRACT FROM AUTHOR]

A theoretical model of laser-induced ion acceleration during the Coulomb explosion of a flat uniform target with two-component ion composition is presented. To the difference from existing models, here, the case of an arbitrary ratio between the total charges of the ion components is considered. Comparison of the result with the numerical solution shows good agreement almost in the entire range of controlling parameters, which allows one to optimize the considered regime of ion acceleration. To this end, conditions were determined for the generation of beams of the best quality. [ABSTRACT FROM AUTHOR]

Coulomb explosion, characterized by Coulomb repulsion between particles with the same charge on the surface of a material, has been used to realize exquisite nano-manipulation, however, researchers usually found only one aspect of the application of Coulomb explosion when they utilized it. Herein, we successfully design a "metal@insulator" based Coulomb explosion process by irradiating oxidized topological crystalline insulator SnTe under an electron beam. The occurrence of Coulomb explosion mainly due to the oxide-encapsulated SnTe retained the metallic surface state, which not only can be positively charged but also can realize charge accumulation through the shielding effect of the insulating oxide layer. By changing experimental conditions and carefully studying various experimental phenomena, we conclude six aspects of the application, namely, speculating the metallic surface state of the oxide-encapsulated SnTe, controllable fabricating nanoplates, observing the PVD (physical vapor deposition) process under low temperature, rapid coating film, unraveling the oriented attachment and self-recrystallization of larger nanocrystals and fabricating hollow structure. Our findings are important for utilizing Coulomb explosion as well as other EBI techniques to conduct nano-manipulation. [ABSTRACT FROM AUTHOR]

We present the results of an integrated laboratory and modeling investigation into the impact of stellar X-rays on cosmic dust. Carbonaceous grains were prepared in a cooled (<200 K) supersonic expansion from aromatic molecular precursors, and were later irradiated with 970 eV X-rays. Silicate (enstatite) grains were prepared via laser ablation, thermally annealed, and later irradiated with 500 eV X-rays. Infrared spectra of the 3.4 ÎĽ m band of the carbon sample prepared with benzene revealed 84% ± 5% band area loss for an X-ray dose of 5.2 Ă—1023 eV.cmâˆ'2. Infrared spectra of the 8â€"12 ÎĽ m Siâ€"O band of the silicate sample revealed band area loss up to 63% ± 5% for doses of 2.3 Ă— 1023 eV.cmâˆ'2. A hybrid Monte Carlo particle trajectory approach was used to model the impact of X-rays and ensuing photoelectrons, Auger and collisionally ionized electrons through the bulk. As a result of X-ray ionization and ensuing Coulomb explosions on surface molecules, the calculated mass loss is 60% for the carbonaceous sample and 46% for the silicate sample, within a factor of 2 of the IR band loss, supporting an X-ray induced mass-loss mechanism. We apply the laboratory X-ray destruction rates to estimate the lifetimes of dust grains in protoplanetary disks surrounding 1 M ⊙ and 0.1 M ⊙ G and M stars. In both cases, X-ray destruction timescales are short (a few million years) at the disk surface, but are found to be much longer than typical disk lifetimes (≳10 Myr) over the disk bulk. [ABSTRACT FROM AUTHOR]

We discuss how Coulomb explosion imaging (CEI), triggered by intense femtosecond laser pulses and combined with laser-induced alignment and covariance analysis of the angular distributions of the recoiling fragment ions, provides new opportunities for imaging the structures of molecules and molecular complexes. First, focusing on gas phase molecules, we show how the periodic torsional motion of halogenated biphenyl molecules can be measured in real time by timed CEI, and how CEI of one-dimensionally aligned difluoroiodobenzene molecules can uniquely identify four structural isomers. Next, focusing on molecular complexes formed inside He nano-droplets, we show that the conformations of noncovalently bound dimers or trimers, aligned in one or three dimensions, can be determined by CEI. Results presented for homodimers of CS2, OCS, and bromobenzene pave the way for femtosecond time-resolved structure imaging of molecules undergoing bimolecular interactions and ultimately chemical reactions. [ABSTRACT FROM AUTHOR]

