Abstract
The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110 fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.
Highlights
The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials
In the present study we demonstrate that the time-resolved Kα X-ray emission spectroscopy (XES) signal of [Fe(bmip)2]2+ shows the signature of vibrational wavepacket dynamics
Three signals were detected simultaneously as a function of pump-probe delay: (1) X-ray solution scattering (XSS) with a scattering range of 0.5–5.0 Å−1, (2) XES over the full Fe Kβ spectral energy range (7035–7070 eV) and (3) intensity at a fixed emitted photon energy of 6404.3 ± 0.2 eV corresponding to the maximum of the Kα1 XES peak (Fig. 1c–d)
Summary
The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Ultrafast optical spectroscopy and quantum dynamics studies requires robust signatures for MLCT and MC excited states and motivates our time-resolved XES and XSS study of the coupled electronic and structural dynamics of [Fe(bmip)2]2+.
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