On the angular dependence of the photoemission time delay in helium
  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. 
I A Ivanov, J M Dahlstr\"om, E Lindroth and A S Kheifets
physics.atom-ph