Double Ionization Of He Research Articles

Evolution of radial and energy distributions in two-photon double ionization of He in near-femtosecond fields

Evolution of radial and energy distributions in two-photon double ionization of He in near-femtosecond fields

Quasifree Photoionization under the Reaction Microscope

We experimentally investigated the quasifree mechanism (QFM) in one-photon double ionization of He and H2 at 800 eV photon energy and circular polarization with a COLTRIMS reaction microscope. Our work provides new insight into this elusive photoionization mechanism that was predicted by Miron Amusia more than four decades ago. We found the distinct four-fold symmetry in the angular emission pattern of QFM electrons from H2 double ionization that has previously only been observed for He. Furthermore, we provide experimental evidence that the photon momentum is not imparted onto the center of mass in quasifree photoionization, which is in contrast to the situation in single ionization and in double ionization mediated by the shake-off and knock-out mechanisms. This finding is substantiated by numerical results obtained by solving the system’s full-dimensional time-dependent Schrödinger equation beyond the dipole approximation.

Electron-impact double ionization of He (1s2s 3 S)

A time-dependent close-coupling method for three-electron atomic systems is used to calculate the electron-impact double ionization of the He (1s2s 3 S) excited state. Partial cross sections are calculated for total angular momenta from . An exponential fitting expression is used to extend the calculated partial cross sections to . The total cross sections at three different incident energies are compared with recent crossed-beams measurements and are found to lie within the experimental error bars. The magnitude of the double ionization cross section is found to be similar to that of the double ionization cross section from the ground state of He.

Small recoil momenta double ionization of He and two-electron ions by high energy photons

We calculate various differential and double differential characteristics of ionization by a single photon for H−, He and for the two-electron ions with Z = 3, 4, 5 in the region of the so-called quasi-free mechanism (QFM) domination. We employ highly accurate wave functions at the electron-electron coalescence line where coordinates of both ionized electrons coincide. We trace the Z dependence of the double differential distributions. For all considered targets we discuss the dependence of the photoelectron energy distribution on the photon energy. Our calculation demonstrated the rapid decrease of QFM contribution with increase of the difference in energy of two outgoing electrons, and with decrease of the angle between two outgoing momenta. As a general feature, we observe the decrease of QFM contribution with nuclear charge growth.

Revealing the two-electron cusp in the ground states of He and H2 via quasifree double photoionization

We report on kinematically complete measurements and ab initio non-perturbative calculations of double ionization of He and H2 by a single 800 eV circularly polarized photon. We confirm the quasifree mechanism of photoionization for H2 and show how it originates from the two-electron cusp in the ground state of a two-electron target. Our approach establishes a new method for mapping electrons relative to each other and provides valuable insight into photoionization beyond the electric-dipole approximation.

Fingerprints of slingshot non-sequential double ionization on two-electron probability distributions

We study double ionization of He driven by near-single-cycle laser pulses at low intensities at 400 nm. Using a three-dimensional semiclassical model, we identify the pathways that prevail non-sequential double ionization (NSDI). We focus mostly on the delayed pathway, where one electron ionizes with a time-delay after recollision. We have recently shown that the mechanism that prevails the delayed pathway depends on intensity. For low intensities slingshot-NSDI is the dominant mechanism. Here, we identify the differences in two-electron probability distributions of the prevailing NSDI pathways. This allows us to identify properties of the two-electron escape and thus gain significant insight into slingshot-NSDI. Interestingly, we find that an observable fingerpint of slingshot-NSDI is the two electrons escaping with large and roughly equal energies.

Parametrization of energy sharing distributions in direct double photoionization of He

We present experimental results on the characteristic sharing of available excess energy, ranging from 11–221 eV, between two electrons in single-photon direct double ionization of He. An effective parametrization of the sharing distributions is presented along with an empirical model that describes the complete shape of the distribution based on a single experimentally determinable parameter. The measured total energy sharing distributions are separated into two distributions representing the shake-off and knock-out parts by simulating the sharing distribution curves expected from a pure wave collapse after a sudden removal of the primary electron. In this way, empirical knock-out distributions are extracted and both the shake-off and knock-out distributions are parametrized. These results suggest a simple method that can be applied to other atomic and molecular systems to experimentally study important aspects of the direct double ionization process.

