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Nanoparticle-mediated laser-induced breakdown (LIB) can be used for Nanoparticle synthesis, cell nanosurgery and laser-induced breakdown spectroscopy (LIBS). 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 fem to 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.

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.

The CO molecular ionization and Coulomb explosion are systematically investigated utilizing a high-resolution time-of-flight mass spectrometer. It is found that the main dissociation channels of hi...

A theoretical study on the coupled electron-nuclear dynamics of HD+ molecular ions under ultrashort, intense laser pulses is performed by employing a well-established quasi-classical model. The influence of the laser carrier-envelope phase on various channel (H + D+, D + H+, and H+ + D+) probabilities is investigated at different laser field intensities. The carrier-envelope phase is found to govern the dissociation (H + D+ and D + H+) and Coulomb explosion (H+ + D+) channel probabilities. The kinetic energy release distributions of the fragments are also found to be sensitive to the carrier-envelope phase of the laser pulse. Our results are in agreement with the previously reported quantum dynamics studies and experiments.

In this paper, we report on the acceleration of protons and oxygen ions from tens of micrometer large water droplets by a high-intensity laser in the range of 1020 W/cm2. Proton energies of up to 6 MeV were obtained from a hybrid acceleration regime between classical Coulomb explosion and shocks. Besides the known thermal energy spectrum, a collective acceleration of oxygen ions of different charge states is observed. 3D PIC simulations and analytical models are employed to support the experiential findings and reveal the potential for further applications and studies.

Ultrafast laser pump-probe methods allow chemical reactions to be followed in real time, and have provided unprecedented insight into fundamental aspects of chemical reactivity. While evolution of the electronic structure of the system under study is evident from changes in the observed spectral signatures, information on rearrangement of the nuclear framework is generally obtained indirectly. Disentangling contributions to the signal arising from competing photochemical pathways can also be challenging. Here we introduce the new technique of three-dimensional covariance-map Coulomb explosion imaging, which has the potential to provide complete three-dimensional information on molecular structure and dynamics as they evolve in real time during a gas-phase chemical reaction. We present first proof-of-concept data from recent measurements on CF3I. Our approach allows the contributions from competing fragmentation pathways to be isolated and characterised unambiguously, and is a promising route to enabling the recording of ‘molecular movies’ for a wide variety of gas-phase chemical processes. Ultrafast laser pump-probe methods allow chemical reactions to be followed in real-time, but to directly show the rearrangement of nuclear frameworks is challenging. Here an improved real-time-resolved 3D covariance-map Coulomb explosion imaging technique for ultrafast gas-phase reactions is presented.

The interaction of an ultra-intense, ultra-short laser pulse with nano-rods is investigated. The nano-rods consist of two materials with different mass. The interaction leads to the ionisation of the nano-rods and their consecutive Coulomb explosion. A number of simulations are performed to understand the nonlinear optical properties of the electronic subsystem interacting with the driver laser. It is shown that the dynamics of the Coulomb explosions can be controlled by changing the cylinder radius, the cylinder density, the gap between the cylinders, and the laser parameters. The simulations are performed with the help of the plasma simulation code (PSC), which is a Particle-In-Cell (PIC) code. The PSC solves the Vlasov-Maxwell system of equations that describes the time evolution of the particle distribution functions.

Low-density patterned Carbon Nanotube (CNT) field emitter is important for applications that require an addressable electron emission source. However, coulomb explosion, that occurs on freestanding CNTs when coulomb repulsion exceeds the limit of van der Waals force, will be induced by low-density patterned CNT field emitters. The coulomb explosion is the main reason cause the short lifetime of low-density patterned CNT field emitters and the direct failure of the CNT field emission (FE) process by inducing a strong arcing between CNT cathode and Indium tin oxide (ITO) anode. Therefore, study the phenomenon of CNT FE induced coulomb explosion and the causes are the keys to increase the lifetime and stability of low-density patterned CNT field emitters.

The potential application of metallic nanoparticles had been attracting attention and interest from different areas of academia and industry. The nanoparticles properties and applications depend heavily on their dimension and shape, thus special interest is aimed to controlling the nanoparticles sizes. Ultrashort laser pulses are known to change metallic nanoparticles characteristics, but the interaction mechanism is still not completely understood. In this work we reduced the dimension of silver nanoparticles with ultrashort pulses and demonstrated that there is a dependence of the particles size on the second order dispersion introduced in the pulses, which became smaller as the dispersion becomes more negative. Based on the results, we propose that the Coulomb explosion that reduces the nanoparticles size is predominantly initiated by multiphotonic ionization for the intensities used (1014 W/cm2).

The effects of laser and plasma parameters on the proton focusing driven by Coulomb explosion in the interaction of a laser pulse with a thin arched carbon-hydrogen target are investigated by two-dimensional particle-in-cell simulations. The influence of the laser polarization, the laser spot size and the geometrical dimension of the arched target has been examined. It is shown that a linearly-polarized laser pulse contributes to the occurrence of the Coulomb explosion in a shorter time, and a larger focal spot is advantageous to produce a homogeneous Coulomb field. The geometrical property of the arched target and the buffer action of heavy ions are appropriate for the forward proton focusing.

We propose a theoretical model describing the expansion of a plasma layer into vacuum for an arbitrary electron plasma component temperature. Comparison with the known limiting cases of the quasi-neutral outflow and the Coulomb explosion, as well as with the results of the 1D electrostatic simulation, has revealed a high accuracy of the proposed model in the description of the spectral–energy and spatial characteristics of ions accelerated during the plasma expansion. The procedure for obtaining the relations between the characteristics of accelerated ions and the laser pulse and target parameters is described with regard to qualitative predictions and the description of the results of numerical kinetic simulation and experiments on the laser and plasma acceleration of ions.

