Figure 2. Laser scans for the two-step ionization scheme.

Panels a and b show laser scans across the two optical transitions at λ₁=216 and λ₁=224 nm, obtained using the ionization path shown in Fig. 1a and α-decay detection of the photo-ionized ¹⁹⁹At. (c) Measurement of the ionization threshold. The count rate of α-particles from the decay of ¹⁹⁹At is registered for different total photon energies of the applied laser beams used in the two-colour schemes {216; λ₂} and {224; λ₂} (cf. Fig. 1b).