Charge transfer from Eu2+ to trivalent lanthanide co-dopants: Systematic behavior across the series.

Electron transfer processes between lanthanide activators are crucial for the functional behavior and performance of luminescent materials. Here, a multiconfigurational ab initio study reveals how direct metal-to-metal charge transfer (MMCT) between the Eu2+ luminescence activator and a Ln3+ co-dopant (Ln3+ = Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+) systematically dictates the luminescence and optical properties of CaF2. The combination of the structures and energies of the electronic manifolds, the vibrational force constants, and the structural properties of the donor and acceptor in the host determines the predictions of five different behaviors of CaF2:Eu2+, Ln3+ co-doped materials after MMCT absorption: formation of stable traps, MMCT emission, emission quenching, Ln3+ emission, and Eu2+ emission.

Broadband infrared LEDs based on europium-to-terbium charge transfer luminescence.

Efficient broadband infrared (IR) light-emitting diodes (LEDs) are needed for emerging applications that exploit near-IR spectroscopy, ranging from hand-held electronics to medicine. Here we report broadband IR luminescence, cooperatively originating from Eu2+ and Tb3+ dopants in CaS. This peculiar emission overlaps with the red Eu2+ emission, ranges up to 1200 nm (full-width-at-half-maximum of 195 nm) and is efficiently excited with visible light. Experimental evidence for metal-to-metal charge transfer (MMCT) luminescence is collected, comprising data from luminescence spectroscopy, microscopy and X-ray spectroscopy. State-of-the-art multiconfigurational ab initio calculations attribute the IR emission to the radiative decay of a metastable MMCT state of a Eu2+-Tb3+ pair. The calculations explain why no MMCT emission is found in the similar compound SrS:Eu,Tb and are used to anticipate how to fine-tune the characteristics of the MMCT luminescence. Finally, a near-IR LED for versatile spectroscopic use is manufactured based on the MMCT emission.

Fine-Tuning the Cr3+ R1-Line by Controlling Pauli Antisymmetry Strength.

Fine-tuning of the Cr3+ zero-phonon R1-line fluorescence (2E-4A2) has recently been achieved by varying the composition of Cr-doped solid solution garnets. Chemical substitutions of Y3+ with Lu3+ and Tb3+ produce R1-line shifts and multiplets that cannot be associated with direct or indirect changes in the Cr-O covalency alone and raise important questions on the nature of the interactions between the CrO6 active center and its environment, hence on the impact of local disorder. In this Letter we show that Pauli antisymmetry interactions with next-nearest neighbor ions, which are commonly omitted from theoretical analyses, induce a nephelauxetic effect on the CrO6 active center that shifts the R1-line. The strength of these interactions increases with the ionic radius and number of codopant ions, all of it resulting in controlled shifts and multiplets of R1-lines. The Pauli antisymmetry induced nephelauxetic effect, combined with the R1-line shift with lattice constant, allows for a very fine-tuning of the Cr3+ R1-line.

Direct Evidence of Intervalence Charge-Transfer States of Eu-Doped Luminescent Materials.

Direct evidence is given for the existence of intervalence charge transfer (IVCT) states of Eu2+/Eu3+ pairs in Eu-doped CaF2, SrF2, and BaF2. They are detected in diffuse reflectance spectra. In doped materials, IVCT states, in which an electron transfer occurs between two metal sites differing only in oxidation state, are rather difficult to observe because the absorption bands are extremely broad and flat, their intensity is low, and no emission follows the IVCT absorptions. Their assignment as IVCT states is provided by state-of-the-art multiconfigurational ab initio calculations. Although IVCT states of lanthanide-doped materials have largely been overlooked so far, they can cause luminescence quenching and even complete luminescence excitation loss. Their direct observation and independent assignment in classical dopant (Eu) and hosts (CaF2, SrF2, BaF2) are very significant: They suggest that the occurrence of IVCT states in other lanthanide-activated materials is very likely overlooked and their impact is ignored.

Color Control of Pr3+ Luminescence by Electron-Hole Recombination Energy Transfer in CaTiO3 and CaZrO3.

