Convergent close-coupling calculations of two-photon double ionization of helium

Kheifets, Anatoli; Ivanov, Igor

doi:10.1088/0953-4075/39/7/015

Cited by 48 publications

(52 citation statements)

References 35 publications

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“…1 clearly show that there is a strong backward-forward asymmetry. The bending of the left and right (symmetric) lobes has already been well documented in previous studies in He [5,11,13,[29][30][31], H − [25], and H 2 [26][27][28]32], and we will only comment briefly on this feature here. Assuming for the moment that the electrons absorb one photon each in the two-photon double ionization event, this would, to a zeroth-order approximation, give rise to a p-lobe structure (oriented along the laser polarization direction) in their respective angular distributions.…”

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confidence: 56%

Two-photon double ionization of helium by attosecond laser pulses: Evidence of highly correlated electron motion

2012

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We apply a recently developed ab initio numerical framework to investigate the angular distributions of the emitted electrons in the immediate proximity of the threshold for the two-photon double ionization of helium. Provided one of the electrons is emitted perpendicular to the laser polarization direction, it is found that the angular distribution of the other electron is characterized by three lobes. The results are similar to those recently reported for the corresponding process in the hydrogen negative ion [R. The problem of direct (nonsequential) two-photon double ionization of helium has been studied extensively in recent years, as exemplified by numerous theoretical [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and experimental [19][20][21][22][23][24] works. This breakup process is fundamental in the sense that it is one of the simplest processes in nature where electron correlations are exhibited, manifested by a rather complex interplay between the electrons. As such, a complete understanding of it will pave the way for further investigations of the role of correlations in few-photon and multiphoton multiple ionization processes in atoms and molecules.In the present Brief Report, we investigate the direct two-photon double ionization process of helium in the near vicinity of the lower threshold (i.e., for 40 eV photons), and with particular emphasis on the direction of ejection of the photo-electrons. In a recent work [25], it was found that the corresponding process in H − is characterized by a strong backward-forward asymmetry in the sense that if one electron is emitted perpendicular to the (linear) laser polarization direction then the other electron is emitted most preferably in the opposite direction, forming three characteristic lobes in the angular distribution. Similar features have also been observed theoretically in H 2 [26][27][28].Solving the time-dependent Schrödinger equation numerically for helium [14], the conditional angular distribution is obtained for both short (500 and 1000 as) and long (4 fs) linearly polarized laser pulses. We examine the case where one of the electrons is emitted perpendicular to the laser polarization direction, and integrate over the energy of both electrons. It is found that the direction of emission of the other electron is characterized by three lobes, concordant with the observations in H − [25]. Furthermore, with increasing pulse duration, the lobe pointing in the backward direction, representing electrons being emitted back-to-back, becomes relatively more important. The "backward" lobe is most distinct at lower photon energies, and already at a photon energy of 42 eV it loses its significance [11]. We therefore anticipate that the presence of the structure at lower photon energies is * sigurd.askeland@ift.uib.no † morten.forre@ift.uib.no a signature of a competing double ionization mechanism that becomes suppressed at higher photon energies.Figure 1 depicts our results for the angular distributions in the double ionization process. I...

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confidence: 56%

Two-photon double ionization of helium by attosecond laser pulses: Evidence of highly correlated electron motion

2012

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“…4(c)], symmetric emission remains dominant, while back-to-back emission is fading in to compete with symmetric emission for ε = 0.01, as suggested by Eq. (17).…”

Section: B Joint Angular Distributionsmentioning

confidence: 99%

“…Comprehensive numerical studies on the DI of helium following the absorption of a few XUV photons were carried out by Parker et al [16] starting 15 years ago. Following the 2005 experiment of Hasegawa et al [7], two-photon DI of helium has been the subject of several theoretical studies [15,[17][18][19][20][21][22][23][24][25][26][27][28]. In particular, Zhang et al [26] calculated joint angular distributions (JADs) for two-photon DI by XUV pulses in both the nonsequential (39.5 eV < ω XUV < 54.4 eV) and the sequential ( ω XUV > 54.4 eV) regimes for different energy sharings of the emitted electrons.…”

Section: Introductionmentioning

confidence: 99%

Laser-assisted XUV double ionization of helium: Energy-sharing dependence of joint angular distributions

Liu

Thumm

2015

Phys. Rev. A

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By numerically solving the time-dependent Schrödinger equation in full dimensionality, we discuss the dependance of joint photoelectron angular distributions on the energy sharing of the emitted electrons for the double ionization of helium atoms by ultrashort pulses of extreme ultraviolet (XUV) radiation in coplanar emission geometry with and without the presence of a comparatively weak infrared (IR) laser pulse. For IR-laser-assisted single-XUV-photon double ionization, our joint angular distributions show that the IR-laser field enhances back-to-back electron emission and induces a characteristic splitting in the angular distribution for electrons that are emitted symmetrically relative to the identical linear polarization directions of the XUV and IR pulse. These IR-pulse-induced changes in photoelectron angular distributions are (i) imposed by different symmetry constraints for XUV-pulse-only and laser-assisted XUV double ionization, (ii) robust over a large range of energy sharings between the emitted electrons, and (iii) consistent with the transfer of discrete IR-photon momenta to both photoelectrons from the assisting IR-laser field. While selection-rule forbidden at equal energy sharing, for increasingly unequal energy sharing we find back-to-back emission to become more likely and to compete with symmetric emission.

