Attosecond photoelectron spectroscopy of helium doubly excited states

TL;DR

This paper introduces a numerical method for simulating attosecond photoelectron spectroscopy of helium, accurately modeling excitation and ionization processes, and analyzing the control of ionization branching ratios near the $2s/2p$ threshold.

Contribution

The paper presents a new close-coupling B-spline based numerical approach for simulating helium's response to ultrafast light pulses, achieving convergence and detailed correlation effects.

Findings

Confirmed control of $2s/2p$ ionization branching ratios.

Demonstrated correlation effects modulate ionization signals.

Linked ionization beatings to doubly excited state interactions.

Abstract

We describe a numerical method that simulates the interaction of the helium atom with sequences of femtosecond and attosecond light pulses. The method, which is based on the close-coupling expansion of the electronic configuration space in a B-spline bipolar spherical harmonic basis, can accurately reproduce the excitation and single ionization of the atom, within the electrostatic approximation. The time dependent Schr\"odinger equation is integrated with a sequence of second-order split-exponential unitary propagators. The asymptotic channel-, energy- and angularly-resolved photoelectron distributions are computed by projecting the wavepacket at the end of the simulation on the multichannel scattering states of the atom, which are separately computed within the same close-coupling basis. This method is applied to simulate the pump-probe ionization of helium in the vicinity of the $2s/2p$ excitation threshold of the He$^+$ ion. This work confirms the qualitative conclusions of one of our earliest publications [L Argenti and E Lindroth, Phys. Rev. Lett. {\bf 105}, 53002 (2010)], in which we demonstrated the control of the $2s/2p$ ionization branching-ratio. Here, we take those calculations to convergence and show how correlation brings the periodic modulation of the branching ratios in almost phase opposition. The residual total ionization probability to the $2s+2p$ channels is dominated by the beating between the $sp_{2,3}^+$ and the $sp_{2,4}^+$ doubly excited states, which is consistent with the modulation of the complementary signal in the $1s$ channel, measured in 2010 by Chang and co-workers~[S Gilbertson~\emph{et al.}, Phys. Rev. Lett. {\bf 105}, 263003 (2010)].