Double Ionization Research Articles

L-Shaped Double Gate Bipolar Impact Ionization MOSFET Based Energy Efficient Leaky Integrate and Fire Neuron for Spiking Neural Network

L-Shaped Double Gate Bipolar Impact Ionization MOSFET Based Energy Efficient Leaky Integrate and Fire Neuron for Spiking Neural Network

Controlling nonsequential double ionization mechanisms of argon atoms by parallel two-color laser pulses

Controlling nonsequential double ionization mechanisms of argon atoms by parallel two-color laser pulses

Yield of Non-Sequential Double Ionization of CO2 Molecules Driven by Intense Laser Fields

Yield of Non-Sequential Double Ionization of CO2 Molecules Driven by Intense Laser Fields

Electron correlation dynamics in atomic Kr excited by XUV pulses and controlled by NIR laser pulses of variable intensity

We have investigated the possibility to track and control correlation dynamics of valence electrons in krypton (Kr) initiated by the absorption of one extreme ultraviolet (XUV) photon. In this investigation, pump-probe experiments have been performed where monochromatized single high-harmonics at photon energies 29.6, 32.8, and 35.9 eV have been used as pump to populate different intermediate excited states. A temporally delayed near-infrared (NIR) pulse probes the population of various decay channels via the detection of Kr2+ ion yields and its transient profiles. We observe that by varying the NIR pulse intensity within a range from to W cm−2, the shape of the Kr2+ transient profile changes significantly. We show that by varying the intensity of the NIR pulse, it is possible—(i) to control the ratio between sequential and non-sequential double ionization of Kr; (ii) to selectively probe quantum beating oscillations between Kr satellite states that are coherently excited within the bandwidth of the XUV pulse; and (iii) to specifically probe the relaxation dynamics of doubly excited (Kr) decay channels. Our studies show that the contribution of different ionization and decay channels (i)–(iii) can be altered by the NIR pulse intensity, thus demonstrating an efficient way to control the ionization dynamics in rare gas atoms.

Ultrafast Two-Electron Orbital Swap in Li Initiated by Attosecond Pulses

A universal mechanism of ultrafast 2-electron orbital swap is discovered through 2-photon sequential double ionization of Li. After a 1s electron in Li is ionized by absorbing an extreme ultraviolet photon, the other 2 bound electrons located on 2 different shells have either parallel or antiparallel spin orientations. In the latter case, these 2 electrons are in the superposition of the singlet and triplet states with different energies, forming a quantum beat and giving rise to the 2-electron orbital swap with a period of several hundred attoseconds. The orbital swap mechanism can be used to manipulate the spin polarization of photoelectron pairs by conceiving the attosecond-pump attosecond-probe strategy and thus serves as a knob to control spin-resolved multielectron ultrafast dynamics.

Atomic autoionization in the photo-dissociation of super-excited deuterated water molecules fragmenting into D+ + O+ + D.

We present the relaxation dynamics of deuterated water molecules via autoionization, initiated by the absorption of a 61 eV photon, producing the very rare D+ + O+ + D breakup channel. We employ the COLd target recoil ion momentum spectroscopy method to measure the 3D momenta of the ionic fragments and emitted electrons from the dissociating molecule in coincidence. We interpret the results using the potential energy surfaces extracted from multi-reference configuration interaction calculations. The measured particle energy distributions can be related to a super-excited monocationic state located above the double ionization threshold of D2O. The autoionized electron energy shows a sharp distribution centered around 0.5 eV, which is a signature of the atomic oxygen autoionization occurring in the direct and sequential dissociation processes of D2O+* at a large internuclear distance. In this way, an O+ radical fragment and a low-energy electron are created, both of which can trigger secondary reactions in their environment.

Ion momentum distributions from sequential double ionization of Ar in elliptically polarized laser fields

In this paper, we utilize a classical ensemble model with Heisenberg-core potential to study sequential double ionization (SDI) of Ar atom by an elliptically polarized laser field. The results show that for random laser phases, as the laser wavelength increases, the ion momentum distribution gradually evolves from a six-band structure at 800 nm to an eight-band structure at 1600 nm. When the laser phase is stable, the ion momentum distribution from 1600 nm laser field exhibits a ten-band structure. These multi-band structures directly reflect the subcycle ionization dynamics of electrons in an elliptically polarized laser field. There is a significant shift among the outer three bands of ion momentum distrbutions from different laser phases, which leads to the fact that only one band is observed in the outer region of the ion momentum distribution for the case of random laser phases. By analyzing the ionization times of the two electrons, it is found that for the case of random phases, the inner bands of the ion momentum distributions originate from those combinations of electron ionization bursts with the ionization time difference of 0.5 cycle, and the outer bands arise from those combinations of ionization bursts with the ionization time difference of 1, 2 and 3 cycles. For 800 nm, the middle band corresponds to those combinations of ionization bursts with the ionization time differences of 1.5 and 2.5 cycles. For 1600 nm, there are two bands in middle regime. One is from the combination with the ionization time difference of 1.5 cycles, and the other is from those combinations with the ionization time difference of 2.5 and 3.5 cycles. These results indicate that in the case of long wavelength and phase-stable laser, the subcycle dynamics in sequential double ionization of atoms is more likely to be observed.

