quantities (total ionization) down to energy-resolved (Photo-Electron Spectra, PES) and angle-resolved (Photo-Angular Distribution, PAD) quantities. Time is also a key quantity as ionization signals can be followed in time, leading to Time-Resolved (TR) results. We briefly describe in this section the various types of observables experimentally accessible, starting from the simplest one, that is the total ionization, to the most elaborate ones (TR-PES and PAD). In terms of cross sections, this means that we go from integrated ones to single-differential and even double-differential ones, all possibly time-resolved. Before discussing these various observables, we briefly introduce key mechanisms of ionization, again focusing the discussion on laser induced ionization.

Ionization mechanisms {#sec:mecha}
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Basic ionization mechanisms are illustrated in Fig. \[fig:ioni\_mecha\].

We start from the simplest case of an atom (left panel in Fig. \[fig:ioni\_mecha\]) to introduce two basic ionization mechanisms. The first one corresponds to a vertical excitation of a bound electron by absorption of one or several ($\nu$) photons (Multi-Photon Ionization or MPI). This mechanisms may spread over several laser cycles and prevails in weak and moderate fields, usually quoted perturbative regime. It is associated to large values of the Keldysh parameter ($\gamma \gg 1$). MPI can promote electrons far 