Coulomb Explosion Model Research Articles

Three-body fragmentation dynamics of BrCNq+ (q=3–6) induced by 1-keV electron impact

The three-body fragmentation dynamics of $\mathrm{BrC}{\mathrm{N}}^{q+} (q=3--6)$ produced by 1-keV electron impact has been investigated using the ion momentum imaging technique. Up to 11 three-body Coulomb explosion channels were identified and analyzed. The corresponding kinetic energy release (KER) distributions were obtained and compared with the predictions of the Coulomb explosion model. By means of the Dalitz plot, Newton diagram, and native frame method, we have studied the concerted and sequential fragmentation mechanisms for channels leading to ${\mathrm{Br}}^{l+}+{\mathrm{C}}^{+}+{\mathrm{N}}^{+} (l=1--3)$. The KER for the intermediate dications $\mathrm{Br}{\mathrm{C}}^{2+}$ and $\mathrm{C}{\mathrm{N}}^{2+}$ from the sequential mechanism were determined and the electronic states of the intermediate molecular ion $\mathrm{C}{\mathrm{N}}^{2+}$ were discussed. For the channels leading to higher charge states of the carbon and nitrogen ions, only the concerted fragmentation mechanism was observed.

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

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

Time-resolved Rayleigh scattering measurements of methane clusters for laser-cluster fusion experiments.

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.

Fragmentation of CF4q+ (q=2,3) induced by 1-keV electron collisions

We report on an investigation on the fragmentation dynamics of $\mathrm{C}{{\mathrm{F}}_{4}}^{q+}$ $(q=2,3)$ induced by 1-keV electron collisions utilizing an ion momentum imaging spectrometer. From the time-of-flight correlation maps five dominating dissociation channels of $\mathrm{C}{{\mathrm{F}}_{4}}^{2+}$ as well as one three-body fragmentation channel of $\mathrm{C}{{\mathrm{F}}_{4}}^{3+}$ are identified. The kinetic energy release (KER) distributions for these channels are obtained and compared with the data available in the literature. The Dalitz-like momentum diagram and the Newton diagram are employed to analyze the breakup mechanism in the three-body fragmentation channel. We found that, for $\mathrm{C}{{\mathrm{F}}_{4}}^{2+}$ dissociation into ${\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_{2}}^{+}+\mathrm{F}$, ${\mathrm{F}}^{+}+\mathrm{C}{\mathrm{F}}^{+}+2\mathrm{F}$, and ${\mathrm{F}}^{+}+{\mathrm{F}}^{+}+{\mathrm{CF}}_{2}$, the concerted breakup is the dominating process. Channel ${\mathrm{C}}^{+}+{\mathrm{F}}^{+}+3\mathrm{F}$ is dominated by the initial charge separation, i.e., $\mathrm{C}{{\mathrm{F}}_{4}}^{2+}\ensuremath{\rightarrow}{\mathrm{F}}^{+}+\mathrm{C}{\mathrm{F}}^{+}+2\mathrm{F}\ensuremath{\rightarrow}{\mathrm{C}}^{+}+{\mathrm{F}}^{+}+3\mathrm{F}$. With the help of the native frame method, we assigned one sequential pathway and two concerted pathways for channel $\mathrm{C}{{\mathrm{F}}_{4}}^{3+}\ensuremath{\rightarrow}{\mathrm{F}}^{+}+{\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_{2}}^{+}$. The branching ratios of these pathways are determined. The momentum correlation of the fragments and the deduced KER distribution indicate that different excited states of $\mathrm{C}{{\mathrm{F}}_{4}}^{3+}$ with different geometries are responsible for these three pathways. The Coulomb explosion model simulation shows that most of the events in this channel are produced by $\mathrm{C}{{\mathrm{F}}_{4}}^{3+}$ ions that have deformed geometries from the neutral ${\mathrm{CF}}_{4}$ molecule.

