De-excitation Processes Research Articles

Unusual emission from 2F5/2 two lowest excited levels of Yb3+-doped LuPO4 nano/micro-crystalline materials

Yb3+-doped inorganic materials are constantly in demand due to a strong interest in fundamental and applied research. The simplicity of the 4f1³ electronic structure minimizes de-excitation processes and enables a quasi-three-level laser emission scheme at 1 μm employed for IR laser devices.In this work, we present an unusual simultaneous emission from two of the lowest levels of the 2F5/2 excited state observed at 77 K, measured for the first time and confirmed by the barycenter law for Yb3+-doped tetragonal LuPO4 revealing the lowest crystal field among Yb3+ ion-activated oxide crystals.Three series of tetragonal lutetium orthophosphate powders, containing various concentrations of Yb3+ ions (0.5–5 mol %), were analyzed in nano and micro-crystalline forms. The ionic liquid-assisted (IL) hydrothermal (HT) method was used for the nano-powders' preparation. This route enabled the fabrication of fine nano-powders with particle sizes of 15 nm in an ethylene glycol (GE) reaction medium and 30 nm in a water (W) medium, each exhibiting the desired well-crystallized single-phase phosphate. For comparative studies, the micro-crystalline samples were synthesized via reaction in the solid phase at a relatively high temperature.The Yb3+ ion also served as a structural probe, its spectroscopy i.e., absorption at 4.2 K, selectively excited luminescence at 77 K, and fluorescence decays were used to assign Yb3+ energy levels in only one D2d center precisely. This assignment was based on exceptional features due to the unique emissions observed from the two lowest levels of the 2F5/2 excited state, split only by 27 cm−1, confirmed by calculation of the crystal field parameters from Zeeman spectra.

Ferroelastic Control of the Multicolor Emission from a Triply Doped Organic Crystal.

Emission from crystalline organic solids is often quenched by nonemissive energy-transfer deexcitation processes. While dispersion of fluorophores in polymers or other hosts has been used to enhance the emission intensity, this strategy results in randomization of guest orientation and optical losses at grain boundaries. Here, we report the doping of inherently nonemissive single crystals of anilinium bromide with three fluorescent organic molecules. The doping process equips the crystal with emission characteristics that tune from blue to deep orange. The emission intensity can be reversibly modulated by ferroelastic twinning, which causes the material to function as a multiemissive force sensor. This approach opens up new pathways in the manipulation of emissive properties in organic crystals and may have substantial implications for optoelectronic devices and sensors.

Production of hypernuclei from antiproton capture within a relativistic transport model

Production yields of single-Λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Lambda $$\\end{document} hypernuclei from simulated peripheral annihilations of antiprotons after capture on various target nuclei are reported. The initial annihilation process and the production of excited hypernuclei are estimated within the GiBUU transport framework, while their deexcitation process is treated with the ABLA++ code. The yield of excited hypernuclei range from ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim $$\\end{document}0.3 % for 16\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}^{16}$$\\end{document}O to ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim $$\\end{document}1.2 % for 132\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}^{132}$$\\end{document}Xe per annihilation, consistent with previous measurements at LEAR, CERN. The yield of specific ground state hypernuclei after deexcitation reaches up to a few 10-4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{-4}$$\\end{document} per annihilation. The hypernuclei are produced in strangeness exchange reactions occuring between a nucleon of the target and the kaons originating from the annihilation in ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim $$\\end{document}80 % of the cases, while the strangeness pair production in secondary pion-nucleon collision contributes to the remaining ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim $$\\end{document}20 %. The simulations indicate that antiproton annihilations at rest on different nuclei could populate a wide range of so-far unexplored hypernuclei.

Luminescence Properties of Al2O3:Ti in the Blue and Red Regions: A Combined Theoretical and Experimental Study.

Using jointly experimental results and first-principles calculations, we unambiguously assign the underlying mechanisms behind two commonly observed luminescence bands for the Al2O3 material. Indeed, we show that the red band is associated with a Ti3+ d-d transition as expected, while the blue band is the combination of the Ti3+ + O- → Ti4+ + O2- and VO•+e- → VO× de-excitation processes. Thanks to our recent developments, which take into account the vibrational contributions to the electronic transitions in solids, we were able to simulate the luminescence spectra for the different signatures. The excellent agreement with the experiment demonstrates that it should be possible to predict the color of the material with a CIE chromaticity diagram. We also anticipated the luminescence signature of Al2O3:Ti,Ca and Al2O3:Ti,Be that were confirmed by experiment.

