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Abstract: The low-temperature single-crystal polarized absorption and the luminescence spectra of Cs2[CrCl2(H2O)4]Cl3 are theoretically analyzed in order to determine the electronic structure of the trans-[CrCl2(H2O)4]+ complex. This study, based on the Racah theory, leads to a good agreement between the theoretical and experimental energy levels. The electronic-exited state 4T2g(Oh) is split into 4Eg and 4B2g components due to the lowering of the symmetry from Oh to D4h. The polarized absorption spectrum and the theoretical energies show that the 4Eg(D4h) state is lower in energy than the 4B2g(D4h) state. The resolved vibronic structure in the luminescence spectrum reveals a progression in a non-totally symmetric b1g mode, a manifestation of a Jahn–Teller effect in the emitting state 4T2g(Oh). Both Jahn–Teller and spin–orbit coupling in the orbitally degenerated 4Eg(D4h) state are necessary to account for the spectroscopic observations. [Copyright &y& Elsevier]

Abstract: The experimental studies have provided evidence of the occurrence of transitions from the 3T1g(3F) ground state to the crystal-field levels 3T2g(3F), 3T1g(3P) and 3A2g(3F) for the V3+ centres in Al2O3 crystal; and from the 3A2g(3F) ground state to the crystal-field levels 3T2g(3F), 3T1g(3F) and 3T1g(3P) for the Ni2+ centres in LiNbO3 crystal (levels are assigned to irreps of the Oh point symmetry group). Using the experimental spectroscopic data, theoretical calculations of the crystal-field levels of V3+:Al2O3 and Ni2+:LiNbO3 are carried out based on the Racah theory. The observed crystalline-field splittings of the V3+ and Ni2+ terms were accounted for using a C3 symmetry Hamiltonian. The spin–orbit interaction was taken into account in this work. The Racah, crystal-field and spin–orbit parameters, which fit experimental and theoretical energy levels, have been reliably obtained. A good agreement between the theoretical and experimental results for the energy levels of V3+:Al2O3 and Ni2+:LiNbO3 has been obtained. [Copyright &y& Elsevier]

Abstract: The polarized absorption spectra of vanadium(III) in Cs3VCl6·4H2O complex are presented in the UV–vis region and correspond to the transition from the ground state 3T1g(3F) to the excited states 3T2g(3F), 3T1g(3P) and 3A2g(3F). Semiempirical calculations of the crystal-field levels of the vanadium(III) with D 4h point group symmetry in Cs3VCl6·4H2O chromophore are carried out, leading to a good agreement between the theoretical and experimental energy levels. [Copyright &y& Elsevier]

Abstract: The polarized absorption spectra of trans -NiCl2(H2O)4 complex were measured by Bussière et al. [Coord. Chem. Rev. 219–221 (2001) 509–543] at low-temperature. Using the experimental spectroscopic data, semiempirical calculations of the crystal-field levels of trans-NiCl2(H2O)4 chromophore are carried out, based on the Racah theory. We used idealized D4h point group symmetry to analyse the observed crystalline-field splitting of this chromophore. As a result, Racah and crystal-field parameters have been reliably obtained. A good agreement between the theoretical and experimental energy levels of trans-NiCl2(H2O)4 complex has been obtained. The region of 3T1g/1Eg(Oh) bands is of great interest and it is useful to use the tetragonal symmetry to understand the features of this spectral region. [Copyright &y& Elsevier]

The absorption and excitation spectra of the tetrahedrally coordinated Cr4+ ions doped in LiAlO2 oxide crystal were measured by Ku¨ck and Hartung at room temperature (Chem. Phys. 240 (1999) 387). These spectra have provided evidence of the occurrence of transitions from the 3A2(3F) ground state to the crystal-field split levels 3T2(3F), 3T1(3F) and 3T1(3P) (levels are assigned to the Td symmetry, i.e., ideal tetrahedron CrO4). The splitting of each band of the Td symmetry is due to the distortion of the ideal tetrahedron. Using the experimentally determined data from the absorption and excitation spectra, a theoretical calculation for the crystal-field levels of Cr4+:LiAlO2 was performed by Ku¨ck and Hartung, based on the angular overlap model theory. The calculation reveals a good agreement between the mean values of the bands, while it underestimates the splitting of each band. A detailed crystal-field analysis of electronic energy levels of Cr4+ doped in LiAlO2 oxide crystal based on the Racah theory is proposed in this work. The observed crystalline-field splitting of the Cr4+ terms was accounted for by using a C2 symmetric Hamiltonian. In turn, reliable crystal-field and Racah parameters have been obtained. This theoretical analysis confirms the observed crystalline-field splittings of the 3F and 3P terms of the Cr4+ ion doped in the oxide crystal LiAlO2. [Copyright &y& Elsevier]

