Single-photon Double Ionization Research Articles

Single-photon double ionization: renormalized-natural-orbital theory versus multi-configurational Hartree–Fock

The N-particle wavefunction has too many dimensions for a direct time propagation of a many-body system according to the time-dependent Schrödinger equation (TDSE). On the other hand, time-dependent density functional theory (TDDFT) tells us that the single-particle density is, in principle, sufficient. However, a practicable equation of motion for the accurate time evolution of the single-particle density is unknown. It is thus an obvious idea to propagate a quantity which is not as reduced as the single-particle density but less dimensional than the N-body wavefunction. Recently, we have introduced time-dependent renormalized-natural-orbital theory (TDRNOT). TDRNOT is based on the propagation of the eigenfunctions of the one-body reduced density matrix, the so-called natural orbitals. In this paper we demonstrate how TDRNOT is related to the multi-configurational time-dependent Hartree–Fock (MCTDHF) approach. We also compare the performance of MCTDHF and TDRNOT versus the TDSE for single-photon double ionization (SPDI) of a 1D helium model atom. SPDI is one of the effects where TDDFT does not work in practice, especially if one is interested in correlated photoelectron spectra, for which no explicit density functional is known.

Dynamic Interference Photoelectron Spectra in Double Ionization: Numerical Simulation of 1D Helium**Supported by the National Natural Science Foundation of China under Grant Nos 61178028, 11674243 and 11674242, and the National Basic Research Program of China under Grant No 2015CB755403.

We report our numerical simulation on the dynamic interference photoelectron spectra for a one-dimensional (1D) He model exposed to intense ultrashort extreme ultraviolet (XUV) laser pulses. The results demonstrate an unambiguous interference feature in the photoelectron spectra, and the interference is unveiled to originate from the dynamic Stark effect. The interference photoelectron spectra are prompted for intense sub-femtosecond XUV laser pulses in double ionization. The stationary phase picture is corroborated qualitatively in the two-electron system. The ability of probing the dynamic Stark effect by the photoelectron spectra in a pragmatic experiment of single-photon double ionization of He may shed light on further investigation on multi-electron atoms and molecules.

Single-photon double and triple ionization of acetaldehyde (ethanal) studied by multi-electron coincidence spectroscopy

Single-photon multiple ionization processes of acetaldehyde (ethanal) have been experimentally investigated by utilizing a multi-particle coincidence technique based on the time-of-flight magnetic bottle principle, in combination with either a synchrotron radiation source or a pulsed helium discharge lamp. The processes investigated include double and triple ionization in the valence region as well as single and double Auger decay of core-ionized acetaldehyde. The latter are studied site-selectively for chemically different carbon core vacancies, scrutinizing early theoretical predictions specifically made for the case of acetaldehyde. Moreover, Auger processes in shake-up and core-valence ionized states are investigated. In the cases where the processes involve simultaneous emission of two electrons, the distributions of the energy sharing are presented, emphasizing either the knock-out or shake-off mechanism.

Double ionization inR-matrix theory using a two-electron outer region

We have developed a two-electron outer region for use within $R$-matrix theory to describe double ionization processes. The capability of this method is demonstrated for single-photon double ionization of He in the photon energy region between 80 and 180 eV. The cross sections are in agreement with established data. The extended $R$-matrix with time dependence method also provides information on higher-order processes, as demonstrated by the identification of signatures for sequential double ionization processes involving an intermediate ${\mathrm{He}}^{+}$ state with $n=2$.

R-matrix with pseudostates study of single photon double ionization of endohedral Be and Mg atoms

We perform R-matrix with pseudostates calculations to investigate single photon double ionization of Be@C60 and Mg@C60. We contrast the present results to our previous studies of He@C60. We find the presence of the confinement resonances in the double photoionization cross sections of endohedral Be and Mg. Keeping the same C60 spherical shell potential in each case for these systems, we explore the change in magnitude and the energy dependence of the resonance features relative to the background cross sections.