Spatio-temporal dynamics of a microsecond pulsed Grimm-type glow discharge

Spatio-temporal dynamics of a PGD, in Ar6.0 at 6 mbar for some of Ar I, Ar II, Cu I, Cu II lines. Analysis of a high current peak during ignition of discharge, excitation and deexcitation processes, development of electron excitation temperature of Ar.

First close-coupling study of the excitation of a large cyclic molecule: collision of c-C5H6 with He.

Recent astronomical observations revealed an increasing molecular complexity in the interstellar medium through the detection of a series of large cyclic carbon species. To correctly interpret these detections, a complex analysis is necessary that takes into account the non-local thermodynamic equilibrium (non-LTE) conditions of the emitting media (e.g. when energy level populations deviate from a Boltzman distribution). This requires proper state-to-state collisional data for the excitation and de-excitation processes of the molecular levels. Cyclopentadiene (c-C5H6), which was recently detected in cold interstellar clouds, is extensively studied in many aspects due to its large importance for chemistry in general. At the same time, there are no collisional data available for this species, which are necessary for a more precise interpretation of the corresponding detections. In this work, we first provide an accurate 3D rigid-rotor interaction potential for the [c-C5H6 + He] complex from high-level of ab initio theories, which has been used to study their inelastic collision by the exact close coupling quantum scattering method. To the best of our knowledge, this is the first study where this method is systematically applied to treat the dynamics of molecular collisions involving more than ten atoms. We also analyse the collisional propensity rules and the differences in contrast to calculations, where the approximate coupled states scattering methods is used.

Assimilating fission-code FIFRELIN using machine learning

This paper presents work that has been done on the FIFRELIN Monte-Carlo code. The purpose of the code is to simulate the de-excitation process of fission fragments. Numerous quantity of insterest are calculated (mass yields, prompt particle spectra, mulitiplicities … ). Up to now the code relies on four free parameters which control the initial excitation and total angular momentum of fission fragment. Finding the good set of the free parameters is a diffucult task. In this work, we have developed an optimization algorithm based on Gaussian Process regression.

Neutrinos from pre-supernova in the framework of TQRPA method

Abstract We propose a new method for calculating spectra and luminosities for (anti)neutrinos produced in the pre-supernova environment by weak processes with hot nuclei. It is based on the thermal quasiparticle random phase approximation (TQRPA), that allows microscopic thermodynamically consistent calculations of the weak-interaction response of nuclei at finite temperatures. For realistic representative pre-supernova conditions from the stellar evolution code MESA, we compute (anti)neutrino luminosities and spectra arising from neutral- and charged-current weak reactions with hot 56Fe and compare them with the contribution of thermal processes. We find that the TQRPA approach produces not only a higher total luminosity of electron neutrinos (mainly born in the electron capture reaction), compared to the standard technique based on the large-scale shell model (LSSM) weak-interaction rates, but also a harder neutrino spectrum. Besides, applying the TQRPA and LSSM, we find that in the context of electron antineutrino generation, the neutral-current nuclear de-excitation (ND) process via neutrino-antineutrino pair emission is at least as important as the electron-positron pair annihilation process. We also show that flavor oscillations enhance the high-energy contribution of the ND process to the electron antineutrino flux. This could potentially be important for pre-supernova antineutrino registration by the Earth’s detectors.

Theoretical study of inelastic processes in collisions of Y and Y+ with hydrogen atom

ABSTRACT Utilizing a simplified quantum model approach, the low-energy inelastic collision processes between yttrium atoms (ions) and hydrogen atoms have been studied. Rate coefficients corresponding to the mutual neutralization, ion-pair formation, excitation, and de-excitation processes for the above collision systems have been provided in the temperature range of 1000–10 000 K. Three ionic states and 73 covalent states are considered in calculations for the collisions of yttrium atoms with hydrogen atoms, which include six molecular symmetries and 4074 partial inelastic reaction processes. For the collisions of yttrium ions with hydrogen atoms, one ionic state and 116 covalent states are included, which are related to three molecular symmetries and 13 572 partial inelastic collision processes. It is found that the rate coefficients for the mutual neutralization process have a maximum at T = 6000 K, which is an order of magnitude higher than those of other processes. Notably, the positions of optimal windows for the collisions of yttrium atoms and ions with hydrogen atoms are found near electronic binding energy −2 eV (Y) and −4.4 eV (Y+), respectively. The scattering channels located in or near these optimal windows have intermediate-to-large rate coefficients (greater than 10−12 cm3 s−1). The reported data should be useful in the study of non-local thermodynamic equilibrium modelling.

