block below the plot indicates typical atomic field strengths related to given laser intensities. \[fig:laser-regimes\] ](laser_regimes){width="0.9\linewidth"}

One can notice the enormous large intensity range of optical lasers. But the range of available conditions is also dramatically extended by FEL, which exist for photons in the IR, VUV and X-ray regime. Also indicated outside the axes are corresponding regions of relevance in atoms and molecules in terms of energy/frequency and field/intensity. The lowest frequencies in the deep IR are associated with molecular vibrations, while the range around visible light belongs to the dynamics of valence electrons and core electrons move at much higher frequencies in X-ray regime. The gray box below the plot indicates typical atomic and molecular field strengths in terms of an equivalent laser intensity.

Laser characteristics have to be considered in relation to the electronic response. This is usually quantified via the ponderomotive potential $U_p$ and the associated Keldysh parameter $\gamma$. $U_p$ represents the electron kinetic energy (averaged over one photon cycle) of a freely oscillating electron (pure quiver motion, no drift velocity) in a laser field. At peak laser intensity, it reads : $$\begin{aligned}
  U_p
  =
  \frac{e^2\, E_0^2}{4\, m_{\rm el}\, \omega_{\rm las}^2} 
  =
  9.33\times10^{-14}{\rm eV}\times\,
  I_0[{\rm W/cm^2}]\,(\lambda_\mathrm{las}[{\rm 