We have deduced the structure of the \ce{bromobenzene}--\ce{I2} heterodimer and the \ce{(bromobenzene)2} homodimer inside helium droplets using a combination of laser-induced alignment, Coulomb explosion imaging, 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 Timepix3 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 \ce{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 Coulomb explosion imaging to determine the structure of large complexes, and point the way toward real-time measurements of bimolecular reactions inside helium droplets.

Inducing and controlling the ultrafast molecular rotational dynamics using shaped laser fields is essential in numerous applications. Several approaches exist that allow following the coherent molecular motion in real-time, including Coulomb explosion-based techniques and recovering molecular orientation from the angular distribution of high harmonics. We theoretically consider a non-intrusive optical scheme for visualizing the rotational dynamics in an anisotropic molecular gas. The proposed method allows determining the instantaneous orientation of the principal optical axes of the gas. The method is based on probing the sample using ultra-short circularly polarized laser pulses and recording the transmission image through a vortex wave plate. We consider two example excitations: molecular alignment induced by an intense linearly polarized laser pulse and unidirectional molecular rotation induced by a polarization-shaped pulse. The proposed optical method is promising for visualizing the dynamics of complex symmetric- and asymmetric-top molecules., Comment: 12 pages, 14 figures

The journal of chemical physics 154(7), 071101 (2021). doi:10.1063/5.0029792, 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., Published by American Institute of Physics, Melville, NY

Understanding and controlling ultrafast charge carrier dynamics is of fundamental importance in diverse fields of (quantum) science and technology. Here, we create a three-dimensional hot electron gas through two-photon photoemission from a copper surface in vacuum. We employ an ultrafast electron microscope to record movies of the subsequent electron dynamics on the picosecond-nanosecond time scale. After a prompt Coulomb explosion, the subsequent dynamics is characterized by a rapid oblate-to-prolate shape transformation of the electron gas, and periodic and long-lived electron cyclotron oscillations inside the magnetic field of the objective lens. In this regime, the collective behavior of the oscillating electrons causes a transient, mean-field lensing effect and pronounced distortions in the images. We derive an analytical expression for the time-dependent focal length of the electron-gas lens, and perform numerical electron dynamics and probe image simulations to determine the role of Coulomb self-fields and image charges. This work inspires the visualization of cyclotron dynamics inside two-dimensional electron-gas materials and enables the elucidation of electron/plasma dynamics and properties that could benefit the development of high-brightness electron and X-ray sources., Excited charge carriers, such as photoelectrons, play an important role in fundamental and technological fields. Here the authors employ an ultrafast electron microscope to directly visualize the cyclotron oscillations and oblate-to-prolate shape change of a photoemitted electron gas from a laser-excited copper surface.

High-precision, time-resolved Coulomb explosion imaging of rotational wave packets in nitrogen molecules created with a pair of time-delayed, polarization-skewed femtosecond laser pulses is presented, providing insight into the creation process and dynamics of direction-controlled wave packets. To initiate unidirectional rotation, the interval of the double-pulse was set so that the second, polarization-tilted pulse hit the molecules at the time when molecules were aligned or antialigned along the polarization vector of the first pulse. During the revival period of the rotational wave packet, pulse intervals around both the full and half revival times were used. The observed molecular wave packet movies clearly show the signatures of quantum rotation, such as angular localization (alignment), dispersion, and revival phenomena, during the unidirectional motion. The patterns are quite different depending on the pulse interval even when the angular distribution at the second pulse irradiation is similar. The observed interval-dependence of the dynamics was analyzed on the basis of the real-time images, with the aid of numerical simulations, and the creation process of the packets was discussed. We show that the observed image patterns can be essentially rationalized in terms of rotational period and alignment parameter. Because the double-pulse scheme is the most fundamental in the creation of direction-controlled rotational wave packets, this study will lead to more sophisticated control and characterization of directional molecular motions.

Rotational dynamics of gas phase carbon disulfide (CS2) dimers were induced by a moderately intense, circularly polarized alignment laser pulse and measured as a function of time by Coulomb explosion imaging with an intense fs probe pulse. For the alignment pulse, two different temporal intensity profiles were used: a truncated pulse with a 150 ps turn-on and a 8 ps turn-off, or a 'kick' pulse with a duration of 1.3 ps. For both types of pulse, rich rotational dynamics with characteristic full and fractional revivals were recorded, showing that the intermolecular carbon-carbon axis periodically aligns along the propagation direction of the laser pulses. The truncated pulse gave the strongest alignment, which we rationalize as being due to a flat relative phase between the components in the rotational wave packet generated. Fourier analysis of the alignment dynamics gave well-spaced peaks which were fit to determine the rotational constant, B, and the centrifugal constant, DJ, for the ground state of the dimer. Our results agree with values from high-resolution IR spectroscopy. Numerical simulations of the alignment accurately reproduced the experimental dynamics when the truncated pulse or a low intensity kick pulse was used, but failed to reproduce the dynamics induced by a high intensity kick pulse. We posit that the discrepancy is due to excitation of the intermolecular torsional motion by the kick pulse.

We present a study on laser-driven proton acceleration from a hydrogen cluster target. Aiming for the optimisation of the proton source, we performed a detailed parametric scan of the interaction conditions by varying different laser and the target parameters. While the underlying process of a Coulomb-explosion delivers moderate energies, in the range of 100 s of keV, the use of hydrogen as target material comes with the benefit of a debris-free, single-species proton acceleration scheme, enabling high repetition-rate experiments, which are very robust against shot-to-shot fluctuations.

