Electron Momentum Spectra Research Articles

Compton ionization momentum spectrum from neutral hydrogenlike atoms

We investigate the ionization momentum spectrum of a bound system composed of two oppositely charged particles, which interact via electromagnetic forces, under nonrelativistic Compton scattering. Our analysis focuses on positronium, a unique system that can be experimentally realized. We find that as the incident photon energy increases, the ionization momentum spectrum of positrons and electrons converges to their respective bound-state momentum distributions. This property distinguishes positronium from the hydrogen atom and can be attributed to the mass ratio ${R}_{m}$ of the two particles in the system. We provide a detailed analysis of the effect of ${R}_{m}$ and demonstrate the existence of hydrogenlike atoms with ${R}_{m}\ensuremath{\ne}1$ that exhibit similar behavior in Compton scattering. Such atoms correspond to exotic systems found in nature.

Simulating the Enrichment of Fossil Radio Electrons by Multiple Radio Galaxies

We simulate the evolution of relativistic electrons injected into the intracluster medium by five radio galaxies. We study the spatial transport and the emission properties of the injected radio plasma over a ∼5 Gyr period, and the sequence of cooling and re-acceleration events experienced by electrons, using a Lagrangian approach joined with a numerical method to model the evolution of momentum spectra of relativistic electrons. When compared with electrons injected by shock waves, electrons injected by radio galaxies (here limited to a single injection event) in our tests are unable to fuel large ∼Mpc-sized radio relics with fossil electrons, as required by current theoretical models, while electrons previously seeded by other shocks can do this. On the other hand, the combination of seeding from radio galaxies and of re-acceleration events from plasma perturbation can produce detectable, small-scale, and filamentary emissions in the proximity (≤100–200 kpc) of radio galaxies.

The nonsequential double ionization of Ar atoms with different initial angular momenta irradiated by a circularly polarized laser pulse

The non-sequential double ionization process of Ar atoms with different initial angular momenta in the 500 nm circularly polarized field is investigated with a classical ensemble model. The numerical results show that, under the circularly polarized laser pulse, the double ionization yield as a function of the laser intensity for Ar atoms with different initial angular momenta exhibit an obvious Knee structure. The ionization yield in the counter-rotating case where the electronic initial rotation direction is opposite to the vector potential of the laser field, significantly higher than that in the co-rotating case where the electronic initial rotation direction is consistent with the vector potential of the laser field. Based on the classical trajectory analysis, it is found that this ionization difference comes from the angle between the direction of the electronic momentum and the force of the laser field at the single ionization instant. The angle in the counter-rotating case is greater than that in the co-rotating case, and the counter-rotating electrons are more likely to return to the parent ion for recollision. Besides, the recollision intants of Ar atoms with the different initial angular momenta are mapped to the electron momentum spectrum. This may provide a method to obtain the electron’s recollision instant of Ar atoms with different initial angular momenta.

Two-dimensional retrieval methods for ultrafast imaging of molecular structure using laser-induced electron diffraction.

Molecular structural retrieval based on electron diffraction has been proposed to determine the atomic positions of molecules with sub-angstrom spatial and femtosecond temporal resolutions. Given its success on small molecular systems, in this work, we pointout that the accuracy of structure retrieval is constrained by the availability of a wide range of experimental data in the momentum space in all molecular systems. To mitigate the limitations, for laser-induced electron diffraction, here we retrieve molecular structures using two-dimensional (energy and angle) electron momentum spectra in the laboratory frame for a number of small molecular systems, which have previously been studied with 1D methods. Compared to the conventional single-energy or single-angle analysis, our 2D methods effectively expand the momentum range of the measured data. Besides utilization of the 2D data, two complementary methods are developed for consistency check on the retrieved results. The 2D nature of our methods also offers a way of estimating the error from retrieval, which has never been explored before. Comparing with results from prior experiments, our findings show evidence that our 2D methods outperform the conventional 1D methods. Paving the way to the retrieval of large molecular systems, in which their tunneling ionization rates are challenging to obtain, we estimate the error of using the isotropic model in place of including the orientation-dependent ionization rate.

