Ionic coherence in resonant above-threshold attosecond ionization spectroscopy

The ionization of atoms with sequences of attosecond pulses gives rise to excited ionic states that are entangled with the emitted photoelectron. Still, the ionic ensemble preserves some coherence that can be controlled through the laser parameters. In helium, control of the 2s/2p He+ coherence is mediated by the autoionizing states below the N = 2 threshold [Phys. Rev. Res. 3, 023233 (2021)]. In the present work we study the role of the resonances both below and above the N = 3 threshold on the coherence of the N = 3 He+ ion, in the attosecond pump-probe ionization of the helium atom, which we simulate using the NEWSTOCK ab initio code. Due to the fine-structure splitting of the N = 3 He+ level, the ionic dipole beats on a picosecond timescale. We show how, from the dipole beating, it is possible to reconstruct the polarization of the ion at its inception.

Automated summary

The ionization of atoms with attosecond pulses generates entangled excited ionic states and photoelectrons, which can be controlled through laser parameters. In helium, the coherence of the 2s/2p He+ state is controlled by autoionizing states below the N = 2 threshold. This study investigates the impact of resonances below and above the N = 3 threshold on the coherence of the N = 3 He+ ion using the NEWSTOCK ab initio code, revealing the polarization of the ion at its inception through the dipole beating on a picosecond timescale.