Strong-field photoionization revisited

Over the past thirty years, extensive studies of strong-field photoionization of atoms have revealed both quantum and classical aspects including above-threshold ionization(1), electron wave-packet drift, quiver and rescattering motions. Increasingly sophisticated spectroscopic techniques(2) and sculpted laser pulses(3) coupled with theoretical advances have led to a seemingly complete picture of this fundamental laser-atom interaction. Here, we describe an effect that seems to have escaped observation: the photoelectron energy distribution manifests an unexpected characteristic spike-like structure at low energy, which becomes prominent using mid-infrared laser wavelengths (lambda > 1.0 mu m). The low-energy structure is observed in all atoms and molecules investigated and thus seems to be universal. The structure is qualitatively reproduced by numerical solutions of the time-dependent Schrodinger equation but its physical origin is not yet identified.