Resonant Structures in the Low-Energy Electron Continuum for Single   Ionization of Atoms in the Tunneling Regime
  We present results of high-resolution experiments on single ionization of He, Ne and Ar by ultra-short (25 fs, 6 fs) 795 nm laser pulses at intensities 0.15-2.0x10^15 W/cm^2. We show that the ATI-like pattern can survive deep in the tunneling 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. 
A. Rudenko, K. Zrost, C.D. Schr\"oter, V.L.B. de Jesus, B. Feuerstein,   R. Moshammer, J. Ullrich