45 research outputs found
A Ten-Fold Solvent Kinetic Isotope Effect for the Nonradiative Relaxation of the Aqueous Ferrate(VI) Ion
Hypervalent iron intermediates have been invoked in the catalytic cycles of many metalloproteins, and thus it is crucial to understand how the coupling between such species and their environment can impact their chemical and physical properties in such contexts. In this 2 work, we take advantage of the solvent kinetic isotope effect (SKIE) to gain insight into the nonradiative deactivation of electronic excited states of the aqueous ferrate(VI) ion. We observe an exceptionally large SKIE of 9.7 for the nanosecond-scale relaxation of the lowest energy triplet ligand field state to the ground state. Proton inventory studies demonstrate that a single solvent O-H bond is coupled to the ion during deactivation, likely due to the sparse vibrational structure of ferrate(VI). Such a mechanism is consistent with that reported for the deactivation of f-f excited states of aqueous trivalent lanthanides, which exhibit comparably large SKIE values. This phenomenon is ascribed entirely to dissipation of energy into a higher overtone of a solvent acceptor mode, as any impact on the apparent relaxation rate due to a change in solvent viscosity is negligible
X-ray induced electron and ion fragmentation dynamics in IBr
Characterization of the inner-shell decay processes in molecules containing
heavy elements is key to understanding x-ray damage of molecules and materials
and for medical applications with Auger-electron-emitting radionuclides. The 1s
hole states of heavy atoms can be produced by absorption of tunable x-rays and
the resulting vacancy decays characterized by recording emitted photons,
electrons, and ions. The 1s hole states in heavy elements have large x-ray
fluorescence yields that transfer the hole to intermediate electron shells that
then decay by sequential Auger-electron transitions that increase the ion's
charge state until the final state is reached. In molecules the charge is
spread across the atomic sites, resulting in dissociation to energetic atomic
ions. We have used x-ray/ion coincidence spectroscopy to measure charge states
and energies of I and Br atomic ions following 1s ionization at
the I and Br \textit{K}-edges of IBr. We present the charge states and kinetic
energies of the two correlated fragment ions associated with core-excited
states produced during the various steps of the cascades. To understand the
dynamics leading to the ion data, we develop a computational model that
combines Monte-Carlo/Molecular Dynamics simulations with a classical
over-the-barrier model to track inner-shell cascades and redistribution of
electrons in valence orbitals and nuclear motion of fragments
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Site-Selective Real-Time Observation of Bimolecular Electron Transfer in a Photocatalytic System Using L-Edge X-Ray Absorption Spectroscopy
Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [IrIII (ppy)2 (bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe3(CO)12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high-resolution Ir L-edge X-ray absorption spectroscopy on the picosecond time scale and restricted active space self-consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance
Quantifying Photoinduced Polaronic Distortions in Inorganic Lead Halide Perovskites Nanocrystals
The development of next generation perovskite-based optoelectronic devices
relies critically on the understanding of the interaction between charge
carriers and the polar lattice in out-of-equilibrium conditions. While it has
become increasingly evident for CsPbBr3 perovskites that the Pb-Br framework
flexibility plays a key role in their light-activated functionality, the
corresponding local structural rearrangement has not yet been unambiguously
identified. In this work, we demonstrate that the photoinduced lattice changes
in the system are due to a specific polaronic distortion, associated with the
activation of a longitudinal optical phonon mode at 18 meV by electron-phonon
coupling, and we quantify the associated structural changes with atomic-level
precision. Key to this achievement is the combination of time-resolved and
temperature-dependent studies at Br K-edge and Pb L3-edge X-ray absorption with
refined ab-initio simulations, which fully account for the screened core-hole
final state effects on the X-ray absorption spectra. From the temporal
kinetics, we show that carrier recombination reversibly unlocks the structural
deformation at both Br and Pb sites. The comparison with the
temperature-dependent XAS results rules out thermal effects as the primary
source of distortion of the Pb-Br bonding motif during photoexcitation. Our
work provides a comprehensive description of the CsPbBr3 perovskites
photophysics, offering novel insights on the light-induced response of the
system and its exceptional optoelectronic properties.Comment: Main: 27 pages, 4 figures SI: 16 pages, 8 figure