Evolution from perturbative to field‐ionization regimes through electron spectroscopy

A study of the electron energy spectra from rare gases ionization by intense femtosecond pulses allows to span the various ionization regimes in strong fields. Using 70 fs, 617 nm pulses, xenon, krypton, and argon produce electron energy spectra characteristic of the perturbative regime, i.e., dominated by resonances and Above‐Threshold Ionization. However, for these atoms, ionization saturates at intensities lower than required for the onset of the tunneling regime. At the highest intensity, helium produces structureless electron energy spectra indicating a very fast ionization. However, the appearance intensity is already too high to observe the perturbative regime. With neon, only the ATI structure remains observable and it seems to stand at the border of the tunneling regime. This trend, from xenon to helium may be viewed as the typical behavior of an atom submitted to increasing intensities if it could be recorded without the limitations of the detection threshold and the saturation intensity.