Hearing the Heartbeat of Atoms: Unveiling Attosecond Horizons

The 2023 Nobel Prize in Physics spotlights the techniques to generate attosecond light pulses. The generation of attosecond pulses heralds a new era in understanding electron dynamics. This perspective traces the evolution of ultrafast science, from early microwave electronics to the recent breakthroughs in attosecond pulse generation and measurement. Key milestones, such as high harmonic generation, the RABBITT method, attosecond streaking camera, etc, illuminate our journey toward capturing the transient electron motions in atoms. Recent discoveries, including zeptosecond delays in H 2 single-photon double ionization and the potential of attosecond “electron” pulses despite challenges, etc., hint at an exciting future for ultrafast studies.

Development of a small Time-Projection-Chamber for the quasi-absolute neutron flux measurement

Accurate actinides fission cross sections around 1 MeV are of primary importance for the safety of generation IV reactors. To have accurate measurements, the neutron flux must also be accurately estimated. This is usually done with respect to the 235U(n,f) cross section. It is however possible to measure the neutron flux with respect to the 1H(n,n)p cross section which is a primary standard, providing an independent and precise measurement. Typically, the usual proton recoil technique uses a silicon detector for neutrons of energy between 1 and 70 MeV. However, the high electron and gamma background due to neutron production under irradiation makes the use of this or any other detector not suitable for an accurate measurement below 1 MeV. To this end, the Gaseous Proton Recoil Telescope is developed and characterized. The goal is to provide quasi-absolute neutron flux measurements with an accuracy better than 3%. It consists of a double ionization chamber with a Micromegas segmented detection plane and the gaseous pressure can be adjusted to protons – and hence neutron – energy. The sensitivity to gamma and electrons background, the intrinsic efficiency as well as the resolution of this detector have been investigated.

Single- and multi-photon-induced ultraviolet excitation and photodissociation of CH3I probed by coincident ion momentum imaging.

The UV-induced photodissociation dynamics of iodomethane (CH3I) in its A-band are investigated by time-resolved coincident ion momentum imaging using strong-field ionization as a probe. The delay-dependent kinetic energy distribution of the photofragments resulting from double ionization of the molecule maps the cleavage of the carbon-iodine bond and shows how the existence of a potential well in the di-cationic potential energy surfaces shapes the observed distribution at small pump-probe delays. Furthermore, the competition between single- and multi-photon excitation and ionization of the molecule is studied as a function of the intensity of the UV-pump laser pulse. Two-photon excitation to Rydberg states is identified by tracking the transformation of the delay-dependent singly-charged iodomethane yield from a pure Gaussian distribution at low intensity to a Gaussian with an exponentially decaying tail at higher intensities. Dissociative ionization induced by absorption of three UV photons is resolved as an additional delay-dependent feature in the kinetic energy of the fragment ions detected in coincidence.

Nonsequential double ionization of atoms driven by spatially inhomogeneous laser fields

<sec>Using a three-dimensional classical ensemble method, we investigate the nonsequential double ionization (NSDI) of xenon atoms from the near infrared wavelength to the mid-infrared wavelength in spatially inhomogeneous laser fields, and compare the results with those from spatially homogeneous laser fields. The results show that the NSDI probability curves from spatially inhomogeneous laser field and spatially homogeneous laser field at short wavelength are similar to each other. With the laser wavelength increasing, NSDI at the high intensities is more and more suppressed for spatially inhomogeneous laser field.</sec><sec>Compared with the result from the spatially homogeneous laser field, the final emission angle of two electrons from the NSDI exhibits a very strongly correlated characteristic in the spatially inhomogeneous field, especially at a longer laser wavelength, the final emission angles of two electrons are almost both concentrated around <inline-formula><tex-math id="M2">\begin{document}${0^\circ }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230548_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230548_M2.png"/></alternatives></inline-formula>, meaning that the two electrons are always emitted into the same direction parallelly. Moreover, effective recollision of the NSDI is always dominated by first return of the first electron from the near infrared to the mid-infrared inhomogeneous laser fields, however, the transition from the first return dominance to the second return dominance occurs in the spatially homogeneous laser fields. Further, we reveal the more details of the ultrafast dynamics of the correlated electrons in the spatially inhomogeneous laser field by back-tracing the classical trajectories of NSDI.</sec>

Double K-Shell Ionization of Ar by 197-MeV/u Xe54+ Ion Impact

We present an experimental study on the double K-shell ionization of argon in single collisions with the Xe54+ ion at 197 MeV/u. The X-ray spectra of multi-ionized argon are measured at the observation angles of 90° and 145° with respect to the projectile beam. The target K X-ray satellite and hypersatellite lines are analyzed with a fitting model and the cross-section ratio of double to single K-shell ionization is derived. The experimental results are compared to the relativistic time-dependent, two-center calculations, and a reasonable agreement is reached.

