Intermediate Excitation Research Articles

Stereospecific radical coupling with a non-natural photodecarboxylase.

Photoenzymes are light-powered biocatalysts that typically rely on the excitation of cofactors or unnatural amino acids for their catalytic activities1,2. A notable natural example is the fatty acid photodecarboxylase (FAP), which uses light energy to convert aliphatic carboxylic acids to achiral hydrocarbons3. Here, we report a way to design a non-natural photodecarboxylase based on the excitation of enzyme-bound catalytic intermediates, instead of relying on cofactor excitation4. Iminium ions5, transiently generated from enals within the active site of an engineered class I aldolase6, can absorb violet light and function as single-electron oxidants. Activation of chiral carboxylic acids, followed by decarboxylation, generates two radicals that undergo stereospecific cross-coupling, yielding products with two stereocenters. Using the appropriate enantiopure chiral substrate, the desired diastereoisomeric product is selectively obtained with complete enantiocontrol. This finding underscores the active site's ability to transfer stereochemical information from the chiral radical precursor into the product, effectively addressing the longstanding problem of rapid racemization of chiral radicals. The resulting 'memory of chirality' scenario7 is a rarity in enantioselective radical chemistry.

Disappearance of rapid photoresponse in ultraviolet illumination of Ag–Sb–S films

The photoconduction in Ag0.5Sb0.5S films changes anomalously with the excitation energy. Although the usual instantaneous generation and recombination of photocarriers appear in the resistance of the films for the illumination at a wavelength of 633 nm, the photoresponse becomes slow with time scales of minutes when the illumination is performed at a wavelength of 280 nm. The rapid and slow phototransients are mixed for an intermediate excitation wavelength of 375 nm. In the simultaneous photoexcitation at multiple wavelengths, the response is complex instead of a superposition of the rapid and slow behaviors, indicating the mutual interaction in the photocarrier transport. The ultraviolet (UV) illumination can thereby block the rapid response that should be caused by the visible light. Moreover, the resistance can even increase during the illumination. Although the adsorption of molecules at the film surface plays an important role for the resistance, the anomalous properties are unaffected by the surface condition. They are thus suggested to be the bulk properties of the films, plausibly caused by the defects generated in the UV irradiation.

The Late-Epoch, Matter-Bounded Spectrum of the Nova V1405 Cassiopeiae

Optical and near-infrared spectra of the classical nova V1405 Cassiopeiae (V1405 Cas) from more than two years after outburst show a still bright nova of intermediate excitation with coronal lines, some high-ionization but non-coronal forbidden lines, and very strong emission features of He ii. However, the common nebular lines of [N ii], [O ii], and [O iii] are absent and the lines of He i are very weak. Analysis with the photoionization code CLOUDY indicates a emission line region that is sharply matter-bounded, truncating before the gas excitation drops sufficiently to support the parent ions of lower excitation lines.

Control of Raman Scattering Quantum Interference Pathways in Graphene.

Graphene is an ideal platform to study the coherence of quantum interference pathways by tuning doping or laser excitation energy. The latter produces a Raman excitation profile that provides direct insight into the lifetimes of intermediate electronic excitations and, therefore, on quantum interference, which has so far remained elusive. Here, we control the Raman scattering pathways by tuning the laser excitation energy in graphene doped up to 1.05 eV. The Raman excitation profile of the G mode indicates its position and full width at half-maximum are linearly dependent on doping. Doping-enhanced electron-electron interactions dominate the lifetimes of Raman scattering pathways and reduce Raman interference. This will provide guidance for engineering quantum pathways for doped graphene, nanotubes, and topological insulators.

The response of a conical flame to a dual-frequency excitation

This work investigates the response of a conical premixed flame to a dual-frequency excitation, based on the integrated CH* signal collected from a photomultiplier tube (PMT), the upstream velocity disturbance measured by a hotwire, and the chemiluminescence signal captured by high-speed imaging. The results show that, in addition to the excitation frequencies, a notable flame response can also be observed at the difference frequency, where the corresponding velocity fluctuation is relatively small. This result means that, at the difference frequency, the velocity fluctuation contributes little to the flame response. Such interacted response generally occurs at intermediate excitation frequencies but tends to disappear as either excitation frequency is below the cut-off frequency. And it increases linearly with the excitation amplitude, with nearly zero dependence on the phase difference. Furthermore, the flame front is extracted based on the chemiluminescence images to analyze the flame area fluctuation. The resultant phase response implies that the fluctuation of the difference frequency propagates downstream convectively, similar to that of the excitation frequencies. Interestingly, the flame area fluctuation at the difference frequency shows significant response and a low-pass characteristic, whereas the CH* fluctuation approaches zero at those low frequencies.