Spectroscopic diagnosis and simulation analysis of tungsten cathode erosion during the start-up process for a magnetoplasmadynamic thruster

Measurement of tungsten density during the start-up process plays a vital role in monitoring real-time cathode mass loss in the study of plasma discharge instrument structure optimization. The relative intensity of the W II 239.71 nm spectral line, collected at the exit by the monochromator, is used to estimate the W II density in combination with the collision radiative model. The model of a small amount of tungsten in high-density plasma takes into account the main kinetic processes including the electron impact excitation process, electron impact de-excitation process, and electron impact ionization process. Additionally, A two-dimensional axisymmetric quasi-neutral fluid model, coupled with a thermal model of the cathode into a self-consistent model, is also constructed to analyze the mass loss mechanism of tungsten cathode during the start-up process. In the initial stage, the cold-state cathode has a large potential drop in the sheath region to accelerate the ion bombardment of the cathode for the requirement of electron emission, and a large number of tungsten atoms removal from the cathode surface due to sputtering and evaporation. In the steady-state operating phase, the primary reason for the mass loss of tungsten cathodes is attributed to thermal evaporation. Both experimental and simulation results indicate that the mass loss rate during the start-up phase is higher than that during steady-state operation phase by 1 to 2 orders of magnitude.

A computational study of CaWO4: Raman spectrum, intrinsic defects, and excited state properties

In this study, we present an ab initio investigation of the optical properties of CaWO4. The crystal structure of the bulk material along with its vibrational (in the ground state) and optical properties (in both ground and excited states) were analyzed. Our simulations accurately identified the observed Raman signature of the material. Additionally, we simulated the intrinsic luminescence properties of CaWO4 and identified the charge transfer that occurs during the de-excitation process. Finally, through the use of ab initio techniques, the intrinsic defect formation energies and their corresponding concentrations were determined providing the first instance of such findings for this material.

Turning the energy channel of side-chain TADF polymers by monomer optimization for high-efficiency solution-processed white OLEDs

Suppressing non-radiative channel in luminescent polymer is the key factor for high device efficiency of organic light-emitting diodes (OLEDs). In this paper, we synthesized three side-chain TADF polymers, PTRZ-TRZ, PTRZ-TPA and PTRZ-Cz, to deeply investigate the relationship between energy transfer and molecular structure. The photophysical measurements show that the copolymerized monomer has significant effect on the exciton deexcitation process of TADF fragment. The rigid carbazole unit can efficiently separate the TADF units to inhibit the aggregation-caused quenching (ACQ) of polymer, while electron-donating triphenylamine unit will induce the severe charge transfer state to form low energy exciplex, which lead to severe energy leakage of the excited TADF. Moreover, according to the single-charge devices test, the ideal separation unit also can improve the capability of charge injection and carrier balance. As a result, the solution-processed OLEDs host with PTRZ-Cz achieved the external quantum efficiency (EQEmax) of 14.2%, which was much higher than that of PTRZ-TPA and PTRZ-TRZ. By doping red phosphorescent emitter Ir(MDQ)2(acac), the hybrid white OLED devices (T-P WOLEDs) based on PTRZ-Cz achieved the maximum current efficiency (CEmax), power efficiency (PEmax) and EQEmax of 46.8 cd/A, 32.6 lm/W and 20.7%, respectively, with a high CRI of 80.6. The outcomes obtained in this study can provide useful insights into the structure−property relationship for further development of efficient TADF polymer hosts for solution-processable OLEDs.

Photophysical and photochemical properties of heterocyclic analogs of 1,4-cyclohexadiene: Experimental and theoretical studies

The photophysical and photochemical properties of a series of heterocyclic analogs of 1,4-cyclohexadiene were studied by theoretical calculation and experiment. With the incorporation of different heteroatoms, the analogs presented distinct photophysical and photochemical properties from 1,4-cyclohexadiene. Their ultraviolet–visible absorption spectra were measured and found to be related to the heteroatoms on the 1,4-cyclohexadiene ring. Calculation of the geometrical and electronic structures of the ground-states revealed that incorporation of the heteroatom affected the photophysical properties of the molecule. The de-excitation processes and electron transition modes were discussed according to the calculated values of the spin–orbit coupling matrix and the electron-hole distributions of the excited states. The reactive sites and geometrical structures showed that incorporation of aheteroatom into 1,4-cyclohexadiene changed the spin density distribution and thus influenced the photochemical properties. These findings are meaningful in explaining the photo-reactivities of these molecules, and provide foundations for further experimental/theoretical investigations and pharmacological applications based on established structure–property correlations.

