Directed Coulomb Explosion Research Articles

Electron transfer in strong-field three-body fragmentation of ArKr2 trimers.

We experimentally studied the three-body fragmentation dynamics of a noble gas cluster (ArKr2) upon its multiple ionization by an intense femtosecond laser pulse. The three-dimensional momentum vectors of correlated fragmental ions were measured in coincidence for each fragmentation event. A novel comet-like structure was observed in the Newton diagram of the quadruple-ionization-induced breakup channel of ArKr2 4+→ Ar+ + Kr+ + Kr2+. The concentrated head part of the structure mainly originates from the direct Coulomb explosion process, while the broader tail part of the structure stems from a three-body fragmentation process involving electron transfer between the distant Kr+ and Kr2+ ion fragments. Due to the field-driven electron transfer, the Coulomb repulsive force of the Kr2+ and Kr+ ions with respect to the Ar+ ion undergoes exchange, leading to changes in the ion emission geometry in the Newton plot. An energy sharing among the separating Kr2+ and Kr+ entities was observed. Our study indicates a promising approach for investigating the strong-field-driven intersystem electron transfer dynamics by using the Coulomb explosion imaging of an isosceles triangle van der Waals cluster system.

Isomerization Dynamics in the Symmetric and Asymmetric Fragmentation of Ethane Dications.

Hydrogen- or proton-migration-induced isomerization has recently been of concern for its critical role in the dissociation of organic molecules of astrophysical or biological relevance. Herein we present a combined experimental and theoretical study of the two-body C-C bond breakdown dissociation of ethane dication. For the asymmetric fragmentation channel CH2+ + CH4+, the kinetic energy release measurements and ab initio quantum chemical calculations demonstrate that the reaction pathway involving hydrogen-migration-induced isomerization of [CH3-CH3]2+ to [CH2-CH4]2+ can be accessed via the lowest triplet state rather than the ground singlet state of ethane dication. Interestingly, it is found that a considerable proportion of the yield of symmetric fragmentation CH3+ + CH3+, which is usually considered from a direct Coulomb explosion and seemingly independent of isomerization, could come from the dissociation of ethane dication in the ground singlet state with the involvement of [CH3-CH3]2+ isomerization to intermediate [H2C(H2)CH2]2+ of the diborane-like double-bridged structure.

Direct Coulomb explosion of N2O2+ induced by monochromatic extreme ultraviolet photons at 38.5 eV**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0402300).

The direct Coulomb explosion of N2O2+ has been investigated experimentally after double-ionization by a single extreme ultraviolet (EUV) photon with an energy of ∼ 38.5 eV. From the ion–ion time-of-flight coincidence spectrum, the de-nitrogenation (N2O2+ → N+ + NO+) and de-oxygenation (N2O2+→ O+ + N2+) photodissociation channels of N2O2+ are unequivocally identified. The measured kinetic energy release (KER) distribution of the de-nitrogenation channel presents a major peak accompanied by a shoulder structure. We find that the major peak can be attributed to the direct photodissociation of the 11Δ and 11Σ+ states, while the shoulder structure should be ascribed to the predissociation of the 11Σ and 11Σ+ states via 13Π state.

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.

Making Sense of Coulomb Explosion Imaging.

A multifaceted agreement between ab initio theoretical predictions and experimental measurements, including branching ratios, channel-specific kinetic energy release, and three-body momentum correlation spectra, leads to the identification of new mechanisms in Coulomb-explosion (CE) induced two- and three-body breakup processes in methanol. These identified mechanisms include direct nonadiabatic Coulomb explosion responsible for CO bond-breaking, a long-range " inverse harpooning" dominating the production of H2+ + HCOH+, a transient proton migration leading to surprising energy partitioning in three-body fragmentation and other complex dynamics forming products such as H2O+ and H3+. These mechanisms provide general concepts that should be useful for analyzing future time-resolved Coulomb explosion imaging of methanol as well as other molecular systems. These advances are enabled by a combination of recently developed experimental and computational techniques, using weak ultrafast EUV pulses to initiate the CE and a high-level quantum chemistry approach to follow the resulting field-free nonadiabatic molecular dynamics.

