  We present and implement a new scheme for independent control of both the
final and the transient population utilizing the symmetry properties of the
system. By proper pulse shaping, utilizing the invariance of the two-photon
absorption to specific phase transformations of the pulse, different time
evolutions of the transient population are photo-induced for a given (fixed)
final state population. The model system is the Na atom. The work is conducted
in the weak-field regime for which the transient two-photon excitation is
described by second-order perturbation theory. One most attractive case is the
extended family of third-order chirp pulses which control the population
build-up duration independently of the final population.
