Coulomb Explosion Research Articles

Dust Destruction by Drift-induced Sputtering in Active Galactic Nuclei

Abstract Recent mid-infrared high spatial resolution observations have revealed that active galactic nuclei (AGNs) may host a polar dust region with the size of several pc, and such dust may be carried by radiation from the central engine. The polar dust emission often exhibits very weak or absence of the silicate 10 μm emission feature. A possible explanation is that the polar dust is dominated by micron-sized large grains because these grains do not show the silicate feature, while it remains unclear how large grains are preferentially supplied to the polar region. Here, we propose a new scenario describing the prevalence of large grains at the polar region. We show that grains are accelerated to the hypersonic drift velocity by the radiation pressure from AGN, and the hypersonic drift results in dust destruction via kinetic sputtering. Sputtering destroys small grains faster than the large ones, and thus larger grains will be preferentially blown over longer distance. Although the hypersonic drift, or kinetic sputtering, tends to be suppressed for very small grains due to the Coulomb drag, they might also be disrupted by Coulomb explosion. Removal of small grains and/or survival of large grains may explain the lack of a silicate 10 μm emission feature in polar dust emission.

Dust Destruction by Charging: A Possible Origin of Gray Extinction Curves of Active Galactic Nuclei

Abstract Observed extinction curves of active galactic nuclei (AGNs) are significantly different from those observed in the Milky Way. The observations require preferential removal of small grains at the AGN environment; however, the physics for this remains unclear. In this paper, we propose that dust destruction by charging, or Coulomb explosion, may be responsible for AGN extinction curves. Harsh AGN radiation makes a dust grain highly charged through photoelectric emission, and grain fission via Coulomb explosion occurs when the electrostatic tensile stress of a charge grain exceeds its tensile strength. We show that Coulomb explosion can preferentially remove both small silicate and graphite grains and successfully reproduce both flat extinction curve and the absence of 2175 Å bump.

From Neutral Aniline to Aniline Trication: A Computational and Experimental Study.

We report density functional theory computations and photoionization mass spectrometry measurements of aniline and its positively charged ions. The geometrical structures and properties of the neutral and singly, doubly, and triply positively charged aniline are computed using density functional theory with the generalized gradient approximation. At each charge, there are multiple isomers closely spaced in total energy. Whereas the lowest energy states of both neutral and cation have the same topology C6H5-NH2, the dication and trication have the C5NH5-CH2 topology with the nitrogen atom in the meta- and para-positions, respectively. We compute the dissociation pathways of all four charge states to NH or NH+ and NH2 or NH2+, depending on the initial charge of the aniline precursor. Dissociation leading to the formation of NH (from the neutral and cation) and NH+ (from the dication and trication) proceeds through multiple transition states. On the contrary, the dissociation of NH2 (from the neutral and cation) and NH2+ (from the dication and trication) is found to proceed without an activation energy barrier. The trication was found to be stable toward abstraction on NH+ and NH2+ by 0.96 and 0.18 eV, respectively, whereas the proton affinity of the trication is substantially higher, 1.98 eV. The mass spectra of aniline were recorded with 1300 nm, 20 fs pulses over the peak intensity range of 1 × 1013 to 3 × 1014 W cm-2. The analysis of the mass spectra suggests high stability of both dication and trication to fragmentation. The formation of the fragment NH+ and NH2+ ions is found to proceed via Coulomb explosion.

The CO2 molecule is never linear

We make an ab initio calculation of the bending distribution functions for low lying vibrational states of the CO2 molecule in its ground electronic state. These functions have their maximum values at a non-linear geometry, and the value zero at linearity, despite the fact that the potential surface has its minimum value at linearity. These functions are in accord with experimental distribution functions inferred by analysis of Coulomb Explosion Imaging experiments. Thus in a femto-second ‘snapshot’ of a room temperature ensemble of gas phase rotating-vibrating CO2 molecules, none would be linear. The same can be said for any triatomic molecule.

Ultrafast Proton Transfer Dynamics on the Repulsive Potential of the Ethanol Dication: Roaming-Mediated Isomerization versus Coulomb Explosion.

If a molecular dication is produced on a repulsive potential energy surface (PES), it normally dissociates. Before that, however, ultrafast nuclear dynamics can change the PES and significantly influence the fragmentation pathway. Here, we investigate the electron-impact-induced double ionization and subsequent fragmentation processes of the ethanol molecule using multiparticle coincident momentum spectroscopy and ab initio dynamical simulations. For the electronic ground state of the ethanol dication, we observe several fragmentation channels that cannot be reached by direct Coulomb explosion (CE) but require preceding isomerization. Our simulations show that ultrafast hydrogen or proton transfer (PT) can stabilize the repulsive PES of the dication before the direct CE and form intermediate H2 or H2O. These neutrals stay in the vicinity of the precursor, and roaming mechanisms lead to isomerization and finally PT resulting in emission of H3+ or H3O+. The present findings can help to understand the complex fragmentation dynamics of molecular cations.

