  We present a model describing the use of ultra-short strong pulses to control
the population of the excited level of a two-level quantum system. In
particular, we study an off-resonance excitation with a few cycles pulse which
presents a smooth phase jump i.e. a change of the pulse's phase which is not
step-like, but happens over a finite time interval. A numerical solution is
given for the time-dependent probability amplitude of the excited level. The
control of the excited level's population is obtained acting on the shape of
the phase transient, and other parameters of the excitation pulse.
