On the angular dependence of the photoemission time delay in helium

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 $ τ_a=τ_W+τ_{cc} $ contains two distinct components: the Wigner time delay $τ_W$ and the continuum-continuum CC) correction $τ_{cc}$. In the case of helium with only one $1s\to Ep$ photoemission channel, the Wigner time delay $τ_W$ does not depend on the photoelectron detection angle relative to the polarization vector. However, the CC correction $τ_{cc}$ shows a noticeable angular dependence. We illustrate these findings by performing two sets of calculations. In the first set, we solve the time-dependent Schrödinger 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.