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Optics Express May 2024The X-ray sources for Compton radiography of ICF experiments are generated by using intense picosecond lasers to irradiate wire targets. The wire diameter must be...
The X-ray sources for Compton radiography of ICF experiments are generated by using intense picosecond lasers to irradiate wire targets. The wire diameter must be designed thin enough, for example ∼ 10 µm in many published works, to comply a high spatial resolution. This results in a low laser-target interception, which limits the photon yield. We investigated a technique of coded-source radiography based on laser-driven annular sources via Monte Carlo and PIC simulations. The annular X-ray source is formed by laser irradiating tube target in which the effect of electron recirculation plays an important role. We proved that this technique has an increased spatial resolution and contrast than that using the Gaussian source produced by wire targets. Therefore, the diameter of the backlighter target can be significantly increased to uplift laser-target interception without compromising on spatial resolution. This contributes towards a reconciliation between the spatial resolution and photon yield for Compton radiography. The results predict the possibility of improving source photon yield by several times in future experiments.
PubMed: 38858878
DOI: 10.1364/OE.512301 -
ACS Applied Materials & Interfaces Jun 2024We present time-resolved Kerr rotation (TRKR) spectra in thin films of CHNHPbI (MAPI) hybrid perovskite using a unique picosecond microscopy technique at 4 K having a...
We present time-resolved Kerr rotation (TRKR) spectra in thin films of CHNHPbI (MAPI) hybrid perovskite using a unique picosecond microscopy technique at 4 K having a spatial resolution of 2 μm and temporal resolution of 1 ps, subjected to both an in-plane applied magnetic field up to 700 mT and an electric field up to 10 V/cm. We demonstrate that the obtained TRKR dynamics and spectra are substantially inhomogeneous across the MAPI films with prominent resonances at the exciton energy and interband transition of this compound. From the obtained quantum beating response as a function of magnetic field in the Voigt configuration, we also extract the inhomogeneity of the electron and hole Lande -values and spin coherence time, *. We also report the TRKR dependence on both the applied magnetic field and electric field. From the change in the quantum beating dynamics, we found that * substantially decreases upon the application of an electric field. At the same time, from the induced spatial TRKR changes, we show that the electric field induced effects are caused by ion migration in the MAPI films.
PubMed: 38857321
DOI: 10.1021/acsami.4c02419 -
Journal of Biomolecular NMR Jun 2024Deuterium (H) spin relaxation of CHD methyl groups has been widely applied to investigate picosecond-to-nanosecond conformational dynamics in proteins by solution-state...
Deuterium (H) spin relaxation of CHD methyl groups has been widely applied to investigate picosecond-to-nanosecond conformational dynamics in proteins by solution-state NMR spectroscopy. The B dependence of the H spin relaxation rates is represented by a linear relationship between the spectral density function at three discrete frequencies J(0), J(ω) and J(2ω). In this study, the linear relation between H relaxation rates at B fields separated by a factor of two and the interpolation of rates at intermediate frequencies are combined for a more robust approach for spectral density mapping. The general usefulness of the approach is demonstrated on a fractionally deuterated (55%) and alternate C-C labeled sample of E. coli RNase H. Deuterium relaxation rate constants (R, R, R, R) were measured for 57 well-resolved CHD moieties in RNase H at H frequencies of 475 MHz, 500 MHz, 900 MHz, and 950 MHz. The spectral density mapping of the 475/950 MHz data combination was performed independently and jointly to validate the expected relationship between data recorded at B fields separated by a factor of two. The final analysis was performed by jointly analyzing 475/950 MHz rates with 700 MHz rates interpolated from 500/900 MHz data to yield six J(ω) values for each methyl peak. The J(ω) profile for each peak was fit to the original (τ, S, τ) or extended model-free function (τ, S, S, τ, τ) to obtain optimized dynamic parameters.
PubMed: 38856928
DOI: 10.1007/s10858-024-00443-w -
Applied Optics Jun 2024The integration of silicon waveguides with low-dimensional materials with excellent optoelectronic properties can enable compact and highly integrated optical devices...
