Rydberg ionization spectroscopy
Rydberg ionization spectroscopy is a spectroscopy technique in which multiple photons are absorbed by an atom causing the removal of an electron to form an ion. == Resonance ionization spectroscopy == The ionization threshold energy of atoms and small molecules are typically larger than the photon energies that are most easily available experimentally.
Source: Wikipedia — Rydberg ionization spectroscopy (CC BY-SA 4.0)