[Show abstract][Hide abstract] ABSTRACT: The polarized reflectivity spectra of CdMoO4 single crystals with a scheelite structure and of CdWO4 with a wolframite structure are measured in the 3–30 eV range by using synchrotron radiation. The spectra of optical constants for the crystallographic axes are derived by a Kramers–Kronig analysis. X-ray photoelectron spectroscopy (XPS) and the calculation of the electronic structure by using a discrete variational Xα method are performed for CdMoO4. The calculation shows that the Cd 4d state is localized at the bottom region of the O 2p valence band, and the Cd 5s state has a significant contribution to the bottom of the conduction band, composed of the Mo 4d state. The XPS spectrum of CdMoO4 resembles that of CdWO4. Unlike the XPS spectrum, the optical spectra of CdMoO4 resemble those of scheelite CaMoO4 rather than CdWO4. An excitonic transition is observed as a weak shoulderlike structure in CdWO4, while it is not appreciable in CdMoO4. These experimental results are discussed in comparison with the theoretical calculation of electronic structures. The remarkable anisotropy of the optical spectra in CdMoO4 is explained by taking into account the presence of the Cd 5s state at the bottom of the conduction band. The dichroism of CdWO4 is discussed in terms of the chain structure of WO66− octahedra in wolframite crystals. The present study indicates that the Cd metal states play a crucial role in the optical properties near the fundamental absorption edge in both CdMoO4 and CdWO4.
Physical Review B 04/2014; 77(15). DOI:10.1103/PhysRevB.77.155118 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is well known that Auger-free luminescence (AFL) is observable when the condition Eg>EVC is satisfied, where Eg is the band-gap energy between the lowest unoccupied band and the highest occupied band and EVC the energy difference between the top of the highest occupied band and the top of the next lower occupied band. From measurements of reflection and X-ray photoelectron spectra, CeF3 is demonstrated to really satisfy this condition. No evidence for AFL is found, nevertheless. The absence of AFL in CeF3 is related to a characteristic nature of its highest and next lower occupied bands, which are quite different from those of previously studied AFL-materials.
Journal of Luminescence 09/2009; 129(9):984-987. DOI:10.1016/j.jlumin.2009.04.011 · 2.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: CeF<sub>3</sub> single crystals heavily doped with Sr<sup>2+</sup> ions are grown by the micro-pulling-down (mu-PD) method. The deviation of the composition in as-grown crystals is found to be entirely negligible. Three dimensional emission-excitation spectra are measured in a wide temperature range of 5-300 K by using synchrotron radiation as an exciting light source. Two intense emission bands are observed at around 320 and 400 nm. The origin of these bands is discussed on the basis of the perturbation effects introduced by Sr doping. It is pointed out that the energy transfer takes place from Ce<sup>3+</sup> sites to defect sites when the crystal is warmed from 5 to 300 K.
[Show abstract][Hide abstract] ABSTRACT: Single crystals of LuAG doped with Pr<sup>3+</sup> ions (0.2 mol%) are grown by the micro-pulling-down (mu-PD) method with an RF heating system. The as-grown crystals are confirmed to be of garnet-type structure with the space group of Ia3d (230). The emission-excitation spectra and luminescence decay kinetics are measured in the temperature range of 5-310 K by using synchrotron radiation as an exciting light source. The intrinsic luminescence is observed at 250 nm at low temperatures. Four emission bands at 308, 325, 360, and 380 nm are ascribed to the 5d-4f transitions of Pr<sup>3+</sup> ions. From the decay kinetics measurements, it is pointed out that there exist two kinds of energy transfer mechanisms from the host LuAG lattice to the Pr<sup>3+</sup> ions. The mechanisms of energy transfer are discussed in connection with the formation of substitution-type defects.
[Show abstract][Hide abstract] ABSTRACT: Spectroscopic properties of CaMoO4 and MgMoO4 crystals were studied in view of their application to cryogenic scintillation detectors. Luminescence spectra and the luminescence decay kinetics were measured over a wide range of temperatures (8–300 K). For the first time we measured time-resolved luminescence spectra of CaMoO4. In addition to the green emission arising from the triplet state of self-trapped excitons (STEs), a new band at around 430 nm with a decay time constant 10 ± 3 ns was observed at T = 8 K. This emission is assigned to the radiative decay of a singlet STE. The relaxation of electronic excitations in the crystals under study is discussed on the basis of our current understanding of their electronic structures and a configuration coordinate model for the radiative decay of STEs. The model includes adiabatic potential energy surfaces (APESs) associated with singlet and triplet states and explains the variation of the luminescence kinetics with temperature as a result of a re-distribution in the population of these states. Thus, judging from the change of the singlet STE emission due to temperature variation, we infer the existence of an energy barrier between the singlet and triplet APESs. The multi-exponential character of the decay of the triplet emission can be understood assuming that the relevant radiative transitions originate from different minima of the triplet APES. Non-radiative energy transfer processes control the population of these states, resulting in thermal variation of the intensities of the different emission components.