Coulomb Explosion Research Articles

Electronic relaxation and dissociation dynamics in formaldehyde: pump wavelength dependence.

The effect of the incident UV pump wavelength on the subsequent excited state dynamics, electronic relaxation, and ultimate dissociation of formaldehyde is studied using first principles simulation and Coulomb explosion imaging (CEI) experiments. Transitions in a vibronic progression in the à ← X̃ absorption band are systematically prepared using a tunable UV source which generates pulses centered at 304, 314, 329, and 337 nm. We find, both via ab initio simulation and experimental results, that the rate of excited state decay and subsequent dissociation displays a prominent dependence on which vibronic transition in the absorption band is prepared by the pump. Our simulations predict that nonadiabatic transition rates and dissociation yields will increase by a factor of >100 as the pump wavelength is decreased from 337 to 304 nm. The experimental results and theoretical simulations are in broad agreement and both indicate that the dissociation rate plateaus rapidly after ≈2 ps following an ultrafast sub-ps rise.

Ultrafast energy transfer between π-stacked aromatic rings upon inner-valence ionization.

Non-covalently bound aromatic systems are ubiquitous and govern the physicochemical properties of various organic materials. They are important to many phenomena of biological and technological relevance, such as protein folding, base-pair stacking in nucleic acids, molecular recognition and self-assembly, DNA-drug interactions, crystal engineering and organic electronics. Nevertheless, their molecular dynamics and chemical reactivity, particularly in electronic excited states, are not fully understood. Here, we observe intermolecular Coulombic decay in benzene dimers, (C6H6)2-the simplest prototypes of noncovalent π-π interactions between aromatic systems. Intermolecular Coulombic decay is initiated by a carbon 2s vacancy state produced by electron-impact ionization and proceeds through ultrafast energy transfer between the benzene molecules. As a result, the dimer relaxes with the emission of a further low-energy electron (<10 eV) and a pair of C6H6+ cations undergoing Coulomb explosion. Coincident fragment-ion and electron momentum spectroscopy, accompanied by ab initio calculations, enables us to elucidate the dynamical details of this ultrafast relaxation process.

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.

Experiment and theory elucidate the pathways for H3+formation in the ultrafast double ionization in methanol

Experiment and theory elucidate the pathways for H₃⁺formation in the ultrafast double ionization in methanol

Enhanced quasi-monoenergetic ions generation: Based on gold nanoparticles application in gas-filled nanosphere targets

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.

Determining the stereo configuration of carbonyl sulfide dimers using Coulomb-explosion imaging

A four-body coincident measurement of carbonyl sulfide dimers is conducted using a femtosecond laser-induced Coulomb explosion. Sequential and concerted processes that occur during the Coulomb explosion are distinguished. Four isomers of carbonyl sulfide dimer are resolved, and their structures are retrieved from the measured coincident momenta of the fragment ions generated in the concerted process, with the help of a classical trajectory simulation. As a result of our study, we can resolve the isomers of a weakly bound molecular cluster and determine their stereo configuration by using the Coulomb explosion. There is also the potential in femtosecond time-resolved imaging for structural evolution of molecular complexes in dissociation.

Nonadiabatic Coupling Effects in the 800 nm Strong-Field Ionization-Induced Coulomb Explosion of Methyl Iodide Revealed by Multimass Velocity Map Imaging and Ab Initio Simulation Studies.

The Coulomb explosion (CE) of jet-cooled CH3I molecules using ultrashort (40 fs), nonresonant 805 nm strong-field ionization at three peak intensities (260, 650, and 1300 TW cm-2) has been investigated by multimass velocity map imaging, revealing an array of discernible fragment ions, that is, Iq+ (q ≤ 6), CHn+ (n = 0-3), CHn2+ (n = 0, 2), C3+, H+, H2+, and H3+. Complementary ab initio trajectory calculations of the CE of CH3IZ+ cations with Z ≤ 14 identify a range of behaviors. The CE of parent cations with Z = 2 and 3 can be well-described using a diatomic-like representation (as found previously) but the CE dynamics of all higher CH3IZ+ cations require a multidimensional description. The ab initio predicted Iq+ (q ≥ 3) fragment ion velocities are all at the high end of the velocity distributions measured for the corresponding Iq+ products. These mismatches are proposed as providing some of the clearest insights yet into the roles of nonadiabatic effects (and intramolecular charge transfer) in the CE of highly charged molecular cations.

