Coulomb Explosion Dynamics Research Articles

Distinguishing the XUV-induced Coulomb explosion dynamics of iodobenzene using covariance analysis

Abstract The primary and secondary fragmentation dynamics of iodobenzene following its ionization at 120 eV were determined using three-dimensional velocity map imaging and covariance analysis. Site-selective iodine 4d ionization was used to populate a range of excited polycationic parent states, which primarily broke apart at the carbon-iodine bond to produce I+ with phenyl or phenyl-like cations (CnHx + or CnHx 2+, with n = 1-6 and x = 1-5). The molecular products were produced with varying degrees of internal excitation and dehydrogenation, leading to stable and unstable outcomes. This further allowed the secondary dynamics of C6Hx 2+ intermediates to be distinguished using native-frame covariance analysis, which isolated these processes in their own centre-of-mass reference frames. The mass resolution of the imaging mass spectrometer used for these measurements enabled the primary and secondary reaction channels to be specified at the level of individual hydrogen atoms, demonstrating the ability of covariance analysis to comprehensively measure the competing fragmentation channels of aryl cations, including those involving intermediate steps.

Two-Dimensional Projected-Momentum Covariance Mapping for Coulomb Explosion Imaging.

We introduce projected-momentum covariance mapping, an extension of recoil-frame covariance mapping for 2D ion imaging studies. By considering the two-dimensional projection of the ion momenta as recorded by the detector, one opens the door to a complex suite of analysis tools adapted from three-dimensional momentum imaging studies. This includes the use of different frames of reference to unravel the dynamics of fragmentation and the application of fragment momentum constraints to isolate specific fragmentation channels. The technique is demonstrated on data from a two-dimensional ion imaging study of the Coulomb explosion of the cis and trans isomers of 1,2-dichloroethene, following strong-field ionization by an intense near-infrared femtosecond laser pulse. Classical simulations are used to guide the interpretation of projected-momentum covariance maps. The results offer a detailed insight into the distinct Coulomb explosion dynamics for this pair of isomers and lay the groundwork for future time-resolved studies of photoisomerization dynamics in this molecular system.

Dynamics of many-body fragmentation of carbon dioxide dimer tetravalent ions produced by intense femtosecond laser fields