The integration of silicon waveguides with low-dimensional materials with excellent optoelectronic properties can enable compact and highly integrated optical devices with multiple advantages for multiple fields. A carbon nanotube (CNT) photodetector integrated on the silicon waveguide has the potential to meet on-chip high-speed optical interconnection systems, based on the outstanding properties of CNTs such as picosecond-level intrinsic photoresponse time, high charge carrier mobility, broad spectral response, high absorption coefficient, and so on. However, the thermal stability of the device may be compromised due to the local suspension in the channel for the height difference between the WG and the substrate. Here, we report a low-cost and low-optical-loss method to achieve the planarized silicon waveguide. After that, the CNT photodetectors integrated on the original and planarized waveguide with asymmetric palladium (Pd)-hafnium (Hf) metal contacts are fabricated. The influence of this planarization method on the performance of devices is analyzed via comparing the dark leakage current, the leakage current rectification ratio (CRR), the series resistances ( ), and the photoelectric response. Finally, a CNT photodetector based on the planarized waveguide with a photocurrent ( ) ∼510.84 , a photoresponsivity ( ) ∼51.04 /, the dark current ∼0.389µ, as well as a 3 dB bandwidth ∼34 at the large reverse voltage -3 is shown.
PubMed: 38856624
DOI: 10.1364/AO.520271 -
International Journal of Dermatology Jun 2024
PubMed: 38853377
DOI: 10.1111/ijd.17290 -
The Journal of Physical Chemistry. A Jun 2024Calculated potential energy structures and landscapes are very often used to define the sequence of reaction steps in an organometallic reaction mechanism and interpret...
Calculated potential energy structures and landscapes are very often used to define the sequence of reaction steps in an organometallic reaction mechanism and interpret kinetic isotope effect (KIE) measurements. Underlying most of this structure-to-mechanism translation is the use of statistical rate theories without consideration of atomic/molecular motion. Here we report direct dynamics simulations for an organometallic benzene reductive elimination reaction, where nonstatistical intermediates and dynamic-controlled pathways were identified. Specifically, we report single spin state as well as mixed spin state quasiclassical direct dynamics trajectories in the gas phase and explicit solvent for benzene reductive elimination from Mo and W bridged cyclopentadienyl phenyl hydride complexes ([MeSi(CMe)]M(H)(Ph), M = Mo and W). Different from the energy landscape mechanistic sequence, the dynamics trajectories revealed that after the benzene C-H bond forming transition state (often called reductive coupling), σ-coordination and π-coordination intermediates are either skipped or circumvented and that there is a direct pathway to forming a spin flipped solvent caged intermediate, which occurs in just a few hundred femtoseconds. Classical molecular dynamics simulations were then used to estimate the lifetime of the caged intermediate, which is between 200 and 400 picoseconds. This indicates that when the η-π-coordination intermediate is formed, it occurs only after the first formation of the solvent-caged intermediate. This dynamic mechanism intriguingly suggests the possibility that the solvent-caged intermediate rather than a coordination intermediate is responsible (or partially responsible) for the inverse KIE value experimentally measured for W. Additionally, this dynamic mechanism prompted us to calculate the / KIE value for the C-H bonding forming transition states of Mo and W. Surprisingly, Mo gave a normal value, while W gave an inverse value, albeit small, due to a much later transition state position.
PubMed: 38836889
DOI: 10.1021/acs.jpca.4c01788 -
The Journal of Physical Chemistry. C,... May 2024Methylammonium lead tribromide perovskite (MAPbBr) is an important material, for example, for light-emitting applications and tandem solar cells. The relevant...
Methylammonium lead tribromide perovskite (MAPbBr) is an important material, for example, for light-emitting applications and tandem solar cells. The relevant photophysical properties are governed by a plethora of phenomena resulting from the complex and relatively poorly understood interplay of excitons and free charge carriers in the excited state. In this study, we combine transient spectroscopies in the visible and terahertz range to investigate the presence and evolution of excitons and free charge carriers at ultrafast times upon excitation at various photon energies and densities. For above- and resonant band-gap excitation, we find that free charges and excitons coexist and that both are mainly promptly generated within our 50-100 fs experimental time resolution. However, the exciton-to-free charge ratio increases upon decreasing the phonon energy toward resonant band gap excitation. The free charge signatures dominate the transient absorption response for above-band-gap excitation and low excitation densities, masking the excitonic features. With resonant band gap excitation and low excitation densities, we find that although the exciton density increases, free charges remain. We show evidence that the excitons localize into shallow trap and/or Urbach tail states to form localized excitons (within tens of picoseconds) that subsequently get detrapped. Using high excitation densities, we demonstrate that many-body interactions become pronounced and effects such as the Moss-Burstein shift, band gap renormalization, excitonic repulsion, and the formation of Mahan excitons are evident. The coexistence of excitons and free charges that we demonstrate here for photoexcited MAPbBr at ultrafast time scales confirms the high potential of the material for both light-emitting diode and tandem solar cell applications.
