Adiabatic and diabatic evolution in the multiphoton ionization of sodium

We have measured the three photon ionization and electron energy spectra of Na from 5-ps laser pulses over the range 16 300 to 17 800 ${\mathrm{cm}}^{\mathrm{-}1}$. Together these measurements suggest that multiphoton ionization can be understood in terms of a Floquet description. Sharp discontinuities in the ionization probability and Rabi splittings in the electron energy spectra are direct consequences of adiabatic and diabatic evolution, respectively, from the initial zero-field ground state.