Two-photon ionization of helium studied with the multiconfigurational time-dependent Hartree–Fock method (original) (raw)

Numerical simulations of single-photon double ionization of the helium dimer

Physical Review A, 2013

We study the energy exchange via electron correlation upon photon absorption over large distances in double photoionization of the helium dimer. Results of numerical simulations of the interaction of a planar helium dimer model with a short light pulse are found to be in good agreement with recent experimental data for the angular distribution of the emitted electron. The double ionization probability is closely related to that of the photoemission of an electron from one of the helium atoms along the internuclear axis. Together with an analysis of the temporal evolution of the two-electron probability distribution this provides direct evidence for the knockoff mechanism by which the photon energy is shared between the electrons over distances of several Angstroms in the dimer.

Two-photon double ionization of helium in the region of photon energies 42-50 eV

Physical Review A - Atomic, Molecular, and Optical Physics, 2007

We report the total integrated cross-section (TICS) of two-photon double ionization of helium in the photon energy range from 42 to 50 eV. Our computational procedure relies on a numerical solution of the time-dependent Schrödinger equation on a square-integrable basis and subsequent projection of this solution on a set of final states describing two electrons in continuum. Close to the threshold, we reproduce results previously known from the literature. The region 47 − 50 eV seems to have been previously unexplored. Our results suggest that TICS, as a function of the photon energy, grows monotonously in the region 42 − 50 eV. We also present fully resolved triple differential cross sections for selected photon energies.

Nonsequential two-photon double ionization of helium

Physical Review A, 2008

We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of ab initio calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process is fully described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime. PACS numbers: 32.80.Rm, 32.80.Fb, 42.50.Hz Correlated dynamical processes in nature poses unique challenges to experiments and theory. A prime example of this is the double ionization of helium by one-photon impact, which has been studied for more than 40 years. However, it is only during the last 15 years or so, that advances in theory, modeling and experiment have enabled scientists to gain a deeper insight into the role of electron correlations in this ionization process . The corresponding problem of two-photon double ionization of helium, in the photon energy interval between 39.4 and 54.4 eV, is an outstanding quantum mechanical problem that has been, and still is, subject to intense research worldwide, both theoretically and experimentally, employing state-of-the-art high-order harmonic [20-22] and free-electron (FEL) light sources . Despite all the interest and efforts that have been put into this research, major fundamental issues remain unresolved. What characterizes this particular three-body breakup process is that the electron correlation is a prerequisite for the process to occur, i.e., it depends upon the exchange of energy between the outgoing electrons, and as such it represents a clear departure from an independent-particle picture.

Two-photon above-threshold ionization of helium

Physical Review A, 2021

Multiphoton ionization provides a clear window into the nature of electron correlations in the helium atom. In the present study, the final state energy range extends up to the region near the N = 2 and N = 3 ionization thresholds, where two-photon ionization proceeds via continuum intermediate states above the lowest threshold. Our calculations are performed using multichannel quantum defect theory (MQDT) and the streamlined R-matrix method. The sum and integration over all intermediate states in the two-photon ionization amplitude is evaluated using the inhomogeneous R-matrix method developed by Robicheaux and Gao. The seamless connection of that method with MQDT allows us to present high resolution spectra of the final state Rydberg resonances. Our analysis classifies the resonances above the N = 2 threshold in terms of their group theory quantum numbers. Their dominant decay channels are found to obey the previously conjectured propensity rule far more weakly for these even parity states than was observed for the odd-parity states relevant to single photon ionization.

Perturbative calculation of two-photon double electron ionization of helium

Journal of Physics B: Atomic, Molecular and Optical Physics, 2008

We report the total integrated cross-section (TICS) of two-photon double ionization of helium in the photon energy range from 40 to 54 eV. We compute the TICS in the lowest order perturbation theory (LOPT) using the length and Kramers-Henneberger gauges of the electromagnetic interaction. Our findings indicate that the LOPT gives results for the TICS in agreement with our earlier non-perturbative calculations.

Attosecond timescale analysis of the dynamics of two-photon double ionization of helium

New Journal of Physics, 2008

We consider the two-photon double ionization (DI) of helium and analyze electron dynamics on the attosecond timescale. We first re-examine the interaction of helium with an ultrashort XUV pulse and study how the electronic correlations affect the electron angular and energy distributions in the direct, sequential and transient regimes of frequency and time duration. We then consider pump-probe processes with the aim of extracting indirect information on the pump pulse. In addition, our calculations show clear evidence for the existence under certain conditions of direct two-color DI processes.

Single Photon Double Ionization of the Helium Dimer

We show that a single photon can ionize the two helium atoms of the helium dimer in a distance up to 10 Å . The energy sharing among the electrons, the angular distributions of the ions and electrons, as well as comparison with electron impact data for helium atoms suggest a knockoff type double ionization process. The Coulomb explosion imaging of He 2 provides a direct view of the nuclear wave function of this by far most extended and most diffuse of all naturally existing molecules.

The role of final state correlation in double ionization of helium: a master equation approach

2010

The process of nonsequential two-photon double ionization of helium is studied by two complementary numerical approaches. First, the time-dependent Schr{\"o}dinger equation is solved and the final wave function is analyzed in terms of projection onto eigenstates of the uncorrelated Hamiltonian, i.e., with no electron-electron interaction included in the final states. Then, the double ionization probability is found by means of a recently developed approach in which the concept of absorbing boundaries has been generalized to apply to systems consisting of more than one particle. This generalization is achieved through the Lindblad equation. A model of reduced dimensionality, which describes the process at a qualitative level, has been used. The agreement between the methods provides a strong indication that procedures using projections onto uncorrelated continuum states are adequate when extracting total cross sections for the direct double ionization process.

A novel estimate of the two-photon double-ionization cross section of helium

Journal of Physics B: Atomic, Molecular and Optical Physics, 2012

In a previous publication, a procedure was proposed for unambiguously extracting the cross sections for double ionization and single ionization from a time-propagated wavepacket, and it was tested on the well-known case of one-photon double ionization of helium successfully. Here, we apply it to the two-photon process for which the numerically predicted double ionization cross section is not completely stabilized yet. Our results confirm the value obtained for this cross section by all but two active groups in the field, they definitely exonerate electron correlations in the final state from any responsibility in this splitting of the published data into two sets, they emphasize the need for a more careful account of reflection effects and propose a tentative explanation for an overestimation of the cross section in the J-matrix method. They also demonstrate the conceptual and computational advantages of the method proposed.

On the use of the Kramers–Henneberger Hamiltonian in multi-photon ionization calculations

Journal of Physics B: Atomic, Molecular and Optical Physics, 2005

We employ the Kramers-Henneberger Hamiltonian for time-independent calculations of multi-photon ionization of atoms with one and two electrons. As compared to the electromagnetic interaction in the length and velocity gauges, the presently employed Kramers-Henneberger gauge has an advantage of the dipole matrix elements for the free-free electron transitions being finite and well-defined quantities. This circumstance simplifies considerably the computations and allows to obtain accurate results for the two-photon ionization of realistic atomic systems.