Ban‐1 Zircon: A New Natural Zircon Reference Material for LA‐MC‐ICP‐MS Zr and Hf Isotopic Determinations

New zircon reference materials for in situ zircon radiogenic Hf isotope and stable Zr isotopic determinations made by laser ablation multi‐collector inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS) are required due to high data productivity and consequently high reference material consumption rate. This study examines a new natural zircon for Zr isotope ratios by double spike thermal ionisation mass spectrometry (TIMS), and for Hf isotopes by bulk solution nebuliser (SN)‐MC‐ICP‐MS with both Zr and Hf determined by LA‐MC‐ICP‐MS. A total of five zirconium isotope measurements from drilled zircons, determined by TIMS, yield a mean δ94/90ZrIPGP‐Zr value of ‐0.09 ± 0.06‰ (2s). Five and eight hafnium isotope measurements for powders from the drilled zircons and Ban‐1‐4 by SN‐MC‐ICP‐MS, yield mean 176Hf/177Hf ratios of 0.282985 ± 0.000011 (2s) and 0.282982 ± 0.000007 (2s), respectively. The mean δ94/90ZrIPGP‐Zr value and 176Hf/177Hf ratio determined by LA‐MC‐ICP‐MS analyses are ‐0.06 ± 0.09‰ (2s, n = 504) and 0.282985 ± 0.000035 (2s, n = 327), respectively. The isotopic homogeneities suggest that the Ban‐1 zircon is a suitable reference material for microbeam Zr and Hf isotopic measurements.

Stable Sr isotope (88Sr/86Sr) fractionation in calcite precious corals

Stable isotope ratios of Sr (88Sr/86Sr) are a novel indicator for identifying terrestrial and oceanic Sr cycling and biological processes. Here we evaluate the temperature dependence of stable Sr isotope ratios (δ88Sr) in the calcite skeletons of precious corals, known as octocorals. High-precision Sr isotope measurements by double spike thermal ionization mass spectrometry were applied to different precious corals (Corallium japonicum, Pleurocorallium elatius, P. konojoi, Corallium sp., Keratoisis sp., and Coralliidae sp.) collected from water depths of 30–1500 m in the Pacific Ocean, corresponding to annual mean water temperatures of 2.5 to 19.5°C. The precious corals showed mean δ88Sr values of 0.101 ± 0.023‰. The magnitude of Sr isotope fractionation between the skeleton and ambient seawater (Δ88Sr) is −0.291 and is about −0.1‰ lower than the value predicted from the precipitation of inorganic calcite. This fractionation is not affected by changes in skeletal Sr/Ca ratio and associated biological effects on δ88Sr are negligible. δ88Sr of precious coral skeleton is an excellent recorder of ambient seawater.

Ellipticity dependence of anticorrelation in the nonsequential double ionization of Ar.

Within the framework of the improved quantitative rescattering (QRS) model, we simulate the correlated two-electron momentum distributions (CMDs) for nonsequential double ionization (NSDI) of Ar by elliptically polarized laser pulses with a wavelength of 788 nm at an intensity of 0.7 × 1014 W/cm2 for the ellipticities ranging from 0 to 0.3. Only the CMDs for recollision excitation with subsequent ionization (RESI) are calculated and the contribution from recollision direct ionization is neglected. According to the QRS model, the CMD for RESI can be factorized as a product of the parallel momentum distribution (PMD) for the first released electron after recollision and the PMD for the second electron ionized from an excited state of the parent ion. The PMD for the first electron is obtained from the laser-free differential cross sections for electron impact excitation of Ar+ calculated using state-of-the-art many-electron R-matrix theory while that for the second electron is evaluated by solving the time-dependent Schrödinger equation. The results show that the CMDs for all the ellipticities considered here exhibit distinct anticorrelated back-to-back emission of the electrons along the major polarization direction, and the anticorrelation is more pronounced with increasing ellipticity. It is found that anticorrelation is attributed to the pattern of the PMD for the second electron ionized from the excited state that, in turn, is caused by the delayed recollision time with respect to the instant of the external field crossing. Our work shows that both the ionization potential of the excited parent ion and the laser intensity play important roles in the process.

