  We propose a technique to prepare coherent superpositions of two
nondegenerate quantum states in a three-state ladder system, driven by two
simultaneous fields near resonance with an intermediate state. The technique,
of potential application to enhancement of nonlinear processes, uses adiabatic
passage assisted by dynamic Stark shifts induced by a third laser field. The
method offers significant advantages over alternative techniques: (\i) it does
not require laser pulses of specific shape and duration and (\ii) it requires
less intense fields than schemes based on two-photon excitation with
non-resonant intermediate states. We discuss possible experimental
implementation for enhancement of frequency conversion in mercury atoms.