Comparison of optimized ion acceleration from thin foils and low-density targets for linearly and circularly polarized laser pulses

A multiparameter comparative analysis of ion acceleration with linearly and circularly polarized relativistically intense laser pulses for solid-density thin foils and low-density planar targets was performed using 3D particle-in-cell (PIC) simulations. Ion acceleration optimization was studied with 3D PIC MANDOR over the laser energy range of three orders of magnitude from three hundred millijoules to three hundred joules in a femtosecond pulse. The optimum target thickness and density was found for a given energy of the laser pulse corresponding to the maximum energy of the accelerated ions. This allows deriving a dependence of the maximum ion energy on laser energy for the optimized solid or low-density targets. The advantage of a circularly polarized laser pulse for generating the most energetic ions was demonstrated, which happens for the regimes of volumetrically heated semitransparent solid foils (directed Coulomb explosion) and of synchronized ion acceleration by slow light from low-density targets. The dependence of the maximum ion energy on laser energy for the optimized targets and both linearly and circularly polarized femtosecond pulses demonstrates a sharper-than-square-root increase.

Optimization of a Laser-Based Proton Source and a New Mechanism of Ion Acceleration

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

Comparative study of ion acceleration by linearly polarized laser pulses from optimized targets of solid and near-critical density

The results of a 3D optimization study of ion acceleration from ultrathin solid density foils (Brantov et al 2015 Phys. Rev. Spec. Top. Accel. Beams 18 021301) are complemented with an improved analytic model of the directed Coulomb explosion. Similarly to optimizing overdense targets, we also optimize low-density targets to obtain maximum ion energy, motivated by progress in producing a new generation of low-density slab targets whose density can be very homogeneous and as low as the relativistic critical density. Using 3D simulations, we show that for the same laser pulse, the ion energy can be significantly increased with low-density targets. A new acceleration mechanism is responsible for such an increase. This mechanism is described qualitatively, and it explains an advantage of low-density targets for high-energy ion production by lasers.

Dynamical coupling of electrons and nuclei for Coulomb explosion of argon trimers in intense laser fields

We have experimentally and theoretically studied the fragmentation dynamics of argon trimer $(\mathrm{A}{\mathrm{r}}_{3})$ in intense laser fields. By coincidently measuring the momentum vectors, we obtained the emission geometry of the three fragmental ions produced in the three-body fragmentation process. In addition to the direct Coulomb explosion channels, we observed the indirect Coulomb explosion channels with Rydberg excitation. We have further developed a classical polyatomic molecular ensemble model, in which all interactions among electrons and nuclei are fully included, to simulate the fragmentation dynamics of argon trimer in intense laser fields. The experimental observations have been reproduced by the model calculation. The simulations show that the Rydberg excitation modifies the kinetic energy release as well as the emission geometry of fragmental ions during the explosion process. The study provides insight into the correlation dynamics of electrons and nuclei of many-body physics driven by intense laser fields.

Control of proton energy in ultra-high intensity laser-matter interaction

Recent breakthroughs in short pulse laser technology resulted in (i) generation of ultra-high intensity (2×1022 W/cm2) and (ii) ultra-high contrast (10−11) short pulses at the Hercules facility of the University of Michigan, which has created the possibility of exploring a new regime of ion acceleration – the regime of Directed Coulomb Explosion (DCE). In this regime of sufficiently high laser intensities and target thicknesses approaching the relativistic plasma skin depth it is possible to expel electrons from the target focal volume by the laser's ponderomotive force allowing for direct laser ion acceleration combined with a Coulomb explosion. That results in greater than 100 MeV protons with a quasi-monoenergetic energy spectrum. The utilization of beam shaping, namely, the use of flat-top beams, leads to more efficient proton acceleration due to the increase of the longitudinal field. According to the results of 2D PIC simulations a 500 TW laser pulse with a super-Gaussian beam profile interacting with 0.1 micron aluminium-hydrogen foil is able to produce monoenergetic protons with the energy up to 240 MeV.

SU-GG-T-462: Observation of Quasi-Monoenergetic Laser Accelerated Proton and Carbon Beams

Purpose: To perform preliminary experiments to achieve the Directed Coulomb Explosion (DCE) regime of proton acceleration to therapeutic energies in high-intensity laser-matter interactions. Method and Materials: Particle-in-Cell (PIC) simulations of the planned experiments at HERCULES laser at the University of Michigan have predicted a new regime of attainable laser-target interactions for proton acceleration. The laser was recently upgraded to 300 TW with Amplified Spontaneous Emission (ASE) intensity contrast ratio of 10−11, allowing intensities of 2×1022 W/cm2 to be achieved in a near diffraction limited, 1.3 micron, focal spot. Dual plasma mirrors have been installed and characterized to reduce the prepulse at < 30 ps (from the uncompensated dispersion of optical elements during the pulse compression) before the main pulse providing 3 orders of magnitude contrast improvement. This allowed experiments on thin foil membranes (50 nm) with 50TW temporally clean pulses without compromising the target. Results: We found for the first time that for all target thicknesses proton spectra exhibit quasi-monoenergetic features, which are more pronounced for ultra-thin (50 nm Si3N4) targets resulted in AE/E∼30%. Moreover for these Si3N4 targets spectra for all the charge states of carbon ions C3+-C6+ are also found to be quasi-monoenergetic. Maximum proton energy drops from 6 MeV for 1 ⌈ m Mylar foil to 4 MeV for 50 nm Si3N4 membranes. Conclusion: Implementation of dual plasma mirrors substantially improved laser contrast and created more favorable proton and carbon flux-energy distributions. Further improvements to the plasma mirrors are required, using better antireflection coatings on glass substrates, to achieve the DCE regime of proton acceleration.