The morphology, structural and optical properties of the Co 2+ ions doped in ZnO nanowires were investigated by S. Geburt et al. (2013) [1] . The cathodoluminescence CL spectrum is composed of the ZnO near-band-edge NBE (ultraviolet emission), green emission band attributed to the recombination of carriers at intrinsic ZnO defects and sharp red emission assigned to the Co 2+ intra-3d luminescence. The photoluminescence excitation PLE spectrum shows the direct transitions from the ground state of Co 2+ occupying T d site to the higher excited states. The visible region provides important information about the electronic structure of the Co 2+ doped zinc oxide nanowires. This work investigates an interpretation of the origin of the ultraviolet NBE and discusses the defects involved in the broad green emission band. A theoretical crystal-field analysis of the visible lines, associated to the Co 2+ ion transition occupying T d site symmetry in ZnO host crystal, is also realized which leads to a satisfactory correlation between experimental and calculated energy levels. The electronic structure was compared to the structure reported for Co 2+ ion doped in ZnO nanocrystalline (nanorods, nanoparticles) and bulk crystals. The quantum confinement effect of ZnO nanowires is interpreted. [ABSTRACT FROM AUTHOR]

The phenomenon of persistent luminescence in the phosphors Zn 2 SnO 4 doped by Cr3+ and codoped by Al3+ or Eu3+ is related to the energy levels of the doping transition ions Cr3+ and rare earth ions Eu3+, the concentration of Al3+ substituting Zn and Sn, the intrinsic defects and the energy bands of the Zn 2 SnO 4 matrix. To fully understand the procedure of persistent luminescence in these phosphors, we firstly determine the role of the Cr3+ ions in this process. For this reason, a crystal-field theory, using the Racah tensor algebra method, of the optical region associated to the transitions between the Stark levels of Cr3+ in these phosphors was performed. We examine then the influence by codoping with Al3+ and Eu3+ on the persistent luminescence process. In the second step, we analyze the role of the traps after discussing some of these properties. Finally, we propose an energy transfer mechanism from PLE, PL spectra and TL curves. • electronic structure Zn 2 SnO 4 :Cr3+ co-doped by Al3+-Eu3+. • Energy transfer mechanism from PLE, PL spectra, and TL is proposed. [ABSTRACT FROM AUTHOR]

The optical properties of the efficient near-infrared long-persistence phosphors Zn 2 SnO 4 : 0.2% Cr3+ spinels were investigated by Yang Li et al. (2015) [1]. These spinels are effective materials to fabricate multi-wavelength, low-cost, NIR phosphorescent phosphors with many potential multifunctional bio-imaging applications. The process of persistent phosphorescence occurs through a delicate interplay between the energy levels of the Cr3+ dopant ions, intrinsic defects and energy bands of the inverse spinel Zn 2 SnO 4 host lattice. To determine the role of the Cr3+ ions in this process, we examines a detailed crystal-field analysis of the visible lines associated to the transitions between the Stark levels of Cr3+ and occupying an O h site symmetry in Zn 2 SnO 4 single crystals. The theoretical study is based on the Racah tensor algebra methods. With reference to this theoretical analysis, the electronic structure of Zn 2 SnO 4 : 0.2% Cr3+ spinels was determined precisely. This study do not confirm the prediction that the band at 330 nm is assigned to the overlap of the 4A 2g (4F) → 4T 1g (4P) and VB → CB transitions. The band at 330 nm is assigned only to VB → CB transitions. The energy level 4T 1g (4P) is located in the conduction band and is not observed experimentally. The band centered at 372 nm is assigned to the transition 4A 2g (4F) → CB. This analysis supports also the assumption that the narrow peak located at 702 nm is associated to the 2E g (2G) → 4A 2g (4F) emission. The exploration of the nature of traps and the evaluation of the trap properties are also essential to understanding the phosphorescence phenomenon. • The theoretical crystal field analyses determines the electronic structure of Zn 2 SnO 4 : 0.2% Cr3+ spinels. • The roles of the center dopant Cr3+ and of the traps in the long persistent phosphorescence are successfully determined. • Energy transfer mechanism from PLE, PL spectra, TL curves and ESR measurements is proposed. [ABSTRACT FROM AUTHOR]

Abstract The optical properties of Co2+ doped α-ZnAl 2 S 4 single crystals were investigated by Anghel et al. (2011) [1,2]. The spectra of this spinel compound consist of electronic transition d-d of Co2+ (3d7) ions. This work examines a detailed crystal-field analysis of the visible lines associated to the transitions between the Stark levels of Co2+, occupying a T d site symmetry in α-ZnAl 2 S 4 single crystals. The theoretical study is based on the Racah tensor algebra methods. With reference to the calculated energy levels of Co2+ doped α-ZnAl 2 S 4 , the electronic structure of Co2+ doped in α-ZnAl 2 S 4 was resolved. The bands appearing in the spectra are reassigned. The calculated and reassigned results are consistent with the experimental data. [ABSTRACT FROM AUTHOR]