Two- and three-photon double ionization of helium by soft x-ray free-electron laser pulses

The multiphoton double ionization of He was investigated using a soft x-ray free-electron laser (SX-FEL), in the perturbative intensity regime (<1013 W cm−2). The photon energy was tuned to the transition between the 1s and 2p levels of He+ ions. At this resonance energy of 40.8 eV, both sequential three-photon double ionization (S3PDI) and two-photon double ionization (2PDI) were measured by electron spectroscopy and by ion time-of-flight mass spectroscopy. We distinguished S3PDI from 2PDI by electron spectroscopy. Based on these experimental data, SX-FEL pulse-energy dependences of electron and ion yields for both S3PDI and 2PDI were discussed. The angular distribution of 2p photoelectrons emitted from He+(2p) ions in the S3PDI process was examined as an example of the simplest photoemission dynamics.

Absolute total cross sections for electron-impact double ionization of He(1s2s3S) and He−(1s2s2p4P)

We report on the experimental determination of absolute, total cross sections for electron-impact double ionization of helium in its metastable 1s 2s 3 S state and of the helium negative ion He− (1s 2s 2p 4 P). Cross sections are measured for impact energies ranging from threshold to 1000 eV using the animated crossed beams technique. The fast, pure beam of metastable helium required for the experiment is produced by photodetachment of fast He− ions. Results for He(1s 2s 3 S) are compared to those for He(1s 2 1 S), and exhibit similar magnitude and behaviour. The cross section for He− is large compared to other negative ions and indicates the predominance of indirect ionization channels at large impact energies.

Multiphoton double ionization of helium using ultraviolet and infrared laser pulses

A time-dependent close-coupling method is used to calculate the multiphoton double ionization of He using a combination of ultraviolet and infrared laser pulses. Total probabilities for the two photon double ionization of He using a laser pulse at 42 eV and 1014 W cm−2 are compared with those in the presence of an additional laser pulse at 1.5 eV and a variety of intensities. Momentum space wavefunction densities calculated for the multiphoton double ionization of He at 42 eV and 1014 W cm−2 are compared with those in the presence of additional laser pulse at 1.5 eV and 1012 W cm−2. Single and triple differential probabilities for the two and three photon double ionization of He using a laser pulse at 42 eV and 1014 W cm−2 are compared with those in the presence of an additional laser pulse at 1.5 eV and 1012 W cm−2.

Single and double over-barrier ionization of He, He+ and Ne system by positron impact

The classical over-barrier ionization model (COBI) method and trajectory calculations were utilized to simulate the ionization of He+ impacted by a positron. The calculated ionization cross sections of He+ agree with other theoretical data. Additionally, we found that the double ionization of He has a definite association with the positron-He+ impact. This result can explain why doubly ionized He seemed to be positron-scattered by the rest of the He+ in our previous study. The COBI model was also extended to study the double ionization caused by positron-Ne impacts. Our theoretical results agree with the experimental data.

Wavelength Dependence of the Two Channels for Double Ionization of He in Intense Laser Field

Wavelength Dependence of the Two Channels for Double Ionization of He in Intense Laser Field

Dynamic Interference Photoelectron Spectra in Double Ionization: Numerical Simulation of 1D Helium**Supported by the National Natural Science Foundation of China under Grant Nos 61178028, 11674243 and 11674242, and the National Basic Research Program of China under Grant No 2015CB755403.

We report our numerical simulation on the dynamic interference photoelectron spectra for a one-dimensional (1D) He model exposed to intense ultrashort extreme ultraviolet (XUV) laser pulses. The results demonstrate an unambiguous interference feature in the photoelectron spectra, and the interference is unveiled to originate from the dynamic Stark effect. The interference photoelectron spectra are prompted for intense sub-femtosecond XUV laser pulses in double ionization. The stationary phase picture is corroborated qualitatively in the two-electron system. The ability of probing the dynamic Stark effect by the photoelectron spectra in a pragmatic experiment of single-photon double ionization of He may shed light on further investigation on multi-electron atoms and molecules.

