Electronic Transitions Research Articles

The x-ray absorption spectrum of the tert-butyl radical: An experimental and computational investigation.

We report the x-ray absorption spectrum (XAS) of the tert-butyl radical, C4H9. The radical was generated pyrolytically from azo-tert-butane, and the XAS of the pure radical was obtained by subtraction of spectra recorded at different temperatures. The bands in the XAS were assigned by abinitio calculations that are in very good agreement with the experimental data. The lowest energy signal in the XAS is assigned to the C1s electron transition from the central carbon atom to the singly occupied molecular orbital (SOMO), while higher transitions correspond to C1s excitations from terminal carbon atoms. Furthermore, we investigated the fragmentation of the radical following resonant C1s excitation by electron-ion-coincidence spectroscopy. Several fragmentation channels were identified. The C1s excitation of the terminal carbons is associated with a stronger fragmentation tendency compared to the lowest C1s excitation of the central carbon into the SOMO. For this core excited state, we still observe an intact parent ion, C4H9+, and a comparatively higher tendency to dissociate into CH3+ + C3H6+.

Four-Dimensional Scaling of Dipole Polarizability: From Single-Particle Models to Atoms and Molecules.

Scaling laws enable the determination of physicochemical properties of molecules and materials as a function of their size, density, number of electrons or other easily accessible descriptors. Such relations can be counterintuitive and nonlinear, and ultimately yield much needed insight into quantum mechanics of many-particle systems. In this work, we show on the basis of single-particle models, multielectron atoms and molecules that the dipole polarizability of quantum systems is generally proportional to the fourth power of a characteristic length, computed from the ground-state wave function. This four-dimensional (4D) scaling is independent of the ratio of bound-to-bound and bound-to-continuum electronic transitions and applies to many-electron atoms when a correlated length metric is used. Finally, this scaling law is applied to predict the polarizability of molecules by electrostatically coupled atoms-in-molecules approach, obtaining approximately 8% absolute and relative accuracy with respect to hybrid density functional theory (DFT) on the QM7-X data set of organic molecules, providing an efficient and scalable model for the anisotropic polarizability tensors of extended (bio)molecules.

Intense Circularly Polarized Luminescence Induced by Chiral Supramolecular Assembly: The Importance of Intermolecular Electronic Coupling.

Herein we report on circularly polarized luminescence (CPL) emission originating from supramolecular chirality of organic microcrystals with a |glum| value up to 0.11. The microcrystals were prepared from highly emissive difluoroboron β-diketonate (BF2dbk) dyes R-1 or S-1 with chiral binaphthol (BINOL) skeletons. R-1 and S-1 exhibit undetectable CPL signals in solution but manifest intense CPL emission in their chiral microcrystals. The chiral superstructures induced by BINOL skeletons were confirmed by XRD analysis. Spectral analysis and theoretical calculations indicate that intermolecular electronic coupling, mediated by the asymmetric stacking in the chiral superstructures, effectively alters excited-state electronic structures and facilitates electron transitions perpendicular to BF2bdk planes. The coupling increases cosθµ,m from 0.05 (monomer) to 0.86 (tetramer) and triggers intense optical activity of BF2bdk. The results demonstrate that optical activity of chromophores within assemblies can be regulated by both orientation and extent of intermolecular electronic couplings.

Electric field-induced electronic structure and intersubband transitions of a hydrogen molecular ion in a gaussian-type quantum ring

In this work, the influence of an external electric field on the electronic structure and intersubband transitions of a singly ionized double-donor system in a GaAs quantum ring defined by Gaussian-type potentials is investigated theoretically. Within the framework of the effective mass approach, the two-dimensional diagonalization method is used for the solution of the Schrödinger equation to obtain the eigen energies and corresponding wave functions. Numerical results reveal that the electronic energy spectrum and the linear optical absorption coefficient of the ring are remarkably affected by the strength of the lateral electric field, internuclear distance and parameters defining the confinement potential. Also, it has been shown that beyond the anti-crossing point, the wave functions exchange their symmetries without mixing, which is a characteristic feature of energy-level anti-crossing.

Modulating Carrier Dynamics in PdSe2: The Role of Pressure in Electronic and Phononic Interactions.

