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Physical Review A

Two-photon double ionization of atomic beryllium with ultrashort laser pulses

Frank L. Yip, Alicia Palacios, Fernando Martı́n, T. N. Rescigno, C. William McCurdy

Published Nov 5, 2015• 9 citations
Publisher

Abstract

We investigate the two-photon double ionization of beryllium atom induced by ultrashort pulses. We use a time-dependent formalism to evaluate the ionization amplitudes and generalized cross sections for the ejection of the $2{s}^{2}$ valence shell electrons in the presence of a fully occupied $1{s}^{2}$ frozen core shell. The relative contributions of the two-photon direct and sequential process are systematically explored by varying both pulse duration and central frequency. The energy and angular differential ionization yields reveal the signatures of both mechanisms, as well as the role of electron correlation in both the single and double ionization continua. In contrast with previous results on the helium atom, the presence of an electronic core strongly affects the final state leading to back-to-back electron emission even in the a priori less correlated two-photon sequential mechanism. In particular, a dominant pathway via excitation ionization through the ${\mathrm{Be}}^{+}(2p)$ determines the profiles and pulse-duration dependencies of the energy and angle differential yields.

DOI
10.1103/physreva.92.053404

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We investigate the two-photon double ionization of atomic magnesium induced by ultrashort pulses. Though the initial and final state symmetries are comparable to the same process in helium, in stark contrast the range of photon energies for which nonsequential ionization is the only open pathway is narrow (less than 1 eV) in magnesium. Thus several sequential ionization pathways feature heavily in these processes. Nonetheless, it is found that for pulse durations between 0.25 and 2.0 fs, the joint angular dependence of the ejected electrons can depend sensitively on pulse length, varying between the strictly back-to-back ejection characteristic of nonsequential ionization to other distributions. The significance of excited-state correlating configurations in representing the initial state of magnesium is discussed in the light of their consequences for the resulting angular distributions at photon energies where sequential ionization can access intermediate states that lie nearby in energy, particularly for longer pulses.
Above 54.4eV54.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}54.4eV, two-photon double ionization of helium is dominated by a sequential absorption process, producing characteristic behavior in the single and triple differential cross sections. We show that the signature of this process is visible in the nuclear recoil cross section, integrated over all energy sharings of the ejected electrons, even below the threshold for the sequential process. Since nuclear recoil momentum imaging does not require coincident photoelectron measurement, the predicted images present a viable target for future experiments with new short-pulse vacuum ultraviolet (vuv) and soft x-ray sources.
In view of recent papers on two-photon double ionization of helium, pertaining to a perceived anomaly in the behavior of the sequential process and its influence on the direct process, as well as the dependence of both on the laser pulse duration, we show that upon the proper formulation, the sequential is well defined and free of divergence, and that the dependence on pulse duration is considerably more intricate. We also argue that the apparent sharp rise of the cross section for the direct process, around 54.4eV54.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}54.4eV, is due to the unintended inclusion of the sequential process, which in any case is not properly describable in terms of a single cross section.
We analyze in this paper the process of double-electron ejection through two-photon absorption from the fundamental state of helium. We focus on the case of photon energies close to 2 a.u., which marks the threshold between direct and sequential double-ionization regimes. We demonstrate the crucial role of two-photon excitation-plus-ionization process of ${\mathit{nlk}}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ Rydberg series. We show that the latter channel must be taken into account in the theory in order to properly describe two-electron ejection. A simple expression is derived for the electron energy spectrum, leading to better insights into the physics underlying two-photon absorption close to the sequential threshold.
We present accurate cross sections for two-photon double ionization of helium at photon energies above and below the threshold for sequential double ionization. Above this threshold (54.4eV)(54.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV})(54.4eV), sequential ionization competes with nonsequential ionization. Remarkably, even below 54.4eV54.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}54.4eV, sequential ionization leaves a clear signature in the magnitude and shape of both the total and energy-sharing cross sections---even though at these energies it is only a virtual process.