Multiphoton double ionization of helium using femtosecond laser pulses

A time-dependent close-coupling method is used to calculate the multiphoton double ionization of He using femtosecond laser pulses. Total double ionization probabilities are calculated for 2, 3, 4, and 5 photon absorption in the photon energy range from 10 to 60 eV. Single and triple differential probabilities are calculated for 2, 3, 4, and 5 photon absorption at energies where the total ionization probability is near a maximum. For circular polarization the total and differential probabilities are consistently smaller compared to linear polarization as the number of photons absorbed is increased while keeping the radiation field intensity constant. For linear polarization, the total and differential probabilities vary substantially as a function of photons absorbed due to the presence of more absorption pathways.

Five-photon double ionization of helium

A time-dependent close-coupling method is used to calculate the five-photon double ionization of He. It is found that the generalized cross section used in the past for two-photon double ionization of He cannot be extended to five-photon double ionization of He. Therefore only five-photon double ionization probabilities that depend on specific radiation field pulses can be calculated.

Virtual Sequential Picture for Nonsequential Two-Photon Double Ionization of Helium.

By using a model based on the second-order time-dependent perturbation theory, we show that the nonsequential two-photon double ionization of He can be understood in a virtual sequential picture: to excite the final double continuum state |k_{1},k_{2}⟩ by absorbing two photons from the ground state |1s^{2},^{1}S_{0}⟩, the single continuum states |1s,k_{1}⟩ and |1s,k_{2}⟩ serve as the dominant intermediate states. This virtual sequential picture is verified by the perfect agreement of the total ionization cross section, respectively, calculated by this model and by the sophisticated numerical solution to the full-dimensional time-dependent Schrödinger equation. This model, without the consideration of the electron correlation in the final double continuum state, works well for a wide range of laser parameters extending from the nonsequential to the sequential regime. The present Letter demonstrates that the electron correlation in the final double continuum state is not important in evaluating the total cross section, while it is indispensable for an accurate computation of a triply differential cross section. In addition, the virtual sequential picture bridges the sequential and nonsequential two-photon double ionization and reveals connections and distinctions between them.

Double ionization inR-matrix theory using a two-electron outer region

We have developed a two-electron outer region for use within $R$-matrix theory to describe double ionization processes. The capability of this method is demonstrated for single-photon double ionization of He in the photon energy region between 80 and 180 eV. The cross sections are in agreement with established data. The extended $R$-matrix with time dependence method also provides information on higher-order processes, as demonstrated by the identification of signatures for sequential double ionization processes involving an intermediate ${\mathrm{He}}^{+}$ state with $n=2$.

Energy differential cross sections for F9+-impact single and double ionization of He

Time-dependent close-coupling methods are used to calculate energy differential cross sections for the single and double ionization of He by impact with F9+ ions at 4.0 MeV amu−1. Single ionization energy differential cross sections using both a one active electron method and a two active electron method are compared with recent experimental results. Double ionization energy differential cross sections using a two active electron method are presented to guide future experiments.

Double ionization of helium by ion impact: second Born order treatment at the fully differential level

In this work, a theoretical study of the double ionization of He by ion impact at the fully differential level is presented. Emphasis is made in the role played by the projectile in the double emission process depending on its charge and the amount of momentum transferred to the target. A Born-CDW model including a second-order term in the projectile charge is introduced and evaluated within an on-shell treatment. We find that emission geometries for which the second-order term dominates lead to asymmetric structures around the momentum transfer direction, a typical characteristic of higher order transitions.

Nonlinear Dichroism in Back-to-Back Double Ionization of He by an Intense Elliptically Polarized Few-Cycle Extreme Ultraviolet Pulse.

Control of double ionization of He by means of the polarization and carrier-envelope phase (CEP) of an intense, few-cycle extreme ultraviolet (XUV) pulse is demonstrated numerically by solving the six-dimensional two-electron, time-dependent Schrödinger equation for He interacting with an elliptically polarized XUV pulse. Guided by perturbation theory (PT), we predict the existence of a nonlinear dichroic effect (∝I^{3/2}) that is sensitive to the CEP, ellipticity, peak intensity I, and temporal duration of the pulse. This dichroic effect (i.e., the difference of the two-electron angular distributions for opposite helicities of the ionizing XUV pulse) originates from interference of first- and second-order PT amplitudes, allowing one to probe and control S- and D-wave channels of the two-electron continuum. We show that the back-to-back in-plane geometry with unequal energy sharing is an ideal one for observing this dichroic effect that occurs only for an elliptically polarized, few-cycle attosecond pulse.