Positron Annihilation Lifetime and Doppler Broadening Spectroscopy of polymers

Positron Annihilation Lifetime and Doppler Broadening spectroscopy of positron annihilation radiation are performed in four different polymers. The size of elementary free volume sites and their chemical environments are determined. It is shown that polymers with different groups of C–H, CO, and C–F can be distinguished by their orbital electron momentum spectrum. Correlation between parameters of the PALS and CDBS techniques in situations involving O-Ps formation at different chemical environments are clarified. The results indicate that both the S and W-parameters show a linear correlation with the size of elementary free volume sites of the polymers. Moreover, for polymers belonging to CO and C–F groups, the effect of positron trapping by polar groups on the probability of O-Ps formation is discussed.

Wavelength-dependent nonsequential double ionization of Ar at laser intensities below the recollision threshold

Using the semiclassical ensemble model, we investigated the correlated electron dynamics in the nonsequential double ionization (NSDI) of Ar driven by a few-cycle laser pulse at a fixed ponderomotive energy Up=0.2 a.u. and over a wide range of wavelength. When the laser wavelength increases from near-infrared to mid-infrared, the kinetic energy of the retuning electrons and the energy transfer efficiency of recollision both decrease and more NSDI events occur in the second half optical cycle after recollision, which leads to the switching of the electron momentum correlation along the laser polarization direction from side-by-side to back-to-back dominant emission. Our results also show that a significant part of NSDI events undergo a “hard recollision” through which the struck electron acquires more energy and ionizes earlier than the returning electron. This type of NSDI event is responsible for the high momentum part, especially the part with momentum higher than 2Up in the correlated electron momentum spectra along the laser polarization direction.

Internal collision induced strong-field nonsequential double ionization in molecules.

Using the classical ensemble method, we have investigated the alignment dependence of the correlated electron dynamics in strong-field nonsequential double ionization (NSDI) of diatomic molecules driven by linearly polarized laser pulses. Our numerical results show that the correlated electron pairs are more likely to emit into the same hemisphere (side-by-side emission) for the parallel aligned molecules at the small internuclear distance, in agreement with previous experimental results. Surprisingly, as the internuclear distance increases, this side-by-side emission is more prevalent for the perpendicularly aligned molecules. Back analyzing of the classical trajectories shows that a considerable part of the NSDI events for the parallel aligned molecules at the large internuclear distances occur through an internal collision, not the well-known recollision. In the internal collision induced NSDI, the first electron tunnels through the inner barrier from the up-field core, moves directly towards the other core, and kicks out the second electron. For this type of NSDI events, the electron pairs are more likely to emit into the opposite hemispheres and thus the correlated electron momentum spectrum exhibits a more dominant back-to-back behavior in the parallel aligned molecules.

High-resolution electron-momentum spectroscopy of the valence orbitals of the benzene molecule

The binding-energy spectrum and electron-momentum spectra of valence orbitals of benzene were measured via a high-resolution electron-momentum spectrometer at incident energies of 600 and 1200 eV plus the binding energy. The experimental momentum profiles were compared with the results from a plane-wave impulse approximation (PWIA) calculation using Dyson orbitals obtained from symmetry-adapted-cluster configuration-interaction calculations. For the ${1e}_{1\mathrm{g}}, {3e}_{2\mathrm{g}}, {3e}_{1\mathrm{u}}$, and ${2b}_{1\mathrm{u}}$ orbitals, the discrepancy between the experimental distributions and the PWIA calculations was tentatively assigned to the distorted wave effects. The current high-resolution electron-momentum spectroscopy results provide the experimental benchmark for rigorous distorted-wave calculations about benzene in the future.

Anomalous ellipticity dependence in nonsequential double ionization of ArXe

Using a three-dimensional classical ensemble method, we present a theoretical study of nonsequential double ionization of ArXe dimer aligned along the minor axis of the elliptically polarized laser pulse. Numerical results show that NSDI probability firstly increases and then decreases with the laser ellipticity increasing, which is different from atoms. Moreover, the correlated electron momentum spectra from elliptical polarization are always asymmetric, and the asymmetry is enhanced as the ellipticity increases. Analysis backward in time indicates that in NSDI of ArXe aligned along the minor axis the recollision occurs via a semi-elliptical trajectory.

Double ionization of neon in elliptically polarized femtosecond laser fields

We present a joint experimental and theoretical investigation of the correlated electron momentum spectra from strong-field double ionization of neon induced by elliptically polarized laser pulses. A significant asymmetry of the electron momentum distributions along the major polarization axis is reported. This asymmetry depends sensitively on the laser ellipticity. Using a three-dimensional semiclassical model, we attribute this asymmetry pattern to the ellipticity-dependent probability distributions of recollision time. Our work demonstrates that, by simply varying the ellipticity, the correlated electron emission can be two-dimensionally controlled and the recolliding electron trajectories can be steered on a subcycle time scale.

