Strong-field photoionization revisited

The discovery of an overlooked but apparently ubiquitous spike in the mid-infrared photoelectron spectra of molecular and atomic gases suggests that we don’t know as much as we thought we did about the ionization of matter in strong fields. 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 ( λ >1.0 μm). 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 Schrödinger equation but its physical origin is not yet identified.