For a symmetric bridge coupled to infinite leads, in the presence of a dipole-coupled external ac field with harmonic mixing, we solve the Schrodinger equation in the time domain using open boundary conditions as well as in the energy domain using Floquet scattering theory. As this potential breaks parity and generalized parity, we find a nonvanishing average current. We then optimize the relative amplitude ratio between the fundamental and the second harmonic, leading to a maximum in the pump current.