Double photoionization of C2+

The time-dependent close-coupling method is used to study the single photon double ionization of C2+ in support of possible experiments at FLASH/DESY using an EBIT. Energy and angle differential cross sections are calculated to fully investigate the correlated motion of the two photoelectrons. Single energy differential as well as total cross sections are calculated for different incident photon energies in the range of 125–225 eV. Good agreement is found between our results and available R-matrix results for the double ionization of C2+. The study is also extended to the double photoionization along the Be-like isoelectronic sequence (Be-F5+), where good agreement is found when compared with available theoretical calculations and experimental measurements.

Single photon double ionization of Helium at 800 eV – observation of the Quasi Free Mechanism

In a kinematically complete experiment we have measured the photo double ionization of Helium at a photon energy of 800 eV and observed He2+ ions with 0 momentum, corresponding to a back-to-back-emission of the two electrons. The results are in good agreement with theoretical calculations.

Atto-second time-delay in single and double photoionization of noble gas atoms

We perform a systematic investigation of the photoelectron group delay in valence shell single ionization of Ne, Ar, Kr and Xe from their respective thresholds to photon energy of 200 eV. This allows us to highlight various important aspects of fundamental atomic physics that can be probed by attosecond time delay measurements and to address several controversies in recent experiments. We also use these data as an input to the time delay calculations of single-photon double ionization (PDI) of noble gas atoms. Various PDI channels are examined and estimates are given for associated time delays due to the shake-off and knock-out processes. These estimates are used to interpret recent measurements of PDI of Xe on the atto-second time scale.

Molecular single photon double K-shell ionization

We have studied single photon double K-shell ionization of small molecules (N2, CO, C2H2n (n=1–3), …) and the Auger decay of the resulting double core hole (DCH) molecular ions thanks to multi-electron coincidence spectroscopy using a magnetic bottle time-of-flight spectrometer. The relative cross-sections for single-site (K−2) and two-site (K−1K−1) double K-shell ionization with respect to single K-shell (K−1) ionization have been measured that gives important information on the mechanisms of single photon double ionization. The spectroscopy of two-site (K−1K−1) DCH states in the C2H2n (n=1–3) series shows important chemical shifts due to a strong dependence on the CC bond length. In addition, the complete cascade Auger decay following single site (K−2) ionization has been obtained.

Ejection of Quasi-Free-Electron Pairs from the Helium-Atom Ground State by Single-Photon Absorption

We investigate the single-photon double ionization of helium at photon energies of 440 and 800eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy. These slow ions are the unique fingerprint of an elusive quasifree photon double ionization mechanism predicted by Amusia etal. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods.

Fully Differential Single-Photon Double Ionization of Neon and Argon

Triply differential cross sections are calculated for one-photon double ionization of neon and argon at various photon energies and electron energy sharings by using a frozen-core treatment to represent the remaining electrons of the residual ion. Angular distributions agree well with all existing experimental data, showing that in spite of its simplicity the method can treat the double ionization of complex targets reliably. A comparison of the cross sections for helium, neon, and argon into the same final state symmetry at the same relative excess energies reveals a distinctive signature of the role of electron correlation in each target.

An R-matrix with pseudo-state approach to the single photon double ionization and excitation of the He-like Li+ ion

The success of the R-matrix with pseudo-state (RMPS) method to model single photoionization processes, benchmarked against dedicated synchrotron light source measurements, is exploited and extended to investigate, single photon double ionization cross-sections for the He-like, Li+ ion. We investigate these processes from both the ground state and the excited 1s2s 1, 3S metastable levels of this He-like system. Comparisons of the results from the RMPS method are made with other state-of-the-art theoretical approaches such as time-dependent close-coupling (TDCC), B-splines and the convergent close-coupling (CCC). Excellent agreement with various other theoretical approaches are achieved but differences occur which are highlighted and discussed. For the ground state the peak of the cross-section is ∼2 kilo-barns (Kb), that for the 1s2s 1S state is ∼6 and ∼1 Kb for the corresponding 1s2s 3S state. All the cross-sections for single photon double ionization are extremely small, being in the region of 2–10 Kb, or less, rendering their experimental determination extremely challenging.

Single-Photon Core Double Ionization in Molecules

Single photon double K-shell ionization of small molecules (N2, O2, CO, CO2) has been observed by coincidence detection of two photoelectrons with two Auger electron and reveals properties of hollow molecules.

