Abstract
A molecular theory of resonant optical rectification via the second-order electric-dipole polarizability related to the magnetic-dipole and electric-quadrupole interaction of electromagnetic radiation with atoms and molecules of an isotropic medium is presented. The magnetic-dipole and electric-quadrupole contributions to dc electric-dipole polarization can be produced by arbitrarily polarized light which has to be at resonance with the molecule. This effect can appear in an isotropic medium composed of molecules with arbitrary symmetries.