High-level theoretical study of the evolution of abundances and interconversion of glycine conformers**Project supported by the Young Scientists Funds of the National Natural Science Foundation of China (Grant Nos. 11404154 and 11304136).

The relative conformer energies of glycine are evaluated by using a focal point analysis expressed as (). The conformer abundances at various temperatures (298−500 K) are calculated based on the relative energies and Boltzmann statistical thermostatistical analysis with and without considering internal hindered rotations. A comparison between the available Raman spectrum and the electron momentum spectrum confirms that the influence of rigid-rotor hindered rotation on the conformational proportions of glycine is considerable, especially for the IIIp structure. The conformational interconversions are discussed. It is found that with increasing temperature, the mole fraction of IIn keeps constant and Ip structure can convert into IVn and IIIp, leading to the decrease in the weight of Ip and the increase in the weights of IVn and IIIp conformers, which is in accordance with experimental observations.

Rabi oscillation in few-photon double ionization through doubly excited states

We theoretically investigate few-photon double ionization of helium in intense XUV laser fields by numerically solving the time-dependent Schr\"odinger equation. Our results show that the familiar single-ring structure in the joint electron momentum spectra is split into the double-ring and previously unobserved triple-ring structures at some specific photon energies. By tracing the electron population evolution of the corresponding states, we found that the triple-ring structure is induced by the coupled Rabi oscillations among the ground, a singly excited, and a doubly excited states. The intermediate detuning causes the asymmetry of the triple-ring structures, which can be controlled by changing the laser intensity and frequency.

Identification of doubly excited states in nonsequential double ionization of Ar in strong laser fields

We use the semiclassical model to study the intensity dependence of nonsequential double ionization (NSDI) of Ar in short strong laser pulses. The contributions to NSDI through sequential ionization of doubly excited states (SIDE) are identified by tracking the energy trajectories of the two outgoing electrons. The correlated electron momentum distributions are calculated from which the longitudinal momentum distributions of the fast and the slow electrons for the side-by-side and the back-to-back emissions are obtained. The simulated momentum distributions of the fast and the slow electrons for NSDI of Ar by linearly polarized fields with a wavelength of 795 nm at an intensity of 7 × 1013 W cm−2 are in good agreement with the experimental measurements of Liu et al (2014 Phys. Rev. Lett. 112 013003). We demonstrate that the process of double ionization through SIDE dominates NSDI only when the laser intensities are below the recollision threshold; nevertheless, for higher intensities the SIDE process still takes place although the contribution to the NSDI yields decreases rapidly as the intensity increases. It has been found that for SIDE at different intensities, both the correlated electron momentum spectra and the momentum distributions of the fast and the slow electrons remain the same.

Correlated electron dynamics in strong-field nonsequential double ionization of Mg.

Using the classical ensemble model, we systematically investigate strong-field nonsequential double ionization (NSDI) of Mg by intense elliptically polarized laser pulses with different wavelengths. Different from the noble atoms, NSDI occurs for Mg driven by elliptically and circularly polarized laser fields. Our results show that in elliptically and circularly polarized laser fields, the NSDI yield is sharply suppressed as the wavelength increases. Interestingly, the correlated behavior in the electron momentum spectra depends sensitively on the wavelengths. The corresponding electron dynamics is revealed by back tracing the classical trajectory.

Universal time delay in the recollision impact ionization pathway of strong-field nonsequential double ionization

With the semiclassical ensemble model, we investigated the correlated electron dynamics in the recollision impact ionization (RII) pathway of strong-field nonsequential double ionization (NSDI) at different laser wavelengths. Our results show that there is a remarkable time delay (about 400 attoseconds) between double ionization and recollision for a considerable part of the RII events. This type of event is responsible for the valley structure in the correlated electron momentum spectra along the laser polarization direction. More interestingly, our results show that this time delay is independent of the wavelength. This universal time delay is of crucial importance for the comprehensive understanding of the correlated electron dynamics in NSDI.

