Excited 4f Research Articles

Antenna‐ and Metal‐Triggered Luminescence in Dense 1,3‐Benzodinitrile Metal–Organic Frameworks ∞3[LnCl3(1,3‐Ph(CN)2)], Ln = Eu, Tb

AbstractLuminescent homoleptic dense 1,3‐benzodinitrile MOFs (metal–organic frameworks) of Eu3+ and Tb3+ were prepared from the anhydrous chlorides LnCl3 (Ln = Eu, Tb) and a melt of the linker ligand 1,3‐benzodinitrile [1,3‐Ph(CN)2=C6H4(CN)2]. The 1,3‐benzodinitrile ligands act as chemical scissors and cut down the LnCl3 structure to 2D sheets of trigonal Cl ion prisms around the rare earth ions and interlinks these nets to form a 3D framework structure of the formula ∞3[LnCl3(1,3‐Ph(CN)2)]. Both compounds exhibit photoluminescence of the trivalent rare earth ions although they are fully concentrated (100 % luminescence centres), and are the sole examples in which a dinitrile linker is utilized as an antenna for rare earth luminescence subsequent to a transfer of the energy from an excited ligand state to an excited 4f state of the rare earth ion. The antenna effect is most efficient for the terbium‐containing framework, whereas the europium spectrum also exhibits metal 4f–4f excitation. The emission can be attributed to 4f–4f transitions of Eu3+ and Tb3+, which give a red emission for europium (intra 4f6 5D0→7F4–0) and a green emission for terbium (intra 4f8 5D4→7F0–6). For Eu3+ a rather uncommon case was observed, in which the 5D4→7F4 transition shows the highest intensity because of a strong ligand‐polarizability‐ and temperature‐dependent dynamic coupling (DC) effect. It is known that luminescence as an intrinsic property of the framework can be triggered in such MOFs by different excitation routes and that both the organic and the inorganic parts are essential for the functionality of these hybrid materials.

Photoionization of tungsten ions with synchrotron radiation

Photoionization experiments were performed with tungsten ions using a photon–ion merged-beams setup at the Advanced Light Source. Cross-sections σq,q+1 for photon-induced single ionization of Wq+ ions (q=1,2,3,5) and σq,q+2 for photon-induced double ionization of W+ ions have been explored in the energy range 20–70 eV. While σq,q+1 for the lower charge states is dominated by broad resonance features in the range of 4f and 5p excitations, the cross-section σ5, 6 shows much narrower peak features. Significant contributions from initial metastable states are evident. Double ionization is substantially enhanced at energies above the threshold for single ionization of a 4f electron in W+.

Enhanced luminescence of Dy 3+ in Y 3Al 5O 12 by Bi 3+ co-doping

In the present paper, phosphors with the composition Y 3− x−y Al 5O 12:Bi 3+ x , Dy 3+ y were synthesized with solid state reactions. The luminescence properties of Bi 3+ and Dy 3+ in Y 3Al 5O 12(YAG) and the energy transfer from Bi 3+ to Dy 3+ were investigated in detail. Bi 3+ in YAG emits one broad band peaking at 304 nm which can be ascribed to the transition from excited states 3P 0, 1 to ground state 1S 0. Dy 3+ in YAG emits two groups of peaks around 484 and 583 nm, respectively, which can be ascribed to the transitions from excited state 4F 9/2 to ground states 6H 15/2 and 6H 13/2. The co-doping of Bi 3+ enhances the luminescent intensity of Dy 3+ by ∼7 times because Bi 3+ can transfer the absorbed energy to Dy 3+ efficiently. The mechanism of energy transfer was also discussed.

MCD study on Ce@C 82 and Ce 2@C 80 in the soft-X-ray region

We have measured the temperature dependence of MCD spectra on Ce@C 82 and Ce 2@C 80 in the Ce 3d–4f excitation region. The results show that Ce@C 82 has antiferromagnetic interaction while Ce 2@C 80 has ferromagnetic interaction between Ce magnetic moments which are much smaller than the Hund-ground-state values.

Influence of External Electric Field on Optical Properties of Dy3+ Embedded in Chalcogenide Glass Modified with Ga and CsBr

In sulfur-based chalcogenide glass, i.e., a covalent amorphous solid, the addition of a small amount of Ga + CsBr switches the nearest neighbors of the Dy3+ dopant from S to Br, which results in significant enhancement in the lifetime of the excited 4f configurational states of Dy3+. The chemical preference between [GaS3Br]- and Dy3+ alters only the local structures of the involved rare earth ion. This atomic restructuring occurs spontaneously at a nanoscale during the melt-quenching process, without the need for further heat treatment, and thus improves the luminescence properties of the activator while keeping the thermal and mechanical properties of the parent host material unchanged. In this study, it is experimentally verified that externally applied DC electric fields can further tune the oscillator strengths of Dy3+ in Ge-S glass that contains Ga + CsBr. This experimental finding proves that mass transports driven by electric fields influence the optical properties of rare earth doped in the chalcogenide glass that is compositionally engineered in this study.

