Wavelength dependence of photoelectron spectra in above-threshold ionization

We present angle- and energy-resolved measurements of photoelectrons produced in strong-field ionization of Xe and Ar using a tunable femtosecond laser in the wavelength range between 600 and 800 nm. Systematic analysis of the experimental data that are quantitatively reproduced by numerical solutions of the time-dependent Schrödinger equation with integration over the laser focal volume demonstrates the dominance of resonance-enhanced ionization. Continuous variation of the laser wavelength allows the identification of a number of consecutive channel-switching effects with a reliable assignment of the intermediate Rydberg states involved. The appearance of the resonant sub-structure in the electron energy spectra is influenced by the presence of a non-resonant contribution. At relatively low laser intensity, a coherent addition of resonant and non-resonant ionization processes is observed. Due to the absence of an intensity dependence in the resonance-enhanced ionization, we observe the persistence of Freeman resonances at the transition to the tunnelling regime.