Globally charged environments can modify the rates of nuclear re- actions at the energies relevant for nuclear fusion energy production. This issue was addressed using the screening potential approach, deriving approximate an- alytical results for some scenarios of interest. The developed model predicts that fusion is hindered for reactions between thermal nuclei, while an enhancement is expected for secondary reactions and cluster Coulomb explosion beam-target reactions.

Ion-coincidence momentum imaging of Coulomb explosion of formaldehyde (H2CO) in femtosecond intense laser fields (800 nm, 1.3 × 1015 W/cm2) is performed with two different laser pulse durations (7 fs and 35 fs). In the 7-fs laser fields, the full fragmentation pathway from H2CO4+ → H+ + H+ + C+ + O+ is identified. The angles between the fragment momenta are well reproduced by a simple Coulomb explosion model from the geometry of neutral formaldehyde, showing that the molecular structure is virtually frozen along the bending coordinates during the multiple ionization. Three-body Coulomb explosion pathways from triply charged states, H2CO3+ → H+ + H+ + CO+ and H+ + CH+ + O+, are observed in both the 7-fs and 35-fs laser fields. Significant changes in the momentum angle distribution and asymmetric energy partitioning between two H+ ions are observed in the 35-fs case, which are attributed to structural deformation prior to the Coulomb explosion in the longer pulse.

In this work, the hypergolic ignition process of 8 recently synthesized ionic liquids (I.L.s) with [BH3(CN)BH2(CN)]− anions with white fuming nitric acid is experimentally investigated by using droplet test method and long distance microscope-high speed photography techniques. Results show that the hypergolic ignition process of the present I.L.s is of a completely different three stage nature and ultra-fast hypergolic ignition is observed for this family of I.L.s ( 1). Subsequently in the second stage, liquid spikes that are ejected into the oxidizer pool during CEDT significantly increase the reactive surface area underneath the liquid surface. As a consequence, local temperature increases, gas phase intermediate product and oxidizer/fuel vapor accumulate due to the continuous liquid phase reaction underneath the surface. When the local pressure overcomes the surface tension, disintegration of the surface in terms of larger ligaments and secondary droplets ejection was observed, together with shooting out of the vapor/smoke. Finally, in the third stage when the local temperature and the vapor concentration increase sufficiently, further gas phase reaction leads to ignition. Fine and stable flame with strong bright and green luminescence continuously evolve from the initial mixing layer. Some red and dark smokes are gradually generated above the burning bright and green flame until finally the flame dies out. The ignition delay time (IDT) decreases with the increase of unsaturation index of the heterocyclic core in the cation, and also the decrease of CEDT. The enthalpy of formation of the I.L.s for different cation structures are correlated with IDT, which represents both the fuel structure chemistry and the ‘coulomb explosion’ enhances mixing effect on the overall reactivity of this diffusive system.

Methyl iodide (CH 3 I) is one of the most studied molecule in the field of photochemistry. This molecule has been investigated by a variety of light sources including lamps, different lasers as well as synchrotron radiation of varying intensity and wavelength. In this review, photoionisation studies of CH 3 I have been summarised for monomer (CH 3 I) and its aggregates i.e., cluster (CH 3 I) n which were excited with a range of photon energy and intensity using different sources. For the case of monomer, ionisation process follows either ionisation followed by dissociation (ladder climbing mechanism) or dissociation followed by ionisation (ladder switching mechanism) depending upon the wavelength and intensity of ionisation source. In case of CH 3 I ionisation by synchrotron radiation, singly and doubly charged molecular ions were observed depending upon the energy of photon. For ionisation using laser sources, singly and multiply charged ions were observed for laser pulses ranging from femtosecond to nanosecond duration. For the case of cluster (CH 3 I) n ionisation with laser sources, multiply charged ions were observed leading to Coulomb explosion phenomena. Lowest possible laser intensity observed to induce Coulomb explosion in CH 3 I clusters was ∼10 9 W/cm 2 using nanosecond laser pulses. Since observation of multiply charged ion at gigawatt intensity is surprising, a detailed description of experimental results has been summarised. A three stage cluster ionisation model which comprises of multiphoton ionization ignited-inverse Bremsstrahlung heating and electron ionization has been adapted to explain experimental observations and to understand nanosecond laser induced Coulomb explosion of CH 3 I clusters.

The effect of laser wavelength on the dissociation mechanisms in methane is examined over a broad range of intensities for both 800 nm and 400 nm laser fields. It is found that, at lower laser intensities, the dissociation pathways identified with the aid of theoretical calculations for the methane cation can account for most of the experimental findings, including the differences observed for irradiation by 800 nm and the 400 nm fields. As the laser intensity increases, the significance of the Coulomb explosion mechanism, along with the contribution of the rescattering process and the concomitant dissociation pathways, is highlighted.

Systematic manipulation of ionic-outcome in laser-cluster interaction process has been realized for studies carried out on tetramethyltin (TMT) clusters under picosecond laser conditions, determined by choice of laser wavelength and intensity. As a function of laser intensity, TMT clusters exhibit gradual enhancement in overall ionization of its cluster constituents, up to a saturation level of ionization, which was distinct for different wavelengths (266, 355, and 532 nm). Simultaneously, systematic appearance of higher multiply charged atomic ions and shift in relative abundance of multiply charged atomic ions towards higher charge state was observed, using time-of-flight mass spectrometer. At saturation level, multiply charged atomic ions up to (C2+, Sn2+) at 266 nm, (C4+, Sn4+) at 355 nm, and (C4+, Sn6+) at 532 nm were detected. In addition, at 355 nm intra-cluster ion chemistry within the ionized cluster leads to generation of molecular hydrogen ion (H2+) and triatomic molecular hydrogen ion (H3+). Generation of multiply charged atomic ions is ascribed to efficient coupling of laser pulse with the cluster media, facilitated by inner-ionized electrons produced within the cluster, at the leading edge of laser pulse. Role of inner-ionized electrons is authenticated by measuring kinetic energy distribution of electrons liberated upon disintegration of excessively ionized cluster, under the influence of picosecond laser pulse. Graphical Abstract ᅟ.

