  A coherent mechanism of robust population inversion in atomic and molecular
systems by a chirped field is presented. It is demonstrated that a field of
sufficiently high chirp rate imposes a certain relative phase between a ground
and excited state wavefunction of a two-level system. The value of the relative
phase angle is thus restricted to be negative and close to 0 or $-\pi$ for
positive and negative chirp, respectively. This explains the unidirectionality
of the population transfer from the ground to the excited state. In a molecular
system composed of a ground and excited potential energy surface the symmetry
between the action of a pulse with a large positive and negative chirp is
broken. The same framwork of the coherent mechanism can explain the symmetry
breaking and the population inversion due to a positive chirped field.