Single-shot imaging of surface molecular ionization in nanosystems

Abstract Using single-shot velocity map imaging technique, explosion imaging of different ion species ejected from 50 nm SiO2 nanoparticles are obtained excitedly by strong near-infrared and ultraviolet femtosecond laser fields. Characteristic momentum distributions showing forward emission of the ions at low excitation intensities and shock wave behaviors at high intensities are observed. When the excitation intensity is close to the dissociative ionization threshold of the surface molecules, the resulting ion products can be used to image the instant near-field distributions. The underlying dynamics of shock formation are simulated by using a Coulomb explosion model. Our results allow one to distinguish the ultrafast strong-field response of various molecular species in nanosystems and will open a new way for further exploration of the underlying dynamics of laser-and-nanoparticle interactions.

Effect of 100 MeV Ni ion irradiation on CdZnTe thin films

Modification of semiconductor thin films by Swift Heavy Ion (SHI) beam irradiation is a unique technique for property improvement. In the present investigation, Cadmium Zinc Telluride (CZT) thin films of thickness 100 nm are deposited on glass substrates by the thermal evaporation method. These films are irradiated with 100 MeV Ni ion beams at a fluence of 1×1012 ions/cm2 and 5×1012 ions/cm2. The SHIinduced surface modifications are studied by Atomic Force Microscopy (AFM). The structural properties are investigated using X-ray diffraction (XRD) technique. Optical characterization is carried out using UV–Vis spectroscopy. The SHI irradiation on CZT films results in grain fragmentation. Phase transformation and widening of bandgap is also observed after SHI irradiation. The SHIinduced property modifications are explained in the framework of ‘coulomb explosion and thermal spike model’.

Efficient generation of nitrogen vacancy centers by laser writing close to the diamond surface with a layer of silicon nanoballs

We proposed a method to effectively fabricate negatively charged nitrogen vacancy (NV−) centers close to the diamond surface by applying femtosecond laser writing technique. With a thick layer of silicon (Si) nanoballs coated, diamond surface was irradiated by high-fluence femtosecond laser pulses. A large number of NV− centers were created around the laser ablation crater area without thermal annealing. The distribution of the NV− centers was expanded to about 50 μm away from the crater center. To demonstrate the function of Si nanoballs, we performed the exactly same laser illumination process on the bare region of the sample surface. In this case, only a few NV− centers were generated around ablation crater. At distance of 32 μm away from crater centers, the NV− density for the case with nanoballs was up to 15.5 times higher compared to the case without nanoballs. Furthermore, we also investigated the influence of laser fluence and pulse number on the NV− density for the case with Si-nanoball layer. Finally, the formation mechanism of NV− centers and the role of Si nanoballs were explained via Coulomb explosion model. The method is demonstrated to be a promising approach to efficiently and rapidly fabricate NV− centers close to the surface of the diamond, which are significant in quantum sensing. Furthermore, the results provide deep insights into complex light-matter interactions.

Multi-particle momentum correlations extracted using covariance methods on multiple-ionization of diiodomethane molecules by soft-X-ray free-electron laser pulses.

Momenta of ions from diiodomethane molecules after multiple ionization by soft-X-ray free-electron-laser pulses are measured. Correlations between the ion momenta are extracted by covariance methods formulated for the use in multiparticle momentum-resolved ion time-of-flight spectroscopy. Femtosecond dynamics of the dissociating multiply charged diiodomethane cations is discussed and interpreted by using simulations based on a classical Coulomb explosion model including charge evolution.

Bright near-surface silicon vacancy centers in diamond fabricated by femtosecond laser ablation

We report the generation of single negatively charged silicon vacancy (SiV-) color centers by focusing a femtosecond (fs) laser on top of a high-purity diamond coated with a layer of Si nanoball. Under the interaction of a high-intensity fs laser, Si atoms were ionized and implanted into the diamond, accompanied with the creation of vacancies. After annealing at 850°C in vacuum for 1h, the photoluminescence spectra of bright spots around the created crater presented a typical strong zero-phonon line at around 737nm of SiV- centers. Bright single SiV- color centers could be observed with a maximum saturating counting rate of 300×103 counts/s. We explain the formation mechanism of SiV- centers in diamond via a Coulomb explosion model. The results demonstrate that fs laser ablation can be utilized as a very promising tool to conveniently fabricate single bright SiV- centers in diamond.

