Magnetic Dipole Transitions Research Articles

Circularly polarized luminescence of Hetero[n]helicenes with 2,1,3-thiadiazole rings at both ends: Design of magnetically-allowed electronic transitions via heteroatom embedding

Design of excellent circularly polarized luminescence (CPL) emitters requires chiral molecules with large transition magnetic dipole moments (TMDM, |m|). In this work [5]-, [7]-, and [9]helicene derivatives with 2,1,3-thiadiazole rings at both ends, TD[5]H, TD[7]H, and TD[9]H, were designed and synthesized. We revealed that TD[9]H showed an excellent CPL with a large glum of +0.04 at 520 nm in toluene, which was significantly larger than that of unsubstituted carbo[7]helicene (glum = +0.008). Density functional theory (DFT) calculations suggested that the remarkable glum of TD[9]H was attributed to the magnetically allowed transition with a large TMDM (|m| = 2.3 × 10−20 erg G−1) for the S1→S0 transition.

Structural and luminescent features of warm reddish-orange light-emitting Sm(III) doped Gd2Si2O7 nanophosphors for near UV-energized LEDs

Solution combustion (SC) synthetic method was used to prepare warm reddish-orange light emitting Gd2-xSi2O7:xSm3+ (x = 1–5 mol%) nanophosphors. Structural analysis was performed to recognize the phase identification. The nanomaterials were structured into triclinic phase with P-1 space group. All the diffraction peaks are fairly matched with the standard data and (h k l) planes have been well indexed. Additionally, the evidence about the site symmetry around Sm3+ ions have been prejudiced by considering the intensity ratio of 4G5/2→6H9/2 (electrical dipole) to 4G5/2→6H5/2 (magnetic dipole) transitions. The highest emission at ∼597 nm is accountable for reddish-orange light emission in the doped nanocrystalline materials. Among different excitation, the most intense excitation peaking at ∼402 nm lies in the near UV range, which specifies that the designated phosphors could be competently energized through NUV based LEDs.

Physical Mechanism of One-Photon Absorption, Two-Photon Absorption, and Electron Circular Dichroism of 1,3,5 Triazine Derivatives Based on Molecular Planarity.

We provide a method to regulate intramolecular charge transfer (ICT) through distorting fragment dipole moments based on molecular planarity and intuitively investigate the physical mechanisms of one-photon absorption (OPA), two-photon absorption (TPA), and electron circular dichroism (ECD) properties of the multichain 1,3,5 triazine derivatives o-Br-TRZ, m-Br-TRZ, and p-Br-TRZ containing three bromobiphenyl units. As the position of the C-Br bond on the branch chain becomes farther away, the molecular planarity is weakened, with the position of charge transfer (CT) on the branch chain of bromobiphenyl changing. The excitation energy of the excited states decreases, which leads to the redshift of the OPA spectrum of 1,3,5-triazine derivatives. The decrease in molecular plane results in a change in the magnitude and direction of the molecular dipole moment on the bromobiphenyl branch chain, which weakens the intramolecular electrostatic interaction of bromobiphenyl branch chain 1,3,5-triazine derivatives and weakens the charge transfer excitation of the second step transition in TPA, leading to an increase in the enhanced absorption cross-section. Furthermore, molecular planarity can also induce and regulate chiral optical activity through changing the direction of the transition magnetic dipole moment. Our visualization method helps to reveal the physical mechanism of TPA cross-sections generated via third-order nonlinear optical materials in photoinduced CT, which is of great significance for the design of large TPA molecules.

Theoretical level energies and transition data for 4p[formula omitted]4d[formula omitted], 4p[formula omitted]4d[formula omitted] and 4p[formula omitted]4d[formula omitted]4f configurations of W[formula omitted] ion

The ab initio quasirelativistic approach developed specifically for the calculation of spectral parameters of highly charged ions has been used to determine transition data for the Mo-like tungsten ion W32+. The configuration interaction method is utilized to include electron correlation effects. The relativistic effects are taken into account in the Breit–Pauli approximation. Level energies, radiative lifetimes τ, Landé g-factors are determined for the ground configuration 4p64d6 and two excited configurations 4p54d7 and 4p64d54f. The radiative transition wavelengths λ, emission transition probabilities A, weighted oscillator strengths gf, and transition line strengths S for the electric dipole, electric quadrupole, electric octupole, magnetic dipole, and magnetic quadrupole transitions among the fine-structure levels of these configurations are produced. The uncertainties of computed spectroscopic parameters are evaluated.