Optical Imaging of Coherent Molecular Rotors

AbstractShort 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 ultrafast 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 nondestructive 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. The technique is illustrated by implementing it with two examples of time‐resolved rotational dynamics: alignment–antialignment 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.

Visualizing the Geometry of Hydrogen Dimers.

The simplest molecular dimer, H2-H2, poses a challenge to both experiment and theory as a system with a multidimensional energy surface that supports only a single weakly bound quantum state. Here, we provide a direct experimental image of the structure of hydrogen dimers [(H2)2, H2-D2, and (D2)2] obtained via femtosecond laser-induced Coulomb explosion imaging. Our results indicate that hydrogen dimers are not restricted to a particular geometry but rather occur as a mixture of all possible configurations. The measured intermolecular distance distributions were used to deduce the isotropic intermolecular potential as well as the binding energies of the dimers.

Formation of CN Radical from Nitrogen and Carbon Condensation and from Photodissociation in Femtosecond Laser-Induced Plasmas: Time-Resolved FT-UV-Vis Spectroscopic Study of the Violet Emission of CN Radical.

Exploring the formation of diatomic radicals in femtosecond plasmas is important to establish the most dominant kinetic pathways following ionization and dissociation of small molecules. In this work, cyano radical formation has been studied from bromoform, acetonitrile, and methanol in nitrogen and argon plasmas created with a focused femtosecond laser beam operating at 100 kHz repetition rate and 1030 nm wavelength with 43 fs pulse length and 250 μJ pulse energy. Time-resolved Fourier transform fluorescence spectroscopy was applied in the ultraviolet-visible (UV-vis) spectral range for the characterization of the rotational and vibrational temperatures of the CN(B) radicals via fitting the experimental data. The high repetition rate of the laser allows efficient coupling with the step-scan Fourier transform spectroscopy method. Coulomb explosion at the very high intensity (∼1016 W/cm2) resulted in the formation of nascent atoms, ions, and electrons. The condensation reactions of carbon and reactive nitrogen species resulted in the formation of CN(B2Σ+) radicals and C2(d3Πg) dicarbon molecules/radicals. The CN(B) radicals were formed at the highest concentration in the case of bromoform because the weak carbon-bromine bonds resulted in reactive carbon atoms and CH radicals, which are reactive precursors for the CN(B) radical formation. In the case of acetonitrile, immediate production of CN(B) is observed with nanosecond resolution, which suggests that the CN is formed either via photodetachment or via roaming reaction associated with the Coulomb explosion of the parent molecule. The nascent rotational temperature was very high (∼6000-8500 K) and rapidly decreased in all instances within 40 ns with bromoform and acetonitrile. The highest vibrational temperature (∼7800 K) was observed in an acetonitrile/Ar mixture that decreased in about 30 ns and then increased in the observed time window. The vibrational temperature increased in all samples between 30 and 200 ns. The time dependence of fluorescence is described with a monoexponential decay in the case of acetonitrile/Ar and with biexponential decays in all other instances in the 0-250 mbar total pressure range. The shorter time constant is close to the radiative lifetime of CN(B) emission (∼60-80 ns), which can be attributed to the CN(B) radicals produced in the first few collisions at lower pressures. The longer CN(B) emission is from CN(B) created by slower chemical reactions involving carbon atoms, C2 radicals, and reactive nitrogen-containing species.

Generation of quasi-monoenergetic ions using optimized hollow nanospheres

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.

Coulomb explosion of CD3I induced by single photon deep inner-shell ionisation

L-shell ionisation and subsequent Coulomb explosion of fully deuterated methyl iodide, CD3I, irradiated with hard X-rays has been examined by a time-of-flight multi-ion coincidence technique. The core vacancies relax efficiently by Auger cascades, leading to charge states up to 16+. The dynamics of the Coulomb explosion process are investigated by calculating the ions’ flight times numerically based on a geometric model of the experimental apparatus, for comparison with the experimental data. A parametric model of the explosion, previously introduced for multi-photon induced Coulomb explosion, is applied in numerical simulations, giving good agreement with the experimental results for medium charge states. Deviations for higher charges suggest the need to include nuclear motion in a putatively more complete model. Detection efficiency corrections from the simulations are used to determine the true distributions of molecular charge states produced by initial L1, L2 and L3 ionisation.