CDBTools - Evaluate Positron Annihilation Coincidence Doppler Broadening Spectrum

CDBTools is a lightweight and easy to use application designed to provide you with an analysis tool for coincidence Doppler broadening (CDB) of the positron annihilation energy spectrum files. This application enables you to analyze the CDB and ratio curves, plots as well as graphs representing the evolution of the orbital electron momentum spectrum. The CDB extraction is provided by selectable filters applied at the diagonal line of the input matrix. To achieve CDB ratio curves with minimal error caused by 511keV peak shift, spectrum curves are recalculated by penalized regression spline.

Correlated electron dynamics in strong-field double ionization

In intense laser-atom/molecule interaction, there are many new phenomena involving multielectron effects. As one of the most important strong-field process, strong-field double ionization of atoms and molecules, especially the strong-field nonsequential double ionization (NSDI), exhibits highly correlated behaviors, providing a clean way to study electron correlation in nature. We review the microscopic electron dynamics of NSDI revealed by recent experimental studies based on the COLTRIMS and the related theory contributions. We summarized the laser intensity-dependent microscopic electron dynamics of NSDI. At low laser intensities, NSDI is dominated by the recollision-excitation with subsequent field ionization, and the time delay between the final ionizations of the two electrons could results in the experimental observed back-to-back emission of the electron pairs. As the laser intensity increases, recollision induced direct ionization dominates NSDI and two electrons ionized almost simultaneously after recollision. In this process the final-state ion-electron attraction and electron repulsion significantly affect the details of the electron correlation pattern. As the laser intensity further increases, the role of final-state ion-electron and electron-electron interactions becomes less important, and the asymmetric energy sharing at recollision is prevalent which accounts for the strange V-like shape in the correlated electron momentum spectra at very high laser intensity. We also introduced the control of the correlated electron dynamics. It has been theoretically predicted that the electron pairs can be controlled to exhibit correlated or anticorrelated behavior with the two-color field, which has been proved by experiment. We also review the progress on strong-field sequential double ionization (SDI). Recent experiments have observed many phenomena which conflicted with the predictions of the previous theoretical models of SDI. Here we introduced a recent developed model of strong-field SDI, which successfully explained the new experimental results and predicted many interesting results.

Strong-field ionization and gauge dependence of nonlocal potentials

Nonlocal potential models have been used in place of the Coulomb potential in the Schrodinger equation as an efficient means of exploring high field laser-atom interaction in previous works. Al- though these models have found use in modeling phenomena including photo-ionization and ejected electron momentum spectra, they are known to break electromagnetic gauge invariance. This paper examines if there is a preferred gauge for the linear field response and photoionization characteristics of nonlocal atomic binding potentials in the length and velocity gauges. It is found that the length gauge is preferable for a wide range of parameters.

Ionization in orthogonal two-color laser fields: Origin and phase dependences of trajectory-resolved Coulomb effects

We report on electron momentum distributions from single ionization of Ar in strong orthogonally polarized two-color (OTC) laser fields measured with the COLTRIMS technique. We study the effect of Coulomb focusing whose signature is a cusp like feature in the center of the electron momentum spectrum. While the direct electrons show the expected strong dependence on the phase between the two colors, surprisingly the Coulomb focused structure is almost not influenced by the weak second harmonic streaking field. This effect is explained by the use of a CTMC simulation which describes the tunneled electron wave packet in terms of classical trajectories under the influence of the combined Coulomb- and OTC laser field. We find a subtle interplay between the initial momentum of the electron upon tunneling, the ionization phase and the action of the Coulomb field that makes the Coulomb focused part of the momentum spectrum apparently insensitive to the weaker streaking field.

Origin of double-line structure in nonsequential double ionization by few-cycle laser pulses.

We investigate nonsequential double ionization (NSDI) of molecules by few-cycle laser pulses at the laser intensity of 1.2-1.5 × 10(14) W/cm(2) using the classical ensemble model. The same double-line structure as the lower intensity (1.0 × 10(14) W/cm(2)) is also observed in the correlated electron momentum spectra for 1.2-1.4 × 10(14) W/cm(2). However, in contrast to the lower intensity where NSDI proceeds only through the recollision-induced double excitation with subsequent ionization (RDESI) mechanism, here, the recollision-induced excitation with subsequent ionization (RESI) mechanism has a more significant contribution to NSDI. This indicates that RDESI is not necessary for the formation of the double-line structure and RESI can give rise to the same type of structure independently. Furthermore, we explore the ultrafast dynamics underlying the formation of the double-line structure in RESI.