Host Sensitized White Luminescence of Dy3 + Activated GdPO4 Phosphors

White light phosphors have been synthesized by solid-state reaction. The phosphors are characterized by X-ray diffraction patterns, room temperature photoluminescence excitation spectra, and room temperature photoluminescence (PL) emission spectra. Under host sensitization and direct 4f–4f excitation, the activator exhibits bright white light. The dependence of PL spectra on the activator doping concentration (from ) and calcining temperature (800, 900, 1000, 1100, and ) are also studied. The study suggests that the phosphor annealed at for with host sensitization has the strongest white light. The calculated chromaticity coordinates with radiation of all the phosphors fall well in the white region of 1931 Commission International de I’Eclairage diagram. These experimental results indicate that the activated is a promising white light material and has potential applications in lighting technology.

Improved RCI techniques for atomic 4f excitation energies: application to Sm I 4f6s2 5DJ levels

We complete the development of a relativistic energy-dependent efficient method by which important pair-correlation effects associated with open subshells can be incorporated into the relativistic configuration interaction (RCI) methodology. We apply this to predict the positions of the 4f6s2 5DJ levels of Sm I. Relative to 5D1, we predict 5D0 lies at −1613 cm−1 and 5D4 at 6589 cm−1. For 5D2 and 5D3, we are 22 cm−1 and 123 cm−1 below the observed difference, respectively. We also calculate magnetic dipole transition rates among these levels and the ground-state 7FJ levels, which may be of interest to future parity nonconservation studies.

Lattice energies and crystal-field parameters of lanthanide monosulphides

Madelung ( U M ) and experimental lattice ( U P ) energies have been determined for the series of lanthanide monosulphides (LnS, Ln=Ce—Lu, excluding Pm). Comparative crystal-field (CF) analysis has been performed for the same series in the point-charge model with inclusion of the ground 4f n and the excited 4f n–1 5d configurations. It has been found that the variations of both U M / U P and the CF parameters for the 4f n–1 5d configuration across the lanthanide series exhibit minima for the magnetic semiconductors SmS, EuS, and YbS.

Preparation and spectroscopic studies of the Mg xSr 1−xAl 2O 4:Eu, Dy ( x = 0.05–0.25) persistent phosphors

Mg x Sr 1− x Al 2O 4:Eu, Dy ( x = 0.05–0.25) phosphors were prepared by the solid-state reaction method; their excitation and emission spectra corresponding to the transitions between the ground 4f 7 and excited 4f 65d 1 configurations of Eu 2+ ion were studied. Broad band UV excited luminescence attributed to Eu 2+ ions was observed around 19,500 cm −1 (513 nm). From the comparison between the excitation and emission spectra, the crystal field splitting of the Eu 2+ 5d states and the parameters of electron–vibrational interaction, such as the Huang–Rhys factor, effective phonon energy, and zero-phonon line position for the studied compound were estimated for the first time.

Luminescence properties and electronic structure of Sm3+-doped YAl3B4O12

The luminescence properties of Sm3+ ions in YAl3B4O12 were studied upon synchrotron excitation in the 3.8–11 eV region. In addition to the 4f → 4f excitation bands, the excitation spectra of the Sm3+ emission contain broad bands at 6.1 and ~7.0 eV. These bands are attributed to charge transfer transition in Sm3+–O2− complexes and 4f → 5d transition of Sm3+ ions, respectively. The optical absorption edge of YAl3B4O12 was determined at 7.3 eV. A comparison with the results of electronic structure calculations on YAl3B4O12 is also made.

Extreme ultraviolet inner shell photoabsorption of iodine and gold ions

Photoabsorption spectra of iodine and gold plasmas were recorded using the dual laser produced plasma technique. In the case of iodine, the spectra were dominated by 4d excitation and contributions from stages up to four times ionized were observed. For gold, the spectra obtained could be assigned to 5p and 4f excitation in Au+ to Au4+. The main spectral features were identified by comparison with atomic structure calculations.

