Classically forbidden recurrences in the photoabsorption spectrum of lithium

We present data on the photoabsorption spectrum of lithium atoms in an electric field at energies between the saddle point of the Stark potential and below the field-free ionization threshold. The spectrum displays a sequence of sharp resonances and a sequence of broad ones. We find that the broad resonances arise from the classically forbidden reflection of waves above a dynamical potential barrier. The recurrence spectrum is also observed and it is dramatically affected by above-barrier reflections. We have developed a semiclassical theory that interprets the spectra using quasiclassical trajectories that undergo above-barrier reflection. © 2000 The American Physical Society.