The angular distribution of the ions coming from Coulomb explosion following femtosecond ionisation of methyl iodide is studied in the spectral range of 600 to 1450 nm. Under these conditions, iodine and methyl fragment ions are generated, both via dissociative ionisation and via Coulomb explosion following the double ionisation of the parent ion. However, the dependence of the angular distribution of the fragment ions on the ionisation wavelength is completely different for those two mechanisms: dissociative ionisation fragments are being ejected with high anisotropy, irrespectively of the wavelength, while the angular distribution of the Coulomb explosion fragments changes from anisotropic, for an excitation wavelength close to 600 nm, to isotropic close to 950 nm, and remain isotropic up to 1450 nm. [ABSTRACT FROM AUTHOR]

Coupling covariance map analysis with ultrafast laser-induced Coulomb explosion imaging (CEI) enabled us to characterise the dissociative ionisation dynamics of methoxycarbonylsulfenyl chloride (H3COCOSCl). We applied a 3D multi-coincidence detection technique which is necessary for covariance imaging analysis. With the aid of such analysis, we found meaningful correlations between several product ion pairs that revealed different fragmentation channels. A weak, narrow correlation in the covariance image of H3COC(O)+/Cl+ was assigned to a concerted three-body dissociation channel. We also observed a broadly distributed correlation in the covariance image of COS+/HCO+, which is ascribed to a multi-body fragmentation channel. Ab initio calculations were carried out to determine the likely starting structures. A very sharp feature in the covariance image of Cl+/ CH3+ indicated that this pair of ions is produced from a two-body-like fragmentation event of CH3Cl2+. We show by direct comparison with CEI of CH3Cl that, contrary to a previously suggested assignment, this ion pair is produced from CH3Cl, which is formed either in the bubbler or gas line before the molecular beam interacts with the laser beam. [ABSTRACT FROM AUTHOR]

We present a time-resolved analysis of Rayleigh scattering measurements to determine the average size of methane clusters and find the optimum timing for laser-cluster fusion experiments. We measure Rayleigh scattering and determine the average size of methane clusters varying the backing pressure (P0) from 11 bar to 69 bar. Regarding the onset of clustering, we estimate that the average size of methane clusters at the onset of clustering is Nc0≅20 at 11 bar. According to our measurements, the average cluster radius r follows the power law of r∝P01.86. Our ion time-of-flight measurements indicate that we have produced energetic deuterium ions with kT = 52±2 keV after laser-cluster interaction using CD4 gas at 50 bar. We find that this ion temperature agrees with the predicted temperature from CD4 clusters at 50 bar with r = 14 nm assuming the Coulomb explosion model. [ABSTRACT FROM AUTHOR]

In this paper, we present an experimental study of charge propensities formed when Argon clusters are ionized with laser pulses of intensities up to 4 × 10 18 Wcm - 2 . The mean cluster size is varied from 1400 to 36,500 atoms to probe changes in ionization propensity at a given laser intensity. We attempted to use the Uniform charge scheme (that was earlier successful in fitting the ion energy spectra) to fit the ion charge state histograms. The difficulty in fitting the measured charge histograms compel us to adopt a non-uniform charging scheme to fit the measured ion charge propensity. Three different empirical weight functions that represent the charge of atoms in different shells of the cluster are used. The parameters of the empirical functions are optimized by least square fit to match the measured ion kinetic energy spectra. The weight function represents the terminal charge states of the ions coulomb exploded from the cluster and gives a better insight into the Coulomb explosion of the charged nanosphere. [ABSTRACT FROM AUTHOR]

The laser intensity dependence of the recoil energies from the Coulomb explosion of small argon clusters has been investigated by resolving the contributions of the individual charge states to the ion recoil energy spectra. Between 10 14 and 10 15 W/cm 2 , the high-energy tail of the ion energy spectra changes its shape and develops into the well-known knee feature, which results from the cluster size distribution, laser focal averaging, and ionization saturation. Resolving the contributions of the different charge states to the recoil energies, the experimental data reveal that the basic assumption of an exploding homogeneously charged sphere cannot be maintained in general. In fact, the energy spectra of the high-q show distinct gaps in the yields at low kinetic energies, which hints at more complex radial ion charge distributions developing during the laser pulse impact. [ABSTRACT FROM AUTHOR]

