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Physical Review A

Theory of two-photon double ionization of helium at the sequential threshold

H. Bachau

Published Mar 9, 2011• 27 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.83.033403

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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 nlk\ensuremath′l\ensuremath′{\mathit{nlk}}^{\ensuremath{'}}{l}^{\ensuremath{'}}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 analyze in this paper the role of the correlations during double-electron ejection through two-photon absorption from the lowest autoionizing state 1^{1}1Se{\mathit{S}}^{\mathit{e}}Se of helium. Emphasis will be put on the pertinence of the notion of simultaneity for double-electron ejection when this process does not require electron interaction.
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.
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.
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 2s22{s}^{2}2s2 valence shell electrons in the presence of a fully occupied 1s21{s}^{2}1s2 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 Be+(2p){\mathrm{Be}}^{+}(2p)Be+(2p) determines the profiles and pulse-duration dependencies of the energy and angle differential yields.
We investigate different processes which may be responsible for ejection of two electrons from the external shell of xenon through the absorption of two and three photons, namely two-photon direct double ionization and three-photon sequential double ionization via the ion in an excited state or in the ground state. We calculate the cross sections of the ionization processes involved, using many-body perturbation theory. These cross sections are then used within a rate equation model for comparing the relative efficiencies of the different double-ionization mechanisms.
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.