A systematic study of photoionization of free lanthanide atoms in the 4d giant resonance region

Charge-separated photoion-yield spectroscopy using monochromatized synchrotron radiation was conducted on free Ce, Nd, Gd, Dy, and Er lanthanide atoms in the 4d giant resonance region to measure the relative partial cross-section curves σ(M n+ ) for the formation of M n+ ( n = 1–4) ions. In addition, photoelectron spectroscopy was conducted on free Nd, Dy, and Er atoms to measure the cross-sections σ( nl) for photoionization of the nl (=4d, 5s, 5p and 4f) subshell electron. Drawing upon the obtained spectra and the results of previous photoion-yield studies on Xe, Cs, Ba, Sm, Eu, and Yb atoms, we discuss the character of photoionization processes in the 4d region. The obtained charge-state distribution shows that as the atomic number Z increases, the 4d giant resonance changes gradually, rather than suddenly, from a shape resonance to an autoionization resonance. One possible explanation for this change is that it is due to the dependence of the potential for the excited 4f electron and the corresponding 4f radial wavefunction on terms in the 4d 94f n+1 configuration. For σ(M +) and σ(4f), which exhibit an asymmetric Beutler–Fano autoionization profile, a curve fitting analysis was conducted to obtain the profile parameters E r, Γ, and q. It was found that for the Gadolinium atom, which has a configuration [Xe]4f 75d6s 2 characterized by a half-filled 4f 7 subshell plus a 5d electron, both the asymmetry parameter q and the charge-state distribution varied peculiarly with the atomic number Z.

Database for inelastic collisions of sodium atoms with electrons, protons, and multiply charged ions

The available experimental and theoretical cross section data for inelastic collision processes of ground (3s) and excited (3p, 4s, 3d, 4p, 5s, 4d, and 4f) state Na atoms with electrons, protons, and multiply charged ions have been collected and critically assessed. In addition to existing data, electron-impact cross sections, for both excitation and ionization, have been calculated using the convergent close-coupling approach. In the case of proton-impact cross section, the database was enlarged by new atomic-orbital close-coupling calculations. Both electron-impact and proton-impact processes include excitation from the ground state and between excited states ( n = 3–5). For electron-impact, ionization from all states is also considered. In the case of proton-impact electron loss, cross sections (the sum of ionization and single-electron charge transfer) are given. Well-established analytical formulae used to fit cross sections, published by Wutte et al. and Schweinzer et al. for collisions with lithium atoms, were adapted to sodium. The “recommended cross sections” for the processes considered have been critically evaluated and fitted using the adapted analytical formulae. For each inelastic process the fit parameters determined are tabulated. We also present the assessed data in graphical form. The criteria for comprehensively evaluating the accuracy of the experimental data, theoretical calculations, and procedures used in determining the recommended cross sections are discussed.

Atomic near-degeneracy for photoemission: Generality of 4f excitations

In a previous study of the 3s X-ray photoelectron spectra, XPS, of Mn, we identified a new intra-atomic many-body effect that lead to an ∼50% increase in the predicted exchange splitting of the main high spin and low spin XPS peaks. The new many-body effect involved the promotion of one electron from the M shell, 3s, 3p, and 3d, into a 4f orbital and a redistribution of the remaining electrons over the M shell orbitals; of particular importance were frustrated Auger configurations. FACs where the 3s shell was filled. In the present work, we demonstrate the general importance of these 4f FACs by showing that they are of comparable importance for increasing the 3s exchange splitting in Ni as they were in Mn.

Eu valence in ultra-thin layers of EuF 3 derived from photon energy dependent photoemission and photoabsorption

X-ray absorption (XAS) and resonant photoemission (RESPE) have been used to study Eu valence in ultra-thin EuF 3 layers grown by MBE. It was shown that resonant photoemission (RESPE) from EuF 3 ultra-thin layers exhibits different features for photon energies close to the 4d–4f threshold (130–150 eV) and the 3d–4f excitation region (1120–1170 eV). For the low energy resonance a clear effect of resonance induced Eu 2+ states (final state 4f 6) has been found whereas no divalent Eu states have been observed when photon energy was tuned to the 3d–4f transition. Our results indicate that Eu valence derived from the XAS and RESPE spectra depends on photon energy and is dominated by the final state effects for the 4d–4f resonance. An explanation is given based on the screening effect and different decay channels of the 4d–4f and 3d–4f resonances.

Enhanced Red Emission in Lithium Doped CaWO<sub>4</sub>:Eu<sup>3+</sup> and CaMoO<sub>4</sub>:Eu<sup>3+</sup> Phosphor

The lithium ions doped red-emitting phosphors of (Ca,Eu)WO4 and (Ca,Eu)MoO4 were synthesized by solid-state reaction method, and their luminescent properties were investigated. The XRD patterns show that the phosphors are isostructural and share a tetragonal scheelite structure, even some lithium ions are doped. The excitation spectra consist of broad charge transfer bands in the short-wave UV region of W6+→O2- and Eu3+→O2- with three sharp lines around 395, 465 and 535 nm of the Eu3+ 4f excitation transitions. The characteristic emissions of WO4 2- and MoO4 2- are quenched absolutely and red light emission of Eu3+ exhibits predominate peak around 615 nm due to the electric dipole energy transition of 5D0→7F2. Excited with 395 nm, the evident enhanced emission of the phosphors were observed when Li+ doped in the host, which implies that the addition of some Li+ is beneficial for the energy transfer from WO4 2- or MoO4 2- to Eu3+.

