Modulation of attosecond beating in resonant two-photon ionization

TL;DR

This paper provides a theoretical analysis of attosecond beating in resonant two-photon ionization, revealing phase shifts and modulation effects near autoionizing states, with a new finite-pulse analytical model validated against helium atom calculations.

Contribution

It introduces a finite-pulse analytical model that accurately describes resonant two-photon ionization phenomena, including phase shifts and beat modulations, with minimal computational effort.

Findings

Peaked phase shifts occur when harmonics cross resonances.

Beating persists without pulse overlap, enabling non-holographic reconstruction.

Model aligns well with ab initio calculations for helium.

Abstract

We present a theoretical study of the photoelectron attosecond beating at the basis of RABBIT (Reconstruction of Attosecond Beating By Interference of Two-photon transitions) in the presence of autoionizing states. We show that, as a harmonic traverses a resonance, its sidebands exhibit a peaked phase shift as well as a modulation of the beating frequency itself. Furthermore, the beating between two resonant paths persists even when the pump and the probe pulses do not overlap, thus providing a sensitive non-holographic interferometric means to reconstruct coherent metastable wave packets. We characterize these phenomena quantitatively with a general finite-pulse analytical model that accounts for the effect of both intermediate and final resonances on two-photon processes, at a negligible computational cost. The model predictions are in excellent agreement with those of accurate ab initio calculations for the helium atom in the region of the N=2 doubly excited states.