Ionic Coherence in resonant ATI attosecond spectroscopy*

The ionization of atoms with sequences of attosecond pulses gives rise to excited ionic ensembles that preserve some coherence, despite their entanglement with the emitted photoelectron. In past theoretical studies of the pump-probe ionization of helium, we have shown that the loss of ionic coherence of the ions with the same principal quantum number N can be controlled by promoting multiphoton ionization from intermediate autoionizing states $N_Ln_\ell$, below the threshold, to the $N_{L'}\epsilon_{\ell'}$ continua~[1]. In the present work, we study the role played by the above-threshold $(N+1)_Ln_\ell$ resonances, which interact with the $N_{L'}\epsilon_{\ell'}$ channels due to correlation. 

We perform \emph{ab initio} simulations using the NewStock code~[2,3] of the attosecond pump-probe ionization of helium above N=3 threshold, either including or excluding the resonances that converge to the N=4 threshold. Due to the fine splitting of the N=3 He$^+$ level, the ionic dipole beats on a picosecond timescale.  From the dipole beating, which is a measurable quantity, we reconstruct the polarization of the ion at its inception and highlight the effect of resonant radiative continuum-continuum transitions on the coherence between ionic states of different parity.

[2] L Argenti and E Lindroth , Phys. Rev. Lett. 105, 053002 (2010).

[3] T Carette et al., Phys. Rev. A 87, 023420 (2013).