Resonant two-photon ionization of helium atoms studied by attosecond interferometry

TL;DR

This paper investigates resonant two-photon ionization of helium atoms using attosecond interferometry, measuring photoelectron wavepacket phases and analyzing electron correlation effects with high-resolution spectrometry.

Contribution

It introduces an experimental approach combining angular and energy resolution to study phase jumps in helium ionization, supported by advanced theoretical calculations.

Findings

Observation of multiple phase jumps at and away from resonance

Correlation effects significantly influence phase behavior

Angular dependence of phase jumps is characterized

Abstract

We study resonant two-photon ionization of helium atoms via the $1s3p$, $1s4p$ and $1s5p^1$P$_1$ states using the 15$^\mathrm{th}$ harmonic of a titanium-sapphire laser for the excitation and a weak fraction of the laser field for the ionization. The phase of the photoelectron wavepackets is measured by an attosecond interferometric technique, using the 17$^\mathrm{th}$ harmonic. We perform experiments with angular resolution using a velocity map imaging spectrometer and with high energy resolution using a magnetic bottle electron spectrometer. Our results are compared to calculations using the two-photon random phase approximation with exchange to account for electron correlation effects. We give an interpretation for the multiple $\pi$-rad phase jumps observed, both at and away from resonance, as well as their dependence on the emission angle.