Hyperfine structure measurements on a forbidden transition of Bi I using fourier transform spectroscopy

Hyperfine structure (hfs) investigations have been performed on a forbidden transition of the Bi I emission spectral line in the visible region at 15,437.49 cm−1, between the levels 6p3 4S3/2 and 6p3 2D5/2, by means of a Fourier Transform spectrometer. Electrodeless discharge lamps (EDL) containing BiI3 were utilized to generate the bismuth plasma. In these observations, all the 6 components of electric quadrupole (ΔF = ±2) transitions have been detected for the first time, in addition to the 12 components of magnetic dipole (ΔF = 0, ±1) transitions. Magnetic dipole constant (A) and electric quadrupole constant (B) have been derived for the combining levels.

Emission Enhancement of Eu3+ Ions in LiY(PO3)4 Phosphate Phosphors via Anionic SO4 2− Substitution

Eu3+-doped LiY(PO3)4(1-x)(SO4)4x (LYSO:Eu) phosphors are synthesized by high-temperature solid-phase method. The crystal structure, microscopic morphology and photo-spectrographic characteristics are analyzed respectively. The phosphor exhibits the strongest excitation peak at 397 nm, which indicates that the sample can be effectively excited by the near-ultraviolet/ultraviolet LED chip. The characteristic emission peaks of Eu3+ ion 5D0→7FJ (J = 0, 1, 2, 3, 4) are observed from the PL spectrum, and the orange emission peak originating from the magnetic dipole transition 5D0→7F1 of Eu3+ is located near 597 nm. When the SO4 2− concentration exceeds 22.5 mol%, the quenching of the phosphor occurs. The Eu3+ emission still retains 78.8% of the initial strength at 495 K, with excellent thermal stability. The emission intensity is decreased with the increase of SO4 2− concentration, indicating that the SO4 2− co-doping leads to an increase in the probability of non-radiative transitions. The best quantum efficiency of the LiY0.95(PO3)2.6(SO4)1.4: 0.05Eu3+ sample is 86.65%, and the CIE chromaticity coordinates of this sample are (0.653, 0.347), which are close to the international standard red color coordinates (0.67, 0.33). These results indicate that LYSO:Eu has a good application in NUV/UV excited white LEDs. Influence of anionic substitutions of SO4 2− on the optical properties is discussed.

Red luminescence from La2O3:Eu3+ nanophosphor prepared by Sol-Gel method

Pure and Eu3+ doped La2O3 nanophosphor was synthesized by sol gel process. PL spectra of undoped La2O3 samples shows broad emission peaks between 400 and 600 nm and Eu3+ doped samples show PL emission peaks at 510, 515, 531, 537, 553, 578, 585, 594, 612 and 624 nm. These peaks are attributed to 5D1 → 7F2, 5D1 → 7F2, 5D0 → 7F0, 5D0 → 7F1,5D0 → 7F2 transitions of Eu3 + ions. The electronic dipole transition corresponding to5D0 → 7F2 (624, 612 nm) is stronger than the magnetic dipole transition of 5D0 → 7F1 (585, 594 nm) of Eu3+ ions. Luminescence behavior of Eu3+ in La2O3 is analyzed by Judd–Ofelt intensity parameters.