Electron Correlations in Sequential Two-Photon Double Ionization of an Ar Atom

Sequential two-photon ionization is a process that is experimentally accessible due to the use of new free-electron laser sources for excitation. For the prototypical rare Ar gas atoms, a photoelectron spectrum (PES) corresponding to the second step of the sequential two-photon double ionization (2PDIII) at a photon excitation energy of 65.3 eV was studied theoretically with a focus on the consequences of electron correlations in the considered process. The calculation predicts many intense lines at low photoelectron energies, which cannot be explained on the basis of a one-electron approximation. The processes that lead to the appearance of these lines include many-electron correlations, either in the first or second step of photoionization. A significant fraction of the intensity of the low-energy part of PES is associated with the Auger decay of the excited states formed at the second step of 2PDI. The shape of the low-energy part of the 2PDIII PES is expected to be dependent on both the energy of photon excitations and the flux of the exciting beam.

Ultrafast Charge and Proton Transfer in Doubly Ionized Ammonia Dimers.

We investigate the ultrafast energy and charge transfer processes between ammonia molecules following ionization reactions initiated by electron impact. Exploring ionization-induced processes in molecular clusters provides us with a detailed insight into the dynamics using experiments in the energy domain. We ionize the ammonia dimer with 200 eV electrons and apply the fragment ions coincident momentum spectroscopy and nonadiabatic molecular dynamics simulations. We identify two mechanisms leading to the doubly charged ammonia dimer. In the first one, a single molecule is ionized. This initiates an ultrafast proton transfer process, leading to the formation of the NH2+ + NH4+ pair. Alternatively, a dimer with a delocalized charge is formed dominantly via the intermolecular Coulombic decay, forming the NH3+·NH3+ dication. This dication further dissociates into two NH3+ cations. The ab initio calculations have reproduced the measured kinetic energy release of the ion pairs and revealed the dynamical processes following the double ionization.

Nonsequential double ionization of Ne with elliptically polarized laser pulses

We show through simulation that the improved quantitative rescattering model (QRS) can successfully predict the nonsequential double ionization (NSDI) process by intense elliptically polarized laser pulses. Using the QRS model, we calculate the correlated two-electron and ion momentum distributions of NSDI in Ne exposed to intense elliptically polarized laser pulses with a wavelength of 788 nm at a peak intensity of $5.0\ifmmode\times\else\texttimes\fi{}{10}^{14}$ W/${\mathrm{cm}}^{2}$. We analyze the asymmetry in the doubly charged ion momentum spectra observed by Kang et al. [Phys. Rev. Lett. 120, 223204 (2018)] in going from linearly to elliptically polarized laser pulses. Our model reproduces the experimental data well. Furthermore, we find that the ellipticity-dependent asymmetry arises from the drift velocity along the minor axis of the elliptic polarization. We explain how the correlated electron momentum distributions along the minor axis provide access to the subcycle dynamics of recollision. From these findings, we expect that we can extend the QRS model for NSDI toward more complicated laser fields in the future.

On the two-body dissociation of dications of water isotopologues

Multiple ionization of water isotopologues is studied using near-IR fs laser beams and asymmetric ω/2ω phase-locked fields, in order to elucidate the involved dissociation mechanics and isotopic effects. The double ionization mechanism of water is identified by the laser polarization dependence of the released, via two body fragmentation, ions. The angular distribution of the H+/D+ fragments, reveal the participation of different molecular orbitals in the ionization process, indicating excitation of both the ground and first few excited states of the dications. The rate of the dissociative processes of the dication is found to be different for the two isotopologues, highlighting an isotopic effect. Inspection of the H2+/D2+ fragments formed prior to dissociation, reveals that they may have been generated from parent cations and parent dications. In the case of dication fragmentation, we find evidence of autoionization. Finally, the formation of the H2+/D2+ ions, is completed within the duration of the pulse.

Two-XUV-photon double ionization of neon

Two-XUV-photon double ionization of Ne, induced by an intense few-pulse attosecond train with a \ensuremath{\sim}4 fs envelope duration is investigated experimentally and theoretically. The experiment is performed at ELI-ALPS (Extreme Light Infrastructure Attosecond Light Pulse Source) utilizing the recently constructed 10 Hz gas phase high-order harmonic generation SYLOS GHHG-COMPACT beamline. A total pulse energy up to \ensuremath{\sim}1 \ensuremath{\mu}J generated in argon in conjunction with high-reflectivity optics in the XUV region allowed the observation of the doubly charged state of Ne induced by 40 eV central XUV-photon energies. The interaction of the intense attosecond pulse train with Ne is also theoretically studied via second-order time-dependent perturbation theory equations of motion. The results of this work, combined with the feasibility of conducting XUV-pump--XUV-probe experiments, constitute a powerful tool for many potential applications. Those include attosecond pulse metrology as well as time-resolved investigations of the dynamics underlying direct and sequential double ionization and their electron correlation effects.