Double Ionization Of Helium Research Articles

Double ionization of helium by fast electron impact

We report on a kinematically complete investigation of He double ionization by 3 keV electron impact. A new experimental approach for experiments on electron impact ionization is applied: in a multi-electron recoil-ion coincidence the momentum vectors of the recoiling ion and of two slowly ejected electrons (E b,c < 20 eV) are determined. The momentum of the fast scattered electron follows from momentum conservation. By examining the double ionization process as a function of the momentum transferred to the target by the scattered projectile different kinematical regimes are separated. For small values of the momentum transfer the angular correlation and the energy partitioning of the slowly ejected electrons exhibit the characteristics of double photoionization. For Bethe ridge kinematics signatures of the TS 1 ionization mechanism are observed.

(e,3e) collisions on He in the quasi-dipolar regime

We report the first measurement of the angular distributions of the two ejected electrons following electron impact double ionization of helium, using a multi-parameter (e,3e) spectrometer. The high incident energy of these experiments allows comparison with photo-double ionization results. Comparison is also made with existing (e,3e) theoretical calculations.

Photo double-ionization of helium : A new approach combining R matrix and semiclassical techniques in an hyperspherical framework

We introduce a new method for computing photo double ionization (PDI) cross sections for two electron atoms. It is formulated in terms of the hyperspherical radius R and relies upon a combination of R matrix techniques in the inner region R≤R0 with a semi classical approximation for the R motion in the outer region. We present a first application of this method to the PDI of He within a model of reduced dimensionality where r1=r2. It demonstrates the validity of our numerical scheme and provides a first quantitative estimate of the energy domain of validity of the Wannier mechanism.

Origin of dips and peaks in the absolute fully resolved cross sections for the electron-impact double ionization of He

Using a multiparameter multicoincidence spectrometer, we have measured the coplanar $(e,3e)$ angular distributions following the double ionization of helium at an incident energy of $\ensuremath{\approx}5.6 \mathrm{keV}$ and under a small projectile's scattering angle of $0.45\ifmmode^\circ\else\textdegree\fi{}$. The two ejected electrons have been detected with equal energies ${E}_{b}{=E}_{c}=10 \mathrm{eV}$. The absolute value of the cross section is determined with an accuracy of $25%$. The origin of dips and peaks in the spectra is exposed by analyzing the corresponding theoretical calculations. These calculations have been done using a four-body final-state wave function for the three electrons moving in the field of ${\mathrm{He}}^{2+}$. The dipolar limit is investigated and the manifestation of the deviation from this limit are pointed out. General features and possible trends for other targets are proposed.

Double Ionization of Helium by Fast Electron Impact

Double ionization of He by 3-keV electron impact is explored in a kinematically complete experiment. The 12-dimensional, 4-particle final-state momentum space is mapped for two slowly ejected electrons $({\mathbf{k}}_{b,c}&lt;1.2\mathrm{a}.\mathrm{u}.)$, the ${\mathrm{He}}^{2+}$ ion $({\mathbf{k}}_{\mathrm{He}}&lt;5\mathrm{a}.\mathrm{u}.)$ and the fast electron for all relative emission angles and energy partitions. Different reaction mechanisms are identified by examining the angular correlation between the slow electrons as a function of the momentum transfer. A fivefold differential cross section is presented for asymmetric energy sharing in coplanar geometry.

Interplay of electron correlation and intense field dynamics in the double ionization of helium

A nonsequential ionization model based on dynamic electron correlation is used to analyze the intensity dependence of the laser-induced single- and double-ionization yields for He at five different laser frequencies. Predictions of the present theoretical model and the recent experimental results are found to agree remarkably well over the entire intensity range. The presence of the ubiquitous ``knee'' structure at the optical wavelengths and its absence at the ultraviolet wavelength are also reproduced by the theory.