PdSe2 is a puckered transition metal dichalcogenide that has been reported to undergo a two-dimensional to three-dimensional structural transition under pressure. Here, we investigated the electronic and phononic evolution of PdSe2 under high pressure using pump-probe spectroscopy. We observed the electronic intraband and interband transitions occurring in the d orbitals of Pd, revealing the disappearance of the Jahn-Teller effect under high pressure. Furthermore, we found that the decay rates of interband recombination and intraband relaxation lifetimes change at 3 and 7 GPa, respectively. First-principles calculations suggest that the bandgap closure slows the decay rate of interband recombination after 3 GPa, while the saturation of phonon-phonon scattering is the main reason for the relatively constant intraband relaxation lifetime. Our work provides a novel perspective for understanding the evolution of the electron and modulation of the carrier dynamics by phonons under pressure.

Benchmarking various nonadiabatic semiclassical mapping dynamics methods with tensor-train thermo-field dynamics.

Accurate quantum dynamics simulations of nonadiabatic processes are important for studies of electron transfer, energy transfer, and photochemical reactions in complex systems. In this comparative study, we benchmark various approximate nonadiabatic dynamics methods with mapping variables against numerically exact calculations based on the tensor-train (TT) representation of high-dimensional arrays, including TT-KSL for zero-temperature dynamics and TT-thermofield dynamics for finite-temperature dynamics. The approximate nonadiabatic dynamics methods investigated include mixed quantum-classical Ehrenfest mean-field and fewest-switches surface hopping, linearized semiclassical mapping dynamics, symmetrized quasiclassical dynamics, the spin-mapping method, and extended classical mapping models. Different model systems were evaluated, including the spin-boson model for nonadiabatic dynamics in the condensed phase, the linear vibronic coupling model for electronic transition through conical intersections, the photoisomerization model of retinal, and Tully's one-dimensional scattering models. Our calculations show that the optimal choice of approximate dynamical method is system-specific, and the accuracy is sensitively dependent on the zero-point-energy parameter and the initial sampling strategy for the mapping variables.

Unravelling the optoelectronic and biological properties of phenanthroimidazo [1,2‐c] quinazoline‐based donor‐acceptor materials

Imidazo[1,2‐c]quinazoline is widely established as biologically spectral active materials, while their optoelectronic properties were seldom investigated in the literature. In this context, this research work introduced two donors of varying strength, such as triphenylamine (TP) and phenothiazine (PZ) units, into the phenanthroimidazo [1,2‐c] quinazoline acceptor unit to form donor‐acceptor type luminescence materials such as TPQZ and PZQZ, respectively and were characterized by NMR and mass spectroscopy. Both these materials exhibited intramolecular charge transfer (ICT) type absorption (∼380−450 nm) and emission (∼540−600 nm) characteristics, which attributed to the electronic transition occurring from the HOMO of the PZ/TP donor to the LUMO+1 and LUMO+2 of the imidazo [1,2‐c] quinazoline acceptor unit, as predicted using DFT calculations. Increasing the electron donor strength was not only limited to fine‐tuning the π→π* based localized (∼400−450 nm) to ICT (∼450−650 nm) emission characteristics in both the solution and solid‐state conditions but also found to improve the zone of inhibition to 16 mm against Staphylococcus aureus/Bacillus subtilis bacterial species. The scope of realizing the luminescence nature of this acceptor unit is further expanded towards tagging biological samples such as E. coli. This work opens up a new paradigm in developing luminescent materials for optoelectronic and biological applications.

Development of an angle-adjustable photonic crystal fluorescence platform for sensitive detection of oxytetracycline.

We pioneered an angle-adjustable photonic crystal fluorescence platform (APC-Fluor) that integrates PCs, an angular resolution spectrometer and a strategically aligned laser source. This configuration, featuring a coaxial rotating swing arm, allows for precise control over the angles of incidence and emission. The presence of photonic crystal microcavities facilitates the dispersion of fluorescent materials and promotes the transition of electrons from the excited state to the lowest vibrational energy level. The optical resonance effect triggered by modulating the alignment of the reflection peaks of the photonic crystals with the emission peaks of the fluorescent materials can significantly enhance the fluorescence intensity. Compared with the single BSA-AuNCs, the optimized fluorescence intensity can be significantly increased by 11.9-fold. The APC-Fluor system showcases rapid and highly sensitive detection capabilities for oxytetracycline (OTC), exhibiting a response across a concentration range from 2 to 1 × 104 nM and achieving a notably low detection limit of 1.03 nM.

Mid-Infrared Photoacoustic Stimulation of Neurons through Vibrational Excitation in Polydimethylsiloxane.

