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.Comment: 11 pages, 3 figure