We study experimentally the three-body Coulomb explosion dynamics of carbon dioxide dimer <inline-formula><tex-math id="M5">\begin{document}${\rm{(CO_2)}}_{2}^{4+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M5.png"/></alternatives></inline-formula> ions produced by intense femtosecond laser field. The three-dimensional momentum vectors as well as kinetic energy are measured for the correlated fragmental ions in a cold-target recoil-ion momentum spectrometer (COLTRIMS). Carbon dioxide dimer is produced during the supersonic expansion of <inline-formula><tex-math id="M6">\begin{document}${\rm{(CO_2)_2}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M6.png"/></alternatives></inline-formula> gas from a 30 μm nozzle with 10 bar backing pressure. The linearly polarized laser pulses with a pulse duration (full width at half maximum of the peak intensity) of 25 fs, a central wavelength of 790 nm, a repetition rate of 10 kHz, and peak laser intensities on the order of <inline-formula><tex-math id="M8">\begin{document}${\rm{8 \times10^{14}}}\;{\rm{W/cm^2}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M8.png"/></alternatives></inline-formula> are produced by a femtosecond Ti:sapphire multipass amplification system. We concentrate on the three-particle breakup channel <inline-formula><tex-math id="M10">\begin{document}${\rm{(CO_2)_2^{4+}}} \rightarrow {\rm{CO}}_{2}^{2+}+{\rm{CO^+}}+ {\rm{O^+}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M10.png"/></alternatives></inline-formula>. The two-particle breakup channels, <inline-formula><tex-math id="M15">\begin{document}${\rm{(CO_2)_2^{4+}}} \rightarrow {\rm{CO}}_{2}^{2+}+ {\rm{CO_{2}}^{2+}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M15.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M15.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M19">\begin{document}${\rm{CO_2^{2+}}\rightarrow CO^++O^+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M19.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M19.png"/></alternatives></inline-formula>, are selected as well for reference. The fragmental ions are guided by a homogenous electric field of 60 V/cm toward microchannel plates position-sensitive detector. The time of flight (TOF) and position of the fragmental ions are recorded to reconstruct their three-dimensional momenta. By designing some constraints to filter the experimental data, we select the data from different dissociative channels. The results demonstrate that the three-body Coulomb explosion of <inline-formula><tex-math id="M20">\begin{document}${\rm{(CO_2)}}_{2}^{4+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M20.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M20.png"/></alternatives></inline-formula> ions break into <inline-formula><tex-math id="M21">\begin{document}${\rm{CO}}_{2}^{2+}+{\rm{CO}}^++{\rm{O}}^+$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M21.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M21.png"/></alternatives></inline-formula> through two mechanisms: sequential fragmentation and non-sequential fragmentation, in which the sequential fragmentation channel is dominant. These three fragmental ions are produced almost instantaneously in a single dynamic process for the non-sequential fragmentation channel but stepwise for the sequential fragmentation. In the first step, the weak van der Waals bond breaks, <inline-formula><tex-math id="M22">\begin{document}${\rm{(CO_2)}}_{2}^{4+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M22.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M22.png"/></alternatives></inline-formula> dissociates into two <inline-formula><tex-math id="M23">\begin{document}${\rm{CO}}_{2}^{2+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M23.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M23.png"/></alternatives></inline-formula> ions; and then one of the C=O covalent bonds of <inline-formula><tex-math id="M24">\begin{document}${\rm{CO}}_{2}^{2+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M24.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M24.png"/></alternatives></inline-formula> breaks up, the <inline-formula><tex-math id="M25">\begin{document}${\rm{CO}}_{2}^{2+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M25.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M25.png"/></alternatives></inline-formula> ion breaks into <inline-formula><tex-math id="M26">\begin{document}${\rm{CO^+}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M26.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M26.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M27">\begin{document}${\rm{O^+}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M27.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M27.png"/></alternatives></inline-formula>. The time interval between the two steps is longer than the rotational period of the intermediate <inline-formula><tex-math id="M28">\begin{document}${\rm{CO}}_{2}^{2+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M28.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M28.png"/></alternatives></inline-formula> ions, which is demonstrated by the circle structure exhibited in the Newton diagram. We find that the sequential fragmentation channel plays a dominant role in the three-body Coulomb explosion of <inline-formula><tex-math id="M29">\begin{document}${\rm{(CO_2)}}_{2}^{4+}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M29.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20230699_M29.png"/></alternatives></inline-formula> ions in comparison of the event ratio of the two fragmentation channels.

General formulation of Coulomb explosion dynamics of highly symmetric charge distributions

We present a theoretical approach to study the dynamics of spherical, cylindrical and ellipsoidal charge distributions under their self-Coulomb field and a stochastic force due to collisions and random motions of charged particles. The approach is based on finding the current density of the charge distribution from the charge-current continuity equation and determining the drift velocities of the particles. The latter can be used either to derive the Lagrangian of the system, or to write Newton’s equation of motion with the Lorentz force. We develop a kinetic theory to include the stochastic force due to random motions of electrons in our model. To demonstrate the efficacy of our method, we apply it to various charge distributions and compare our results to N-body simulations. We show that our method reproduces the well-known emittance term in the envelope equation of uniform spherical and cylindrical charge distributions with correct coefficients. We use our model for the gravitational collapse of an ideal gas as well as the cyclotron dynamics of a cylindrical charge distribution in a uniform magnetic field and propose a method to measure the emittance of electron beams.

Coulomb Explosion Dynamics of Chlorocarbonylsulfenyl Chloride.

The Coulomb explosion dynamics following strong field ionization of chlorocarbonylsulfenyl chloride was studied using multimass coincidence detection and covariance imaging analysis, supported by density functional theory calculations. These results show evidence of multiple dissociation channels from various charge states. Double ionization to low-lying electronic states leads to a dominant C-S cleavage channel, while higher states can alternatively correlate to the loss of Cl+. Triple ionization leads to a double dissociation channel, the observation of which is confirmed via three-body covariance analysis, while further ionization leads primarily to atomic or diatomic fragments whose relative momenta depend strongly on the starting structure of the molecule.

Imaging multiphoton ionization dynamics of CH3I at a high repetition rate XUV free-electron laser

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

Time-resolved dissociative ionization and double photoionization of CO2.

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

Coulomb explosion imaging for gas-phase molecular structure determination: An ab initio trajectory simulation study.