Atomic data on inelastic processes in boron–hydrogen collisions with accounting for fine structure

ABSTRACT This paper reports the results of investigations of inelastic processes in B + H and B+ + H− collisions with accounting for the fine structure. The calculations of the cross-sections and rate coefficients for excitation, de-excitation, ion-pair formation, and mutual neutralization processes (240 in total) are performed by using the quantum asymptotic method, which is modified in this paper for accounting of the fine structure, and by the multichannel approach within the framework of the Born–Oppenheimer formalism. It is found that accounting for the fine structure may change the rate coefficients significantly, and the rates calculated with accounting for the fine structure cannot be obtained by a simple redistribution of the non-relativistic rates. The processes important for non-local thermodynamic equilibrium modelling of stellar atmospheres are designated.

Recent developments in the modeling of (n,γ) reactions with FIFRELIN

Nowadays, the accuracy of experimental results relies more and more on detailed simulations. In the STEREO experiment, the interaction of neutrinos in the liquid scintillator is signed by a n-capture on a Gd atom. The FIFRELIN predictions of the Gd γ-cascades were shown to significantly improve the Data/MC agreement. In the CRAB method, lately proposed to calibrate cryogenic particle detectors at low energy (100 eV), the FIFRELIN cascades of the W and Ge isotopes played a central role in the feasibility study of the method. The FIFRELIN code employs a Monte Carlo Hauser-Feshbach framework based on Bečvár’s algorithm. A sample of nuclear level schemes is generated for a specific isotope of interest, taking into account the uncertainties from nuclear structure. In this work, new improvements on the FIRELIN de-excitation process are reported. Angular correlations of γ-rays in the de-excitation process have been implemented in order to provide a more accurate description of the γ-ray cascades. The anisotropy of the γ-rays with respect to the axis of a previously emitted γ-ray is modeled using the angular correlation formalism, which requires input of the spins and multipolarities of the states involved in the FIFRELIN cascade. Furthermore, the simulation of primary γ-rays emitted from (n,γ) reactions has been updated using the EGAF database.

Revealing and Tuning the Photophysics of C=N Containing Photothermal Molecules: Excited State Dynamics Simulations.

Molecular photothermal conversion materials are recently attracting increasing attention for phototherapy applications. Herein we investigate the excitation and de-excitation processes of a photothermal molecule (C1TI) that is among the recently developed class of small-molecule-based photothermal imines with superb photothermal conversion efficiencies (PTCEs) up to 90% and a molecule (M2) that is constructed by replacing the amino group of C1TI with an H atom, via excited-state dynamics simulations based on the time-dependent density functional theory (TD-DFT). The simulations reveal fast (<150 fs of average time) nonradiative decays of the lowest excited singlet (S1) state to a conical intersection (CI) with the ground (S0) state in high yields (C1TI: 93.9% and M2: 87.1%). The fast decays, driven by C=N bond rotation to a perpendicular structural configuration, are found to be barrierless. The slight structural difference between C1TI and M2 leads to drastically different S0-S1 energy surfaces, especially M2 features a relatively much lower CI (0.8 eV in energy) and much more decay energy (1.0 eV) to approach the CI. This work provides insights into the de-excitation mechanisms and the performance tuning of C=N enabled photothermal materials.

Constraints from isoscaling on the source size in energetic heavy ion collisions

In the framework of the statistical multifragmentation model, the nuclear isoscaling analysis is extended to constrain the ratio between the sizes of the decaying sources formed in a collision between two heavy ions. It is found that the ratio between the probabilities of observing $n$ fragments in each event, for each of the sources, follows a scaling law, similar to the traditional nuclear isoscaling. However, the corresponding slope is sensitive to the source sizes. This property is explained analytically using the grand-canonical ensemble. The extent to which our findings are affected by finite size effects and by the deexcitation of the hot primary fragments is investigated. The scaling turns out to be robust and weakly affected by effects implied by these two aspects. We also find that the Poisson distribution is a fairly good approximation to the above mentioned probabilities, associated with both the primordial fragments, produced at the breakup stage, and the final ones, found at the end of the fragment deexcitation process.