Slower Auger Recombination in 12-Faceted Dodecahedron CsPbBr3 Nanocrystals.

Over the past two decades, intensive research efforts have been devoted to suppressions of Auger recombination in metal-chalcogenide and perovskite nanocrystals (PNCs) for the application of photovoltaics and light emitting devices (LEDs). Here, we have explored dodecahedron cesium lead bromide perovskite nanocrystals (DNCs), which show slower Auger recombination time compared to hexahedron nanocrystals (HNCs). We investigate many-body interactions that are manifested under high excitation flux density in both NCs using ultrafast spectroscopic pump-probe measurements. We demonstrate that the Auger recombination rate due to multiexciton recombinations are lower in DNCs than in HNCs. At low and intermediate excitation density, the majority of carriers recombine through biexcitonic recombination. However, at high excitation density (>1018 cm-3) a higher number of many-body Auger process dominates over biexcitonic recombination. Compared to HNCs, high PLQY and slower Auger recombinations in DNCs are likely to be significant for the fabrication of highly efficient perovskite-based photonics and LEDs.

Excited State Absorption Upconversion Induced by Structural Defects for Photocatalysis with a Breakthrough Efficiency

AbstractThe nitrogen‐deficient graphitic carbon nitride (g‐C3N4) has been prepared, a new excited state absorption (ESA) up‐conversion mode is discovered, which is directly induced by structural defects, showing distinct chemical characteristics from those based on lanthanide ions and triplet states chromophores. The abundant N2C vacancies in g‐C3N4 nanosheets work as the crucial intermediate excitation states, which lead to g‐C3N4 upconverted emitting at the wavelength of 436 nm excited by the light with the wavelength of 800 nm. This process is proven to proceed via a two‐photon involved ESA mode with a breakthrough quantum efficiency of 0.64 %. Further, we combine N2C vacancies enriched g‐C3N4 with In2S3 and CdS, and successfully achieved an infrared light driven photocatalytic reactions. These findings offered a new family of up‐conversion materials; more semiconductors with various structural defects are potential complementary members.

Exited State Absorption Upconversion Induced by Structural Defects for Photocatalysis with a Breakthrough Efficiency.

The nitrogen-deficient graphitic carbon nitride (g-C3 N4 ) has been prepared, a new excited state absorption (ESA) up-conversion mode is discovered, which is directly induced by structural defects, showing distinct chemical characteristics from those based on lanthanide ions and triplet states chromophores. The abundant N2C vacancies in g-C3 N4 nanosheets work as the crucial intermediate excitation states, which lead to g-C3 N4 upconverted emitting at the wavelength of 436 nm excited by the light with the wavelength of 800 nm. This process is proven to proceed via a two-photon involved ESA mode with a breakthrough quantum efficiency of 0.64 %. Further, we combine N2C vacancies enriched g-C3 N4 with In2 S3 and CdS, and successfully achieved an infrared light driven photocatalytic reactions. These findings offered a new family of up-conversion materials; more semiconductors with various structural defects are potential complementary members.

Superexchange and spin-orbit coupling in monolayer and bilayer chromium trihalides

We build a microscopic model to study the intra- and inter-layer superexchange due to electrons hopping in chromium trihalides ($\mathrm{CrX}_3$, X= Cl, Br, and I). In evaluating the superexchange, we identify the relevant intermediate excitations in the hopping. In our study, we find that the intermediate hole-pairs excitations in the $p$-orbitals on X ion play a crucial role in mediating various types of exchange interactions. In particular, the inter-layer antiferromagnetic exchange may be realized by the hole-pair-mediated superexchange. Interestingly, we also find that these virtual hopping processes compete with each other leading to weak intra-layer ferromagnetic exchange. In addition, we also study the spin-orbit coupling effects on the superexchange and investigate the Dzyaloshinskii-Moriya interaction. Finally, we extract the microscopic model parameters from density functional theory for analyzing the exchange interactions in a monolayer $\mathrm{CrI}_3$.

