Resonant structures in the low-energy electron continuum for single ionization of atoms in the tunnelling regime

We present high-resolution fully differential experimental data on single ionization of He, Ne and Ar by ultra-short (25 fs, 6 fs) 795 nm laser pulses at intensities 0.15–2.0 × 10 15 W cm −2 . We show that the ATI-like pattern can survive deep in the tunnelling regime and that the atomic structure plays an important role in the formation of the low-energy photoelectron spectra even at high intensities. The absence of ponderomotive shifts, the splitting of the peaks and their degeneration for few-cycle pulses indicate that the observed structures originate from a resonant process.