X-ray Transient Absorption Studies of Exciton Self-Trapping*

We present optical pump / X-ray probe studies to probe changes in electronic distribution and local structure upon formation of a self-trapped exciton. The experiments were carried out on the mixed-valence halide-bridged transition metal linear chain material [Pt(en₂)][Pt(en₂)Cl₂](ClO₄)₄, a quasi-one-dimensional Peierls insulator with a strong electron-phonon coupling that drives the self-trapping process. Time-resolved measurements were carried out at the Advanced Photon Source to probe changes in the Pt L_III XANES spectrum on a picosecond time scale following excitation of the optical intervalence charge transfer transition. Ab initio FEFF9 modeling of the spectra and the associated angular momentum projected density of states were used to interpret the results. We find photoinduced changes reflecting electronic redistribution and local lattice distortions that correspond to exciton localization in the small polaron limit.