Direct optical observation of attosecond electron wavepacket interference

Recently, the ionization probability of helium atoms with APT photon energies below the ionization threshold in presence of a time-delayed infrared (IR) field was studied and was found to modulate with twice the IR frequency. The result was explained by the interference of transiently bound electron wavepackets (EWP). We investigated this physical system with attosecond time-resolved transient absorption spectroscopy, The different observable probed by this all-optical technique compared to the experiment by Johnsson et al. allows us to verify the plausibility of the previously introduced intuitive theoretical model.