3D non-LTE iron abundances in FG-type dwarfs

Iron is one of the most important elements in-stellar astrophysics. However, spectroscopic measurements of its abundance are prone to systematic modelling errors. We present three dimensional non-local thermodynamic equilibrium (3D non-LTE) calculations across 32 STAGGER-grid models with effective temperatures from 5000 K to 6500 K, surface gravities of 4.0 dex and 4.5 dex, and metallicities from −3 dex to 0 dex, and we study the effects on 171 Fe i and 12 Fe ii optical lines. In warm metal-poor stars, the 3D non-LTE abundances are up to 0.5 dex larger than 1D LTE abundances inferred from Fe i lines of an intermediate excitation potential. In contrast, the 3D non-LTE abundances can be 0.2 dex smaller in cool metal-poor stars when using Fe i lines of a low excitation potential. The corresponding abundance differences between 3D non-LTE and 1D non-LTE are generally less severe but can still reach ±0.2 dex. For Fe ii lines, the 3D abundances range from up to 0.15 dex larger to 0.10 dex smaller than 1D abundances, with negligible departures from 3D LTE except for the warmest stars at the lowest metallicities. The results were used to correct 1D LTE abundances of the Sun and Procyon (HD 61421), and of the metal-poor stars HD 84937 and HD 140283, using an interpolation routine based on neural networks. The 3D non-LTE models achieve an improved ionisation balance in all four stars. In the two metal-poor stars, they removed excitation imbalances amounting to 250 K to 300 K errors in effective temperature. For Procyon, the 3D non-LTE models suggest [Fe/H] = 0.11 ± 0.03, which is significantly larger than literature values based on simpler models. We make the 3D non-LTE interpolation routine for FG-type dwarfs publicly available, in addition to 1D non-LTE departure coefficients for standard MARCS models of FGKM-type dwarfs and giants. These tools, together with an extended 3D LTE grid for Fe ii from 2019, can help improve the accuracy of stellar parameter and iron abundance determinations for late-type stars.

Rydberg atomic antenna in strongly driven multielectron atoms

We study the role of intermediate excitations of Rydberg states as an example of Kuchiev's ``atomic antenna'' in above-threshold ionization of xenon, in particular their effect on the coherence between the spin-orbit-split states of the ion. We focus on the case of a laser frequency close to resonant with the spin-orbit splitting, where a symmetry (parity) argument would preclude any coherence being directly generated by strong-field ionization. Using ab initio simulations of coupled multielectron spin-orbit dynamics in strong laser fields, we show how field-driven rescattering of the trapped Rydberg electrons introduces efficient coupling between the spin-orbit-split channels, leading to substantial coherences, exceeding 10% for some photon energies.

Comparing the Excited State Dynamics of CH2OO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation.

Ab initio molecular dynamics studies of CH2OO molecules following excitation to the minimum-energy geometry of the strongly absorbing S2 (1ππ*) state reveal a much richer range of behaviors than just the prompt O-O bond fission, with unity quantum yield and retention of overall planarity, identified in previous vertical excitation studies from the ground (S0) state. Trajectories propagated for 100 fs from the minimum-energy region of the S2 state show a high surface hopping (nonadiabatic coupling) probability between the near-degenerate S2 and S1 (1nπ*) states at geometries close to the S2 minimum, which enables population transfer to the optically dark S1 state. Greater than 80% of the excited population undergoes O-O bond fission on the S2 or S1 potential energy surfaces (PESs) within the analysis period, mostly from nonplanar geometries wherein the CH2 moiety is twisted relative to the COO plane. Trajectory analysis also reveals recurrences in the O-O stretch coordinate, consistent with the resonance structure observed at the red end of the parent S2-S0 absorption spectrum, and a small propensity for out-of-plane motion after nonadiabatic coupling to the S1 PES that enables access to a conical intersection between the S1 and S0 states and cyclization to dioxirane products.

Calculation Method of Core Loss Under Intermediate Frequency Excitation

Based on Brockhaus magnetic measurement system, the magnetic properties of GT100 ultra-thin silicon steel under middle and high frequency sinusoidal excitation are measured. Based on the Preisach model, the static hysteresis and loss characteristics are simulated accurately. In addition, an improved loss model was proposed. The formula of eddy current loss was improved according to skin effect principle, and the statistical parameter V0 of abnormal loss was adjusted by iterative algorithm. The simulation results of sinusoidal dynamic hysteresis loop and iron loss at different frequencies and different flux densities are compared with the measured results, which verifies the accuracy and versatility of the improved model. In addition, the influence of increasing frequency and flux density on total loss is also studied.

