Electron correlation in two-photon double ionization of helium from attosecond to XFEL pulses

Abstract

Abstract. We investigate the role of electron correlation in the two-photon double ionization of helium for ultrashort XUV pulses with durations ranging from a hundred attoseconds to a few femtoseconds. We perform time-dependent ab initio calculations for pulses with mean frequencies in the so-called "sequential" regime ( ω > 54.4 eV). Electron correlation induced by the time correlation between emission events manifests itself in the angular distribution of the ejected electrons, which strongly depends on the energy sharing between them. We show that for ultrashort pulses two-photon double ionization probabilities scale non-uniformly with pulse duration depending on the energy sharing between the electrons. Most interestingly we find evidence for an interference between direct ("nonsequential") and indirect ("sequential") double photo-ionization with intermediate shake-up states, the strength of which is controlled by the pulse duration. This observation may provide a route toward measuring the pulse duration of FEL pulses.

Attosecond chronoscopy of photoemission

“…For photon energies above 2 each electron can be ejected by one photon independent of the proximity to and energy sharing with the other electron. Signatures of the sequential and non-sequential character in the energy dependence of double ionization rate and in the energy sharing and angular correlations of the emitted electrons have been the focus of a large number of theoretical (see e.g., Laulan and Bachau, 2003;Ishikawa and Midorikawa, 2005;Nikolopoulos and Lambropoulos, 2007;Palacios et al, 2009;Feist et al, 2008Feist et al, , 2009bPazourek et al, 2011;Horner et al, 2007;Foumouo et al, 2010;Nepstad et al, 2010, and references therein) and experimental (Nabekawa et al, 2005;Sorokin et al, 2007;Antoine et al, 2008;Rudenko et al, 2008;Kurka et al, 2010) studies. However, the direct observation of the timing of the ionization steps implied by the notion of (non) sequentiality has not yet been achieved.…”

Two-photon double ionization of helium: Evolution of the joint angular distribution with photon energy and two-electron energy sharing

“…Simultaneously, attosecond light sources using high-order harmonic generation have produced even shorter pulses at a number of different photon energies, including even single-cycle attosecond pulses [13,14]. Owing to the increasing availability of attosecond pulses and the promise of near-future increases in their intensities as well * liangyou.peng@pku.edu.cn † qhgong@pku.edu.cn as the near-future promise of even subfemtosecond FEL light pulses, it is now becoming feasible to study electron correlation dynamics on its natural time scale [15,16]. Moreover, using the ultrashort pulses of these xuv laser sources, truly direct multiple excitation or ionization of several electrons bound in atoms or molecules becomes possible [17].…”

“…Indeed, it has become a subfield of its own, with many theoretical investigations [15,16, and several experimental studies [50][51][52][53]. In the long-pulse-duration limit, TPDI of helium can be classified into two types: "sequential" and "nonsequential."…”

Finite elements and the discrete variable representation in nonequilibrium Green's function calculations. Atomic and molecular models

“…[28]. There, the accuracy of the DVR [46] on the one hand and the sparse character of FEs on the other led to an efficient TDSE code which is well parallelizable.…”