Spectroscopic properties and intense red-light emission of (Ca, Eu, M)WO 4 (M = Mg, Zn, Li)

The red-emitting phosphors of (Ca, Eu, M)WO 4 (M = Mg, Zn, Li) were prepared through solid-state reactions, and their spectroscopic properties were studied. After the addition of a small amount of Mg 2+, Zn 2+ or Li + in (Ca, Eu)WO 4, the red-light emission intensity of Eu 3+ increases obviously. In the luminescence spectra of the phosphors, the predominant transition emission is 5D 0 → 7F 2 (616 nm), whereas the other emissions are very weak. The excitation spectra are composed of interweaved ligand-to-metal charge-transfer bands (CTB) of W 6+–O 2− and Eu 3+–O 2−, and a few 4f excitation transitions of Eu 3+. Among the 4f excitation transitions of Eu 3+, there are three strong excitation lines corresponding to 7F 0 → 5L 6, 7F 0 → 5D 2 and 7F 0 → 5D 1 transitions , whose relative excitation intensity ratio is seriously affected when Li + doped in the host. The new phosphors may be applied as red-emitting phosphors for white light emitting diodes.

Spectroscopic properties of Dy 3+ ions in NaF–B 2O 3–Al 2O 3 glasses

This paper reports the optical properties of Dy 3+ in sodium fluoroborate glasses of the type XNaF·(89 − X)B 2O 3·10 Al 2O 3·1Dy 2O 3 (where X = 8, 12, 16, 20 and 24). Judd–Ofelt intensity parameters ( Ω 2, Ω 4, Ω 6) are derived from the absorption spectra. The Judd–Ofelt theory has been applied to interpret the local environment of Dy 3+ ions and bond covalency of RE–O bond. These parameters have been used to calculate radiative transition probabilities ( A rad), lifetimes ( τ R) and branching ratios ( β R) for the excited level 4F 9/2. The predicted values of τ R are compared with the measured values for 4F 9/2 level for five glass compositions (Glass (A–E)). The stimulated emission cross-section σ( λ P) are also evaluated for the 4F 9/2 → 6H J ( J = 11/2, 13/2, and 15/2) transitions.

Theoretical studies of electron-vibrational 4f N–4f N−1 5d spectra in LiYF 4:RE 3+ crystals

The UV and VUV inter-configuration 4f N –4f N−1 5d spectra of rare-earth ions in insulators consist of broad electron-vibrational bands which sometimes have a resolved fine structure. In the present work, the low-temperature absorption band shapes of the impurity Ce 3+, Pr 3+ and Nd 3+ ions in LiYF 4 crystals have been simulated in the framework of the microscopic theory operating with the real phonon spectrum of the host crystal lattice. The energy levels and wave functions of the ground (4f N ) and excited (4f N−1 5d) electronic configurations of rare-earth ions were obtained from the numerical diagonalization of the Hamiltonian containing energies of electrostatic Coulomb and exchange interactions between electrons, spin–orbit interactions and the crystal field interaction. Crystal field parameters and electron–phonon coupling constants were treated in the framework of the exchange charge model. Form-functions of the spectral bands proportional to the calculated integral intensities of electric dipole transitions are obtained as a sum of convolutions of spectral densities of multiphonon correlation functions and form-functions of zero-phonon lines with the widths determined by distributions of random lattice strains and non-radiative transition probabilities. The calculated values of the Huang–Rhys parameters of crystal field states in mixed 4f N−1 5d configurations vary in the range from 0.1 to 15 and correspond to intermediate or strong electron–phonon interactions. Results of simulations of the spectral envelopes agree satisfactorily with the experimental data available from the literature.

Field isotope shifts in the 6s-6p and 4f-5d transitions from the 4f^6 6s^2 ^7F_J ground multiplet of Sm I

Multiconfiguration Dirac-Fock (MCDF) calculations are used to interpret the field shift contributions to isotope shifts of Sm I in visible and near-UV transition lines. The investigated lines correspond to the transitions between the ground configuration 4f^6 6s^2 and the excited configurations 4f^6 6s6p and 4f^5 5d6s^2. For the 6s-6p transition the field shift is negative as a result of the reduced 6s electron occupation, whereas for the 4f-5d transition the field shift is positive because of the decreased screening of 6s and inner-shell electrons. MCDF calculations for the electron charge density at the origin that include the configuration mixing effect agree well with the electronic factor ratios derived from experimental data with the exception of a few cases of 6s-6p transitions.