PubMed: 38835933
DOI: 10.1021/acs.jpcc.3c08509 -
Nature Communications Jun 2024Classical photochemistry requires nanosecond excited-state lifetimes for diffusion-controlled reactions. Excited radicals with picosecond lifetimes have been implied by...
Classical photochemistry requires nanosecond excited-state lifetimes for diffusion-controlled reactions. Excited radicals with picosecond lifetimes have been implied by numerous photoredox studies, and controversy has arisen as to whether they can actually be catalytically active. We provide direct evidence for the elusive pre-association between radical ions and substrate molecules, enabling photoinduced electron transfer beyond the diffusion limit. A strategy based on two distinct light absorbers, mimicking the natural photosystems I and II, is used to generate excited radicals, unleashing extreme reduction power and activating C(sp)-Cl and C(sp)-F bonds. Our findings provide a long-sought mechanistic understanding for many previous synthetically-oriented works and permit more rational future photoredox reaction development. The newly developed excitation strategy pushes the current limits of reactions based on multi-photon excitation and very short-lived but highly redox active species.
PubMed: 38834625
DOI: 10.1038/s41467-024-49006-5 -
ACS Nano Jun 2024Spin transition (ST) materials are attractive for developing photoswitchable devices, but their slow material transformations limit device applications. Size reduction...
Spin transition (ST) materials are attractive for developing photoswitchable devices, but their slow material transformations limit device applications. Size reduction could enable faster switching, but the photoinduced dynamics at the nanoscale remains poorly understood. Here, we report a femtosecond optical pump multimodal X-ray probe study of polymeric nanorods. Simultaneously tracking the ST order parameter with X-ray emission spectroscopy and structure with X-ray diffraction, we observe photodoping of the low-spin-lattice within ∼150 fs. Above a ∼16% photodoping threshold, the transition to the high-spin phase occurs following an incubation period assigned to vibrational energy redistribution within the nanorods activating the molecular spin switching. Above ∼60% photodoping, the incubation period disappears, and the transition completes within ∼50 ps, preceded by the elastic nanorod expansion in response to the photodoping. These results support the feasibility of ST material-based GHz optical switching applications.
PubMed: 38833689
DOI: 10.1021/acsnano.3c10042 -
The Journal of Physical Chemistry. B Jun 2024The versatile functions of carotenoids in biological systems are associated with the extended π-electron conjugation system. Strong visible absorption resulting from...
The versatile functions of carotenoids in biological systems are associated with the extended π-electron conjugation system. Strong visible absorption resulting from the optically allowed S (1B) state and the low-lying optically forbidden S (2A) state examined. Carotenoids also exhibit an absorption band in the ultraviolet-B region; however, the origin of this band (hereafter referred to as S state) is not well characterized. The S state is a candidate for the destination level of the well-known S → S transient absorption; however, an obvious energy mismatch has been observed. In this study, we examined the steady-state and picosecond transient absorption spectra of lycopene in various solvents. The S absorption of carotenoids with diverse conjugation lengths was also examined. The dependence of the energies on solvent polarizability and conjugation length revealed that both S and S are the "second" B state. The absorption spectrum for lycopene at 200 K revealed an additional vibrational band, which may be the vibrational origin of the S → S band. Considering the slow vibrational relaxation of the 2A state, the S → S transition may represent the 2A ( = 1) → 2B ( = 0) transition, and the energetic contradiction can be resolved.
PubMed: 38833602
DOI: 10.1021/acs.jpcb.4c02212