PH assisted modulation in the binding affinity for BODIPY-benzimidazole conjugate with anionic cyclodextrin

Modulation in the properties of chromophoric guest dyes involving non-covalent supramolecular interactions of various macrocyclic hosts are the intense topics of current research. In this study we report interaction of a sulfobutylether-β-cyclodextrin (SBEβCD) host in modulating the photophysical as well as prototropic properties of an inhouse synthesized BODIPY-benzimidazole conjugate dye, abbreviated as BDZ. The SBEβCD host displays substantially different binding strengths for the protonated and unprotonated forms of the prototropic BDZ dye, inducing a large upward pKa shift for the dye. Furthermore, structural confinement of the dye upon binding with the host leads to substantial enhancement in the fluorescence intensity and lifetime for both the prototropic forms of the dye. This can be attributted to the reduction in the non-radiative deexcitation process for the bound dye. Fluorescence enhancements are also accompanied with large increase in fluorescence anisotropy decay times, substantiating dye-host binding. Since unprotonated BDZ in free state is extremely weak in emission due to its intramolecular charge transfer (ICT) character, its fluorescence enhancement on binding to SBEβCD is found to be more pronounced than protonated BDZ. Importantly, emission of BDZ–SBEβCD system appears in the green region of visible spectrum, implying the immense prospect of the system for fluorescence-based sensor and imaging applications in different biological studies.

Accurate Calibration of Nuclear Recoils at the 100 eV Scale Using Neutron Capture

Searches for light dark matter (DM) and studies of coherent elastic neutrino-nucleus scattering (CEvNS) imply the detection of nuclear recoils in the 100 eV range. However, an absolute energy calibration in this regime is still missing. The CRAB project proposes a method based on nuclear recoils induced by the emission of an MeV gamma following thermal neutron capture. A detailed feasibility study has shown that this method yields distinct nuclear recoil calibration peaks at 112 eV and 160 eV above background for tungsten. In the first phase, the CRAB project foresees to perform a nuclear recoil calibration of cryogenic CaWO4 detectors read-out by TES, similar to the detectors used in CRESST and NUCLEUS. The low-power TRIGA reactor in Vienna provides a clean beam of thermal neutrons well suited for such a measurement. Newly developed and compact sub-keV calibration sources based on x-ray fluorescence (XRF) provide an absolute energy calibration during operation at the research reactor as well as in the DM/CEvNS experiments. In the second phase, additional tagging of the photons produced in the de-excitation process will allow extending the calibration method to even lower energies and to a wider range of detector materials, such as Ge. Combined with the XRF source, CRAB may allow measuring energy quenching in the sub-keV regime.

Electron impact excitation cross section calculations of the fine structure transitions of Mo and their applications in the diagnostics of the laser induced Mo plasma

The current work reports a detailed calculation of electron impact excitation cross sections for the fine structure transitions of Mo from the manifolds 4d 55s, 4d 45s 2, 4d 6 and 4d 55p to the manifolds 4d 55s, 4d 45s 2, 4d 6, 4d 55p, 4d 45s5p, 4d 55d, 4d 56s, 4d 45s6s, and 4d 57s, using the relativistic distorted wave approximation for the applications in plasma modelling. Multi-configurational Dirac–Fock wave functions are used in the calculations. The oscillator strength and cross section results are compared with the previous calculations and measurements. A comprehensive collisional radiative (CR) model is developed and used to characterize laser induced molybdenum plasma to ensure that the calculated cross sections can be used for various plasma modelling applications. The current CR model has taken into account the electron impact excitation and de-excitation processes using the calculated consistent cross sections. The electron induced processes are dominant kinetic processes in the laser induced plasma. Furthermore, the diagnostics of the laser induced Mo plasma is done by coupling the current CR model with the experimental laser induced breakdown spectroscopic measurements of Mal et al (2021 Appl. Phys. B 127 52). The plasma parameter, i.e., electron temperature has been calculated using nine measured intensities of the emission lines of Mo, with wavelengths 406.9, 423.3, 438.2, 453.7, 476.0, 550.7, 553.3, 557.0 and 592.9 nm. The results are also compared with the values reported in the Boltzmann plot at various delay times ranging from 0.5 to 5.0 μs.