Theory of the Rydberg blockade with multiple intermediate-state excitations

We present a detailed theory of the Rydberg blockade, including contributions from multiple intermediate-state excitations. Two fields drive transitions between ground and Rydberg levels via an off-resonance intermediate state. Assuming a perfect blockade, we calculate the probability to excite fully symmetric collective states having either zero or one Rydberg excitation, but an arbitrary number of intermediate-state excitations. Both ``bare'' state and ``dressed'' state approaches are used for (1) constant amplitude driving fields and (2) adiabatic pulse driving fields. It is shown that a dressed state approach offers distinct advantages when multiple intermediate-state excitations occur. In the case of fixed amplitude fields, the multiple intermediate excitations can result in comblike modulated populations of individual states having one Rydberg excitation and $n\ensuremath{\ll}N$ intermediate-state excitations. However, when summed over all such state populations, most of the modulation disappears and the system is described to a good approximation by an effective two-level model. In the case of adiabatic, pulsed fields, there is no such modulation and an effective two-level model (in the dressed basis), corrected for light shifts, can be used to model the system. In addition to solving this problem using conventional methods, we show that similar results could be obtained using a form of the Holstein-Primakoff transformation.

Mechanistic Analysis of Micro-Neurocircuits Underlying the Epileptogenic Zone in Focal Cortical Dysplasia Patients.

We aim to explore the microscopic neurophysiology of focal cortical dysplasia (FCD) induced epileptogenesis in specific macroscopic brain regions, therefore mapping a micro-macro neuronal network that potentially indicates the epileptogenic mechanism. Epileptic and relatively non-epileptic temporal neocortex specimens were resected from FCD and mesial temporal lobe epilepsy (mTLE) patients, respectively. Whole-cell patch-clamping was performed on cells from the seizure onset zone (SOZ) and non-SOZ inside the epileptogenic zone (EZ) of FCD patients, as well as the non-epileptic neocortex of mTLE patients. Microscopic data were recorded, including membrane characteristics, spontaneous synaptic activities, and evoked action potentials. Immunohistochemistry was also performed on parvalbumin-positive (PV+) interneurons. We found that SOZ interneurons exhibited abnormal neuronal expression and distribution as well as reduced overall function compared with non-SOZ and mTLE interneurons. The SOZ pyramidal cells experienced higher excitation but lower inhibition than the mTLE controls, whereas the non-SOZ pyramidal cells exhibited intermediate excitability. Action potential properties of both types of neurons also suggested more synchronized neuronal activity inside the EZ, particularly inside the SOZ. Together, our research provides evidence for a potential neurocircuit underlying SOZ epileptogenesis and non-SOZ seizure susceptibility. Further investigation of this microscopic network may promote understanding of the mechanism of FCD-induced epileptogenesis.

Deformation parameter changes in fission mass yields within the systematic statistical scission-point model

The fission fragment mass-yields are evaluated for pre-actinide and actinide isotopes using a systematic statistical scission point model. The total potential energy of the fissioning systems at the scission point is presented in approximate relations as functions of mass numbers, deformation parameters and the temperature of complementary fission fragments. The collective temperature, T coll, and the temperature of fission fragments, T i , are separated and the effect of collective temperature on mass yields results is investigated. The fragment temperature has been calculated with the generalized superfluid model. The sum of deformation parameters of complementary fission fragments has been obtained by fitting the calculated results with the experimental data. To investigate the transitions between symmetric and asymmetric modes mass yields for pre-actinide and heavy actinides are calculated with this model. The transition from asymmetric to symmetric fission is well reproduced using this systematic statistical scission point model. The calculated results are in good agreement with the experimental data with T coll = 2 MeV at intermediate excitation energy and with T coll = 1 MeV for spontaneous fission. Despite the Langevin model, in the scission point model, a constraint on the deformation parameters of fission fragments has little effect on the results of the mass yield.