Controlling luminescence in phosphors able to produce several emissions from different stable excited states determines their use in optical devices. We investigate the color control mechanism that quenches the greenish-blue emission in favor of the red one in the archetype phosphor CaTiO3:Pr3+. State-of-the-art ab initio calculations indicate that direct host-to-dopant energy transfer (released by electron-hole recombination following the interband excitation and structural reorganization) selectively populates the 1D2 red luminescent state of Pr3+ and bypasses the 3P0 greenish-blue emitter. Local defects can modulate the electron-hole recombination energy and therefore increase the red emission efficiency, as experimentally observed. The selection of red emission does not happen in CaZrO3:Pr3+ because the electron-hole recombination energy is much higher. The calculations could not support the widely accepted color control mechanism based on metal-to-metal charge transfer states. The conclusion sets new points of view for the color control of lanthanide activated inorganic phosphors.

X-ray Excitation Triggers Ytterbium Anomalous Emission in CaF2:Yb but Not in SrF2:Yb.

Materials that luminesce after excitation with ionizing radiation are extensively applied in physics, medicine, security, and industry. Lanthanide dopants are known to trigger crystal scintillation through their fast d-f emissions; the same is true for other important applications as lasers or phosphors for lighting. However, this ability can be seriously compromised by unwanted anomalous emissions often found with the most common lanthanide activators. We report high-resolution X-ray-excited optical (IR to UV) luminescence spectra of CaF2:Yb and SrF2:Yb samples excited at 8949 eV and 80 K. Ionizing radiation excites the known anomalous emission of ytterbium in the CaF2 host but not in the SrF2 host. Wave function-based ab initio calculations of host-to-dopant electron transfer and Yb2+/Yb3+ intervalence charge transfer explain the difference. The model also explains the lack of anomalous emission in Yb-doped SrF2 excited by VUV radiation.

New Insights in 4f(12)5d(1) Excited States of Tm(2+) through Excited State Excitation Spectroscopy.

Optical excitation of ions or molecules typically leads to an expansion of the equilibrium bond lengths in the excited electronic state. However, for 4f(n-1)5d(1) excited states in lanthanide ions both expansion and contraction relative to the 4f(n) ground state have been reported, depending on the crystal field and nature of the 5d state. To probe the equilibrium distance offset between different 4f(n-1)5d(1) excited states, we report excited state excitation (ESE) spectra for Tm(2+) doped in CsCaBr3 and CsCaCl3 using two-color excited state excitation spectroscopy. The ESE spectra reveal sharp lines at low energies, confirming a similar distance offset for 4f(n-1)5d(t2g)(1) states. At higher energies, broader bands are observed, which indicate the presence of excited states with a different offset. On the basis of ab initio embedded-cluster calculations, the broad bands are assigned to two-photon d-d absorption from the excited state. In this work, we demonstrate that ESE is a powerful spectroscopic tool, giving access to information which cannot be obtained through regular one-photon spectroscopy.

Metal-to-metal charge transfer between dopant and host ions: Photoconductivity of Yb-doped CaF2 and SrF2 crystals.

Dopant-to-host electron transfer is calculated using ab initio wavefunction-based embedded cluster methods for Yb/Ca pairs in CaF2 and Yb/Sr pairs in SrF2 crystals to investigate the mechanism of photoconductivity. The results show that, in these crystals, dopant-to-host electron transfer is a two-photon process mediated by the 4f(N-1)5d excited states of Y b(2+): these are reached by the first photon excitation; then, they absorb the second photon, which provokes the Y b(2+) + Ca(2+) (Sr(2+)) → Y b(3+) + Ca(+) (Sr(+)) electron phototransfer. This mechanism applies to all the observed Y b(2+) 4f-5d absorption bands with the exception of the first one: Electron transfer cannot occur at the first band wavelengths in CaF2:Y b(2+) because the Y b(3+)-Ca(+) states are not reached by the two-photon absorption. In contrast, Yb-to-host electron transfer is possible in SrF2:Y b(2+) at the wavelengths of the first 4f-5d absorption band, but the mechanism is different from that described above: first, the two-photon excitation process occurs within the Y b(2+) active center, then, non-radiative Yb-to-Sr electron transfer can occur. All of these features allow to interpret consistently available photoconductivity experiments in these materials, including the modulation of the photoconductivity by the absorption spectrum, the differences in photoconductivity thresholds observed in both hosts, and the peculiar photosensitivity observed in the SrF2 host, associated with the lowest 4f-5d band.

