Over the past thirty years, extensive studies of strong-field photoionization of atoms have revealed both quantum and classical aspects including above-threshold ionization1, electron wave-packet drift, quiver and rescattering motions. Increasingly sophisticated spectroscopic techniques2 and sculpted laser pulses3 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.

Nature Phys.

Blaga, C. I., Catoire, F., Colosimo, P., Paulus, G. G., Muller, H. G., Agostini, P., & DiMauro, L. F. (2009). Strong-field photoionization revisited. Nature Phys., 5(5), 335–338. doi:10.1038/nphys1228