Coupled-channel calculations of ionization and excitation cross sections for ion-helium collisions

Using the semiclassical approximation we calculate cross sections for excitation, single and double ionization of helium in collisions with , and in the energy range between and . The method is the solution of time-dependent coupled-channel equations for the amplitudes of two-electron basis states. The basis states are obtained by diagonalizing the Hamiltonian of the helium atom in a finite basis of antisymmetrized products of hydrogen-like Slater functions. The projectile-electron interaction takes the retardation effects into account. The results are compared with experimental data and other theoretical approaches.

Double ionization of helium in fast ion collisions: the role ofmomentum transfer

Double ionization of helium in the perturbative regime has been explored in a kinematically complete collision experiment using 100 MeV/u C6+ ions. Different ionization mechanisms are identified by inspecting the angular distribution of the electrons as a function of the momentum transfer q to the target by the projectile. For q < 1.2 au, both electrons are distributed uniformly in the plane perpendicular to the projectile axis, and distinct similarities with photoionization are identified. For q > 1.2 au, the faster electron resulting from a binary encounter with the projectile is emitted along the direction of momentum transfer, while the other electron is distributed uniformly. Experimental data are compared with various model calculations based on the Bethe-Born approximation with shake-off. Surprisingly, the effect of the final state interaction is found to depend decisively on the choice of the initial state wavefunction.

Double Ionization of Helium by Fast Fully Stripped Ions

A complete momentum-space map of the double ionisation events in the collision of 100 MeV/u C6+ ions with helium has been obtained. From this map the angular distribution of two ejected electrons is generated as a function of the momentum transferred by the projectile to the atom. Analysis of the angular distribution of the fragments in the plane transverse to the projectile axis (the azimuthal plane) shows a separation of events into two domains, depending on the momentum transferred by the projectile to the target. For momentum transfers smaller than 1.2 a.u., both electrons are distributed independent of the azimuthal angle. For momentum transfers larger than 1.2 a.u., the electron with larger energy is distinctly emitted along the direction of momentum transfer, and the one with smaller energy is distributed isotropically. The value of 1.2 a.u. is approximately equal to the mean value of the momenta of the bound electrons in the helium atom and demarcates indirect, soft collisions and direct, hard collisions respectively. For soft collisions, the electron angular distribution shows certain similarities with the angular distributions in photo-double ionization.

Photo-double ionization of molecular hydrogen

The angular distribution of the correlated electron pair emitted in single-photon double ionization of the hydrogen molecule is analysed and calculated using a variety of approximations. Attention is directed particularly towards the differences between the molecular angular distribution and that of the corresponding `united atom', i.e. that arising from the double ionization of helium. Qualitative agreement is obtained with recent experiments on photo-double ionization of the hydrogen molecule. The major effects arising from the two-centre nuclear field of the molecule and the orientation of the axis at the moment of photon absorption are exposed in the simpler problem of photoionization of the H2+ ion.

Ionization of Helium and Neon by fast Ion Impact

Single and double ionization of helium and neon by 1 GeV/u U92+ ion impact was explored in a kinematically complete experiment using multi-electron recoil-ion momentum spectroscopy. Cross sections, the emission characteristics of ions and electrons as well as the momentum balance are quantitatively discussed in terms of photoionization of the atom in the field of virtual photons generated by the projectile (Weizsäcker-Williams 1934). Electron-electron correlation after double ionization of neon as well as a first result from the recoil-ion momentum spectrometer at the ESR of GSI are presented.

Calculation of electron- and photon-impact ionization via a close-coupling approach

We discuss the applications of the Convergent Close-Coupling method to electron-impact ionization and photon-impact double ionization of helium.

Strong Suppression of the Positronium Channel in Double Ionization of Noble Gases by Positron Impact

Positron-induced double ionization of helium and neon has been studied at energies from threshold to 900 eV. A remarkable difference between the near-threshold behavior of the single and double ionization cross sections is found: Single ionization is dominated by positronium (Ps) formation, while for double ionization the Ps channel is absent or strongly suppressed. Absolute cross sections for the two targets have been derived and are compared to a modified Rost-Pattard parametrization. Good agreement is found, showing that this type of model can be extended from single to double ionization.

