Physical Review A

Two-photon double photoionization of atomic Mg by ultrashort pulses: Variation of angular distributions with pulse length

Roger Y. Bello, Frank L. Yip, T. N. Rescigno, Robert R. Lucchese, C. William McCurdy

Published Nov 6, 2020• 4 citations

Abstract

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.

DOI: 10.1103/physreva.102.053107

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Physical Review A

Two-photon double photoionization of atomic Mg by ultrashort pulses: Variation of angular distributions with pulse length

Roger Y. Bello, Frank L. Yip, T. N. Rescigno, Robert R. Lucchese, C. William McCurdy

Published Nov 6, 2020• 4 citations

Publisher

Abstract

DOI: 10.1103/physreva.102.053107

Related papers

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

Frank L. Yip, Alicia Palacios, Fernando Martı́n +2 more

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.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.

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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.

Above

54.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}

, 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.

7p^{2}P_{\frac{1}{2},\frac{3}{2}}

and

8p^{2}P_{\frac{1}{2},\frac{3}{2}}

intermediate states and three-photon ionization via the two-photon-allowed

8d^{2}D_{\frac{5}{2},\frac{3}{2}}

resonant intermediate states are reported. The photoelectrons are energy analyzed with a spherical-sector electrostatic energy analyzer with an energy resolution of ~ 0.1 eV and angular resolution of ~ \ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}. A straightforward calculation based upon a finestructure scheme gives excellent agreement with the measured angular distributions for the

^{2}P_{\frac{1}{2}}

states. The laser pulse duration is comparable to the hyperfine precession period which allows the hyperfine coupling to partially destroy the anisotropy initially produced in the

^{2}P_{\frac{3}{2}}

resonant intermediate states. Quantitative calculations including hyperfine coupling take this into account and provide an expression which gives a reasonable fit to the

^{2}P_{\frac{3}{2}}

data.

Phase-shift differences and amplitude ratios of the outgoing

s

and

d

continuum wave packets generated by two-photon ionization of helium atoms are determined from the photoelectron angular distributions obtained using velocity map imaging. Helium atoms are ionized with ultrashort extreme-ultraviolet free-electron laser pulses with a photon energy of 20.3, 21.3, 23.0, and 24.3 eV, produced by the SPring-8 Compact SASE Source test accelerator. The measured values of the phase-shift differences are distinct from scattering phase-shift differences when the photon energy is tuned to an excited level or Rydberg manifold. The difference stems from the competition between resonant and non-resonant paths in two-photon ionization by ultrashort pulses. Since the competition can be controlled in principle by the pulse shape, the present results illustrate a new way to tailor the continuum wave packet.