Electron-induced delayed desorption of solid argon doped with methane

The total yield of particles desorption from solid Ar doped with CH4 under irradiation with an electron beam was studied at 5 K. The measurements were carried out at a CH4 concentration of 1 and 5%. The effect of explosive delayed desorption from the surface of argon matrix was discovered in both mixtures. With a higher concentration of CH4, it appeared at lower doses and was more pronounced. Two types of self-oscillations were observed: long-period bursts (on a time scale of about 25 min) and short-period oscillations (of about 10 s). In pure solid Ar delayed desorption was not observed despite the accumulation of a significant number of excess electrons, exceeding their number in mixtures of Ar and CH4 as it was found by measurements of thermally stimulated exoelectron emission. This finding discards the model of Coulomb explosion for the phenomenon detected. In this paper we focused on the role of hydrogen (one of the radiolysis products) in delayed desorption. The formation of atomic hydrogen in the matrix was traced via cathodoluminescence by the emission band of the excimer Ar2H* at 166 nm. Desorption of excited hydrogen atoms in the excited state was detected by the Ly-α emission line. A decrease of the Ar2H* band intensity at higher concentration of CH4 was found evidencing bleaching these centers likely due to recombination of H atoms with energy release and formation of molecular hydrogen. The data obtained give additional evidence in favor of the hypothesis that the exothermic reactions of radiolysis products serve as a stimulating factor for delayed desorption.

High-energy 120 MeV Au9+ ion beam-induced modifications and evaluation of craters in surface morphology of SnO2 and TiO2 nanocomposite thin films

The electronic excitation and modification in the properties of the target material occur due to the kinetic energy loss of incident ions mainly through the inelastic collision with atomic electrons in the process of electronic energy stopping. Nanocomposite tin oxide (SnO2) and titanium dioxide (TiO2) thin film were grown on silicon and ITO substrates using RF magnetron-sputtering process. These thin films were irradiated by 120 MeV Au9+ Swift Heavy Ion (SHI) beam with varying fluence from 5 × 1011 ions cm−2 to 2 × 1013 ions cm−2 to induce modifications in thin films by dense electronic excitation. Surface morphology and depth profile of craters (approx. 85 nm) have been characterized by multimode atomic force microscopy (AFM). Grain mapping of AFM images provided the variation in grain size in the range of 72–279 nm and RMS roughness increased (4.01–23.6 nm) with increasing the ion fluence. Scanning Electron Microscopy (SEM) was also carried out to confirm the formation of craters in the irradiated film. X-ray diffraction (XRD) technique has been used for the structural analysis of virgin and irradiated thin films. XRD patterns established the formation of film in mixed (rutile and gamma) phase. Further, the crystallite size was evaluated using the Debye–Scherrer and Williamson–Hall method. It was found that crystallite size lie in the range of 36–54 nm which increased for the lower fluence and then decreased for higher fluence. The ion irradiation-induced strain has been explained by Thermal spike and Coulomb explosion model. UV–Vis study showed that the optical band gap reduced from 3.87 to 3.72 eV with increasing the ion fluence which can be ascribed to the formation of intermediate states and free radicals between the valance and band conduction band. Rutherford Backscattering Spectrometry (RBS) ensured that the film thickness decreased from 210 to 183 nm with increasing the ion fluence. The simulation of the RBS spectra provides the elemental composition of Sn (0.37), Ti (0.65) and O (0.85) in SnO2 and TiO2 nanocomposite thin films.

Fragmentation dynamics of CS2 in collisions with 1.0 keV electrons.

The dissociation dynamics of CS2 molecules in collisions with 1.0 keV electrons is studied. We observe a series of two- and three-body fragmentation channels which are identified from the correlation map between fragment ions. For all of the channels, the kinetic energy release (KER) distributions are obtained. The Dalitz plot and Newton diagram are adopted to analyze the fragmentation dynamics of the three-body dissociation channels. For and , both the concerted and sequential fragmentation mechanisms are observed where the concerted mechanism dominates. For , 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.

