Realization of adiabatic population transfer in a three-level system by using LR Hermitian invariants theory

We have studied the population transfer in a three-state system driven by laser beams in terms of the theory of Lewis－Riesenfeld Hermitian invariants for explicitly time-dependent Hamiltonian. The exact solutions obtained allow us to recover two existing schemes producing complete population transfer in the literature, i.e. the \pi-pulse and stimulated Raman adiabatic passage methods in our unified description. We present a strict formulation of adiabatic conditions and a more accurate expression of the \pi-pulse method according to the exact time-evolution of state vector.