Nonmonotonic excitation power dependence of the UV photoluminescence rate from large ZnO nanoparticle assemblies

The photoluminescence from solid-state ZnO nanoparticles (NPs) assemblies was studied as a function of the number on NPs in the assembly. With increasing number of NPs, the UV emission band broadened and redshifted, while the UV/visible emission rate ratio decreased. Furthermore, while samples with few NPs displayed a typical linear dependence for the UV emission rate as a function of the excitation power, samples with large number of deposited NPs showed unusual non-linear, nonmonotonic dependences with maxima at intermediate excitation powers. Also, for samples with large number of NPs, the UV band became broader as the excitation power was increased. These unusual results can be understood by considering the interplay between photon self-absorption effects, interface trap states between NPs and illumination power dependent potential barriers at NP/NP interfaces. The exposure of samples with large number of NPs to ethanol vapor during the photoluminescence measurements resulted in partial removal of the non-monotonic behavior of the UV emission rate dependence on excitation power.

Deformation Parameters and Collective Temperature Changes in Photofission Mass Yields of Actinides Within the Systematic Statistical Scission Point Model

The photofission fragment mass yields of actinides are evaluated using a systematic statistical scission point model. In this model, all energies at the scission point are presented as a linear function of the mass numbers of fission fragments. The mass yields are calculated with a new approximated relative probability for each complementary fragment. The agreement with the experimental data is quite good, especially with a collective temperature Tcol of 2 MeV at intermediate excitation energy and Tcol = 1 MeV for spontaneous fission. This indicates that the collective temperature is greater than the value obtained by the initial excitation energy. The generalized superfluid model is applied for calculating the fragment temperature. The deformation parameters of fission fragments have been obtained by fitting the calculated results with the experimental values. This indicates that the deformation parameters decrease with increasing excitation energy. Also, these parameters decrease for fissioning systems with odd mass numbers.

Hyperon weak radiative decay * *Supported by the National Natural Science Foundation of China (11425525, 11521505), DFG and NSFC funds to the Sino-German CRC 110 “Symmetries and the Emergence of Structure in QCD” (NSFC) (11261130311), Strategic Priority Research Program of Chinese Academy of Sciences (XDB34030302), National Key Basic Research Program of China (2015CB856700); Q.W. is also supported by the research

We revisit the hyperon weak radiative decays in the framework of the non-relativistic constituent quark model. This study confirms the nonlocal feature of the hyperon weak radiative transition operators, which are dominated by the pole terms, and an overall self-consistent description of the available experimental data for the Cabibbo-favored hyperon weak radiative decays is presented. It provides a natural mechanism for evading the Hara theorem, where sizeable parity-violating contributions can come from the intermediate orbital excitations. Cancellations between pole terms also explain the significant SU(3) flavor symmetry breaking manifested by the experimental data. We also discuss several interesting selection rules arising from either the electromagnetic or the weak interaction vertices. These features suggest nontrivial relations among various hyperon decays.

Experimental and Theoretical Investigation of the 3sp(d) Rydberg States of Fenchone by Polarized Laser Resonance-Enhanced-Multiphoton-Ionization and Fourier Transform VUV Absorption Spectroscopy.

The VUV absorption spectrum of fenchone is re-examined using synchrotron radiation Fourier transform spectrometry, revealing new vibrational structure. Picosecond laser (2+1) resonance enhanced multiphoton ionization (REMPI) spectroscopy complements this, providing an alternative view of the 3spd Rydberg excitation region. These spectra display broadly similar appearance, with minor differences that are largely explained by referring to calculated one- and two-photon electronic excitation cross-sections. Both show good agreement with Franck-Condon simulations of the relevant vibrational structures. Parent ion REMPI ionization yields with both femtosecond and picosecond excitation laser pulses are studied as a function of laser polarization and intensity, the latter providing insight into the relative two-photon excitation and one-photon ionization rates. The experimental circular-linear dichroism observed in the parent ion yields varies strongly between the 3s and 3p Rydberg states, in good overall agreement with the calculated two-photon excitation circular-linear dichroism, while corroborating other evidence that the 3pz sub-state plays no more than a very minor role in the (2+1) REMPI spectrum. Vibrationally resolved photoelectron spectra are recorded with picosecond pulse duration (2+1) REMPI at selected intermediate vibrational excitations. The 3s intermediate state displays a very strong Δv=0 propensity on ionization, but the 3p intermediate evidences more complex vibronic dynamics, and we infer some 3p→3s internal conversion prior to ionization.