It was recently shown that nanostructured targets with largely spaced gold ultrasmall nanoparticles (NPs) show outstanding performances in enhancing the laser-driven ions' acceleration process due to the higher laser-to-target energy absorption [Vallières et al., Phys. Rev. Accel. Beams 22, 091303 (2019)]. Based on this structure, here, an alternative nanostructured design is proposed to promote light/heavy ions' acceleration quality. The scheme relies on using a gold NP layered nanosphere filled with a low-density argon gas. The nanosphere has an inner layer of vanadium and an outer layer of proton–carbon (1:1) mixture. The validity of this suggestion has been simulated by the two-dimensional particle-in-cell code (EPOCH). Simulation results indicate that the interaction of ultra-intense laser (∼ 4.61 × 1019 W/cm2) with a gas-filled gold NP layered nanosphere can positively decrease the aggregation of electrons stated inside the target, leading to higher Coulomb repulsion between charged ions. Therefore, we can expect the generation of quasi-monoenergetic H+, C6+, V20+, and Au49+, as well as Ar15+ (cutoff energy of ∼ 0.49 MeV/u and relative divergence angle of 2.9°) at the end of the interaction. From simulations, as the interaction terminates, for a gas-filled gold NP layered nanosphere with an optimal gap space of 80 nm, a cutoff energy increase of roughly 19% for H+, 16.4% for C6+, and rather equal percent of 15.9% for medium-heavy ions (V20+ and Au49+) is obtained with respect to a hollow gold NP layered nanosphere. Moreover, a relative divergence angle decrease of up to nearly 0.29–1.91 times will be calculated for the accelerated ions. Overall, the results verify that a gas-filled gold NP layered nanosphere can be regarded as a candidate for the generation of quasi-monoenergetic ions through the spherical Coulomb explosion regime. [ABSTRACT FROM AUTHOR]

The fragmentations of iodine cyanide ions created with 2 to 8 positive charges by photoionization from inner shells with binding energies from 59 eV (I 4d) to ca. 900 eV (I 3p) have been examined by multi-electron and multi-ion coincidence spectroscopy with velocity map imaging ion capability. The charge distributions produced by hole formation in each shell are characterised and systematic effects of the number of charges and of initial charge localisation are found. [ABSTRACT FROM AUTHOR]

We apply a three-fold covariance imaging method to analyse previously acquired data [C. S. Slater et al., Phys. Rev. A 89, 011401(R) (2014)] on the femtosecond laser-induced Coulomb explosion of spatially pre-aligned 3,5-dibromo-3',5'-difluoro-4'-cyanobiphenyl molecules. The data were acquired using the "Pixel Imaging Mass Spectrometry" camera. We show how three-fold covariance imaging of ionic photofragment recoil trajectories can be used to provide new information about the parent ion's molecular structure prior to its Coulomb explosion. In particular, we show how the analysis may be used to obtain information about molecular conformation and provide an alternative route for enantiomer determination. [ABSTRACT FROM AUTHOR]

Twin graphene can introduce stable and extended defects to energy band engineering, giving unique electron transport properties, which is expected to have potential applications in the fields of magnetism, spin transport or photoluminescence. This study proposes a method of using electric-field-assisted temporally-shaped femtosecond laser ablation liquid (ETLAL) of graphene dispersion to prepare graphene quantum dots (GQDs) with an average particle size of 2–3 nm and oxygen-containing functional groups modified surface, which realizes the controllable preparation of single crystal and twin GQDs (5-fold twin). This method controls the crystallinity of GQDs from two aspects: (1) The intervention of an electric field can rapidly command the directional motion of the cavitation bubbles and the nanoparticles contained therein to collide and crystallize at higher temperatures and pressures, which is the key to the formation of twin GQD; (2) Adjusting temporally-shaped femtosecond laser pulse delay could control the proportion of the Coulomb explosion during the ablation process, which increases the carbon cluster supplied by the cavitation bubble which is the key of polyploid number in twin GQDs. This research provides a fast, green strategy for the preparation of unprecedented twin GQDs, which is of great significance to the applications in the field of 2D material defect engineering. [Display omitted] [ABSTRACT FROM AUTHOR]