The authors consider a finite range ac-biased front gate acting on a quantum channel with Rashba and Dresselhaus spin-orbit interaction effects. The ac-biased gate, giving rise to a dynamical Rashba coupling, causes spin-resolved coherent resonant inelastic scattering. A pure dc spin current is subsequently generated without accompanying charge current. In the presence of Dresselhaus effect, the dc spin current is suppressed in the regime but is assisted in the high kinetic energy regime. (C) 2008 American Vacuum Society.