  We investigate the generation of even and odd harmonics using an intense
laser and a weak second harmonic field. Our theoretical approach is based on
solving the saddle-point equations within the Strong Field Approximation. The
phase of the even harmonic oscillation as a function of the delay between
fundamental and second harmonic field is calculated and its variation with
energy is found to be in good agreement with recent experimental results. We
also find that the relationship between this phase variation and the group
delay of the attosecond pulses, depends on the intensity and wavelength of the
fundamental field as well as the ionization potential of the atom.
