142 research outputs found
Correction of complex nonlinear signal response from a pixel array detector
The pulsed free-electron laser light sources represent a new challenge to photon area detectors due to the intrinsic spontaneous X-ray photon generation process that makes single-pulse detection necessary. Intensity fluctuations up to 100% between individual pulses lead to high linearity requirements in order to distinguish small signal changes. In real detectors, signal distortions as a function of the intensity distribution on the entire detector can occur. Here a robust method to correct this nonlinear response in an area detector is presented for the case of exposures to similar signals. The method is tested for the case of diffuse scattering from liquids where relevant sub-1% signal changes appear on the same order as artifacts induced by the detector electronics
Dynamics of the photoinduced insulator-to-metal transition in a nickelate film
The control of materials properties with light is a promising approach
towards the realization of faster and smaller electronic devices. With phases
that can be controlled via strain, pressure, chemical composition or
dimensionality, nickelates are good candidates for the development of a new
generation of high performance and low consumption devices. Here we analyze the
photoinduced dynamics in a single crystalline NdNiO film upon excitation
across the electronic gap. Using time-resolved reflectivity and resonant x-ray
diffraction, we show that the pump pulse induces an insulator-to-metal
transition, accompanied by the melting of the charge order. Finally we compare
our results to similar studies in manganites and show that the same model can
be used to describe the dynamics in nickelates, hinting towards a unified
description of these photoinduced phase transitions.Comment: 17 pages, 6 figure
Imaging ultrafast excited state pathways in transition metal complexes by X-ray transient absorption and scattering using X-ray free electron laser source
This report will describe our recent studies of transition metal complex structural dynamics on the fs and ps time scales using an X-ray free electron laser source, Linac Coherent Light Source (LCLS). Ultrafast XANES spectra at the Ni K-edge of nickel(II) tetramesitylporphyrin (NiTMP) were successfully measured for optically excited state at a timescale from 100 fs to 50 ps, providing insight into its sub-ps electronic and structural relaxation processes. Importantly, a transient reduced state Ni(I) (π, 3d(x2−y2)) electronic state is captured through the interpretation of a short-lived excited state absorption on the low-energy shoulder of the edge, which is aided by the computation of X-ray transitions for postulated excited electronic states. The observed and computed inner shell to valence orbital transition energies demonstrate and quantify the influence of electronic configuration on specific metal orbital energies. A strong influence of the valence orbital occupation on the inner shell orbital energies indicates that one should not use the transition energy from 1s to other orbitals to draw conclusions about the d-orbital energies. For photocatalysis, a transient electronic configuration could influence d-orbital energies up to a few eV and any attempt to steer the reaction pathway should account for this to ensure that external energies can be used optimally in driving desirable processes. NiTMP structural evolution and the influence of the porphyrin macrocycle conformation on relaxation kinetics can be likewise inferred from this study
Ultrafast Excited State Relaxation of a Metalloporphyrin Revealed by Femtosecond X-ray Absorption Spectroscopy
Photoexcited Nickel(II) tetramesitylporphyrin
(NiTMP), like many
open-shell metalloporphyrins, relaxes rapidly through multiple electronic
states following an initial porphyrin-based excitation, some involving
metal centered electronic configuration changes that could be harnessed
catalytically before excited state relaxation. While a NiTMP excited
state present at 100 ps was previously identified by X-ray transient
absorption (XTA) spectroscopy at a synchrotron source as a relaxed
(d,d) state, the lowest energy excited state (<i>J. Am. Chem.
Soc.</i>, <b>2007</b>, <i>129</i>, 9616 and <i>Chem. Sci.</i>, <b>2010</b>, <i>1</i>, 642),
structural dynamics before thermalization were not resolved due to
the ∼100 ps duration of the available X-ray probe pulse. Using
the femtosecond (fs) X-ray pulses of the Linac Coherent Light Source
(LCLS), the Ni center electronic configuration from the initial excited
state to the relaxed (d,d) state has been obtained via ultrafast Ni
K-edge XANES (X-ray absorption near edge structure) on a time scale
from hundreds of femtoseconds to 100 ps. This enabled the identification
of a short-lived Ni(I) species aided by time-dependent density functional
theory (TDDFT) methods. Computed electronic and nuclear structure
for critical excited electronic states in the relaxation pathway characterize
the dependence of the complex’s geometry on the electron occupation
of the 3d orbitals. Calculated XANES transitions for these excited
states assign a short-lived transient signal to the spectroscopic
signature of the Ni(I) species, resulting from intramolecular charge
transfer on a time scale that has eluded previous synchrotron studies.
These combined results enable us to examine the excited state structural
dynamics of NiTMP prior to thermal relaxation and to capture intermediates
of potential photocatalytic significance
Performance of a beam-multiplexing diamond crystal monochromator at the Linac Coherent Light Source
Atomistic characterization of the active-site solvation dynamics of a model photocatalyst
The interactions between the reactive excited state of molecular photocatalysts and surrounding solvent dictate reaction mechanisms and pathways, but are not readily accessible to conventional optical spectroscopic techniques. Here we report an investigation of the structural and solvation dynamics following excitation of a model photocatalytic molecular system [Ir 2 (dimen) 4 ] 2+, where dimen is para-diisocyanomenthane. The time-dependent structural changes in this model photocatalyst, as well as the changes in the solvation shell structure, have been measured with ultrafast diffuse X-ray scattering and simulated with Born-Oppenheimer Molecular Dynamics. Both methods provide direct access to the solute-solvent pair distribution function, enabling the solvation dynamics around the catalytically active iridium sites to be robustly characterized. Our results provide evidence for the coordination of the iridium atoms by the acetonitrile solvent and demonstrate the viability of using diffuse X-ray scattering at free-electron laser sources for studying the dynamics of photocatalysis
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