We present a conceptually new, slit-based slice imaging technique for ion-imaging experiments, offering a way for high slice resolution imaging in the polarization plane of an ionizing laser pulse. In the present method, a mechanically adjustable slit is installed in the drift region of the flight of the ions so that only a thin central slice of a three-dimensionally expanding ion cloud (Newton sphere) passes through the slit. The sliced cloud is then projected onto a two-dimensional position-sensitive ion detector installed parallel to the slice plane. Compared to the conventional two-dimensional imaging, the present “space-slice imaging” scheme has two principle novelties: (1) The slit acts as an ideal gate for the slicing, and a slice resolution of 1% or higher can be achieved, in principle, using submillimeter slit width for a typical a few-centimeter ion cloud. (2) The imaging plane can be automatically parallel to the polarization plane of a laser pulse regardless of the state of polarization, resulting in a hitherto unrealized “camera angle.” We developed a space-slice ion imaging apparatus to realize and validate the present scheme. To evaluate its performance, we carried out the Coulomb explosion imaging of the N2 molecule. By adjusting slit width, slicing up to approximately 0.33% was achieved without remarkable image distortion. The polarization-dependent imaging shows that the ejection angles of ions can be directly read from the observed images obtained with any polarization states. The present imaging measurements in the laser polarization plane opens new avenues for the study of laser-induced dynamics; these dynamics cannot be fully characterized with the existing two-dimensional setups. As an example, we applied the present approach to the time-resolved imaging of a laser-driven rotational wave packet of N2, using a circularly polarized exploding pulse as an isotropic probe in the imaging plane. We successfully observed clear time-dependent images containing full spatiotemporal information of the wave packet dynamics. Details of the concept, design, and operation of our apparatus are presented in the present paper.

Carbon nanotube properties can be modified by ion irradiation; therefore it is important to know the manner in which ions deposit energy (how much and where) in the nanotubes. In this work, we have studied, experimentally and with a simulation code, the irradiation of multi-walled carbon nanotubes (MWCNT), supported on a holey amorphous carbon (a-C) substrate, with low energy (2–10 keV/u) H+ and H2+ molecular beams, impinging perpendicularly to the MWCNT axis. The energy distribution of protons traversing the nanotubes (either from the H+ beam or dissociated from the H2+ beam) was measured by the transmission technique in the forward direction. Two well-differentiated peaks appear in the experimental energy-loss distribution of the fragments dissociated from the molecular H2+ beam, in correspondence to the ones detected with the proton beam. One is the low-energy loss peak (LELP), which has a symmetric width; the other is the high-energy loss peak (HELP), which shows an asymmetric broadening towards larger energy loss than the corresponding proton energy distribution. A semi-classical simulation, accounting for the main interaction processes (both elastic and inelastic), of the proton trajectories through the nanotube and the supporting substrate has been done, in order to elucidate the origin of these structures in the energy spectra. Regarding the H+ energy spectrum, the LELP corresponds to projectiles that travel in quasi-channelling motion through the most outer walls of the nanotubes and then pass through the substrate holes, whereas the HELP results mostly from projectiles traversing only the a-C substrate, with the asymmetry broadening being due to a minor contribution of those protons that cross the a-C substrate after exiting the nanotube. The broadening of the peaks corresponding to dissociated fragments, with respect to that of the isolated protons, is the result of vicinage effects between the fragments, when travelling in quasi-channelling conditions through the outer layers of the nanotube, and Coulomb explosion just after exiting the target. The excellent agreement between the measured and the simulated energy spectra of the H+ beam validates our simulation code in order to predict the energy deposited by ion beams in carbon nanotubes. This work has been financially supported by Fondecyt 1100759, Fondecyt 1121203 and USM-DGIP 11.11.11, Anillo ACT1108, Proyecto Basal FB0821 - CONICYT, the Spanish Ministerio de Economía y Competitividad and European Regional Development Fund (Projects FIS2014-58849-P and PGC2018-096788-B-I00), and Fundación Séneca (Project No. 19907/GERM/15).

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.

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.

Graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs) can be used in different applications such as optoelectronic and biomedical applications, respectively. Hence, the selective synthesis of GQDs and GOQDs is highly desirable but challenging. Here, we present GQDs and GOQDs selectively prepared by an easy and simple pulsed laser ablation in liquid (PLAL) method by controlling the laser wavelength. The obtained GQDs and GOQDs showed a significantly different optoelectronic nature mainly due to the existence of surface oxygen-rich functional groups (e.g. carboxyl or hydroxy groups). Also, we described a possible mechanism for the formation of oxygen functional groups during the PLAL process based on the Coulomb explosion model, which can give further insight for designing functional carbon materials.

Recent advances in ultrafast laser technology hint at the possibility of using shaped pulses to generate deracemization via selective enantiomeric conversion; however, experimental implementation remains a challenge and has not yet been achieved. Here, we describe an experiment that can be considered an accessible intermediate step on the road towards achieving laser induced deracemization in a laboratory. Our approach consists of driving a racemic mixture of 3D oriented 3,5-difluoro-3', 5'-dibromobiphenyl (F2H3C6–C6H3Br2) molecules with a simple train of Gaussian pulses with alternating polarization axes. We use arguments related to the geometry of the field/molecule interaction to illustrate why this will increase the amplitude of the torsional oscillations between the phenyl rings while simultaneously breaking the inversion symmetry of the dynamics between the left- and right-handed enantiomeric forms, two crucial requirements for achieving deracemization. We verify our approach using numerical simulations and show that it leads to significant and experimentally measurable differences in the internal enantiomeric structures when detected by Coulomb explosion imaging.