Quasi free mechanism in single photon double ionization of Helium

We have kinematically complete measured photo double ionization of Helium at Eγ=800 eV and observed recoiling ions with nearly 0 momentum that agree well with theoretical predictions of a new photo double ionization mechanism.

Single-photon double ionization of He with generalized Sturmian basis

In this work we present the application of the generalized Sturmian basis to the single photon - double ionization of atoms.

Single-photon double ionization of H2away from equilibrium: A showcase of two-center electron interference

We demonstrate the effect of two-center interference on single-photon double ionization [double photoionization (DPI)] of the aligned H${}_{2}$ molecule when it shrinks or expands from the equilibrium internuclear distance. This interference affects the first stage of the DPI process in which the primary photoelectron is ejected predominantly along the polarization axis of light and its geometrical interference factor is most sensitive to the internuclear distance in the parallel ($\ensuremath{\Sigma}$) orientation of the internuclear and polarization axes. This effect is responsible for strong modification of the DPI amplitude in the parallel orientation while the corresponding amplitude for the perpendicular ($\ensuremath{\Pi}$) orientation is rather insensitive to the internuclear distance. The combination of these two factors explains the profound kinetic energy release effect on the fully differential cross sections of DPI of H${}_{2}$.

Nuclear-recoil differential cross sections for the double photoionization of helium

The time-dependent close-coupling method for the double photoionization of atoms is extended to compute fully differential nuclear-recoil cross sections. Excellent agreement is found between our calculated double-photoionization total cross sections of He and the measurements at all photon energies. Differential cross-section results are presented for the single-photon double ionization of He at photon energies of 99, 125, and 225 eV. The single-differential cross-section results at 99 eV agree with previous theory and experiment. Symmetric momentum distributions are found in the plane perpendicular to the polarization direction, while dipolelike momentum distributions are found in the other two planes. The variation of the nuclear-recoil triple-differential cross sections of He${}^{2+}$ with the nuclear-recoil momenta are presented. The total cross sections for the nuclear-recoil of He${}^{2+}$ match the corresponding ones for the double electron ejection of He for all the studied photon energies.

Time-dependent formalism of double ionization of multielectron atomic targets

A time-dependent formalism for treating double ionization of atomic targets with core electrons is presented. It relies on the solution of the time-dependent Schrödinger equation with a combined orbital and grid-based representation of the time-evolving wave packet. Total and differential double ionization probabilities and cross sections are then obtained through a exterior complex scaling implementation. The method is demonstrated for single-photon double ionization of the valence electrons of beryllium atom using 500 attosecond pulsed lasers, with the inner-shell electrons held fixed in the frozen-core orbitals. The extracted cross sections are compared to existing experimental and time-independent theoretical results. The accuracy of the formalism is demonstrated in furtherance of investigation of physical processes induced by ultrashort pulses in multielectron atoms that cannot be achieved in a time-independent framework.

Energy sharing in the two-electron attosecond streak camera

Using the recently developed concept of the two-electron streak camera (see Emmanouilidou et al 2010 New J. Phys.12 103024), we studied the energy sharing between the two ionizing electrons in single-photon double ionization of He(1s2s). We found that the most symmetric and asymmetric energy sharings correspond to different ionization dynamics with the ion's Coulomb potential significantly influencing the latter. This different dynamics for the two extreme energy sharings gives rise to different patterns in asymptotic observables and different time delays between the emission of the two electrons. We show that the two-electron streak camera resolves the time delays between the emission of the two electrons for different energy sharings.

Lifetime and kinetic energy release of metastable dications dissociation

A new method for the determination of dynamical features of the molecular dication dissociation processes, following the single photon double ionization, investigated by time-of-flight mass spectrometry technique has been developed. The method is based on an extension of the generalized simulated annealing statistical methodology, previously applied in other fields. Here it is described and applied, as an example, to the case of the dissociation of the CO22+ dication giving CO++O+ ion fragments. The results are consistent with previous determination of the metastable lifetime of the dication, but the analysis also provides additional information about the dynamics of the reaction.