Configuration coordinate energy level diagrams of intervalence and metal-to-metal charge transfer states of dopant pairs in solids.

Configuration coordinate diagrams, which are normally used in a qualitative manner for the energy levels of active centers in phosphors, are quantitatively obtained here for intervalence charge transfer (IVCT) states of mixed valence pairs and metal-to-metal charge transfer (MMCT) states of heteronuclear pairs, in solid hosts. The procedure relies on vibrational frequencies and excitation energies of single-ion active centers, and on differences between ion-ligand distances of the donor and the acceptor, which are attainable empirically or through ab initio calculations. The configuration coordinate diagrams of the Yb(2+)/Yb(3+) mixed-valence pair in Yb-doped YAG and the Ce(3+)/Yb(3+) heteronuclear pair in Ce,Yb-codoped YAG, are obtained and described. They are drawn from empirical data of the single-ions and their usefulness is discussed. The first diagram suggests that IVCT states of Yb(2+)/Yb(3+) pairs may play an important role in the quenching of the Yb(3+) emission and it provides the details of the quenching mechanism. The second diagram supports the interpretation recently given for the energy transfer from Ce(3+) to Yb(3+) in Ce,Yb-codoped YAG via a MMCT Ce(4+)-Yb(2+) state and it provides the details. The analyses of the two diagrams suggest the formation of Yb(2+)/Yb(3+) pairs after the Ce(3+)-to-Yb(3+) MMCT, which is responsible for the temperature quenching of the Yb(3+) emission excited via Ce(3+) (4f → 5d) absorption in Ce,Yb-codoped YAG.

Intervalence charge transfer luminescence: interplay between anomalous and 5d - 4f emissions in Yb-doped fluorite-type crystals.

In this paper, we report the existence of intervalence charge transfer (IVCT) luminescence in Yb-doped fluorite-type crystals associated with Yb(2+)-Yb(3+) mixed valence pairs. By means of embedded cluster, wave function theory ab initio calculations, we show that the widely studied, very broad band, anomalous emission of Yb(2+)-doped CaF2 and SrF2, usually associated with impurity-trapped excitons, is, rather, an IVCT luminescence associated with Yb(2+)-Yb(3+) mixed valence pairs. The IVCT luminescence is very efficiently excited by a two-photon upconversion mechanism where each photon provokes the same strong 4f(14)-1A1g→ 4f(13)((2)F7/2)5deg-1T1u absorption in the Yb(2+) part of the pair: the first one, from the pair ground state; the second one, from an excited state of the pair whose Yb(3+) moiety is in the higher 4f(13)((2)F5/2) multiplet. The Yb(2+)-Yb(3+) → Yb(3+)-Yb(2+) IVCT emission consists of an Yb(2+) 5deg → Yb(3+) 4f7/2 charge transfer accompanied by a 4f7/2 → 4f5/2 deexcitation within the Yb(2+) 4f(13) subshell: [(2)F5/25deg,(2)F7/2] → [(2)F7/2,4f(14)]. The IVCT vertical transition leaves the oxidized and reduced moieties of the pair after electron transfer very far from their equilibrium structures; this explains the unexpectedly large band width of the emission band and its low peak energy, because the large reorganization energies are subtracted from the normal emission. The IVCT energy diagrams resulting from the quantum mechanical calculations explain the different luminescent properties of Yb-doped CaF2, SrF2, BaF2, and SrCl2: the presence of IVCT luminescence in Yb-doped CaF2 and SrF2; its coexistence with regular 5d-4f emission in SrF2; its absence in BaF2 and SrCl2; the quenching of all emissions in BaF2; and the presence of additional 5d-4f emissions in SrCl2 which are absent in SrF2. They also allow to interpret and reproduce recent experiments on transient photoluminescence enhancement in Yb(2+)-doped CaF2 and SrF2, the appearance of Yb(2+) 4f-5d absorption bands in the excitation spectra of the IR Yb(3+) emission in partly reduced CaF2:Yb(3+) samples, and to identify the broadband observed in the excitation spectrum of the so far called anomalous emission of SrF2:Yb(2+) as an IVCT absorption, which corresponds to an Yb(2+) 4f5/2 → Yb(3+) 4f7/2 electron transfer.