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“…Two-photon double ionization is much more difficult to detect than singlephoton double ionization due to its extremely small total cross section (≈10 −52 cm 4 s) [17][18][19]. It was first measured in 2005 [18], followed by many theoretical studies [16,17,[19][20][21][22][23][24][25].…”

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confidence: 99%

“…However, as pointed out by Kheifets and Ivanov [21], two-photon double-ionization angular distributions consist of five terms that include electronic correlation effects as similar Gaussian factors and can be divided into symmetrical and antisymmetrical components, in analogy to the single-photon double-ionization angular distributions discussed above. , separately normalized to their respective angleintegrated yields.…”

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confidence: 99%

Criterion for Distinguishing Sequential from Nonsequential Contributions to the Double Ionization of Helium in Ultrashort Extreme-Ultraviolet Pulses

Liu

Thumm

2015

Phys. Rev. Lett.

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We quantify sequential and nonsequential contributions in two-photon double ionization of helium atoms by intense ultrashort extreme-ultraviolet pulses with central photon energies ℏω ctr near the sequential double-ionization threshold. If the spectrum of such pulses overlaps both the sequential (ℏω > 54.4 eV) and nonsequential (ℏω < 54.4 eV) double-ionization regimes, the sequential and nonsequential doubleionization mechanisms are difficult to distinguish. By tracking the double-ionization asymmetry in joint photoelectron angular distributions, we introduce the two-electron forward-backward-emission asymmetry as a measure that allows the distinction of sequential and nonsequential contributions. Specifically, for ℏω ctr ¼ 50 eV pulses with a sine-squared temporal profile, we find that the sequential double-ionization contribution is the largest at a pulse length of 650 as, due to competing temporal and spectral constraints. In addition, we validate a simple heuristic expression for the sequential double-ionization contribution in comparison with ab initio calculations. DOI: 10.1103/PhysRevLett.115.183002 PACS numbers: 32.80.Rm, 33.80.Rv, 42.50.Hz In 1975, the mechanism of nonsequential double ionization was revealed in the photoionization of alkaline earth atoms [1]. It is enabled by strong electronic correlation and thus clearly distinct from the sequential double-ionization mechanism. Nonsequential double ionization was observed for noble gas atoms in 1982 [2] and received rapidly increasing attention from both experimentalists [3-9] and theorists [10][11][12][13].Photoelectron angular distributions of single-photon double ionization were found to owe their structure partly to (dipole) selection rules [11,14,15] and to consist of symmetrical and antisymmetrical contributions (with regard to electron exchange), that each can be written as the product of an angular and a correlation factor [11]. For coplanar emission geometry, where the emitted-electron momenta and polarization axis of the linearly polarized extremeultraviolet (XUV) pulse lie in a plane, and for equal energy sharing (equal asymptotic kinetic energies E 1 and E 2 of the photoelectrons), the angular factor becomes j cos θ 1 þ cos θ 2 j 2 , where θ 1 and θ 2 are photoelectron emission angles relative to the polarization direction of the ionizing light, while the electron correlation factor follows as expf−4 ln 2½ðθ 12 − πÞ=θ 1=2 2 g, with the mutual emission angle θ 12 ¼ jθ 1 − θ 2 j. The adjustable parameter, θ 1=2 , is related to the significance of correlation in the double-ionization process [11]. The antisymmetrical contribution and back-to-back electron emission vanish at equal energy sharing but become progressively more prominent for increasingly nonequal energy sharing, as the antisymmetrical contribution j cos θ 1 − cos θ 2 j 2 gradually appears in joint photoelectron angular distributions [10,11,16].

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Convergent close-coupling calculations of two-photon double ionization of helium

Cited by 48 publications

References 35 publications

Two-photon double ionization of helium by attosecond laser pulses: Evidence of highly correlated electron motion

Two-photon double ionization of helium by attosecond laser pulses: Evidence of highly correlated electron motion

Laser-assisted XUV double ionization of helium: Energy-sharing dependence of joint angular distributions

Criterion for Distinguishing Sequential from Nonsequential Contributions to the Double Ionization of Helium in Ultrashort Extreme-Ultraviolet Pulses

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