Two-photon above-threshold ionization of helium (original) (raw)

One photon double ionization of helium: threshold behaviour

Zeitschrift für Physik D: Atoms, Molecules and Clusters, 1990

In order to study the dynamics of double photoionization of helium, we report new coincidence measurements between low energy electrons and doubly charged ions, from 78 to 95 eV photon energy. We show that the range of validity of the Wannier theory depends upon the observable. For the exponent n of the threshold law, this range amounts to some 3 eV above onset, thus confirming previous published experimental work. In contrast, the energy distribution of the two outgoing electrons is found flat, within 20%, in agreement with the theoretical predictions, but in a 15 eV energy range above threshold.

Resonant and nonresonant multiphoton ionization of helium

Physical Review A, 1994

We have investigated the multiphoton ionization of helium at wavelengths between 310 and 330 nm at intensities between 8X10' and 5X10' W/cm and at 630 nm at intensities of 1X10' W/cm . We characterize the ionization processes from photoelectron energy and angular distributions observed concurrently with photoion spectra. At the shorter wavelengths we find that resonant enhancement via the ac Stark shifted six-photon resonant states (1s3d and 1s3s) is a dominant ionization path as described previously by Kulander [Phys. Rev. Lett. 63, 1058 (1989)] and by Rudolph et al. [Phys. Rev. Lett. 66, 3241 (1991)]. At intensities above those required for resonant enhancement, and at wavelengths longer than those required for six-photon resonance, we observe that nonresonant sevenphoton ionization dominates. This process gives rise to continuous distributions of low-energy electrons with characteristic angular distributions that peak near 0 and 60' relative to the laser polarization. At yet higher intensities, above the threshold where the nonresonant seven-photon channel closes, the dominant ionization path occurs via seven-photon resonant states with odd parity. This path gives rise to an-gu1ar distributions characteristic of intermediate states with f character.

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.

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.

Rydberg and autoionizing triplet states in Helium up to the N = 5 threshold

Atomic Data and Nuclear Data Tables, 2008

Energy levels of highly excited bound Rydberg states, the position and widths of autoionizing states, and oscillator strengths are calculated for He 3 S, 3 P e , 3 P o , 3 D e and 3 D o symmetries up to the N = 5 He + excitation threshold. The calculations are performed with the Kmatrix B-spline method with maximum orbital angular momentum ' max = 8. Reliable doubly excited-state parameters up to the n = 20 multiplet below each ionization threshold are presented. One thousand and six hundred newly identified bound and metastable states, seven times those available in literature, fill many gaps, reveal a dozen intruder states, and allow new speculations on propensity rules and radiative decays of triplet Rydberg states.

A quantum-field theory approach to the calculation of energy levels in helium-like Rydberg atoms

Annals of Physics, 1987

We discuss the tine structure splitting of the energy levels in Rydberg states of helium or helium-like ions on the basis of quantum electrodynamics, using time-independent perturbation theory and the radiation gauge. For the zero-order description of the states we use products of Dirac-type wavefunctions. with shielding for the outer electron. The perturbing interaction includes the residual electrostatic potential, the interaction coming from the exchange of virtual photons, and the creation of virtual electron-positron pairs. It is shown that the level shifts for low-Z ions are given to high accuracy by a procedure followed previously.

Radiative and Relativistic Effects in the Decay of Highly Excited States in Helium

Physical Review Letters, 2000

A recent experimental study [J.-E. Rubensson et al., Phys. Rev. Lett. 83, 947 (1999)] measured a significant fluorescence yield of the He(2lnl 0 ) photoexcited resonances, showing major qualitative differences from nonrelativistic predictions. We present a further theoretical study of these states, and perform R-matrix multichannel quantum defect theory calculations to extract fluorescence and ionization cross sections. These theoretical results are in excellent agreement with newer, higher-resolution measurements. Radiative and spin-orbit effects are quantified and shown to play an important role in the overall characterization of highly excited states. PACS numbers: 31.50. + w, 32.80.Fb, 32.80.Rm The doubly excited 2lnl 0 photoionization spectrum of helium has been studied extensively over the past three and a half decades, beginning with the pioneering experimental work of Madden and Codling [1] and the corresponding theoretical work of Cooper, Fano, and Prats [2]. While enormous advances in both experimental and theoretical capabilities have led to a detailed understanding of these states , apparently it had never been necessary to include higher-order effects in their characterization, such as alternate radiative decay channels or relativistic interactions.

Selection rules in the few-photon double ionization of the helium atom

Journal of Physics B: Atomic, Molecular and Optical Physics, 2011

We analyse selection rules for the emission of two electrons from the helium atom following the absorption of a few photons in an intense laser field. The rules arise, as generalization of the well-studied one-photon case, due to the symmetries of the accessible final states in the two-electron continuum. We show, in particular, that an increase in the number of absorbed photons leads to alternating suppression and non-suppression of the back-to-back emission of the two electrons. Results of numerical simulations using a model of the helium atom are in agreement with the theoretical predictions.