Photoacoustic (PA) emitters are emerging ultrasound sources offering high spatial resolution and ease of miniaturization. Thus far, PA emitters rely on electronic transitions of absorbers embedded in an expansion matrix such as polydimethylsiloxane (PDMS). Here, it is shown that mid-infrared vibrational excitation of C─H bonds in a transparent PDMS film can lead to efficient mid-infrared photoacoustic conversion (MIPA). MIPA shows 37.5 times more efficient than the commonly used PA emitters based on carbon nanotubes embedded in PDMS. Successful neural stimulation through MIPA both in a wide field with a size up to a 100µm radius and in single-cell precision is achieved. Owing to the low heat conductivity of PDMS, less than a 0.5°C temperature increase is found on the surface of a PDMS film during successful neural stimulation, suggesting a non-thermal mechanism. MIPA emitters allow repetitive wide-field neural stimulation, opening up opportunities for high-throughput screening of mechano-sensitive ion channels and regulators.

Nitrogen-vacancy centers in diamond: discovery of additional electronic states

Abstract Nitrogen-vacancy (NV) defect centers in diamond are key to applications in quantum sensing and quantum computing. They create localized electronic states in the diamond lattice with distinct population relaxation pathways following photoexcitation that ultimately enable its unique properties. The defect is known to exist in two charge states: neutral and negative, with respectively one and
two known optically-active electronic transitions. Here, we report on the observation of a large number of hitherto undiscovered excited electronic states in both charge states as evidenced by distinct optical transitions in the infrared to ultraviolet part of the spectrum. These transitions are observed by monitoring the electronic relaxation of NV centers after photoexcitation using transient absorption
spectroscopy, directly probing transient phenomena occurring on timescales from femtoseconds to microseconds. We also for the first time probed the electron transfer dynamics from the 3E state of NV− to nearby single-substitutional nitrogen defects hat leads to the well-known effect of NV photoluminescence quenching.

Verification and validation of electrostatic turbulence simulation for numerical Lie transform code

Abstract This paper reports the verification and validation of a new gyrokinetic code, numerical Lie transform (NLT), for electrostatic turbulence simulation in tokamak plasmas. Based on the standard Cyclone Base Case (CBC), NLT has been verified against two typical gyrokinetic codes, gyrokinetic toroidal code (GTC) and gyrokinetic numerical experiment of tokamak (GKNET), by simulating the ion temperature gradient mode (ITG) and trapped electron mode (TEM). The linear dispersion relation and mode structure agree well among all these codes. In nonlinear simulation, the time evolution of the ion heat diffusivity obtained from NLT is consistent with the GTC result. Furthermore, utilizing experimental equilibria with obvious turbulence features from HL-2A shot #22388, the validation of NLT for real tokamak simulation has been carried out. The profiles of the ion turbulent thermal diffusivity calculated by NLT match well with experimental results.

Defect Pairs in Boron Nitrides That Exhibit Strong Electronic Transitions in the Infrared

Defect Pairs in Boron Nitrides That Exhibit Strong Electronic Transitions in the Infrared

Emergence and evolution of electronic modes with temperature in spin-gapped Mott and Kondo insulators

Electronic modes emerge within the band gap at nonzero temperature in strongly correlated insulators such as Mott and Kondo insulators, exhibiting momentum-shifted magnetic dispersion relations from the band edges. As the temperature increases, the emergent modes gain considerable spectral weights and form robust bands that differ from the zero-temperature bands. Here, the origin of the emergent modes, their relation to doping-induced modes, and how their spectral weights increase with temperature are clarified in the ladder and bilayer Hubbard models and one- and two-dimensional Kondo lattice models using effective theory for weak inter-unit-cell hopping and numerical calculations. The results indicate that the temperature-driven change in the band structure reflecting spin excitation, including the change in the number of bands, can be observed in various strongly correlated insulators even with a spin gap, provided that the temperature increases up to about spin-excitation energies. The controlled analyses developed in this study significantly contribute to the fundamental understanding of the band structure of strongly correlated insulators at nonzero temperature. Published by the American Physical Society 2024

Emergence and evolution of electronic modes with temperature in spin-gapped Mott and Kondo insulators

