  We calculated third order non-linear polarization to estimate the two-photon
absorption of non-interacting two-level molecules in the transmission-type
degenerate pump-probe geometry. The spectral intensity and the phase changes of
the laser pulses when passing through a thin dielectric slab composed of the
molecules were considered. We also investigated the effect of the decay rate of
the molecules and the chirp of the pulses on their spectral intensity and phase
changes.
