## 5 Time-Resolved Coherent Excitation and Decay of Strongly Correlated Autoionizing States
### Relative Time-Delay in the Photoemission of Electrons from Different Subshells  
#### 6.2.1 Introduction  
In 2008, experimentalists at the department of attosecond physics of the Max Planck Institute of Quantum Optics in Garching, Germany, headed by [PERSON], were in the process of carrying out a spectacular pump-probe experiment of fundamental significance. This involved the implementation of their attosecond streaking technique for the photoionization of neon. Their goal was to measure whether, upon photoemission of the electrons from the \(2s\) and \(2p\) subshells of neon by an attosecond pulse with photon energy centered around 106 eV, the electrons are ejected absolutely simultaneously, as the conventional wisdom was at the time, or with a time difference, i.e., with a _relative time delay_ (TD).  
In view of that experimental project, [PERSON] of the Garching institute and the Athens team started a collaboration with the purpose of developing a many-electron theory that could determine whether such a TD could indeed occur in neon photoionization, and, if so, what its sign and magnitude ought to be. This possibility would be unique in the study of atomic physics, since there are no other collision processes (e.g., other atoms) that could interfere.  
At that time we were in the dark, since this new problem of time-dependent many-electron quantum mechanics had never been considered and understood theoretically, let alone solved quantitatively. The only certainty was that if this were a real phenomenon, the corresponding information ought to be contained in the METDSE describing the synchronized pump (EUV attosecond pulse) plus probe (IR laser pulse) experiment, in terms of the Hamiltonian \(\mathbf{H}_{tot}(t)=\mathbf{H}_{Ne}+\mathbf{V}_{\text{int}}(t)\). \(\mathbf{H}_{Ne}\) is the Hamiltonian of the field-free neon and \(\mathbf{V}_{\text{int}}(t)=\mathbf{V}_{IR}(t)+\mathbf{V}_{EUV}(t-\Delta t)\), where \(\Delta t\) is the controlled time delay between the application of the two pulses in the attosecond streaking experiment [11].  
The theoretical treatment and results were published in [11], including its SOM (see also [17] and below), together with the experimental findings of [PERSON] et al. [11]. Both the experiment and the theory introduced into atomic and molecular physics a new and observable concept in attosecond electron dynamics--namely the relative TD in the photoemission from atoms--and methods in terms of which quantitative results can be obtained.  
After the 2010 publication [11], a large number of publications, experimental as well as theoretical, have dealt with aspects of this problem, not only for neon but also for other cases. This body of work was recently reviewed comprehensively by [PERSON], [PERSON], and [PERSON] [55]. Section V of [55], titled \"Time-resolved photoionization of many-electron atoms\", contains a discussion related to the seminal work on neon. As regards theory, even time-independent treatments have been implemented, reducing the understanding of a phenomenon where the concept of time is inevitably involved (e.g., quantum mechanical time of free N-electron wavepacket formation), to the calculation of photoionization Eisenbud-Wigner-Smith (EWS) time delays in terms of energy derivatives of scattering phases.