Electronic modes emerge within the band gap at nonzero temperature in strongly correlated insulators such as Mott and Kondo insulators, exhibiting momentum-shifted magnetic dispersion relations from the band edges. As the temperature increases, the emergent modes gain considerable spectral weights and form robust bands that differ from the zero-temperature bands. Here, the origin of the emergent modes, their relation to doping-induced modes, and how their spectral weights increase with temperature are clarified in the ladder and bilayer Hubbard models and one- and two-dimensional Kondo lattice models using effective theory for weak inter-unit-cell hopping and numerical calculations. The results indicate that the temperature-driven change in the band structure reflecting spin excitation, including the change in the number of bands, can be observed in various strongly correlated insulators even with a spin gap, provided that the temperature increases up to about spin-excitation energies. The controlled analyses developed in this study significantly contribute to the fundamental understanding of the band structure of strongly correlated insulators at nonzero temperature. Published by the American Physical Society 2024

Reconciling mean-squared radius differences in the silver chain through improved measurement and ab initio calculations

Nuclear charge radius differences in the silver isotopic chain have been reported through different combinations of experiment and theory, exhibiting a tension of two combined standard errors. This study investigates this issue by combining high-accuracy calculations for six low-lying states of atomic silver with an improved measurement of the 5s2S1/2−5p2P3/2 transition optical isotope shift. Our calculations predict measured electronic transition energies in Ag at the 0.3% level, the highest accuracy achieved in this system so far. We calculate electronic isotope shift factors by employing analytical response relativistic coupled-cluster theory and find that a consistent charge radius difference between Ag107,109 is returned when combining our calculations with the available optical isotope shift measurements. We therefore recommend an improved value for the mean-squared charge radius difference between Ag107 and Ag109 as 0.207(6) fm2, within one combined error from the value derived from muonic Ag experiments, and an updated set of charge radii differences across the isotopic chain. Published by the American Physical Society 2024

Novel BiOI/LaOXI〈IX〉 heterojunction with enhanced visible-light driven photocatalytic performance: unveiling the mechanism of interlayer electron transition.

Improving visible light absorption plays an important role in the utilization of solar power for photocatalysis. Using first-principles calculations within the HSE06 functional, we propose that the semiconductor heterojunction BiOI/LaOXI〈IX〉 extends the optical absorption to the near-infrared range, boosts the absorption coefficient from 1.28 × 105 cm-1 to above 2.20 × 105 cm-1 in the visible light range, and increases the conversion efficiency of solar power up to 9.48%. The enhanced optical absorption derives from the significant interlayer transition and excitonic effect which benefit from polarized LaOXI with a flat band in the highest valence band (VB). In BiOI/LaOClI〈ICl 〉, the electrostatic potential difference (ΔΦ) modifies the band edge positions to meet the requirements for photocatalytic overall water splitting, while the polarized electric field (Ep) accelerates the separation of photogenerated carriers and regulates the overpotentials of photogenerated carriers following a direct Z-scheme strategy. In addition, BiOI/LaOXI〈IX〉 is dynamically and thermodynamically stable. Furthermore, only a low external potential is needed to drive the redox reaction. Our theoretical results suggest that BiOI/LaOXI〈IX〉 could be a potential photocatalyst for overall water splitting with enhanced visible light absorption.

Reconciling mean-squared radius differences in the silver chain through improved measurement and ab initio calculations

Nuclear charge radius differences in the silver isotopic chain have been reported through different combinations of experiment and theory, exhibiting a tension of two combined standard errors. This study investigates this issue by combining high-accuracy calculations for six low-lying states of atomic silver with an improved measurement of the 5s2S1/2−5p2P3/2 transition optical isotope shift. Our calculations predict measured electronic transition energies in Ag at the 0.3% level, the highest accuracy achieved in this system so far. We calculate electronic isotope shift factors by employing analytical response relativistic coupled-cluster theory and find that a consistent charge radius difference between Ag107,109 is returned when combining our calculations with the available optical isotope shift measurements. We therefore recommend an improved value for the mean-squared charge radius difference between Ag107 and Ag109 as 0.207(6) fm2, within one combined error from the value derived from muonic Ag experiments, and an updated set of charge radii differences across the isotopic chain. Published by the American Physical Society 2024

Investigation of the optical properties of Pb1-xCdxTe films using spectroscopic ellipsometry