Propagation of short and ultraintense laser pulses in a semi-infinite space of overdense hydrogen plasma is analyzed via fully relativistic, real geometry particle-in-cell (PIC) simulations including radiation friction. The relativistic transparency and hole-boring regimes are found to be sensitive to the transverse plasma field, backward light reflection, and laser pulse filamentation. For laser intensities approaching I ∼ 1024 W/cm2, the direct laser acceleration of protons, along with ion Coulomb explosion, results in their injection into the acceleration phase of the compressed electron wave at the front of the laser pulses. The protons are observed to be accelerated up to 10–20 GeV with densities around a few times the critical density. The effect qualitatively depends on initial density and laser intensity, disappearing with the initial density increase or intensity decrease.

Physics reports 808, 1 - 74 (2019). doi:10.1016/j.physrep.2019.02.002, Free-electron lasers produce extremely brief, coherent, and bright laser-like photon pulses that allow to image matter at atomic resolution and at timescales faster than the characteristic atomic motions. In pulses of about 50 femtoseconds duration they provide as many photons as one gets in 1 s from modern storage ring synchrotron radiation facilities. FLASH, the Free-Electron Laser at DESY in Hamburg was the first FEL in the XUV/soft X-ray spectral range, started operation as a user facility in summer 2005, and was for almost 5 years the only short wavelength FEL facility worldwide. Hence, most of the technological developments as well as the scientific experiments performed by the user community were new and unique as outlined below. FLASH was driving FEL science and technology and paved the way for many new ideas. Because of using a linear accelerator in superconducting RF technology FLASH combines the extreme peak brightness characteristic for FELs with very high average brightness. It also was the prototype for the European XFEL located in the Hamburg metropolitan area, which started user operation in summer 2017.The present review provides an overview of the progress made with accelerator science and technology at FLASH for the production of stable beams of well characterized electron pulses, reduction of the pulse jitter to the femtosecond level, generation of ultra-short photon pulses, adequate synchronization of the machine parameters with the experiment, and demonstrating advanced FEL schemes using variable gap undulators. Much of this was done in the very exciting early days of FEL science when it was even not clear if the FEL concept could be realized for X-rays. The development and the operation of the FLASH user facility is described, as well as the techniques developed to make use of the new type of X-ray beams including photon beam diagnostics and damage studies of the optical elements. The review emphasizes breakthrough experiments which demonstrated that many of the ideas collected in the world-wide discussion of the scientific case of free-electron lasers could indeed be realized and they often produced unexpected results. The first experiment on Coulomb explosion of Xe clusters performed in 2002 was a clear demonstration of the feasibility of experiments with free-electron laser beams and opened a lively discussion in the atom, molecular and optical physics community (AMO).Time resolved single-shot single-particle imaging, summarized in the slogan “Take movies instead of pictures”, was one of the most popular science drivers for the construction of free-electron X-ray lasers. As a first step in this direction experiments using a highly focused beam of FLASH demonstrated that pictures of 2 dimensional objects could be reconstructed from single-shot single-particle diffraction patterns. Explosion dynamics of nano-size particles hit by an intense FEL pulse were studied. This method, called “diffraction before destruction”, is now very successfully applied with hard X-rays and, to a large extent, solves the radiation damage problem in structural biology. A long term goal is to determine the 3 dimensional structure of a large molecule from a single-shot diffraction pattern. Along these lines the 3D architecture of free Ag nanoparticles could be determined from one diffraction pattern only using soft X-rays from FLASH.To understand light–matter interactions in this new parameter space a number of pioneering AMO experiments have been performed including non-linear interactions in atoms, molecules and clusters. Multiphoton photoionization processes in the presence of intense optical fields have been studied, as well as photo-absorption of XUV photon energies on molecular ions important for astrophysics. The nature of formation and breaking of molecular bonds was investigated in VUV pump–VUV probe experiments using a reaction microscope and a specific delay line. As an example the process of ultrafast isomerization of acetylene molecules C$_2$H$_2$ triggered by single photon excitation has been studied. The structural changes during the isomerization process were visualized and an isomerization time of 52 +/- 15 fs was found.Clusters of variable size, which can be produced routinely, allow distinguish between inter- and intra-atomic effects and are considered model systems for the investigation of light–matter interactions in multi-atom objects. As an example such experimental studies provided instructive data for benchmarking theoretical models describing cluster ionization in intense short-wavelength laser pulses. The combination of single-shot single-particle imaging for determination of the cluster size with spectroscopy was crucial for success of these experiments. The investigations could later be extended to very large Xe clusters providing new insights into the nanoplasma formation and explosion dynamics of such large systemsFrom early on, studies of high energy density plasmas and warm dense matter have been one of the most prominent research fields in building the scientific case for X-ray free-electron lasers. A good understanding of this complex regime between cold solids and hot dilute plasmas is important for high pressure studies, applied materials studies, inertial fusion, and planetary interiors. With the first observation of saturable absorption of an L-shell transition in Aluminum and pioneering studies of warm dense hydrogen FLASH kicked off research of matter in extreme conditions with free-electron lasers.In condensed matter experiments the emphasis is not so much on the peak power of the FEL beam and extreme focusing, but on beam properties like polarization and pulse duration. The sample has to stay intact in the beam over hours and the number of photons per pulse impinging on the sample has to be limited to avoid space charge effects. After demonstrating the possibility to record single-shot resonant magnetic scattering images with FELs the first time-resolved demagnetization study using a pump–probe approach with an IR-pump pulse and an XUV probe pulse to record a resonant magnetic scattering pattern as a function of pump–probe delay was also performed at FLASH.Free-electron lasers offer the possibility to extend the well-established X-ray spectroscopic techniques for the investigation of the static electronic structure of matter to probing the evolution of the electronic structure in the time domain after controlled excitation. At FLASH first time resolved core level photoemission (TR-XPS) experiments have been performed which are element specific and provide information on the dynamics of the local charge state around a specific center. Using 198 eV photons in a surface study at Ir single crystals it was possible to separate surface and bulk contributions in the Ir 4f levels with sufficient instrumental resolution. Time and angular resolved photoelectron spectroscopy (TR-ARPES) is a very powerful tool to study non-equilibrium electron dynamics of condensed matter systems, since it offers the possibility to follow the dynamics of the full band structure of a material. In another pioneering experiment the photo-induced dynamics of the Mott insulator 1T-TaS$_2$ was studied at FLASH by investigating the dynamics of the Ta 4f photoemission. The formation of a commensurate charge density wave (CCDW) leads to a splitting of the Ta 4f level which decreases first on a sub-picosecond time scale due to electronic melting of the CCDW and afterwards on a picosecond lifetime due to electron–phonon coupling. This leads to transfer of energy from the electronic system to the lattice and a partial melting of the periodic lattice distortions accompanying the periodic charge arrangement in the CCDW phase.In materials science X-ray absorption and emission spectroscopy are among the most powerful spectroscopies to study the electronic structure of matter. The wavelength of the radiation is scanned over certain element specific resonances which at FLASH 1 can only be done by scanning the electron energy. This is time consuming and makes the experiments difficult. Nevertheless, the first time-resolved X-ray emission spectroscopy (XES) experiment was done at FLASH 1 in order to study non-thermal melting of a silicon sample. From a comparison of the observed valence electronic structure at different times after the photoexcitation it became clear that in the melting process in the first few ps a non-equilibrium low density liquid state is reached. The existence of such a metastable low density liquid state had been postulated for many systems that show tetragonal bonding in the crystalline phase like water for example, but spectroscopically the time-resolved silicon XES data taken at FLASH verified its existence for the first time. FLASH 2 has tunable undulators and it was demonstrated that scanning of the wavelength is very easy there., Published by Elsevier Science, North-Holland, Amsterdam [u.a.]