Quasiparticle random-phase approximation calculations forM1transitions with the noniterative finite-amplitude method and application to neutron radiative capture cross sections

We derive the equations of quasiparticle random-phase approximation (QRPA) based on the finite amplitude method (FAM) with the Hartree-Fock+BCS (HF+BCS) single-particle states, and calculate the magnetic dipole (M1) transition for deformed gadolinium isotopes. Our QRPA calculation shows both large spin-flip transitions in the 5 to 10 MeV excitation energy and the low energy orbital transition that would correspond to the M1 scissors mode observed in nuclear experiments. Then, we calculate neutron capture reactions based on the statistical Hauser-Feshbach theory with the photoabsorption cross sections given by QRPA. We find that the capture cross section is enhanced due to the contribution from the low energy M1 transition although the calculated capture cross section still underestimates the experimental data. This issue in the calculated capture cross section could be improved by uncertainties of low energy E1 transition neglected in our QRPA calculation.

A novel orange-red Ba3MgTa2O9:Eu3+ phosphor synthesis and luminescence properties in WLEDs

A new orange-red phosphor Ba3MgTa2O9:xEu3+ (0.05 ≤ x ≤ 0.60) was successfully prepared through the high-temperature solid-state reaction. A study of its structure, morphology, photoluminescence and fluorescence lifetime was conducted. By exposing the phosphor to near-ultraviolet (n-UV) light, orange-red light is generated as a result of the 7F0→5D1 magnetic dipole transition. The emission intensity is maximal at a doping concentration of x = 0.4, and concentration quenching occurs at higher concentrations. At 423 K, Ba3MgTa2O9:0.4Eu3+ phosphor luminescence intensity remained at 71% in reference to that at 303 K, showing good thermal stability. The CIE coordinates of the phosphor with the optimal doping concentration at room temperature are located at (0.6008, 0.3942) with a color purity of 98.27%. A high color rendering index is achieved by the prepared WLEDs, with Ra = 93.8 and CCT = 5671 K, which are located in the white light region of CIE with the coordinates of (0.3286, 0.3487). It is shown that the prepared Ba3MgTa2O9:Eu3+ phosphor is a potential orange-red phosphor material for WLEDs.

Theoretical investigation of Sb-like sequence: Sb I, Te II, I III, Xe IV, and Cs V

We aim to investigate atomic properties of Sb-like sequence: Sb I, Te II, I III, Xe IV, and Cs V. The multiconfiguration Dirac–Hartree–Fock and relativistic configuration interaction methods, which are implemented in the general-purpose relativistic atomic structure package GRASP2018, are used in the present work. The lowest energy levels of the 5s25p3, 5s5p4, 5p5, 5s5p35d, and 5s25p2{6s,7s,6p,7p,5d,6d,4f} configurations and electric dipole, magnetic dipole, and electric quadrupole transitions between states of these configurations are computed. Accuracy of energy levels are evaluated by comparing it with the National Institute of Standards and Technology Atomic Spectra Database recommended values and with other methods. Accuracy of transitions data are investigated using quantitative and qualitative evaluation method.

Multipole theory of optical spatial dispersion in crystals

Natural optical activity is the paradigmatic example of an effect originating in the weak spatial inhomogeneity of the electromagnetic field on the atomic scale. In molecules, such effects are well described by the multipole theory of electromagnetism, where the coupling to light is treated semiclassically beyond the electric-dipole approximation. That theory has two shortcomings: it is limited to bounded systems, and its building blocks -the multipole transition moments- are origin dependent. In this work, we recast the multipole theory in a translationally-invariant form that remains valid for crystals. Working in the independent-particle approximation, we introduce "intrinsic" multipole transition moments that are origin independent and transform covariantly under gauge transformations of the Bloch eigenstates. Electric-dipole transitions are given by the interband Berry connection, while magnetic-dipole and electric-quadrupole transitions are described by matrix generalizations of the intrinsic magnetic moment and quantum metric. In addition to multipole-like terms, the response of crystals at first order in the wavevector of light contains band-dispersion terms that have no counterpart in molecular theories. The full response is broken down into magnetoelectric and quadrupolar parts, which can be isolated in the static limit where electric and magnetic fields become decoupled. The rotatory-strength sum rule for crystals is found to be equivalent to the topological constraint for a vanishing chiral magnetic effect in equilibrium, and the formalism is validated by numerical tight-binding calculations.

Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties.