Energetic electron and ion generation from interactions of intense laser pulses with laser machined conical targets

The generation of energetic electron and proton beams was studied from the interaction of high intensity laser pulses with pre-drilled conical targets. These conical targets are laser machined onto flat targets using 7–180 µJ pulses whose axis of propagation is identical to that of the main high intensity pulse. This method significantly relaxes requirements for alignment of conical targets in systematic experimental investigations and also reduces the cost of target fabrication. These experiments showed that conical targets increase the electron beam charge by up to 44 ± 18% compared with flat targets. We also found greater electron beam divergence for conical targets than for flat targets, which was due to escaping electrons from the surface of the cone wall into the surrounding solid target region. In addition, the experiments showed similar maximum proton energies for both targets since the larger electron beam divergence balances the increase in electron beam charge for conical targets. 2D particle in cell simulations were consistent with the experimental results. Simulations for conical target without preplasma showed higher energy gain for heavy ions due to ‘directed coulomb explosion’. This may be useful for medical applications or for ion beam fast ignition fusion.

MO‐EE‐A2‐05: Experimental Implementation of the Directed Coulomb Explosion Regime of Laser‐Proton Acceleration

Purpose: To quantify the laser pulse requirements and target parameters required to achieve the Directed Coulomb Explosion (DCE) regime of laser‐target interaction for the acceleration of protons to therapeutic energies. Method and Materials: Particle‐in‐Cell (PIC) simulations of the planned experiments along the funded upgrade path of the HERCULES laser at the University of Michigan have predicted a new regime of attainable laser‐target interactions for proton acceleration. The laser was recently upgraded to 300 TW and a temporal pulse contrast ratio of 10−11, allowing intensities of 2×1022 W/cm2 to be achieved in a near diffraction limited, 1.3 micron, focal spot. The 2 ns long amplified spontaneous emission (ASE) pre‐pulse was suppressed by a factor 10−3 through the implementation of a cross‐polarized wave (XPW) pulse cleaner to prevent pre‐plasma creation on the front surface and preserve the physical integrity of the thin‐film target. Dual plasma mirrors are being characterized to reduce the prepulse at &lt; 30 ps before the main pulse (caused by variations in the index of refraction through the optic path) below a contrast ratio of 10−11. This will allow experiments on thin foil targets (&lt; 100 nm) up to 300TW with no significant pre‐pulse to compromise the target. Results: Preliminary measurements show an additional reduction of the contrast ratio of the ASE pre‐pulse by a factor of 10−3 after the addition of dual plasma mirrors. Additional work is required to optimize the setup and parameters of the plasma mirrors to account for polarization effects and wave front distortions and variable intensity levels. Conclusion: Implementation of dual plasma mirrors is progressing with promising results and will soon allow experimental implementation of the laser pulse characteristics required to test the DCE regime of proton acceleration.

Determination of structural parameters from advanced molecular electronic spectroscopy: The double ionization of nitrous oxide by synchrotron radiation

The double photoionization of N2O molecules, in the 30–50 eV energy range, has been studied by synchrotron radiation. In the whole energy range, dissociative ionization producing N+ + NO+ or N2+ + O+ has been observed. These two processes appear to occur also below the vertical thresh-old of 35.8 eV, where the double ionization should be indirect. In the range between 35.8 and 38.5 eV. The two processes occur instead by direct coulomb explosion of the N2O2+ dication. Above 38.5 eV, the dissociation leading to NO+ + N+ is also promoted by the formation of a dication metastable state, which decays by fluorescence to the ground state and then dissociates.

Accelerating monoenergetic protons from ultrathin foils by flat-top laser pulses in the directed-Coulomb-explosion regime.

We consider the effect of laser beam shaping on proton acceleration in the interaction of a tightly focused pulse with ultrathin double-layer solid targets in the regime of directed Coulomb explosion. In this regime, the heavy ions of the front layer are forced by the laser to expand predominantly in the direction of the pulse propagation, forming a moving longitudinal charge separation electric field, thus increasing the effectiveness of acceleration of second-layer protons. The utilization of beam shaping, namely, the use of flat-top beams, leads to more efficient proton acceleration due to the increase of the longitudinal field.