Sensitivity of Anatase and Rutile Phases of TiO2 to ion irradiation: Examination of the applicability of Coulomb Explosion and Thermal Spike Models

Sensitivity of the anatase and rutile phases of titanium dioxide to Swift Heavy Ion (SHI) irradiation was experimentally probed and compared with the predictions of the Coulomb explosion, analytical and inelastic thermal spike models of ion-matter interaction. Conforming to the predictions of all these models, our study indicated higher sensitivity of anatase to these ions than the rutile phase. A detailed examination however revealed that Coulomb explosion model cannot explain either the nature of variation of the interaction cross section of SHI with the energy deposited by these ions, Se to the target electrons, or the relative values of the threshold electronic energy loss, Seth of anatase and rutile. The analytical thermal spike (a-TS) model, using the available physicochemical data for this oxide, predicted that tracks cannot form either in anatase or in rutile by 297 MeV and 511 MeV Ni ions, while inelastic thermal spike (i-TS) model predicted formation of ion tracks by 297 MeV Ni ions and their absence with 511 MeV Ni ions in both anatase and rutile. Our observation agreed with the predictions of i-TS model albeit with a difference in the radius of the tracks. In addition, we observed halo of defect ridden crystalline region of much larger radius around the ion track. Interestingly, the radius of the halo scales with the velocity of the ions, which is opposite to the conventionally observed velocity effect.

Coincidence momentum imaging of four- and three-body Coulomb explosion of formaldehyde in ultrashort intense laser fields

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.

Overrun phenomenon and neutron yield in Coulomb explosion of deuterated alkane clusters driven by intense laser field**Project supported by the National Natural Science Foundation of China (Grant No. 11005080).

By using a simplified Coulomb explosion model, the laser-driven Coulomb explosion processes of three deuterated alkane clusters, i.e., deuterated methane (CD4)N, ethane (C2D6)N and propane (C3D8)N clusters are simulated numerically. The overrun phenomenon that the deuterons overtake the carbon ions inside the expanding clusters, as well as the dependence of the energetic deuterons and fusion neutron yield on cluster size, is discussed in detail. Researches show that the average kinetic energy of deuterons and neutron yield generated in the Coulomb explosion of (C2D6)N cluster are higher than those of (CD4)N cluster with the same size, in qualitative agreement with the reported conclusions from the experiments of (C2H6)N and (CH4)N clusters. It is indicated that (C2D6)N clusters are superior to (CD4)N clusters as a target for the laser-induced nuclear fusion reaction to achieve a higher neutron yield. In addition, by comparing the relevant data of (C3D8)N cluster with those of (C2D6)N cluster with the same size, it is theoretically concluded that (C3D8)N clusters with a larger competitive parameter might be a potential candidate for improving neutron generation. This will provide a theoretical basis for target selection in developing experimental schemes on laser-driven nuclear fusion in the future.

Water fragmentation induced by ion impact: Fragment-ion-energy determination at different ZP/v regimes

This work focuses on the energy distributions of positive water ionic fragments produced by ion impact at MeV impact energies. An improved Coulomb explosion model coupled to a classical trajectory Monte Carlo simulation is used to provide energy centroids of the fragments for the dissociation channels resulting from the removal of two to five electrons from the water molecule. This model explicitly includes the post-collisional interaction of the projectile with the resulting ionic fragments affecting their kinetic energy release spectra especially at low impact energies. Theoretical data are benchmarked against recent data collected for 220keV Xe22++H2O collisions which corresponds to a large ZP/v collision parameter. To extend our tests to the low ZP/v regime, fragment species as a function of emission energy and time-of-flight were recorded in 3MeV Li3+ collisions by using an electrostatic spectrometer and a time-of-flight mass spectrometer, respectively. Present experimental data reveals the existence of multiple-ionization processes leading to charge state up to 4+.

Identification of absolute geometries of cis and trans molecular isomers by Coulomb Explosion Imaging.

An experimental route to identify and separate geometric isomers by means of coincident Coulomb explosion imaging is presented, allowing isomer-resolved photoionization studies on isomerically mixed samples. We demonstrate the technique on cis/trans 1,2-dibromoethene (C2H2Br2). The momentum correlation between the bromine ions in a three-body fragmentation process induced by bromine 3d inner-shell photoionization is used to identify the cis and trans structures of the isomers. The experimentally determined momentum correlations and the isomer-resolved fragment-ion kinetic energies are matched closely by a classical Coulomb explosion model.