Intervalence charge transfer luminescence: the anomalous luminescence of cerium-doped Cs2LiLuCl6 elpasolite.

The existence of intervalence charge transfer (IVCT) luminescence is reported. It is shown that the so called anomalous luminescence of Ce-doped elpasolite Cs2LiLuCl6, which is characterized mainly by a very large Stokes shift and a very large band width, corresponds to an IVCT emission that takes place in Ce(3+)-Ce(4+) pairs, from the 5de(g) orbital of Ce(3+) to 4f orbitals of Ce(4+). Its Stokes shift is the sum of the large reorganization energies of the Ce(4+) and Ce(3+) centers formed after the fixed-nuclei electron transfer and it is equal to the energy of the IVCT absorption commonly found in mixed-valence compounds, which is predicted to exist in this material and to be slightly larger than 10,000 cm(-1). The large band width is the consequence of the large offset between the minima of the Ce(3+)-Ce(4+) and Ce(4+)-Ce(3+) pairs along the electron transfer reaction coordinate. This offset is approximately 2√3 times the difference of Ce-Cl equilibrium distances in the Ce(3+) and Ce(4+) centers. It is shown that the energies of the peaks and the widths of IVCT absorption and emission bands can be calculated ab initio with reasonable accuracy from diabatic energy surfaces of the ground and excited states and that these can be obtained, in turn, from independent calculations on the donor and acceptor active centers. We obtained the energies of the Ce(3+) and Ce(4+) active centers of Ce-doped Cs2LiLuCl6 by means of state-of-the-art wave-function-theory spin-orbit coupling relativistic calculations on the donor cluster (CeCl6Li6Cs8)(11+) and the acceptor cluster (CeCl6Li6Cs8)(12+) embedded in a quantum mechanical embedding potential of the host. The calculations provide interpretations of unexplained experimental observations as due to higher energy IVCT absorptions, and allow to reinterpret others. The existence of another IVCT emission of lower energy, at around 14,000-16,000 cm(-1) less than the 5dt(2g) emission, is also predicted.

Blue absorption and red emission of Bi2+ in solids: strongly spin-orbit coupled 6p levels in low symmetry fields.

Wave function embedded cluster ab initio calculations on a (BiO8)(14-) cluster under the effects of a high symmetry Oh confinement potential are used to study the energies of the (2)P1/2, (2)P3/2(1), and (2)P3/2(2) spin-orbit coupling levels of the 6s(2)6p configuration of Bi(2+) in Oh, D4h, D2h, D4, D2d, D2, S4, C4v, C4, C3v, C2v, C2, Cs, and C1 fields, together with the (2)P1/2→(2)P3/2(1) and (2)P1/2→(2)P3/2(2) absorption oscillator strengths and the (2)P3/2(1) radiative lifetime. These levels are responsible for the blue absorptions and the red-orange emissions produced when Bi(2+) is doped in borates, phosphates, sulphates, and other hosts. It is found that the splitting of (2)P3/2 is mainly due to the tetragonal D4h and orthorhombic D2h components of the actual field. It is enhanced by Bi going towards two or four ligands. The intensities of the (2)P1/2→(2)P3/2(1) and (2)P1/2→(2)P3/2(2) absorptions are mostly induced by the Bi displacements and by tetragonal scalenoidal D2d fields. The most favorable fields for a large splitting of the (2)P3/2 level that can drive a red shift of the (2)P3/2(1) →(2)P1/2 emission are the C2v and Cs fields resulting from the combination of D2h orthorhombic fields and Bi approaching two or four ligands on the main orthorhombic planes.

Large splittings of the 4f shell of Ce3+ in garnets.

