X‐ray Absorption Spectroscopy for the Characterization of Photoinduced Linkage Isomers in Single Crystals: Experimental Setup and XANES Results**

Photoinduced linkage isomers (PLI) of the nitrosyl ligand in single crystals Na2[Fe(CN)5NO] ⋅ 2H2O (SNP, sodium nitroprusside) have been studied by X-ray absorption spectroscopy (XAS) at low temperatures, using a cryostat allowing in-situ polarized LASER irradiation and XAS measurements. In XANES (X-ray near edge structure) region significant modification pre-edge peak intensities is observed after ground state (GS) NO-κN isomer with light blue infrared spectral range, used for generation isonitrosyl NO-κO (MS1) side-on bound κ2N,O (MS2), respectively. These observations are interpreted respect to changes nature frontier (unoccupied) molecular orbitals different configurations, expected from earlier calculations.