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

Four-body Coulomb explosion of acetylene in collision with highly charged ions

The dynamics of four-body Coulomb explosion of acetylene in collision with 50 keV/u ${\mathrm{Ne}}^{8+}$ are investigated by detecting all four ionic fragments and the scattered projectile in quintuple coincidence. Six different breakup channels of ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{q+}$ cations with $4\ensuremath{\leqslant}q\ensuremath{\leqslant}8$ are identified. The kinetic energy (KE) distributions of all ionic fragments as well as the total kinetic energy release (KER) distributions are obtained and compared with the calculation results of the point-charge (PC) approximation model for each channel. For symmetric charge distribution channels, the calculations of PC model, in general, agrees well with the peak positions of the measured KER and KE distributions for all three channels with the largest deviation appearing in the breakup of ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{4+}$. However, for asymmetric charge distribution channels, although the KER values calculated by the PC model for the breakup of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}^{5+}$ and ${\mathrm{C}}_{2}{\mathrm{H}}_{2}^{7+}$ fit very well with experiment, the KEs of the two protons and the two C cations exhibit noticeable discrepancy between experiment and PC model. In addition, it is found that the fragmentation of ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{6+}$ which may lead to either two ${\mathrm{C}}^{2+}$ cations or one ${\mathrm{C}}^{+}$ and one ${\mathrm{C}}^{3+}$ cation shows a strong preference for symmetric charge distribution between the two C cations.

N-NO & NN-O bond cleavage dynamics in two- and three-body Coulomb explosion of the N2O2+ dication.

Triatomic Coulomb explosion dynamics are initiated by single-photon double ionization of N2O with an ultrafast EUV pulse and are probed by delayed near-IR pulses. The triatomic benchmark system exhibits competing two- and three-body dissociation dynamics that are reflected in the time resolved branching ratios and in the co-linear three-body momentum correlation spectra. Both the N-NO and the NN-O bond dissociation channels result in vibrationally excited molecular products. Channel resolved kinetic energy release (KER) spectra exhibit shifts emerging at long probe delays of hundreds of femtoseconds. The asymptotic shifts, towards lower KER indicate that the long-range Coulomb repulsion is effectively screened at bond-distances above ∼16 Å, at which the Rydberg electron is localized on one of the dissociating fragments. Thus, revealing up to a 0.9 eV gap that develops between the molecular Rydberg ion state and its core at long bond distance.

Ultrafast proton migration and Coulomb explosion of methyl chloride in intense laser fields

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

Study on Coulomb explosions of ion mixtures

The paper presents a theoretical work on the dynamics of Coulomb explosion for spherical nanoplasmas composed by two different ion species. Particular attention has been dedicated to study the energy spectra of the ions with the larger charge-to-mass ratio. The connection between the formation of shock shells and the energy spread of the ions has been the object of a detailed analysis, showing that under particular conditions the width of the asymptotic energy spectrum tends to become very narrow, which leads to a multi-valued ion phase space. The conditions to generate a quasi-monoenergetic ion spectrum have been rigorously demonstrated and verified by numerical simulations using a technique that, exploiting the spherical symmetry of the problem, allows one to obtain very accurate and precise results.

Complete dissociation branching fractions and Coulomb explosion dynamics of SO2 induced by excitation of O 1s pre-edge resonances.

Fragmentation processes of SO2 following excitation of the six main O 1s pre-edge resonances, as well as above the ionization threshold and below the resonances, are studied using a position-sensitive time-of-flight ion imaging detector, and the associated dissociation branching ratios and break-up dynamics are determined. In order to distinguish between the O(+) and S(2+) fragments of equal mass-to-charge ratio, the measurements have been performed with the isotopically enriched S(18)O2 sample. By analysis of the complete set of the fragment momentum vectors, the β values for the fragments originating from the SO(+) + O(+) break-up and the kinetic energy release for fragmentation channels of both SO2 (2+) and SO2 (3+) parent ions are determined. We also present results on the three-body break-up dynamics.

Comparing Coulomb explosion dynamics of multiply charged triatomic molecules after ionization by highly charged ion impact and few cycle femtosecond laser pulses

Recent experiments using highly charged ions (HCI) at Tokyo Metropolitan University and few cycle laser pulses at the advanced laser light source have centered on multiply ionizing carbonyl sulfide to form charge states from 3 + to 7 + . By measuring the kinetic energy release during subsequent break up and comparing with previous results from HCI impact on CO2 we can see a pattern emerging which implies that shorter laser pulses than the current sub 7 fs standard could lead to higher kinetic energy release than expected from Coulomb explosion.

