Attosecond Delays in Resonant Photoionization

Attosecond delays in the photoionization of atomic states have been evidenced in recent experiments performed in the 2010's [1, 2]. The delays were associated to the emission of photoelectron wave packets ejected from different atomic states, in the combined presence of attosecond pulses of XUV radiation and of a synchronized IR laser pulse, the latter being used as a reference ``clock'' [3]. These experiments were performed at XUV frequencies connecting the ground state to a ``flat'' continuum. Theoretical treatments were able to relate the measured delays to Wigner's definition of time delays in terms of the energy derivative of the phase-shift attached to the continuum wave functions of the photoelectrons [4]. Attention has recently shifted towards the case of resonant photoionization in the course of which the XUV frequency is tuned close to a resonance of the target system. The case of a transition towards an autoionizing states of the target is particularly interesting as it makes evident the role of electronic correlations [5]. Here, we shall present recent advances realized in the theoretical interpretation of this new class of experiments. \\[4pt] [1] M. Schultze et al. Science \textbf{328}, 1658-1662 (2010).\\[0pt] [2] K. Kl\"{u}nder, et al. Phys. Rev. Lett. \textbf{106}, 143002 (2011) 5 p.\\[0pt] [3] A. Maquet, J. Caillat, and R. Ta\"{i}eb, J. Phys. B: At. Mol. Opt. Phys. \textbf{47,} 204004 (2014) 13 p.\\[0pt] [4] E. P. Wigner, Phys. Rev. \textbf{98}, 145-7 (1955).\\[0pt] [5] Christian Ott, et al. $ s$Nature \textbf{516}, 374-378 (2014).