Helical bilayer nanographenes (HBNGs) are chiral π-extended aromatic compounds consisting of two π-π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π-π interactions between the two layers), being 26, 14, and 10 benzene rings, respectively, according to the X-ray analysis. Unexpectedly, the electrochemical study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of gabs and glum (3.6 × 10-2) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its electric and magnetic dipole transition moments. Furthermore, spectroelectrochemical studies as well as the fluorescence spectroscopy support the aforementioned experimental findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.

A New Look at the Explanation of Differences in Properties between La3+ and Lu3+ Titanate Hosts Occupied by Eu3+

In order to improve the quality of WLEDs, considerable efforts have been devoted to investigate red phosphor. In this study, two double perovskites La2MgTiO6 and La2ZnTiO6 and two novel pyrochlores Lu2MgTiO6 and Lu2ZnTiO6 doped with 1%Eu3+ were synthesized by the co-precipitation method. The crystal structure, absorption, excitation, and emission spectra of all samples were investigated in detail. Dissimilar luminescent behaviors of the studied samples were observed. Due to the different site symmetry of Eu3+, each compound exhibits a different emission spectrum. The emission of the magnetic dipole transition is dominant in all the spectra of the Lu-based hosts, while the forced electric dipole transition is the strongest one in the spectra of the others. Besides, the ones crystallized in double perovskite structures possess larger bandgaps and show a more intense emission from the 5D1 level of Eu3+ as compared to the pyrochlore compounds. Furthermore, the distinct mechanism of the cross-relaxation of four titanate compounds was investigated. The above findings contribute to the understanding of the two novel pyrochlore hosts.

Explorations of pseudo-Dirac dark matter having keV splittings and interacting via transition electric and magnetic dipole moments

Explorations of pseudo-Dirac dark matter having keV splittings and interacting via transition electric and magnetic dipole moments

Physical Mechanism of Nonlinear Spectra in Triangene.

In this work, we theoretically investigate the linear and nonlinear optical absorption properties of open triangulene spin chains and cyclic triangulene spin chains in relation to their lengths and shapes. The physical mechanism of local excitation within the triangular alkene unit and the weak charge transfer between the units are discussed. The uniformly distributed electrostatic potential allows the system to have a small permanent dipole moment that blocks the electronic transition in the light excitation such that the electronic transition can only be carried out between adjacent carbon atoms. The one-photon absorption (OPA) spectra and two-photon absorption (TPA) spectra are red-shifted with the addition of triangulene units compared to N = 3TSCs (triangulene spin chains, TSCs). Here, TPA is mainly caused by the first step of the transition. The length of the spin chain has a significant adjustment effect on the photon cross-section. TSCs of different lengths and shapes can control chirality by adjusting the distribution of the electric dipole moment and transition magnetic dipole moment. These analyses reveal the photophysical properties of triangulene and provide a theoretical basis for studying the photophysical properties of triangulene and its derivatives.

Relaxation echo

The possibility of generation of a microwave echo-signal due to irreversible phase relaxation in a system of equidistant triplets with a cascade scheme of magnetic-dipole transitions is predicted. The disappearance of relaxation entails the disappearance of this echo-signal. The mechanism of this effect lies in the destructive interference of quantum transitions, which radiate in anti-phase with the same eigen-frequencies. The difference in phase relaxation times for these transitions leads to incomplete damping of the resulting coherence. As a result an echo-signal is generated.

Naphthalimide-Annulated [n]Helicenes: Red Circularly Polarized Light Emitters

Two [n]heliceno-bis(naphthalimides) 1 and 2 (n = 5 and 6, respectively) where two electron-accepting naphthalimide moieties are attached at both ends of helicene core were synthesized by effective two-step strategy, and their enantiomers could be resolved by chiral stationary-phase high-performance liquid chromatography (HPLC). The single-crystal X-ray diffraction analysis of enantiopure fractions of 1 and 2 confirmed their helical structure, and together with experimental and calculated circular dichroism (CD) spectra, the absolute configuration was unambiguously assigned. Both 1 and 2 exhibit high molar extinction coefficients for the S0-S1 transition and high fluorescence quantum yields (73% for 1 and 69% for 2), both being outstanding for helicene derivatives. The red circularly polarized luminescence (CPL) emission up to 615 nm for 2 with CPL brightness (BCPL) up to 66.5 M-1 cm-1 demonstrates its potential for applications in chiral optoelectronics. Time-dependent density functional theory (TD-DFT) calculations unambiguously showed that the large transition magnetic dipole moment |m| of 2 is responsible for its high absorbance dissymmetry (gabs) and luminescence dissymmetry (glum) factor.