Charge migration and decay of doubly charged ammonia clusters

The photo-fragmentation of ammonia molecular clusters ionized with soft x-rays is studied for photon energies near the N-1s threshold. The fragmentation is studied with a 3D momentum mass spectrometer to access the energy and angular correlations between fragments. By choosing the cluster sizes below or above the critical size of stable dication, we investigate the coulomb explosion dynamics and the interplay between charge delocalization and mobility of molecules in the clusters.

Coulomb explosion dynamics of triatomic molecules in laser pulses ranging from 7 to 200fs

Femtosecond laser pulses from 7fs to 200fs have been used to explore the ionization process and dissociation dynamics of triatomic molecules. Time and position sensitive detection allows us to detect each fragment ion in coincidence. We observe and characterize which ionization channels are Coulombic and which are non-Coulombic, and we observe concerted and stepwise processes which involve metastable fragment ions.

Influence of a strong laser field on the Coulomb explosion and the stopping power of fast C60clusters in plasmas

The influence of a high-intensity laser field on the Coulomb explosion and stopping power for a swift C${}_{60}$ cluster ion in a plasma target is studied by means of the molecular dynamics method based on the linearized Vlasov-Poisson theory. In the presence of the laser field, the general expressions for the induced potential in the target among the ions within the cluster are derived. Based on the numerical solution of the equations of motion for the constituent ions, the Coulomb explosion patterns and the cluster's stopping power are discussed for a range of laser and plasma parameters. Numerical results show that the laser field affects the correlation between the ions and contributes to weakening the wake effect and the stopping power as compared to the laser-free case. On the other hand, the vicinage effects in the cluster Coulomb explosion dynamics and the stopping power are strongly affected by the variations in the laser parameters, cluster speed, and plasma parameters. For example, Coulomb explosions are found to be hindered in the longitudinal expansion with the increasing of laser intensity and laser angle, and the Coulomb explosions proceed faster for lower plasma densities and higher electron temperatures.

First-principles molecular-dynamics simulation of biphenyl under strong laser pulses by time-dependent density-functional theory

The femtosecond laser reaction dynamics of the 3,5-difluoro-3\ensuremath{'},5\ensuremath{'}-dibromo-biphenyl (DFDBrBPh) molecule is investigated using time-dependent density-functional theory combined with molecular-dynamics (TDDFT-MD) simulation. This work is based on a recent experiment that monitored torsional motion of the DFDBrBPh molecule by femtosecond time-resolved Coulomb explosion imaging [Madsen et al., Phys. Rev. Lett. 102, 073007 (2009)]. The results confirm that the probe pulse triggers a Coulomb explosion and the kick pulse induces the torsional motion of two phenyl rings, using the experimental settings of the lasers. The Coulomb explosion dynamics simulation verifies that the F and Br atoms dissociate to the ion detector while maintaining their initial alignment with respect to the phenyl rings, which is the fundamental basis of Coulomb explosion imaging of molecular torsion. Furthermore, the period and amplitude of the torsional motion obtained by the simulation are consistent with the experimental values. This validates the ability of the TDDFT-MD method to reveal the underlying mechanism of experimentally observed molecular torsional dynamics.

Dynamics of Coulomb explosion of Xe clusters in an ultrafast high-intensity laser field

Under the classical particle-dynamics simulation, Coulomb explosion of small Xe clusters in an intense femtosecond laser field and energies of the generated particles are studied. Our results show that the average kinetic energy of the generated ions increases with the cluster size. The dependence of the average kinetic energy of the product ions on the number of atoms inside Xe clusters and the change of average ions charge states from different shells of Xe561 within the interactional time are studied. The oscillation of the mass center of electrons along the direction of the laser electric field is calculated and the time evolutions of the total potential energy and the total kinetic energy of all the product particles are presented here. The total kinetic energy of Xe ions and free electrons are also calculated as functions of the interactional time.

Influence of laser frequency chirp on deuteron energy from laser-driven deuterated methane cluster expansion

The simulations of three-dimensional particle dynamics are carried out to investigate the Coulomb explosion dynamics of deuterated methane clusters under the irradiation of an ultrashort intense laser pulse. The final kinetic energy of deuterons produced from the cluster explosion is calculated as a function of the pulse width, the laser intensity and the pulse chirp. It is found that the deuteron energy obtained in an intense laser pulse with negative chirp is higher than that with positive chirp, which agrees qualitatively with the experimental results reported by Fukuda et al. [Y. Fukuda et al., Phys. Rev. A 67, 061201 (2003)].