Comparison of Strong-field Ionization Models in the Wavelength-scaling of High Harmonic Generation.

We report the use of wavelength-tuneable laser pulses from an optical parametric amplifier to generate high-order harmonics in a range of noble gases. The variation of the harmonic cut-off wavelength and phasematching pressure with gas species and fundamental wavelength were recorded. The experimental results are compared to a phenomenological model of the harmonic generation process, incorporating two separate models of photo-ionization. While the calculated phasematching pressure is generally insensitive to the ionization model, for the harmonic cut-off we obtain superior agreement between experiment and theory when the Yudin-Ivanov (YI) ionization model is used, compared to the commonly utilised Ammosov-Delone-Krainov (ADK) model.