Magnetic dipole and quadrupole transitions in the ν2 + ν3 vibrational band of carbon dioxide.

This paper aims at the theoretical study of the CO2 magnetic-dipole ν2 + ν3 rovibrational absorption band that was recently detected in the Martian atmosphere. Specific characteristics of the magnetic dipole operator are carefully examined. Our evaluation of the magnetic-dipole line intensities is based on the variational calculations and the use of molecular properties is determined through specially performed abinitio quantum chemical calculations. The comparison of our simulated magnetic-dipole spectrum with available laboratory taken data also requires the knowledge of line intensities in the quadrupole band, which partially overlaps with that magnetic-dipole. Quadrupole intensities, once reconsidered, are permitted to correct previously reported values of the integrated intensity as well as the intensity of selected branches. The sum of our calculated magnetic-dipole and quadrupole rovibrational lines is shown to be in good agreement with both sets of presently available data from FTIR and OFCEAS laboratory observations.

Synthesis and emission characteristics of rare earth ions (Eu3+ and Tb3+) doped calcium orthogermanate phosphors

In this study, rare earth ions (Eu3+ and Tb3+) doped calcium orthogermanate (Ca2GeO4) phosphors were synthesized through the high temperature solid-state route. PL (Photoluminescence), FTIR (Fourier transform infrared spectroscopy), SEM (Scanning electron microscopy), EDS (Energy dispersive spectroscopy) and PXRD (Powder X-ray diffraction) techniques were used to characterize all the prepared powder samples. The PXRD results of the samples give an orthorhombic phase system. The SEM-EDS measurements are used to confirm the doping of europium and terbium ions into the Ca2GeO4 lattice. The vibrational characteristics of the proposed phosphors are investigated from FTIR spectra. As per the PL studies, the Eu3+ and Tb3+ activated Ca2GeO4 phosphors are exhibited a prominent red emission at 610 nm and bright green emission at 547 nm under 244 nm and 229 nm excitation wave lengths respectively. The obtained results are ascribed to the ED (Electric Dipole) transition and MD (Magnetic Dipole) transitions 5D0 → 7F2 and 5D4 → 7F5 of europium and terbium ions respectively. The CIE diagram illustrates the emission colors of the synthesized phosphors which are given as red and green emission regions. Based on obtained results, the prepared phosphors may be suitable for solid state light and display devices.

Hamiltonian sensitivity in calculating the electromagnetic moments and electroexcitation form factor for the sd nuclei using shell model with Skyrme–Hartree–Fock

In this work, the nuclear electromagnetic moments for the ground and low-lying excited states for sd shell nuclei have been calculated, resulting in a revised database with 56 magnetic dipole moments and 41 electric quadrupole moments. The shell model calculations are performed for each sd isotope chain, considering the sensitivity of changing the sd two-body effective interactions USDA, USDE, CWH and HBMUSD in the calculation of the one-body transition density matrix elements. The calculations incorporate the single-particle wave functions of the Skyrme interaction to generate a one-body potential in Hartree–Fock theory to calculate the single-particle matrix elements. For most sd shell nuclei, the experimental data are well reproduced, except for those spans near the island of inversion. In order to interpret the structure of low-lying excited states, the electric quadrupole and magnetic dipole transition form factors and the corresponding reduced transition probabilities in the sd shell nuclei have also been calculated, for which the experimental data are available. The present results demonstrate the nuclear electromagnetic moments’ sensitivity to many forms of the understanding of nucleon–nucleon interactions and provide a crucial baseline for future improvements in conceptual calculations.