On the angular dependence of the photoemission time delay in helium

TL;DR

This paper explores how the photoemission time delay in helium varies with emission angle, revealing that the Wigner delay is angle-independent while the continuum-continuum correction depends on angle, supported by theoretical calculations and experiments.

Contribution

It demonstrates the angular dependence of the continuum-continuum correction in helium's photoemission time delay, combining experimental data with two theoretical approaches.

Findings

Wigner delay in helium is angle-independent.

Continuum-continuum correction shows noticeable angular dependence.

Theoretical calculations agree with experimental measurements.

Abstract

We investigate an angular dependence of the photoemission time delay in helium as measured by the RABBITT (Reconstruction of Attosecond Beating By Interference of Two-photon Transitions) technique. The measured time delay $ \tau_a=\tau_W+\tau_{cc} $ contains two distinct components: the Wigner time delay $\tau_W$ and the continuum-continuum CC) correction $\tau_{cc}$. In the case of helium with only one $1s\to Ep$ photoemission channel, the Wigner time delay $\tau_W$ does not depend on the photoelectron detection angle relative to the polarization vector. However, the CC correction $\tau_{cc}$ shows a noticeable angular dependence. We illustrate these findings by performing two sets of calculations. In the first set, we solve the time-dependent Schr\"odinger equation for the helium atom ionized by an attosecond pulse train and probed by an IR pulse. In the second approach, we employ the lowest order perturbation theory which describes absorption of the XUV and IR photons. Both calculations produce close results in a fair agreement with experiment.