Based on the combination of computational fluid dynamics (CFD), fluid–solid coupling theory, double-temperature model, C ++ programming and OpenFOAM simulation, this paper investigated the reflection and stratified absorption phenomenon of 10 ps pulsed laser on the surface of materials, calculated the laser reflectivity and stratified absorption coefficient as variables with electron and lattice temperature then innovatively added the calculating module of these variables to the laser processing simulation model. This paper also designed an functional module to simulate the multiple reflection and absorption phenomenon of deep-hole topography to pulsed laser and add it to the simulation model. Furthermore, the energy distribution algorithm in the model which aims at calculating the occupying ratio of energy used in Coulomb explosion and thermal effects has been also modified. The research results combined with experimental verification present that the modified simulation model has a higher accuracy than the original one, which proves that the laser-solid interactions mentioned above have large impacts on the precision of laser processing simulation. It is also proved that the modified energy distribution algorithm is more effective on improving the simulation accuracy than the original one. To sum up, a much more reliable simulation model which has the potential to guide the actual laser processing has been established.

We examine proton acceleration from spherical carbon-hydrogen targets irradiated by a relativistic laser pulse. Particle-In-Cell (PIC) simulations are carried out in 2 and 3 dimensions (2D and 3D) to compare fast proton spectra. We find very different final kinetic energies in 2D and 3D simulations. We show that they are caused by the different Coulomb fields in 2D and 3D. We propose a correction scheme for the proton energies to test this hypothesis. In the case of sub-focus diameter targets comparison of corrected 2D energies with 3D results show good agreement. This demonstrates that caution is required when modeling experiments with simulations of reduced dimensionality.

The electron–ion dynamics of the linear lithium carbide molecule under femtosecond laser pulses have been investigated in the framework of Ehrenfest molecular dynamics, in which valence electrons are treated quantum mechanically by time-dependent density functional theory (TDDFT) and ions are described classically. The on- and off-resonant multiphoton ionization processes have been induced by regulating laser frequency and laser intensity. The laser pulse with on-resonant frequency induces pronounced enhancement in electron ionization, bond length vibration, and energy absorption. Moreover, the coulomb explosion is preferred to occur in the on-resonant case, which is in qualitative agreement with previous theoretical investigations. The subtle relations between escaped electron number and absorbed photon number are well discussed with the increasing of laser intensity. Finally, the effect of self-interaction error is analyzed by comparing escaped electron number calculated with LDA and LDA-ADSIC. And the revTPSS-meta-GGA, a currently more accurate nonempirical exchange–correlation energy functional from a point of static density functional theory, is introduced to display its capability for the description of ionization process within nonlinear and the nonperturbative regime of isolated systems.

The 3-D particle-in-cell simulations with the code MANDOR demonstrate effective ion acceleration from the interaction of intense ultrashort linearly polarized laser pulses with both ultrathin solid dense foils and low-density targets when the laser energy ranges from several millijoules to tens of joules. The optimum foil thickness and the corresponding maximum energy of the accelerated ions for a given energy of the laser pulse were found. Different mechanisms of ion acceleration, such as target normal sheath acceleration, directed Coulomb explosion, and ponderomotive acceleration, are involved. We discuss the transition from one acceleration regime to another when the target thickness and density and the laser pulse intensity change and show that reducing the target density significantly increases the ion energy. We present some examples of isotope production as a possible application of multimegaelectronvolt proton beams on the joule scale of a laser.