Ab initio embedded cluster calculations on Ce(3+)-doped Y3Al5O12, Lu3Al5O12, Gd3Al5O12, Y3Ga5O12, Lu3Ga5O12, and Gd3Ga5O12, which do not make use of any adjustable parameter, support recent assignments of the seventh 4f level of Ce(3+) in garnets [Przybylinska et al., Appl. Phys. Lett., 2013, 102, 241112] and that the splitting of the 4f shell of Ce(3+) in these materials is slightly smaller than 4000 cm(-1) and much larger than the 2000-2500 cm(-1) commonly assumed in analyses of 5d → 4f emission bands. Why this wrong assumption has been working well so far is explained by the fact that the intensity of the emission to the seventh level of the 4f(1) configuration is found to be only one hundredth of the integrated intensity of the emissions to the other six levels, which group themselves into two sets of three individual levels separated by 2000-2500 cm(-1). The effective field splitting and the spin-orbit coupling splitting are found to be of the same size. From a strong field coupling point of view, the first six levels result from the interactions between (2)T(2u) and (2)T(1u) cubic levels and the higher, isolated seventh level comes directly from the cubic (2)A(2u). From a weak field coupling point of view, the first three levels result from the splitting of (2)F(5/2), the second three levels from the splitting of (2)F(7/2) and the seventh level from a strong, cubic field driven interaction between (2)F(7/2) and (2)F(5/2) components [Herrmann et al., J. Appl. Phys., 1966, 37, 1312].

Ab initio theoretical study on the 4f(2) and 4f5d electronic manifolds of cubic defects in CaF2:Pr3+.

Wave function-based embedded cluster ab initio calculations have been carried out in order to study the 4f(2) and 4f5d energy levels of the cubic substitutional defect of Pr-doped CaF2. The 4f(2) → 4f5d absorption spectrum and 4f5d → 4f(2) emission spectrum have been calculated. The 4f(2 1)S0 level is found to be immersed in the 4f5d(eg) manifold, so that no quantum cutting from (1)S0 can occur and only strong 4f5d(eg) → 4f(2) emission is predicted, which supports previous assumptions made in order to explain results in CaF2:Pr(3+). The details of the 4f(2) → 4f5d(eg) and 4f(2) → 4f5d(t2g) bands of the absorption spectrum are interpreted and assignments are made. The lowest level of the 4f5d(eg) configuration is found to have 80% of singlet character, in opposition to Hund's Rules, and the issue is discussed in detail. The comparison between the experimental 4f5d(eg) → 4f(2) high resolution emission spectrum of the cubic site of CaF2:Pr(3+) and the calculated emission spectra from the two lowest 4f5d(eg) states 1T2u((1)T2u) and 1Eu(1(3)T2u) suggests the possibility that the experimental emission of the cubic Pr defect of CaF2:Pr(3+) is in fact a multiple emission.

Host effects on the optically active 4f and 5d levels of Ce3+ in garnets.

When Ce(3+) is doped in a garnet, it substitutes for some cations. The effect of the substitution on the optically active 4f and 5d levels of Ce(3+) can be analyzed in terms of an undistorted substitution followed by a structural relaxation, but, whereas the contribution of the undistorted substitution can be predicted/calculated using the crystallographic structure of the pure garnet, which is at hand, that of the structural relaxation demands the detailed local structure around the Ce(3+) impurity, which is hard to know. Hence the importance of knowing the role of the undistorted substitution. What we study in this paper is the role of the unrelaxed host effects on the 4f and 5d levels of Ce(3+)-doped garnets (i.e. the interactions between Ce and the second and more distant neighbors). When they are added to the (previously studied) unrelaxed first-neighbor effects, they give the contributions of the undistorted substitutions. We performed spin-orbit coupling, relativistic, embedded cluster, wave function based ab initio calculations on the (CeO8)(13-) cluster under the effects of the embedding potentials of 21 selected garnets of Si, Al, and Ga, which have been obtained in this work, using experimental unrelaxed structures of the pure garnets. The calculations reveal that the unrelaxed host effect: (1) produces a red shift of the first 4f → 5d transition and (2) is an important contribution to this transition which plays a very important role in differentiating the values it has in different garnet families. The unrelaxed host effect is found to be responsible for the low value of the first 4f → 5d transition in Ce(3+)-doped Lu2CaMg2Si3O12. It is analyzed in terms of 5d and 4f centroid energy contributions and crystal field splitting contributions in the 21 doped garnets. The effect on the splittings of the 4f and 5d levels is also discussed. The undistorted host approximation is found to give reasonable comparison with experiments, so that it represents a relatively fast way to provide reliable ab initio information on the 4f and 5d levels of Ce(3+) in garnets.