Direct Coulomb-explosion imaging of coherent nuclear dynamics induced by few-cycle laser pulses in light and heavy hydrogen

We followed fast evolution of coherent nuclear wave packets in ${\text{H}}_{2}$ and ${\text{D}}_{2}$ molecules after their interaction with 8-fs 800-nm laser pulses. The molecules were probed by another few-cycle pulse time delayed for up to 10 ps with respect to the pump. For neutral molecules we observed coherent rotational dynamics characterized by periodic revivals without noticeable decoherence within the 10 ps time scale. For heavy hydrogen up to four rotational states were involved in the wave packets for each of the two spin isomers. In light hydrogen the resulting dynamics was dominated by beating of just two rotational states. For neutral molecules the experimental results are in excellent agreement with our numerical simulations obtained by solving the time-dependent Schr\"odinger equation. By measuring time-dependent yields for singly ionized rotating ${\text{D}}_{2}$ molecules, we conclude that for an 8-fs $3\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.2em}{0ex}}\text{W}/{\text{cm}}^{2}$ pulse the ionization probability is nearly independent of the angle between the molecular axis and the electric field. For those molecules that were ionized by the pump pulse we observed both vibrational and rotational dynamics. In molecular ions coherent vibrational wave packets evolving on the bound ${\ensuremath{\sigma}}_{g}$ potential surface also exhibit revivals. Time-dependent angular distributions for the molecular ions exhibit transient alignment only soon after the pulse (18 fs for $\text{H}_{2}{}^{+}$ and 35 fs for $\text{D}_{2}{}^{+}$) with no consequent revivals within the next 10 ps due to broad distribution of active vibrational states with different rotational constants.

Accelerating protons to therapeutic energies with ultraintense, ultraclean, and ultrashort laser pulses

Proton acceleration by high-intensity laser pulses from ultrathin foils for hadron therapy is discussed. With the improvement of the laser intensity contrast ratio to 10(-1) achieved on the Hercules laser at the University of Michigan, it became possible to attain laser-solid interactions at intensities up to 10(22) W/cm2 that allows an efficient regime of laser-driven ion acceleration from submicron foils. Particle-in-cell (PIC) computer simulations of proton acceleration in the directed Coulomb explosion regime from ultrathin double-layer (heavy ions/light ions) foils of different thicknesses were performed under the anticipated experimental conditions for the Hercules laser with pulse energies from 3 to 15 J, pulse duration of 30 fs at full width half maximum (FWHM), focused to a spot size of 0.8 microm (FWHM). In this regime heavy ions expand predominantly in the direction of laser pulse propagation enhancing the longitudinal charge separation electric field that accelerates light ions. The dependence of the maximum proton energy on the foil thickness has been found and the laser pulse characteristics have been matched with the thickness of the target to ensure the most efficient acceleration. Moreover, the proton spectrum demonstrates a peaked structure at high energies, which is required for radiation therapy. Two-dimensional PIC simulations show that a 150-500 TW laser pulse is able to accelerate protons up to 100-220 MeV energies.

Double photoionization of N 2O molecules in the 28–40 eV energy range

The double photoionization of N 2O molecules, in the 28–40 eV energy range, has been studied by synchrotron radiation. In the whole energy range, dissociative ionization producing N + + NO + or N 2 + + O + has been observed. Below ∼35.5 eV, these two processes involve the formation of some autoionizing states. In the range between ∼35.5 and ∼38.5 eV the two processes occur instead by a direct coulomb explosion of the N 2O 2+ dication. Above ∼38.5 eV the dissociation leading to NO + + N + is also promoted by the formation of a dication metastable state which decays by fluorescence to the ground state and then dissociates.

Ionization/dissociation of thiazole and thiazolidine induced by strong laser fields

The interaction of thiazole and thiazolidine with a strong (∼ 2×10 16 W/cm 2 ) fs laser field has been studied at λ=790 nm by means of time-of-flight (TOF) mass spectrometry. The observed relative abundance of the doubly charged intact parent ion in thiazolidine is higher than that of thiazole, while the laser-molecule coupling strength is found to be much more efficient for the aromatic (thiazole) than the nonaromatic (thiazolidine) molecule. The mass spectra of thiazolidine are attributed to a combination of field ionization with subsequent multiphoton processes. It is also concluded, that direct Coulomb explosion within the transient multiply charged parent ions leads to the production of multiply charged atomic ions.