Strong Electron Correlation Identified in Planetary Atomic Structure

Abstract

Investigating the two-electron correlation of doubly excited states (DESs) in the planetary atomic structure poses multiple conceptual and methodological challenges. Such highly correlated dynamics in a three-body Coulomb system can leave distinct signatures in the unique nonsequential above-threshold double ionization (NS-ATDI) exposed in a coherent laser field. Herein, we perform kinematically complete studies for multi-photon double ionization of cold strontium atoms. Our results reveal strongly favored NS-ATDI characterized by a series of band structures that demonstrate the energy and angular correlations of emitted two-electron pairs. These unprecedented characteristics reinforce the fact that both electrons are excited and ionized via the transition DESs, such that exotically strong two-electron correlations associated with atomic structure are relayed in the laser-driven time-dependent three-body system. The present findings transcend the traditional landscape in multi-photon double ionization and reshape our fundamental understanding of electron correlation in nature.

Figures (3)

• Figure 1: Schematic of the experimental setup. The continuum cold atomic beam is delivered to the reaction chamber by a red-shifted 461 nm pushing laser and further ionized by femtosecond lasers. The photoelectrons and recoil Sr$^{2+}$ ion are guided to detectors at opposite ends by homogeneous magnetic and electric fields. The inset illustrates that a pair of electrons is ionized with correlated kinetic energies and emitted back-to-back by the proposed NS-ATDI mechanism via intermediate DESs.
• Figure 2: Electron energy spectra and correlated angular distributions (CADs). (A to C) Electron-electron JES at $I=$ 6 TW/cm$^2$, 18 TW/cm$^2$, and 60 TW/cm$^2$. In (B), the white dashed line presents the zeroth to third ATDI where kinetic energy-sum of two electrons are constant and 12-15 indicate the number of photons absorbed. (D) Electron energy spectra for SI at $I=$ 3 TW/cm$^2$, 6 TW/cm$^2$, and 18 TW/cm$^2$. The peaks I, III, IV and II correspond to electrons ionized from $5s^2\ ^1S_0$ with absorption of four-photon, the five-photon, six-photon and from $5s5p\ ^3P_2$ with three-photon absorption, respectively. The spectra are normalized to maxima of the peak I. (E) CADs of the second electron with $E_{e2}=1.0-1.5$ eV as the first electron with $E_{e1}=1.0-1.5$ eV emitted along the positive x-axis, corresponding to nonsequential ATDI electrons marked within the yellow dashed box in (B). (F) Same as (E) but for sequential ATDI with $E_{e2} = 2.2 - 2.6$ eV and $E_{e1} = 0 - 0.2$ eV, corresponding to electrons marked within the green dashed circle in (B).
• Figure 3: Same as Fig. \ref{['fig2']}, A to C but for linearly polarized 400 nm pulse at 18 TW/cm$^2$. (A) The experimental electron-electron JES; (B) Full-dimensional TDSE simulations.