A study of the two-step mechanism in the double ionization of helium by fast electrons

In view of recent experiments it can be suggested that the two-step process is not negligible even at high impact energy. An ab initio calculation has been performed on the basis of the second Born approximation using different correlated wavefunctions for the initial bound state. This calculation also shows that the contribution of the two-step process must be included at any impact energy.

Ionization of Helium in the Attosecond Equivalent Light Pulse of 1 GeV/NucleonU92+Projectiles

Single and double ionization of helium by 1GeV/nucleon U{sup 92+} impact was explored in a kinematically complete experiment. The relativistic ion generates a subattosecond (10{sup {minus}18} s ) superintense (I{gt}10{sup 19} W/cm {sup 2} ) electromagnetic pulse, which is interpreted as a field of equivalent photons (Weizs{umlt a}cker-Williams method). Cross sections, the emission characteristics of ions and electrons as well as momentum balances, are quantitatively discussed in terms of photoionization of the atom in this broadband, ultrashort virtual photon field. {copyright} {ital 1997} {ital The American Physical Society}

Rmatrix with pseudostates calculation for single and double ionization of helium by photon impact

We have applied the $R$ matrix with pseudostates (RMPS) method to calculate single and double ionization of helium by photon impact. Using the velocity gauge of the dipole operator, excellent agreement with high-precision experimental data is obtained for photon energies between threshold and 200 eV. While some discrepancies remain between results from different gauges, we demonstrate that the RMPS method can produce satisfactory results if either the length, velocity, or acceleration form of the dipole operator is used.

Equivalent-photon method study of differential cross sections for double ionization of helium by relativistic heavy-ion impact

We have calculated the probability for double ionization of helium by fast heavy-ion impact in the equivalent-photon picture. We find that the {ital shape} of differential cross sections is significantly affected by the inclusion of the electron-electron interaction in both the initial and the final channel, but is rather insensitive to the projectile charge and impact velocity. {copyright} {ital 1997} {ital The American Physical Society}

Energetic electrons in strong-field ionization.

A strong-field theory is extended to the description of energetic photoelectron spectra. Experimental spectra are accurately described for both linear and circular polarizations, subject only to a single adjustment of reported peak laser intensities. Examined are single and (sequential) double ionization of helium, to energies of about 1 keV. An approximation of the spectrum as a Maxwellian distribution yields a temperature dependent on the spectrometer viewing angle. Final-state Coulomb effects in circular polarization are examined.

Double Ionization of Helium and Neon for Fast Heavy-Ion Impact: Correlated Motion of Electrons from Bound to Continuum States.

Double ionization of He and Ne by charged particle impact was explored in kinematically complete experiments measuring the final momenta of the two emerging electrons $({\mathbf{P}}_{e1}^{f},{\mathbf{P}}_{e2}^{f})$ and of the recoiling target ion ${\mathbf{P}}_{R}^{f}$. The momentum transfer from the $3.6\mathrm{MeV}/u$ ${\mathrm{Se}}^{28+}$ projectile to the electrons is found to be negligibly small. Thus distinct patterns observed in the ${\mathbf{P}}_{e1}^{f}$-versus- ${\mathbf{P}}_{e2}^{f}$ spectra sensitively reflect the details of the electrons' correlated motion. Classical calculations reasonably describe the data only when the $(e\ensuremath{-}e)$ interaction during the ionization reaction is included.

Consistency test of double ionization of helium by photons and charged particles.

We have performed a consistency test of recent data for the ratios of cross sections of double to single ionization of helium by photons and by fast charged particles. It is based on a relationship that connects the dipole and nondipole components of cross sections by charged particle impact to photoionization and Compton scattering, respectively. We find the high energy ratio {ital R}{sub {ital Z}} for charged particles to be a sensitive probe of the photoionization data near threshold. Using recent experimental data for photoionization, the asymptotic ratio {ital R}{sub {ital Z}} is found to range from 0.240{percent} to 0.295{percent}, as compared with direct experimental measurement of 0.26{percent}. {copyright} {ital 1996 The American Physical Society.}