Wavelength dependence of the angular distribution of the Coulomb explosion in the femtosecond ionisation of methyl iodide

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.

Coulomb explosion dynamics of methoxycarbonylsulfenyl chloride by 3D multimass imaging

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.

Charging and ion ejection dynamics of large helium nanodroplets exposed to intense femtosecond soft X-ray pulses

Ion ejection from charged helium nanodroplets exposed to intense femtosecond soft X-ray pulses is studied by single-pulse ion time-of-flight (TOF) spectroscopy in coincidence with small-angle X-ray scattering. Scattering images encode the droplet size and absolute photon flux incident on each droplet, while ion TOF spectra are used to determine the maximum ion kinetic energy, \(E_{\text {kin}}\), of \(\hbox {He}_{j}^{+}\) fragments (j = 1–4). Measurements span \(\hbox {He}_N\) droplet sizes between \(N\sim 10^{7}\) and \(\sim 10^{10}\) (radii \(R_0\) = 78–578 nm), and droplet charges between \(\sim 9\times 10^{-5}\) and \(\sim 3\times 10^{-3}\) e/atom. Conditions encompass a wide range of ionization and expansion regimes, from departure of all photoelectrons from the droplet, leading to pure Coulomb explosion, to substantial electron trapping by the electrostatic potential of the charged droplet, indicating the onset of hydrodynamic expansion. The unique combination of absolute X-ray intensities, droplet sizes, and ion \(E_{\text {kin}}\) on an event-by-event basis reveals a detailed picture of the correlations between the ionization conditions and the ejection dynamics of the ionic fragments. The maximum \(E_{\text {kin}}\) of He\(^{+}\) is found to be governed by Coulomb repulsion from unscreened cations across all expansion regimes. The impact of ion-atom interactions resulting from the relatively low charge densities is increasingly relevant with less electron trapping. The findings are consistent with the emergence of a charged spherical shell around a quasineutral plasma core as the degree of ionization increases. The results demonstrate a complex relationship between measured ion \(E_{\text {kin}}\) and droplet ionization conditions that can only be disentangled through the use of coincident single-pulse TOF and scattering data.

Statistical analysis of correlations in the x-ray induced Coulomb explosion of iodopyridine

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

Laser-induced Coulomb explosion imaging of (C6H5Br)2 and C6H5Br–I2 dimers in helium nanodroplets using a Tpx3Cam

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

Revisiting neutron yield in table-top nuclear fusion driven by an intense femtosecond laser pulse interaction with the gas clusters

With the development of femtosecond (fs) laser technology, an fs laser pulse with 10s of Joule, even 100s of Joule energy is available and the focused laser intensity can be expected to induce the pure Coulomb explosion of the cluster with a much larger average radius than before. Meanwhile, the production of gas cluster with an average radius of upto 10s of nanometer has been possible. In this case, it is necessary to reinvestigate the feasibility of 109 n/shot neutron yield for the practical application in the intense fs laser-driven nuclear fusion. In this work, the neutron yield from the explosions of the D2 clusters of 6–20 nm average radius at the 0.5–100 J pulse energy and the CD4 clusters was investigated theoretically. It is found that the optimum neutron yield of 109 n/shot can be obtained at the laser energy and the cluster radius currently available. However, a clustered-gas jet with a large cross-section is demanded to match the proper plasma diameter.

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.

Photofragmentation of large dimers into singly charged ions via the knockout mechanism

We study the fragmentation of very large dimers into two singly charged ions caused by absorption of a photon. In this process a photo electron emitted from one atom of the dimer has certain chances to hit the other atom knocking out one of its electrons. This results in the production of two singly charged ions and the consequent Coulomb explosion of the residual doubly charged system. We develop a theory of this process and apply it to calculate the fragmentation of $^4$He$_2$, $^7$Li - $^4$He and $^6$Li - $^4$He dimers. Our results for the helium dimer are in good agreement with available experimental data. For the Li-He dimers (where experimental data are still absent) our results predict that this fragmentation mechanism becomes much more efficient than in the case of He$_2$. We also show that, provided the recoil effects are of minor importance, the so called reflection approximation is very accurate also for large dimers.

