Non-equilibrium-state x-ray absorption spectroscopy: a local structure study of photo-induced phase transition

Abstract

We describe non‐equilibrium‐state x‐ray absorption spectroscopy focusing on local structure of photo‐excited states trapped at low temperature. For this purpose, a novel Ge 100 pixel array detector with a packing density of 88% was developed. The local structure of photo‐induced phase of Fe(II) spin crossover complex, [Fe(2‐pic)3]Cl2EtOH (2‐pic=2‐aminomethyl pyridine), was investigated at low temperature (T <150 K). The use of pixel array detector and high‐flux synchrotron x‐ray source (multipole wiggler) successfully provided x‐ray absorption spectra with high quality, in‐situ, during the photo‐excitation. It was found that the photo‐induced phase under optical pumping at low temperature (T < 50 K) has an octahedral geometry with the elongated Fe‐N distance (2.16 ± 0.01 Å), stabilizing the high spin state (S=2) configuration. No indication of symmetry breaking of FeN6 clusters upon LS↔HS spin‐state switching was observed. It was demonstrated that the technique is a promising means to probe the local structure of non‐equilibrium state such as trapped excited states or metastable states.