Coulomb explosion of CO 2 , CO 2 → CO 2 2 + + 2e − → CO + + O + + 2e − , in phase-locked two-color ultrashort intense laser fields (800 nm and 400 nm, ∼10 14 W/cm 2 ) has been investigated by coincidence momentum imaging. The momentum images of the O + and CO + fragments show asymmetric distributions along the laser polarization direction depending on the relative phase of the two-color laser fields. The fragment asymmetry becomes most prominent (∼4%) at the relative phases providing the largest asymmetry of the electric field amplitude, with the O + fragments preferentially ejected to the stronger field side. The mechanism of the asymmetric bond breaking of the two equivalent C O bonds is discussed in terms of the nuclear dynamics in CO 2 2 + .

International audience; Short laser pulses are widely used for controlling molecular rotational degrees of freedom and inducing molecular alignment, orientation, unidirectional rotation and other types of coherent rotational motion. To follow the ultra-fast rotational dynamics in real time, several techniques for producing molecular movies have been proposed based on the Coulomb explosion of rotating molecules, or recovering molecular orientation from the angular distribution of high-harmonics. The present work offers and demonstrates a novel non-destructive optical method for direct visualization and recording of movies of coherent rotational dynamics in a molecular gas. The technique is based on imaging the time-dependent polarization dynamics of a probe light propagating through a gas of coherently rotating molecules. The probe pulse continues through a radial polarizer, and is then recorded by a camera. We illustrate the technique by implementing it with two examples of time-resolved rotational dynamics: alignment-anti-aligment cycles in a molecular gas excited by a single linearly polarized laser pulse, and unidirectional molecular rotation induced by a pulse with twisted polarization. This method may open new avenues in studies on fast chemical transformation phenomena and ultrafast molecular dynamics caused by strong laser fields of various complexities.

For ultra-high-intensity lasers irradiating nanometer-sized targets, Coulomb explosion (CE) is one of the main ion acceleration schemes. Previous studies have shown that the CE of solid nanospheres can produce quasi-monoenergetic ions. However, the development of optimized hollow nanospheres has yet to be achieved. Currently, the technology for the production of various types of hollow nanospheres has already been established. Still, the applications of hollow nanospheres are more inclined toward energy storage. This study shows that CE-based ion acceleration is another plausible application for these hollow nanospheres. Different nanosphere designs were analyzed using simple analytical models, which showed the possibility of using them to produce quasi-monoenergetic ions. This was then confirmed using one-dimensional particle–particle simulations in spherical coordinates. Overall, the results showed that hollow nanospheres are viable targets that can be used for the production of quasi-monoenergetic ions via spherical CE. Furthermore, the new proposed target design substantially improved the energy coupling efficiency.

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 corresponding covariance maps reveal the presence of a barrel-shaped structure (as in gas phase) but also other structures not present in the gas phase, most notably a linear chain structure.

We demonstrate the experimental realization of impulsive alignment of carbonyl sulfide (OCS) molecules at the Low Density Matter Beamline (LDM) at the free-electron laser FERMI. OCS molecules in a molecular beam were impulsively aligned using 200 fs pulses from a near-infrared laser. The alignment was probed through time-delayed ionization above the sulphur 2p edge, resulting in multiple ionization via Auger decay and subsequent Coulomb explosion of the molecules. The ionic fragments were collected using a time-of-flight mass spectrometer and the analysis of ion-ion covariance maps confirmed the correlation between fragments after Coulomb explosion. The analysis of the CO+ and S+ channels allowed us to extract the rotational dynamics, which is in agreement with our theoretical description as well as with previous experiments. This result opens the way for a new class of experiments at LDM within the field of coherent control of molecules with the possibilities that a precisely synchronized optical-pump XUV-probe laser setup like FERMI can offer., Comment: 7 pages, 6 figures

This chapter begins with a discussion of what constitutes a relativistic system at high energy density and how to produce such systems. The primary focus is on lasers intense enough to create relativistic electron motion. The chapter proceeds to discuss the motion of individual electrons in laser pulses and then the problem of producing the interaction of such laser pulses with sold targets , as opposed to with plasma blown off their surfaces. Topics discussed further related to laser irradiation of solids include absorption, harmonic generation, and induced transparency. The discussion then turns to particle acceleration. Wakefield acceleration of electrons is discussed at length. Ion acceleration by target sheaths, by laser pistons, and by Coulomb explosions are discussed next. After that, the chapter analyzes hole drilling by such lasers and the collisionless shocks that may result. The chapter ends with a brief review of several other phenomena, including magnetic-field generation, betatron X-ray production, positron production, nuclear reactions, and phenomena involving intense beams.

Enhanced acceleration of protons to high energy by relatively modest high power ultra-short laser pulses, interacting with snow micro-structured targets was recently proposed. A notably increased proton energy was attributed to a combination of several mechanisms such as localized enhancement of the laser field intensity near the tip of $1~\unicode[STIX]{x03BC}\text{m}$ size whisker and increase in the hot electron concentration near the tip. Moreover, the use of mass-limited target prevents undesirable spread of absorbed laser energy out of the interaction zone. With increasing laser intensity a Coulomb explosion of the positively charged whisker will occur. All these mechanisms are functions of the local density profile and strongly depend on the laser pre-pulse structure. To clarify the effect of the pre-pulse on the state of the snow micro-structured target at the time of interaction with the main pulse, we measured the optical damage threshold (ODT) of the snow targets. ODT of $0.4~\text{J}/\text{cm}^{2}$ was measured by irradiating snow micro-structured targets with 50 fs duration pulses of Ti:Sapphire laser.

