The double ionization cusp in the threshold photoelectron spectrum of helium

Cvejanović, S; Shiell, Ralph C.; Reddish, T J

doi:10.1088/0953-4075/28/23/002

Cited by 19 publications

(20 citation statements)

References 26 publications

(14 reference statements)

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“…We remark here that the Wannier treatment supported by experimental evidence [3,11,32] indicates a basically flat energy distribution at threshold. In contrast, the CDS2C and 3C models predict a sharply peaked energy distribution around equal-energy sharing of the outgoing electrons.…”

Section: A Total Cross Sections and Excess-energy Distributionssupporting

confidence: 72%

A comparative study on the spin asymmetry and integrated cross sections for the electron-impact ionization of atomic hydrogen

Berakdar

O’Mahony

Mota-Furtado

1997

Zeitschrift f�r Physik D Atoms, Molecules and Clusters

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The spin asymmetry and integrated spin-unresolved cross sections for the electron-impact ionization of atomic hydrogen are studied. The influence of individual final-state interactions on these quantities is elucidated by investigating the collision process within the plane-wave impulse approximation, the first Born approximation and within an independent Coulomb particle model. The predictions of the latter approximation for the shape and magnitude of the spin asymmetry are in good agreement with experimental data over the entire measured energy range. The effects of final-state electronic correlations are investigated by the inclusion of the electronic Coulomb density-of-states factor as well as by employing a three-body Coulomb wave-function for the description of the final state. For ionization of hydrogenlike ions the calculated asymmetry parameter has a positive slope near threshold indicating that such an energy dependence is due to the electron-nucleus interactions. At threshold the results are analyzed in light of the Wannier theory of threshold ionization. PACS: 34.80.Dp I IntroductionElectron-impact ionization of atomic hydrogen is the simplest process leading to three continuum charged particles. Hence, this reaction provides an ideal testing ground for theoretical models treating the motion of Coulomb particles in the continuum. Full information on this collision process is obtained by measuring the ionization rate for given vector momenta and spin states of the two escaping electrons. Various integrated cross sections test different aspects of the ionization dynamics. For example, it has been shown by Wannier [1] that at threshold the analytical dependence of the total ionization cross section on the total excess energy derives from the phase-space volume available for double escape. Ever since an immense amount of experimental and classical, semi-classical and quantum mechanical theoretical investigations (see e.g. [1][2][3][4][5][6][7][8][9][10][11][12][13]) have been carried out to test the Wannier prediction of the slope of the total cross section. The original Wannier theory assumes SC-states only for the outgoing electrons. Thus, the Wannier theory provides no estimate of the spin states occupied by the electrons in the final channel. A measure of the relative magnitudes of singlet to triplet scattering is given by the spin asymmetry A [14-17]. Upon extension of the Wannier treatment to arbitrary¸, S and (total angular momentum, total spin and parity) [6,18], it has been concluded that all¸-states have the same energy dependence at threshold and nearly all¸S -states (in particular singlet and triplet states) possess the same threshold law. These findings allow one to conclude that A does not depend on the excess energy near threshold although an exact value of A is not provided. Recently, however, an inspection of near threshold experimental data for the spin asymmetry revealed a slightly positive slope for A with increasing excess energy [19]. This behaviour has not been confirmed by recent c...

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Section: A Total Cross Sections and Excess-energy Distributionssupporting

confidence: 72%

A comparative study on the spin asymmetry and integrated cross sections for the electron-impact ionization of atomic hydrogen

Berakdar

O’Mahony

Mota-Furtado

1997

Zeitschrift f�r Physik D Atoms, Molecules and Clusters

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“…Intensive investigations of the DPI cross section of He were undertaken by detecting one of the ejected photoelectrons (see, e.g., [25][26][27][28][29]). The measured Wannier exponent was in agreement with the theoretical value of 1.056 assuming a flat energy distribution between the ejected electrons.…”

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confidence: 99%

Oscillations in the Double-Photoionization Cross Section of Li near Threshold

2002

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The threshold region of the double-photoionization cross section of lithium was investigated using monochromatized synchrotron radiation and ion time-of-flight spectrometry. While the overall energy dependence can be described by the Wannier power law, we found oscillations in the cross section which are in good agreement with a modulated threshold law as proposed by Temkin [Phys. Rev. Lett. 49, 365 (1982)]]. This behavior may be due to the unequal binding energies of the electrons involved in the double-photoionization process.

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“…Together the probabilities represent 3-body events where both electrons have to participate in the scattering process simultaneously. Experimentally such events have been detected by measuring extremely slow ("zero kinetic energy", ≈ 0) electrons produced in electron-atom collisions [10] or in photoionization [13]. Assuming that the energy distribution of the electron dσ/d is almost flat for energies E ≈ 0 [10], [16] the probability for three-body events, as discussed above, is related to the slow-electron spectrum through The constant of proportionality σ ± is expected to be different for negative and positive energies E. The actual ratio σ − /σ + depends on experimental parameters (such as pressure in the gas cell) and is not yet fully understood [13].…”

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confidence: 99%

What happens to two-electron resonances when their energy approaches the break-up threshold?

Rost

Wintgen

1996

Europhys. Lett.

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PACS. 34.80Dp -Atomic excitation and ionization by electron impact. PACS. 03.65Sq -Semiclassical theories and applications. PACS. 31.10+z -Theory of electronic structure, electronic transitions, and chemical binding.Abstract. -Using semiclassical S-matrix theory in a simulated electron-hydrogen scattering experiment and converged quantum calculations for the widths of high-lying resonances in H − we show that resonance formation (or the lifetime of resonances) below the fragmentation threshold E = 0 and ionization above threshold have the same energy dependence |E| 1.127 for |E| → 0. Hence, series of resonances which have been characterized by approximate quantum numbers will disappear towards threshold.

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The double ionization cusp in the threshold photoelectron spectrum of helium

Cited by 19 publications

References 26 publications

A comparative study on the spin asymmetry and integrated cross sections for the electron-impact ionization of atomic hydrogen

A comparative study on the spin asymmetry and integrated cross sections for the electron-impact ionization of atomic hydrogen

Oscillations in the Double-Photoionization Cross Section of Li near Threshold

What happens to two-electron resonances when their energy approaches the break-up threshold?

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