The ionization dynamics of circular Rydberg states in strong circularly polarized infrared (800 nm) laser fields is studied by means of numerical simulations with the time-dependent Schrödinger equation. We find that at certain intensities, related to the radius of the Rydberg states, atomic stabilization sets in, and the ionization probability decreases as the intensity is further increased. Moreover, there is a strong dependence of the ionization probability on the rotational direction of the applied laser field, which can be understood from a simple classical analogy.

Multiphoton ionization of helium is investigated in the superintense field regime, with particular emphasis on the role of the electron-electron interaction in the ionization and stabilization dynamics. To accomplish this, we solve ab initio the time-dependent Schrödinger equation with the full electron-electron interaction included. By comparing the ionization yields obtained from the full calculations with the corresponding results of an independent-electron model, we come to the somewhat counterintuitive conclusion that the single-particle picture breaks down at superstrong field strengths. We explain this finding from the perspective of the so-called Kramers-Henneberger frame, the reference frame of a free (classical) electron moving in the field. The breakdown is tied to the fact that shake-up and shake-off processes cannot be properly accounted for in commonly used independent-electron models. In addition, we see evidence of a change from the multiphoton to the shake-off ionization regime in the energy distributions of the electrons. From the angular distribution, it is apparent that the correlation is an important factor even in this regime.

Abstract:In this paper, we present theoretical results for the process of non-sequential two-photon double ionization of helium at the photon energy 42 eV. Our approach is based on solving the time-dependent Schrödinger equation in a B-spline based numerical framework. Information about the process is obtained by extracting the double-ionized component by means of uncorrelated final states. The total (generalized) cross section for the process is extracted, as well as differential cross sections resolved in electron energies and ejection angles. We focus on the impact the final-state correlation has on the accuracy of the cross sections.PACS (

We apply a recently developed ab initio numerical framework to investigate the angular distributions of the emitted electrons in the immediate proximity of the threshold for the two-photon double ionization of helium. Provided one of the electrons is emitted perpendicular to the laser polarization direction, it is found that the angular distribution of the other electron is characterized by three lobes. The results are similar to those recently reported for the corresponding process in the hydrogen negative ion [R. The problem of direct (nonsequential) two-photon double ionization of helium has been studied extensively in recent years, as exemplified by numerous theoretical [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and experimental [19][20][21][22][23][24] works. This breakup process is fundamental in the sense that it is one of the simplest processes in nature where electron correlations are exhibited, manifested by a rather complex interplay between the electrons. As such, a complete understanding of it will pave the way for further investigations of the role of correlations in few-photon and multiphoton multiple ionization processes in atoms and molecules.In the present Brief Report, we investigate the direct two-photon double ionization process of helium in the near vicinity of the lower threshold (i.e., for 40 eV photons), and with particular emphasis on the direction of ejection of the photo-electrons. In a recent work [25], it was found that the corresponding process in H − is characterized by a strong backward-forward asymmetry in the sense that if one electron is emitted perpendicular to the (linear) laser polarization direction then the other electron is emitted most preferably in the opposite direction, forming three characteristic lobes in the angular distribution. Similar features have also been observed theoretically in H 2 [26][27][28].Solving the time-dependent Schrödinger equation numerically for helium [14], the conditional angular distribution is obtained for both short (500 and 1000 as) and long (4 fs) linearly polarized laser pulses. We examine the case where one of the electrons is emitted perpendicular to the laser polarization direction, and integrate over the energy of both electrons. It is found that the direction of emission of the other electron is characterized by three lobes, concordant with the observations in H − [25]. Furthermore, with increasing pulse duration, the lobe pointing in the backward direction, representing electrons being emitted back-to-back, becomes relatively more important. The "backward" lobe is most distinct at lower photon energies, and already at a photon energy of 42 eV it loses its significance [11]. We therefore anticipate that the presence of the structure at lower photon energies is * sigurd.askeland@ift.uib.no † morten.forre@ift.uib.no a signature of a competing double ionization mechanism that becomes suppressed at higher photon energies.Figure 1 depicts our results for the angular distributions in the double ionization process. I...

The influence of intermediate resonant dissociative channels in the few-photon ionization dynamics of H + 2 is demonstrated in a pump-probe scenario. Two-photon ionization of H + 2 by two sequentially applied pump and probe vuv/fs 10 11 W/cm 2 laser pulses is reported. The kinetic energy distribution of the ejected protons is calculated by solving the time-dependent Schrödinger equation within the Born-Oppenheimer approximation, including the electronic three-dimensional and vibrational one-dimensional motion. Population is effectively transferred from the 1sσg to the 3pσu potential surface, via resonant one-photon absorption, by applying a chirped pump pulse. The molecule is ultimately ionized by the probe pulse. It is found that a double-peaked structure appears in the resulting kinetic energy release spectra of the nuclear fragments. A corresponding modulated structure also appears in the dissociative channels, merely demonstrating that the double-peaked structure in the spectra originates from the molecular 1sσg-3pσu electronic dynamics, and an inherent twocenter interference in the underlying electric dipole coupling. It turns out that the dipole coupling between the 1sσg and 3pσu electronic states vanishes at an internuclear distance of 2.25 a.u., i.e., close to the equilibrium internuclear separation. The resulting node in the R-dependent dipole coupling imposes a minimum in the corresponding kinetic energy release spectra of the dissociating nuclei. By creating a negative or positive chirp in the applied pump pulse, it is found that the modulations can be made more or less salient.