Effective Hamiltonian parameters for ab initio energy-level calculations of SrCl2:Yb2+ and CsCaBr3:Yb2+.

Calculated energy levels from recent ab initio studies of the electronic structure of SrCl2:Yb(2+) and CsCaBr3:Yb(2+) are fitted with a semi-empirical 'crystal-field' Hamiltonian, which acts within the model space 4f(14) + 4f(13)5d + 4f(13)6s. Parameters are obtained for the minima of the potential energy curves for each energy level and also for a range of anion-cation separations. The parameters are compared with published parameters fitted to experimental data and to atomic calculations. The states with significant 4f(13)6s character give a good approximation to the impurity-trapped exciton states that appear in the ab initio calculations.

Radial correlation effects on interconfigurational excitations at the end of the lanthanide series: a restricted active space second order perturbation study of Yb2+ and SrCl2:Yb2+.

At the end of the lanthanide series, 4f → 5d and other interconfigurational transitions, in which one electron is excited from a tight 4f orbital to a much more diffuse one, occur with a break of many f-f pairs, which make the electron correlation effects dominant. For instance, the large energy gap of 25 000 cm(-1) (∼29 500 cm(-1) without spin-orbit coupling) above the 4f(14) ground state of the SrCl2:Yb(2+) material is mostly due to electron correlation. In effect, a minimal multiconfigurational restricted active space (RASSCF) calculation that includes only the 4f(14) ground and 4f(13)5d and 4f(13)6s open-shell excited configurations gives a very small gap (5400 cm(-1)), whereas the correlation corrections to the 4f(14) → 4f(13)5d(eg) transition energies at the second order perturbation theory (RASPT2) level are very large: 35 599 ± 439 cm(-1), in average, for all excited states. These corrections are too large to be accurate at second order perturbation level. When a second f-shell is also included in the active space and single and double excitations to the 5d, 6s, and 5f shells are treated variationally, the (extended) RASSCF energy gap above the ground state and the electronic transitions increase by 22 038 ± 120 cm(-1) and the RASPT2 correlation energy corrections become small (-721 ± 571 cm(-1)), as it is desirable for a second order perturbation. A comparative analysis of both RASPT2 results reveals that the lack of the second f-shell accounts for 12 700 cm(-1) of the 14 223 ± 80 cm(-1) overestimation of interconfigurational transitions energies by the minimal RASPT2 calculation, which indicates an inaccurate calculation of the differential radial correlation between the 4f(14) and 4f(13)5d configurations by second order perturbation theory. In order to establish practical and accurate procedures for the calculation of 4f → 5d and other interconfigurational transitions at the end of the lanthanide series, the above and other RASSCF/RASPT2 calculations on the ionization potential of Yb(2+) in gas phase and in SrCl2 have been benchmarked in this paper against coupled cluster (coupled cluster singles and doubles and triples ) calculations, and RASSCF/RASPT2 calculations on the absorption spectrum of SrCl2:Yb(2+) have been compared with experiment. The results support that variational calculation of SD 4f → 5f excitations prior to RASPT2 calculations can be a realistic, accurate, and feasible choice to model radial correlation effects at the end of the lanthanide series.

Electronic spectra of Yb2+-doped SrCl2.

