  We study the Cooper minimum in high harmonic generation from argon atoms
using long wavelength laser pulses. We find that the minimum in high harmonic
spectra is systematically shifted with respect to total photoionization cross
section measurements. We use a semi-classical theoretical approach based on
Classical Trajectory Monte Carlo and Quantum Electron Scattering methods
(CTMC-QUEST) to model the experiment. Our study reveals that the shift between
photoionization and high harmonic emission is due to several effects: the
directivity of the recombining electrons and emitted polarization, and the
shape of the recolliding electron wavepacket.
