  The ultrafast dynamic evolution of an atomic system under medium-strength
laser fields is studied by performing transient absorption measurement. An
analytical model developed from perturbation theory with a modified transition
dipole moment is presented to explain the spectral features of the multilevel
system. By fitting the measured absorption spectra to the model, the system's
dynamic evolution is quantified by different amplitude and phase modulation
factors in the pump--probe and probe--pump scenarios. This study provides a way
to understand laser--matter interaction in the transition area between the
strong-field and weak-field regimes.