The absorption and emission spectra of Yb(2+)-doped SrCl(2) have been calculated on the basis of ab initio quantum chemical calculations which consider recently found, unexpected excited states with double-well energy curves and complex electronic structure, resulting from avoided crossings between Yb-trapped excitons and Yb impurity states, which influence prominent spectral features. The root mean square deviation and largest absolute error of the calculated energy levels are 394 and -826 cm(-1), respectively. The YbCl(8) moiety breathing mode vibrational frequencies and bond lengths of the lowest states are consistent with observed vibrational progressions and energy shifts induced by uniaxial compression. Photoionization is predicted above 49,000 cm(-1) as a consequence of the spin-orbit induced spreading of the Yb-trapped exciton character in the upper part of the spectrum and three new emission bands are predicted with origins at about 33,800, 36,400, and 43,600 cm(-1). The electron correlation methods used overestimate the relative stabilization of the 4f(14) ground state and this leads to a constant error of the whole absorption spectrum of about 3500 cm(-1) (23%-7%). Although this energy shift is customarily considered an adjustable parameter, it is a nonparametric, direct product in an ab initio route which shows the limitations on the proper representation of differential correlation between the 4f(N) and 4f(N-1)5d (or similar) configurations and the need for theoretical improvement.

Yb2+-doped SrCl2: electronic structure of impurity states and impurity-trapped excitons.

First-principles electronic structure calculations of the excited states of Yb(2+)-doped SrCl(2) crystals up to 65,000 cm(-1) reveal the existence of unexpected excited states with double-well potential energy surfaces and dual electronic structure lying above and very close in energy to the 4f(13)5d manifold, with which they interact strongly through spin-orbit coupling. The double-well energy curves result from avoided crossings between Yb-trapped exciton states (more stable at short Yb-Cl distances) and 4f(13)6s impurity states (more stable at long Yb-Cl distances); the former are found to be preionization states in which the impurity holds the excited electron in close lying empty interstitials located outside the YbCl(8) moiety. Spin-orbit coupling between the double-well states and the lower lying 4f(13)5d impurity states spreads the dual electronic structure character to lower energies and, hence, the instability of the divalent oxidation state is also spread. To some extent, the dual electronic structure (impurity-trapped exciton-impurity state) of some excited states expresses and gives support to hypotheses of interaction between Yb(2+) and Yb(3+) pairs proposed to understand the complex spectroscopy of the material and conciliates these hypotheses with interpretations in terms of the existence of only one type of Yb(2+) defect. The results presented confirm the presence of impurity states of the 4f(13)6s configuration among the 4f(13)5d manifolds, as proposed in literature, but their energies are very different from those assumed. The Yb-trapped excitons found in this chloride host can be seen as precursors of the luminescent Yb-trapped excitons characterized experimentally in the isomorphous SrF(2) crystals.

Energy gaps in the 4f(13)5d(1) manifold and multiple spontaneous emissions in Yb(2+)-doped CsCaBr(3).

Multiple spontaneous 4f(13)5d(1) --> 4f(14) emissions are predicted in Yb(2+)-doped CsCaBr(3) crystals by ab initio quantum chemical calculations. Four emission bands are found at 23,900, 26,600, 34,600, and 43,900 cm(-1) that should be experimentally observable at low temperatures. The first, third, and fourth bands are slow, electric dipole forbidden emissions that can be described as spin-forbidden. The second band is a fast, electric dipole-allowed emission that cannot be described as spin-allowed, but as spin-enabled; its radiative emission lifetime is 400 ns. Large energy gaps (23 900, 4600, 4000 cm(-1), respectively), relative to the maximum local phonon energies calculated (around 185 cm(-1)), are found below the emitting levels of the slow bands, which indicates that these states should be significantly stable and multiphonon relaxation to the lower states should be negligible. A smaller gap (2600 cm(-1)) separates the states of the fast band, which should result in a temperature dependent competition between radiative and nonradiative decay. Differential correlation between 4f-4f and 4f-5d pairs, splitting of the 5d shell by interactions with the host, and spin-orbit effects within the 4f(13) subshell, are found to be responsible for the existence of the gaps, which, in turn, split the absorption spectrum into four groups of separate bands, three of which could lie below the host absorption threshold. The quantum chemical methods employed make use of explicit wave functions expanded in terms of flexible basis sets, multiconfigurational self-consistent-field and multireference second-order perturbation methods to account for nondynamic and dynamic electron correlation, scalar and relativistic terms in the (YbBr(6))(4-) defect cluster Hamiltonian, and quantum mechanical embedding potentials to represent the host crystal.