The electronic states of many Mott insulators, including iridates, are often conceptualized in terms of localized atomic states such as the famous "J eff = 1/2 state". Although, orbital hybridization can strongly modify such states and dramatically change the electronic properties of materials, probing this process is highly challenging. In this work, we directly detect and quantify the formation of dimer orbitals in an iridate material Ba5AlIr2O11 using resonant inelastic x-ray scattering (RIXS). Sharp peaks corresponding to the excitations of dimer orbitals are observed and analyzed by a combination of density functional theory (DFT) calculations and theoretical simulations based on a Ir-Ir cluster model. Such partially delocalized dimer states lead to a re-definition of the angular momentum of the electrons and changes in the magnetic and electronic behaviors of the material. We use this to explain the reduction of the observed magnetic moment with respect to prediction based on atomic states. This study opens new directions to study dimerization in a large family of materials including solids, heterostructures, molecules and transient states.Many of the most interesting phases in correlated quantum materials occur in systems with strong Coulomb repulsion U , which tends to drive electron localization via the Mott insulating mechanism [1]. Due to this, we often conceptualize the electronic and magnetic properties of these systems in terms of localized states [2], even though many of the most interesting cases occur when there is strong competition between U and electron hopping t. Of particular interest in this regard is the localized "J eff = 1/2 state" in the iridates, which is the conceptual building block for a host of fascinating proposed and observed states including frustrated magnets [3-7], topological insulators [8,9], and possibly even unconventional superconductors [10]. Hopping between IrO 6 octahedra delocalizes the electrons and is expected to be relevant in many classes of iridate (or other heavy d-electron materials) with edge-sharing or face-sharing octahedra [11][12][13][14][15][16]. This can heavily modify, or even destroy, the J eff = 1/2 state motivating arguments about how best to conceptualize the electronic state of iridates [11,17,18]. Directly probing these states is therefore highly desirable. In the simplest case of dimerization between neighboring pairs of IrO 6 octahedra one expects the formation of quasi-localized dimer orbitals, which are difficult to probe by photo-emission due to the absence of dispersive bands and hard to probe optically as dipole optical selection rules means that transitions within a orbital manifold are nominally forbidden. RIXS, on the other hand, has been shown as an particularly incisive probe of on-site localized transitions in the iridates, but as far as we are aware, has never definitively isolated an excitation asso-ciated with dimerization [19,20].In this Letter, we establish that RIXS can directly measure peaks associated with orbital dimerization...

Although ultrafast manipulation of magnetism holds great promise for new physical phenomena and applications, targeting specific states is held back by our limited understanding of how magnetic correlations evolve on ultrafast timescales. Using ultrafast resonant inelastic X-ray scattering we demonstrate that femtosecond laser pulses can excite transient magnons at large wavevectors in gapped antiferromagnets and that they persist for several picoseconds, which is opposite to what is observed in nearly gapless magnets. Our work suggests that materials with isotropic magnetic interactions are preferred to achieve rapid manipulation of magnetism.