Abstract: A method and an apparatus for selectively cancelling the effect of the active center of a molecule are provided. The method comprises illuminating a target molecule with two synchronized ultrashort X-ray pulses using a laser, the two synchronized ultrashort X-ray pulses having different central photon energies the subtraction of which matches the photon energy of a peak of the core spectrum of the target molecule, such that a core state of an atom of the target molecule, and also of identical surrounding molecules, is selectively excited by the re-DFG effect as a result of the illumination. The method is implementable for simple or complex molecular systems and bulk materials.