Smallest Organic Tetracation in the Gas Phase: Stability of Multiply Charged Diiodoacetylene Produced in Intense Femtosecond Laser Fields.

Coulomb explosion imaging, which is the reconstruction of a molecular structure by measuring the three-dimensional momenta of atomic ions formed by a Coulomb explosion of multiply charged molecular cations (MMCs), has been utilized widely. In contrast, intact MMCs, whose properties and reactions are interesting from both fundamental and applied scientific perspectives, themselves have been little explored to date. This study demonstrates that the four-atom molecule diiodoacetylene (DIA) can survive as a long-lived species in the gas phase after the removal of four electrons in intense femtosecond laser fields. The electron configurations of the equilibrium structures of the electronic ground states calculated by the complete active space self-consistent field (CASSCF) method reveal the stability of multiply charged DIA. The dissociation energies are estimated to be 3.01, 3.59, 2.57, 1.82, and 1.61 eV for neutral, cation radical, dication, trication radical, and tetracation, respectively. A fairly deep potential well suggests that a DIA tetracation is metastable toward dissociation, whereas the repulsive potential of a pentacation radical confirms its absence in the mass spectrum. With their sufficiently long lifetimes, minimum number of atoms, and simple dissociation paths, DIA MMCs are promising candidates for further experimental and theoretical investigations of multiply charged ion chemistry.

Investigation of the size of nanoparticles formed during femto- and nanosecond laser ablation of zircon

Laser ablation of zircon can be used to analyze its composition for geological history. However, the effect of laser properties on nanoparticle size has not been studied extensively. The effect of laser fluence and pulse duration on the diameter of zircon nanoparticles was analyzed using field-emission scanning electron microscopy and energy dispersive spectroscopy. The results showed that the diameters of the zircon nanoparticles induced by a femtosecond laser increased with increasing laser fluence, and that these particles were smaller than those induced by a nanosecond laser with the same laser fluence. Furthermore, the mechanism of zircon nanoparticle formation induced by laser ablation has been discussed. The explosion mechanism is the primary mechanism of nanoparticle generation. In particular, the zircon nanoparticles induced by the femtosecond laser were the result of Coulomb explosion, while phase explosion contributed to the zircon nanoparticles induced by the nanosecond laser. Therefore, the nature of zircon nanoparticles induced by laser ablation is mainly determined by the pulse duration.

Dissociation dynamics in chloroform molecule induced by ion impact

The ion impact multiple ionization and subsequent dissociation of Chloroform (CHCl3) is investigated with Ar7+ ion beam having an energy of 1 MeV. Using a linear time of flight mass spectrometer coupled with a position sensitive detector, complete as well as incomplete Coulomb explosion pathways for CHCl32+ and CHCl33+ ions are identified and studied. Possible modes of fragmentation pathway for all the identified channels are investigated with the help of kinetic energy release and momentum distributions of fragmented ions. Although concerted fragmentation is the most dominant pathway within the identified channels, we have discussed the possibilities of sequential processes which may occur. A positive slope between the coincidence partners of H+ and C+ probably features sequentiality in the breakup dynamics but remain inconclusive due to lack of complete information. Formation of Cl2+ from the doubly charged CHCl32+ is a direct evidence of bond rearrangement reaction following a structural change during charge separation.

Suppression of X-ray-Induced Radiation Damage to Biomolecules in Aqueous Environments by Immediate Intermolecular Decay of Inner-Shell Vacancies.

The predominant reason for the damaging power of high-energy radiation is multiple ionization of a molecule, either direct or via the decay of highly excited intermediates, as, e.g., in the case of X-ray irradiation. Consequently, the molecule is irreparably damaged by the subsequent fragmentation in a Coulomb explosion. In an aqueous environment, however, it has been observed that irradiated molecules may be saved from fragmentation presumably by charge and energy dissipation mechanisms. Here, we show that the protective effect of the environment sets in even earlier than hitherto expected, namely immediately after single inner-shell ionization. By combining coincidence measurements of the fragmentation of X-ray-irradiated microsolvated pyrimidine molecules with theoretical calculations, we identify direct intermolecular electronic decay as the protective mechanism, outrunning the usually dominant Auger decay. Our results demonstrate that such processes play a key role in charge delocalization and have to be considered in investigations and models on high-energy radiation damage in realistic environments.

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.