Anomalous effect of ion velocity on track formation in GeS

Systematic experiments were performed for studying the effect of the projectile velocity (velocity effect, VE) in GeS which has a highly anisotropic conductivity. The prethinned specimens were irradiated by Bi, Au, W, Xe, Ag, Kr, Ni and Fe ions of about E≈1MeV/nucleon energy. Track radii were measured by transmission electron microscopy. Compared to previous experiments performed with high velocity projectile, there is a marked VE for Se>20keV/nm (Se – electronic stopping power). However, the VE is gradually reduced and finally disappears as Se decreases. This effect is described for the first time. The predictions according to the Analytical Thermal Spike Model are in excellent quantitative agreement with the experiments in the range Se>20keV/nm. The anomalous behavior of track evolution at lower values of Se is attributed to the combination of semiconducting and insulating properties. An explanation of the VE is given based on the Coulomb explosion model.

Water fragmentation by bare and dressed light ions with MeV energies: Fragment-ion-energy and time-of-flight distributions

The energy and time-of-flight distributions of water ionic fragments produced by impact of fast atoms and bare and dressed ions; namely, ${\mathrm{H}}^{+}, {\mathrm{Li}}^{0--3+}, {\mathrm{C}}^{1+}$, and ${\mathrm{C}}^{2+}$ are reported in this work. Fragment species as a function of emission energy and time-of-flight were recorded by using an electrostatic spectrometer and a time-of-flight mass spectrometer, respectively. An improved Coulomb explosion model coupled to a classical trajectory Monte Carlo (CTMC) simulation gave the energy centroids of the fragments for the dissociation channels resulting from the removal of two to five electrons from the water molecule. For the energy distribution ranging up to 50 eV, both the experiment and model reveal an isotropic production of multiple charged oxygen ions, as well as hydrogen ions. From the ion energy distribution, relative yields of the dissociation resulting from multiple ionization were obtained as a function of the charge state, as well as for several projectile energies. Multiple-ionization yields with charge state up to 4+, were extracted from the measurements of the time-of-flight spectra, focused on the production of single and multiple charged oxygen ions. The measurements were compared to ion-water collision experiments investigated at the keV energy range available in the literature, revealing differences and similarities in the fragment-ion energy distribution.

Femtosecond charge and molecular dynamics of I-containing organic molecules induced by intense X-ray free-electron laser pulses.

We studied the electronic and nuclear dynamics of I-containing organic molecules induced by intense hard X-ray pulses at the XFEL facility SACLA in Japan. The interaction with the intense XFEL pulse causes absorption of multiple X-ray photons by the iodine atom, which results in the creation of many electronic vacancies (positive charges) via the sequential electronic relaxation in the iodine, followed by intramolecular charge redistribution. In a previous study we investigated the subsequent fragmentation by Coulomb explosion of the simplest I-substituted hydrocarbon, iodomethane (CH3I). We carried out three-dimensional momentum correlation measurements of the atomic ions created via Coulomb explosion of the molecule and found that a classical Coulomb explosion model including charge evolution (CCE-CE model), which accounts for the concerted dynamics of nuclear motion and charge creation/charge redistribution, reproduces well the observed momentum correlation maps of fragment ions emitted after XFEL irradiation. Then we extended the study to 5-iodouracil (C4H3IN2O2, 5-IU), which is a more complex molecule of biological relevance, and confirmed that, in both CH3I and 5-IU, the charge build-up takes about 10 fs, while the charge is redistributed among atoms within only a few fs. We also adopted a self-consistent charge density-functional based tight-binding (SCC-DFTB) method to treat the fragmentations of highly charged 5-IU ions created by XFEL pulses. Our SCC-DFTB modeling reproduces well the experimental and CCE-CE results. We have also investigated the influence of the nuclear dynamics on the charge redistribution (charge transfer) using nonadiabatic quantum-mechanical molecular dynamics (NAQMD) simulation. The time scale of the charge transfer from the iodine atomic site to the uracil ring induced by nuclear motion turned out to be only ∼5 fs, indicating that, besides the molecular Auger decay in which molecular orbitals delocalized over the iodine site and the uracil ring are involved, the nuclear dynamics also play a role for ultrafast charge redistribution. The present study illustrates that the CCE-CE model as well as the SCC-DFTB method can be used for reconstructing the positions of atoms in motion, in combination with the momentum correlation measurement of the atomic ions created via XFEL-induced Coulomb explosion of molecules.