Two-photon double ionization of helium above and below the threshold for sequential ionization

Horner, D. A.; Morales, Felipe; Rescigno, T. N.; Martín, Fernando; McCurdy, C. William

doi:10.1103/physreva.76.030701

Cited by 111 publications

(145 citation statements)

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“…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.…”

supporting

confidence: 73%

“…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.…”

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confidence: 99%

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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: 99%

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

2012

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“…In the data for 84 eV and 99 eV features an order of magnitude smaller are visible, which occur at momenta corresponding to a two photon process (shown by the arrows in figure 4) where the bound He + electron is excited to either the 2s or 2p state before being ionised by the second photon (as discussed in detail in [25,26]). While these processes should also be present in the 125 eV, 150 eV and 180 eV spectra, they occur at angles where the 1s process dominates, making them difficult to observe.…”

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confidence: 99%

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

2015

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We have developed a two-electron outer region for use within R-matrix theory to describe double ionisation processes. The capability of this method is demonstrated for single-photon double ionisation of He in the photon energy region between 80 eV to 180 eV. The cross sections are in agreement with established data. The extended RMT method also provides information on higherorder processes, as demonstrated by the identification of signatures for sequential double ionisation processes involving an intermediate He + state with n = 2.The development of laser sources capable of generating ultra-short light pulses or intense XUV and X-ray laser light [1][2][3] has put additional emphasis on the investigation of multiple ionisation processes [4]. To complement recent experimental developments, there is a need for new computational techniques able to describe the multiple ionisation of general atoms. Although progress has been made in the description of double photoionisation of general atoms [5], further progress is needed to enable investigation of these processes in a time-dependent manner. This is of particular importance in experiments involving ultra-short light pulses.One possible route to develop capability for the description of double-ionisation processes is R-Matrix theory. Time-independent R-matrix theory has already been extended for this purpose. The R-matrix with pseudostates technique (RMPS) utilises a large basis set in the inner region, which includes residual-ion states in the continuum. Although the first electron can escape, the second must remain bound. Nevertheless, information on double ionisation can be obtained through evaluating the probability that the residual ion is left in an excited state above the ionisation threshold [6]. A second R-matrix approach for double ionisation is intermediate-energy Rmatrix (IERM) theory, in which two electrons are allowed to escape the inner region. This leads to a 2-dimensional propagation of the R-matrix over a large distance where the R-matrix is matched to asymptotic solutions. This approach was initially applied to scattering [7,8], and has recently been applied to double photoionisation [9].One promising computational method for solving the time-dependent Schrödinger equation (TDSE) for many electron atoms in laser fields is R-Matrix with timedependence (RMT) [10][11][12]. With electron-electron interactions fully incorporated, RMT has been used to successfully model a number of features of many-electron atoms undergoing photo-ionisation [13,14]. In RMT, an R-Matrix basis set inner region [15] is attached to an outer-region finite difference grid [16] with the ability to describe an ionised electron, enabling the physical processes in each region to be modelled with an appropriate numerical method.While previous applications of RMT to doubleionisation [17] have calculated accurate two-photon cross sections, the range of observables obtainable with this method is narrowed by an upper limit on the radial distance travelled by the inner ionising electron,...

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“…The simulation uses spectroscopic data 7,15,16 and takes into account our experimental parameters (see Supplementary Methods). The excellent agreement between experimental and simulated spectra allows us to unambiguously identify the majority of the observed features.The infrared field required for our interferometric measurement opens new processes for two-electron ejection [17][18][19][20][21] . Ionization takes place in a 'non-sequential' process with the absorption of two photons from the atomic ground state or via a 'sequential' process, where the XUV photon is absorbed by the neutral atom, and the infrared photon by the ion in an excited state.…”

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confidence: 99%

“…The infrared field required for our interferometric measurement opens new processes for two-electron ejection [17][18][19][20][21] . Ionization takes place in a 'non-sequential' process with the absorption of two photons from the atomic ground state or via a 'sequential' process, where the XUV photon is absorbed by the neutral atom, and the infrared photon by the ion in an excited state.…”

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confidence: 99%

Double ionization probed on the attosecond timescale

et al. 2014

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Double ionization following the absorption of a single photon is one of the most fundamental processes requiring interaction between electrons 1-3 . Information about this interaction is usually obtained by detecting emitted particles without access to real-time dynamics. Here, attosecond light pulses 4,5 , electron wave packet interferometry 6 and coincidence techniques 7 are combined to measure electron emission times in double ionization of xenon using single ionization as a clock, providing unique insight into the two-electron ejection mechanism. Access to many-particle dynamics in real time is of fundamental importance for understanding processes induced by electron correlation in atomic, molecular and more complex systems.The emergence of attosecond science (1 as = 10 −18 s) in the new millennium opened an exciting area of physics bringing the dynamics of electron wave functions into focus. The important goal of real-time visualization of the interplay between electrons and their role in molecular bonding now seems to be in reach. After a decade where attosecond light sources 4,5 were characterized and their potential demonstrated, the next phase will include the exploration of correlated electron dynamics in complex systems. A series of ground-breaking studies on single ionization (SI) in atoms using attosecond light pulses sheds light on the escaping electron and its interaction with the residual ion 6,8 , and the resulting coherent superposition of neutral bound states 9,10 . Double ionization (DI) by absorption of a single photon is an inherently more challenging phenomenon, both experimentally and theoretically 1-3 . The two-electron ejection can be understood only through interactions between electrons, and is usually discussed in terms of different mechanisms 11 . In the knockout mechanism, the electron excited by interaction with the light field (the photoelectron) collides with another electron on its way out, resulting in two emitted electrons. In the shake-off mechanism, orbital relaxation following the creation of a hole ionizes a second electron. Electron correlations may also lead to indirect DI processes via highly excited states of the singly-charged ion 12 . One-photon experimental investigations with the pair of electrons detected in coincidence can provide a fairly complete DI description without, however, following the dynamics of the electron correlation in real time. Multiphoton experimental investigations have been performed both on the femtosecond and attosecond timescales 13,14 , but DI in strong laser fields does not require electron correlation.In this work, we study DI of xenon in the near-threshold region using attosecond extreme ultraviolet (XUV) pulses for excitation and multi-electron coincidence techniques to disentangle SI and DI events. Using an interferometric technique with a weak infrared laser field, we demonstrate the existence of different ionization mechanisms and get new insight into the quantum dynamics of one-photon DI with evidence for inter-shell correlation e...

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Two-photon double ionization of helium above and below the threshold for sequential ionization

Cited by 111 publications

References 16 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

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

Double ionization probed on the attosecond timescale

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