Attosecond transient absorption spectroscopy of helium above the N=2 ionization threshold

Attosecond transient absorption spectroscopy (ATAS) allows for the study of electron dynamics in atoms and molecules with attosecond time resolution. Previous works reported in the literature have made use of ATAS to image and control such dynamics in the single-channel ionization continuum of helium; in particular, in the vicinity of the doubly excited autoionizing states lying between the N=1and N=2 thresholds. In this work, we have extended these studies to autoionizing states lying above the N=2 threshold, where several ionization channels are open. From an accurate solution of the time-dependent Schrödinger equation, we predict the appearance of pronounced one-photon beatings between the 3snp states and the adjacent ¹S^e and ¹D^e resonances, as well as, more surprisingly, two-photon beatings between the 3s3p  doubly excited state and the ¹P^o nonresonant continuum. Both effects lead to a significant distortion of the 3snp Fano profiles and to a strong variation of these profiles with the pump-probe delay, thus demonstrating control of the corresponding multichannel two-electron correlated wave packets, in the same way as reported for resonances lying below the N=2 threshold.