Coulomb Explosion in Nanosecond Laser Fields

We report experimental observations of Coulomb explosion using a nanosecond laser at 532 nm with intensities less than 1012 W/cm2. We observe multiply charged atomic ions Arn+ (1 ≤ n ≤ 7) and Cn+ (1 ≤ n ≤ 4) from argon clusters doped with molecules containing aromatic chromophores. The yield of Arn+ depends on the size of the cluster, the number density, and the photostability of the dopant. We propose that resonant absorption of ArN+ achieves a high degree of ionization, and the highly positively charged cluster has the capability to strip electrons from the evaporating Ar+ on the surface of the cluster prior to Coulomb explosion, forming Arn+.

Breakdown of the Coulomb-explosion imaging technique induced by the ultrafast rotation of fragments

Coulomb-explosion imaging is a broadly employed technique to reconstruct the geometry of molecules from the direct multibody breakups of its ions. However, we reveal that this technique fails for a large class of systems, such as (${\mathrm{CO})}_{2}{}^{3+}$ and ${\mathrm{ArCO}}^{3+}$, since the events of the ``direct breakup channel'' are not the real direct breakups but the rapid sequential breakups with a short-lived and ultrafast-rotational fragment. Using Ar-CO as a prototype, we have investigated theoretically its Coulomb-explosion process. We find that due to the interfield between the metastable ${\mathrm{CO}}^{2+}$ and ${\mathrm{Ar}}^{+}$, ${\mathrm{ArCO}}^{3+}$ changes from its initial T shape into the more stable linear shape within 100 fs; such a rotation of ${\mathrm{CO}}^{2+}$ is hundreds of times faster than the field free case. Further, the advanced three-body surface hopping simulations indicate that the angle ${\ensuremath{\angle}}_{(\mathrm{Ar},\mathrm{CO})}$ has rotated from an initial ${95}^{\ensuremath{\circ}}$ to $110.6{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$ on average before the Coulomb explosion, which agrees well with the one (${115}^{\ensuremath{\circ}}$) detected by Gong et al. [Phys. Rev. A 88, 013422 (2013)]. Furthermore, we point out that correct geometry of ArCO can be obtained from the three-body breakups with high kinetic-energy release ($>20$ eV).

Electron transfer mediated decay of alkali dimers attached to He nanodroplets.

Alkali metal dimers attached to the surface of helium nanodroplets are found to be efficiently doubly ionized by electron transfer mediated decay (ETMD) when photoionizing the helium droplets. This process is evidenced by detecting in coincidence two energetic ions created by Coulomb explosion and one low-kinetic energy electron. The kinetic energy spectra of ions and electrons are reproduced by simple model calculations based on diatomic potential energy curves, and are in agreement with ab initio calculations for the He-Na2 and He-KRb systems. This work demonstrates that ETMD is an important decay channel in heterogeneous nanosystems exposed to ionizing radiation.

On the Feasibility of a Coulomb Explosion in a Micropinch

In this paper we present experimental data on the generation of ions with MeV-energy in a Z-pinch discharge in a heavy-element medium. The calculations show that the escape of runaway electrons from the constriction region can lead to the production of a positive volume charge and then to the Coulomb explosion of plasma, which generates high-energy particles.

Direct imaging of direction-controlled molecular rotational wave packets created by a polarization-skewed double-pulse.

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.

Resolving and imaging the stereo-configuration of OCS dimers in gas phase

SynopsisPolar, non-polar and cross-shaped isomers of OCS dimer are observed and the corresponding structures are resolved using fs laser-induced Coulomb explosion. These three structures are confirmed in the simulations, and demonstrate that the dimer structures can be reconstructed from the measured momenta distributions. Our study will promote the real time imaging the evolution of molecular dimer at fs time scale after excitation and ionization.

How do charged environments affect low-energy fusion rates? Screening effects in laser-induced non-neutral plasmas

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.

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.

Geometrically reconstructing laser Coulomb-exploded molecules

SynopsisGeometrical reconstructions are performed for Carbonyl Sulfide (OCS) through Coulomb Explosion Imaging (CEI) employing intense femtosecond laser ionization with pulse durations 7 fs, 30 fs, 60 fs, 100 fs, and 200 fs, using two methods that significantly differ in approach. Firstly, a look up table is employed to reconstruct OCS (2, 2, 2) channel and highlight the presence of degenerate regions. Following this a more sophisticated constrained nonlinear optimization approach improves on precision and gives an indication of uncertainty. Results are compared with equilibrium geometry.

N-NO & NN-O bond cleavage dynamics in the N2O2+

SynopsisN2O Coulomb explosion is initiated by single-photon double ionization with an ultrafast EUV pulse and is probed by delayed near-IR pulses. The competing two- and three-body dissociation dynamics are reflected in the time resolved branching ratios and in the three-body momentum correlation spectra. Channel resolved kinetic energy release spectra exhibit up to a 0.9 eV shift as a function of time delay – interpreted in terms of the long-bond distance difference between the potentials of a Rydberg ion state and its dication core.