Exchange effects on time-resolved photoemission

The $1s2s$ singlet and triplet metastable states of the helium atom, which have remarkably long lifetimes, can be conveniently generated in a helium gas sample by discharge currents. In this work, we study theoretically the effect of exchange parity on the time-resolved photoionization of the atom from these two metastable states. In particular, we compare the non-resonant photoemission delays to the $1s\varepsilon_\ell$ and $2\ell\varepsilon_{\ell'}$ channels, as well as the resonant pump-probe ionization to the $2\ell\varepsilon_{\ell'}$ channels, mediated by the autoionizing doubly-excited states that converge to the $2s/2p$ He$^+$ threshold.