Using spectroscopic ellipsometry, we studied the optical properties of four Pb1-xCdxTe (0 ≤ x ≤ 0.20) films. The Pb1-xCdxTe films, deposited on silicon substrates using an electron-beam deposition technique, show a rock-salt structure, with their lattice constants decreasing as a function of Cd concentration. Ellipsometry measurements, which covered a wide spectral range between 0.1 eV to 4.1 eV, determined the index of refraction and the extinction coefficient of the films. As the Cd concentration increases from 0 % to 20 % in the Pb1-xCdxTe films, the index of refraction decreases by ∼10 %, across the entire energy range. Similarly, there seems to be a decrease in the extinction coefficient with the increase of Cd concentration. An oscillator model, depicting the optical functions of each Pb1-xCdxTe film, allowed us to obtain the band gap of each film which blue-shifts as the Cd concentration is increased. Besides the fundamental band gap, we recovered the higher-order electronic transitions that occur in the Brillouin zone of the Pb1-xCdxTe lattice.

Characterizing of PVA/CMC/MWCNTs nanocomposite films doped Zn0.9Ni0.1S nanoparticles for comprehensive γ-ray radiation shielding performance

Nanocomposite films were prepared from Carboxymethylcellulose (CMC), Polyvinyl alcohol (PVA) polymers and multi-walled carbon nanotubes (MWCNTs) using the casting technique. The resulting PVA/CMC/MWCNTs nanocomposite films have been incorporated with various concentrations of the Zn0.9Ni0.1S nanoparticles. The γ-ray radiation shielding capabilities of the films were discussed in details for various gamma sources using the Phy-X/PSD software in the energy range (0.015–15 MeV). The linear (LAC) and mass (MAC) attenuation parameters, the half-value layer (HVL), the tenth value layer (TVL), the effective atomic number (Zeff) were determined. The γ-ray buildup factors, were also determined for different penetration depths in the range (0.5–40 MFP). Results showed that, the shielding parameters are greatly affected by the density of the samples and the PVA/CMC/MWCNTs samples have the highest shielding ability in the energy region ranges (0.015–0.2 MeV) and (5–15 Mev). The structural and chemical characteristics of the films and the nanoparticles were examined using X-ray diffraction technique. The films elemental composition and morphology were discussed using energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscope (SEM) techniques, respectively. The linear and nonlinear optical properties, including the extinction coefficient (K), the energy gap (Eg), the refractive index (n), the dielectric parameters, nonlinear susceptibilities(χ(1), χ(3)) and nonlinear refractive index (n2), were discussed by UV–Vis spectroscopy's absorbance and transmittance spectra. A reduction in Eg values from 4.55 eV to 4.5 eV and 4.41 eV for films doped with 1 wt% and 3 wt% of Zn0.9Ni0.1S. The uniform distribution of the added nanoparticles at 5 wt% has been confirmed. The energy dispersion parameters confirmed a higher probability of electronic transitions at low energy due to the intermolecular modifications.

Copper (II) and cobalt (II) bis(hexafluoroacetylacetonate) coordination complexes with N-styrylbenzimidazole

The study considers metal complexes based on N-styrylbenzimidazole as compounds having significant pharmacological properties The work is aimed at examining the crystal structure and electronic structure of transition metal bis(hexafluoroacetylacetonate) complexes (copper (II) (complex A) and cobalt (II) (complex B)) with N-styrylbenzimidazole using X-ray diffraction analysis and ultraviolet spectroscopy. The X-ray diffraction analysis was used to prove bipyramidal coordination in copper (II) and cobalt (II) bis(hexafluoroacetylacetonate) complexes with N-styrylbenzimidazole. The atoms of copper (II) and cobalt (II) in the complexes exhibit an unusual for β-diketonate complexes distorted square-planar coordination, while the chelate cycles in M(hfacac)2L are characterized by anomalously large kink angles. Thus, for the copper (II) bis(hexafluoroacetylacetonate) complex, the kink angle of the O3∙∙∙O4 interaction for the equatorially positioned ligand is 29.47°, while for the axially positioned ligand, the kink angle of the O1∙∙∙O2 interaction is 19.13°. For the cobalt (II) bis(hexafluoroacetylacetonate) complex, these angles are 22.10 and 19.50°, respectively. Electron spectroscopy was used to examine the electronic structure of the specified complexes. The following types of electronic transitions were identified: π→π*-transitions primarily localized on ligands, as well as transitions caused by electron transfer from the p-orbital of the hetero nitrogen atom of the styrylbenzimidazole cycle to the d-orbital of metal ions, and n→π transition localized on the imidazole ring. For each of the complexes, d-d* transitions between the molecular orbitals of the corresponding metal ion were localized in the long wavelength part of the spectrum.