Electron angular distributions in two-photon double ionization of helium

Makris, M. G.; Nikolopoulos, L. A. A.; Lambropoulos, P.

doi:10.1209/epl/i2001-00312-y

Cited by 13 publications

(8 citation statements)

References 18 publications

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“…Lambropoulos et al [24] reviewed investigations of intense laser interactions with two-electron systems up to around 1997. Since that review, the number of theoretical investigations of intense laser-atom processes involving He has increased greatly, not only for HHG [25][26][27][28][29][30][31][32][33][34] but also for multiphoton ionization processes [25,[35][36][37][38][39][40][41][42] and, especially, for two-photon double-ionization processes [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60].…”

Section: Introductionmentioning

confidence: 99%

Evidence of the2s2p(1P) doubly excited state in the harmonic generation spectrum of helium

Djiokap

Starace

2011

Phys. Rev. A

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By solving the two-active-electron time-dependent Schrödinger equation in an intense, ultrashort laser field, we investigate evidence of electron correlations in the high-order harmonic generation spectrum of helium. As the frequency of the driving laser pulse varies from 4.6 to 6.6 eV, the 13th, 11th, and 9th harmonics sequentially become resonant with the transition between the ground state and the isolated 2s2p( 1 P ) autoionizing state of helium, which dramatically enhances these harmonics and changes their profiles. When each of the 9th and 13th harmonics are in resonance with this autoionizing state, there is also a low-order multiphoton resonance with a Rydberg state, resulting in a particularly large enhancement of these harmonics relative to neighboring harmonics. When the 11th harmonic is in resonance with the 2s2p( 1 P ) autoionizing state, the 13th harmonic is simultaneously in resonance with numerous higher-energy autoionizing states, resulting in a competition between these two harmonics for intensity. These results demonstrate that even electron correlations occurring over a narrow energy interval can have a significant effect on strong-field processes such as harmonic generation.

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Section: Introductionmentioning

confidence: 99%

Evidence of the2s2p(1P) doubly excited state in the harmonic generation spectrum of helium

Djiokap

Starace

2011

Phys. Rev. A

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“…We show that the PES provides the cleanest signature of the process. An inflection (knee) in the laser power dependence of double ionization is also discernible, within a window of intensities which depends on the pulse duration and cross sections PACS numbers: 32.80.WrThe issue of direct versus sequential double ionization has in the last few years emerged in a new context, namely two-photon double ionization of Helium under XUV radiation, and in particular photon energies of about 45 eV [1,2,3,4,5,6,7]. Although until very recently, sources of radiation in that wavelength range, mostly synchrotrons, could not provide the needed intensity (more than 10 12 W/cm 2 ), the situation has now changed.…”

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confidence: 99%

Signatures of direct double ionization under xuv radiation

2005

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In anticipation of upcoming two-photon double ionization of atoms and particularly Helium, under strong short wavelength radiation sources (45 eV), we present quantitative signatures of direct twophoton double ejection, in the photoelectron spectrum (PES) and the peak power dependence, that can be employed in the interpretation of related data. We show that the PES provides the cleanest signature of the process. An inflection (knee) in the laser power dependence of double ionization is also discernible, within a window of intensities which depends on the pulse duration and cross sections PACS numbers: 32.80.WrThe issue of direct versus sequential double ionization has in the last few years emerged in a new context, namely two-photon double ionization of Helium under XUV radiation, and in particular photon energies of about 45 eV [1,2,3,4,5,6,7]. Although until very recently, sources of radiation in that wavelength range, mostly synchrotrons, could not provide the needed intensity (more than 10 12 W/cm 2 ), the situation has now changed. It is conceivable that further developments and optimization of High Order Harmonic Generation (HOHG) might succeed [8]. The upcoming second phase of the FEL XUV source at DESY [9], however, is expected to easily satisfy that requirement. Thus it is a matter of probably short time that the first experimental data on this process will be obtained. When that happens, it is important to have available unequivocal and quantitative signatures of the process and this is the purpose of this paper.What is it that makes this process interesting? Recall that single-photon double ionization, especially in Helium, has been studied in great detail, both theoretically and experimentally [10]. It is basically well understood, although interesting details, especially near the threshold keep coming up [11]. It could also be argued that this process is fundamentally two-step, in the sense that the single available photon can only interact with one of the electrons and it is only through electron-electron correlation that double ejection is possible. As often said, correlation either in the initial or the final state is necessary [12]. To stress the point, let us note that the process would be impossible for non-interacting electrons. The same is true for the other extreme case of double ionization, namely long-wavelength (∼780 nm) high intensity and short pulse duration. The mechanism for that process, other than the sequential, is also explicitly understood as two-step [13]. Specifically, the theoretical interpretation rests on the physical picture of one electron pulled out by the strong oscillating field, set into oscillation and liberating the other electron -with a probability depending on the intensity -as it is driven back to the vicinity of the nucleus. Despite the enormous number of photons streaming through the atomic diameter, at those intensities, these photons cannot act simultaneously and separately on each of the electrons, with a probability of any significance. The reason is scree...

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“…And this is the unique feature of the combination of atomic system with wavelength. A number of subtle features are reflected in the photoelectron angular distribu-tion, in analogy to those found earlier in helium [5], but space does not allow their discussion here. It will suffice to mention that, as already alluded to, this process offers the possibility to study photoelectrons in the double continuum having the same orbital angular momentum (p) and nearly equal or very unequal kinetic energies, thus providing a glimpse into otherwise difficult to access aspect of correlation in the continuum.…”

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confidence: 87%

“…Recent work on the two-electron (three-body) problem has revealed new layers of physics in the context of fewphoton double ionization [1][2][3][4][5][6][7]. In particular, two-photon double ionization of helium has been shown to exhibit features not possible in single-photon double ionization and not easy, if possible at all, to disentangle in strongfield multiphoton ionization under long-wavelength radiation.…”

mentioning

confidence: 99%

“…The picture that appears to be emerging suggests that double ionization, and hence the underlying behavior of the two-electron system, under single-photon excitation on the one hand and multiphoton of high order on the other, represent two extremes with their own features, while the intermediate regime possesses a vast richness of unexplored physics. For example, under two-photon ionization, the judicious choice of wavelength offers the possibility to separate direct from sequential double ionization [2], whose photoelectron energy and angular distribution [5] have shown counterintuitive behavior. Until quite recently, the theoretical analysis of these processes, owing to the lack of sources of appropriate intensity, could have been considered of only academic interest.…”

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confidence: 99%

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