Stimulated x-ray Raman scattering for selective preparation of dark states bypassing optical selection rules

We present an x-ray based stimulated Raman approach to control the preparation of populations and coherent superpositions of (valence) electronic states in generic molecular systems, including optically dark states. Leveraging on the unique properties of core-level excitations as intermediates of the Raman process, we demonstrate that optically forbidden singlet-singlet and singlet-triplet transitions are made accessible. We explore the role of both intrinsic molecular properties (transition dipole moments and involved molecular orbitals) and external field parameters to explain the efficiency and selectivity of the process towards specific classes of electronic states, and to control it towards the amplification of desired populations or superpositions. While the reported mechanism naturally applies to virtually all organic and bio-organic molecules and can straightforwardly be extended to a broader class of molecules, atoms and materials, detailed results for two molecular systems are here reported as a testbed of the proposed approach, for which experimental feasibility is eventually discussed