We use a computational model to study the ablation mechanism of metal target irradiated by femtosecond pulse laser. It is confirmed that the Coulomb explosion can occur during femtosecond laser ablation of metal. The influence of thermal ablation and Coulomb explosion on the ablation depth is respectively investigated. Comparing the calculated results with the experimental ones, we find that the theoretical results which consider the thermal ablation only agree well with the experimental ones at high laser fluence, and those which take the Coulomb explosion into account fit well with the experimental ones at lower laser fluence, which exactly explains the ablation mechanism. In contrast with the previous theoretical results which only consider the thermal ablation, our theoretical simulation describes the ablation mechanism straightforward by making comparison of ablation depth, and provides a more reasonable explanation that fits with the actual ablation process.

We demonstrate that broadband impulsive stimulated Raman scattering with transient grating experimental geometry is capable of recording vibrational spectra in time and frequency domains to reveal the dissociation mechanism of liquid CH3I. The repulsive 3Q0 potential energy surface is modified to a quasi-bound surface in the liquid CH3I due to the solvent cage, making the wave packets to get trapped. A small portion of the wave packet passes through the solvent cage, resulting in the C I bond breakage with the product I2. This work provides a first glimpse of immense potential of BB-ISRS in studying photodissociation dynamics in liquids.

Fragment ions produced after the ionization of the acetic acid dimer (CH3COOH)2 in a near-infrared intense femtosecond laser field were measured using time-of-flight mass spectrometry. The two-body Coulomb explosion (CE) process (CH3COOH)22+ → 2CH3COOH+ with a kinetic energy release of 3.4 eV, similar to the case of (HCOOH)22+ (Hoshina et al., 2012), was identified. The two-body CE of (CH3COOH)22+ was induced when the laser polarization direction was perpendicular to the C⋯C intermolecular axis in contrast to the case of (HCOOH)22+. The preferential configurations may be those that deform dimer structures close to dimer dications during the double-ionization process.

We present a brief review of recent theoretical and numerical simulation results on the acceleration of ions from various targets irradiated by high-power femtosecond laser pulses. The results include: the optimization of the laser-plasma acceleration of ions over the thickness of a solid target; a new dependence of the energy of accelerated protons from a semi-transparent foil on the incident pulse energy; a theoretical model of plasma layer expansion in the vacuum for a fixed temperature of heated electrons, describing arbitrary regimes of particle acceleration, from the quasineutral flow of a plasma to Coulomb explosion; analytic theories of the relativistic Coulomb explosion of a spherical microtarget and the radial ponderomotive acceleration of ions from a laser channel in a transparent plasma; and calculations optimizing the production of isotopes for medicine using next-generation lasers.

Alkali metals can react explosively with water and it is textbook knowledge that this vigorous behaviour results from heat release, steam formation and ignition of the hydrogen gas that is produced. Here we suggest that the initial process enabling the alkali metal explosion in water is, however, of a completely different nature. High-speed camera imaging of liquid drops of a sodium/potassium alloy in water reveals submillisecond formation of metal spikes that protrude from the surface of the drop. Molecular dynamics simulations demonstrate that on immersion in water there is an almost immediate release of electrons from the metal surface. The system thus quickly reaches the Rayleigh instability limit, which leads to a ‘coulomb explosion’ of the alkali metal drop. Consequently, a new metal surface in contact with water is formed, which explains why the reaction does not become self-quenched by its products, but can rather lead to explosive behaviour. The explosion of alkali metals in water is a typical high-school chemistry experiment, but its mechanism is not fully understood. Using high-speed cameras and molecular simulations it is now shown that a key early step in this reaction is the migration of electrons from the alkali metal into water, leading to a charging of the metal's surface and subsequent Coulomb explosion.

The multi-photon dissociation and Coulomb explosion of ethyl bromide C2H5Br under near-infrared (800 nm) femtosecond laser field are experimentally investigated by a DC-sliced ion imaging technique. The sliced images of fragment ions C2H5+, Br+, CH3+, CH2Br+, H2+ and C2H3Br+ are obtained, and their dissociative pathways are assigned by observing their corresponding kinetic energy release (KER) and angular distribution. It is shown that low-KER components of these fragment ions result from multi-photon dissociation of singly charged parent ion C2H5Br+, while high-KER components come from Coulomb explosion of doubly charged parent ion C2H5Br2+. It is also shown that the precursor species [C2H5+…Br+] has a longer lifetime than [C2H3Br+…H2+] and [CH3…CH2Br+]. In addition, the probable H2 and H2+ elimination channels are theoretically simulated by Gaussian 09 software packages, and the results show that the former is an asynchronous process while the latter is a synchronous process.

The study of metallic clusters of Sodium (Na), containing 5.10 atoms per cluster, subjected to intense femtosecond laser fields showed that the irradiation of these clusters leads to high excitation energy that can be the source of highly energetic electrons and ions and fragmentation process. In this context and based on the nanoplasma model adapted to our study, we have studied the dependence on the femtosecond laser wave lengths and intensities the different parameters of the nanoplasma. It was found that the energy of the emitted electrons after explosion of the cluster is of the order of 93KeV. We have found also an equation which expresses the variation of the expansion velocity of the metallic cluster. It was found that the Coulomb pressure is smaller than the hydrodynamic pressure, and then the dynamic pressure is responsible of the dynamics of expansion and explosion of the cluster.