J.-C. Krupa

UV and VUV optical excitations in wide band gap materials doped with rare earth ions: 4f–5d transitions

Abstract From the point of view of luminescence applications, large band gap materials such as fluoride compounds are suitable matrices into which a small concentration of foreign ions called activators can be incorporated. To improve the knowledge of the optical properties of the overall system, the investigation of the different interactions of UV and VUV electromagnetic radiations with the condensed matter was undertaken. The excitation of the luminescence induced in rare earth ion activators doped in LiYF4, LaF3 and YF3 was recorded in a wide continuous spectral region from 5 to 15 eV using synchrotron radiation as a photon source. The main interest of this work was focused on the localization in energy of the different photon absorption mechanisms occurring in these fluoride compounds doped with rare earth ions.

Preparation and size effect on concentration quenching of nanocrystalline Y2SiO5:Eu

Abstract Nanocrystalline Y2SiO5:Eu was prepared by the sol-gel technique. The photoluminescence spectra and quenching concentration of nanocrystalline Y2SiO5:Eu and normal Y2SiO5:Eu synthesized by the high-sintering method were measured and compared. Higher quenching concentration and stronger luminescent intensity in nanocrystalline Y2SiO5:Eu than in normal Y2SiO5:Eu was observed. This is ascribed to the influence of a confinement effect on resonant energy transfer in nanosized particles. These properties show that nanocrystalline Y2SiO5:Eu has important potential applications.

Luminescence concentration quenching of 1D2 state in YPO4:Pr3+

Under selective excitation of the 3P0 level of the Pr3+ ion in YPO4, the emission spectra and fluorescence decay curves were measured at different concentrations and temperatures. The origin of the spectral line located at 16 318 cm-1 (612.8 nm) was discussed and attributed to the 1D2→ 3H4 transition. The process of concentration quenching for the 1D2 state was also studied. Using the Inokuti-Hirayama model, the non-exponential fluorescence decay curves of the 1D2 level were fitted. The result shows that dipole-quadrupole interaction between Pr3+ ions, which causes 1D2→ 1G4 and 3H4→ 3F4 cross-relaxation, results in the quenching of 1D2 emissions.

Luminescence of nanometric scale Y2SiO5:Eu3+

Abstract We report what is to our best knowledge the first results of spectroscopic properties of X1-Y2SiO5:Eu3+. The sample under UV or dye laser excitation at room temperature exhibits a strong orange-red fluorescence. By using selective excitation at low temperature, dissimilar photoluminescence (PL) spectra from different sites were obtained. The fluorescence spectra, the excitation spectra of the fluorescence and the radiative lifetime were studied.

SITE SELECTIVELY EXCITED LUMINESCENCE AND ENERGY TRANSFER OF X1-Y2SIO5:EU AT NANOMETRIC SCALE

Two different types of structure called X1 and X2 are existing in Y2SiO5 at normal conditions. In X1 type, Y3+ ions occupy two sites where they are surrounded respectively by nine and seven oxygen ions, while in X2 structure, only six and seven oxygen ions are involved. Nanometric scale X1-Y2SiO5 crystals were prepared by sol-gel method with particle size around 50 nm. Site selective excitation at low temperature has shown four different luminescent centers. Two of them belong to Eu3+ embedded in X1-Y2SiO5, the other two are attributed to Y2O3:Eu phase and to a particular site on the surface. The occurrence of the latter site may be related to the nanometric size of the sample. A pronounced excitation energy transfer from site 2 to site 1 was also observed on excitation spectra. The energy transfer rate increases rapidly with increasing Eu3+ concentration and for x=0.7 in Y2−xSiO5:Eux, no fluorescence takes place in site 2 at 15 K. The lifetimes of the 5D0 levels of Eu3+ in the two sites were measured as ...

Investigation of charge transfer O2− → Ln3+ and F− → Ln3+ in LaF3:(Ln3+, O2−) and YF3:(Ln3+, O2−) systems

Excitation spectra of the visible luminescence of Eu 3+ , Dy 3+ , Er 3+ ions doped into two fluoride matrices, LaF 3 and YF 3 , holding some traces of O 2- ions were recorded in the UV and VUV range. Charge transfer bands O 2- →Ln 3+ (Ln 3+ =Eu 3+ , Dy 3+ , Er 3+ ) and F - →Eu 3+ appear clearly on excitation spectra recorded at low temperature. Using the experimental data and Jorgensens theoretical developments, it becomes possible to predict the energy of charge transfer O 2- →Ln 3+ and F - →Ln 3+ for all the trivalent ions of the lanthanide series. The time-dependent theory was used to simulate the CT absorption profile

Investigation in the VUV range of the excitation efficiency of the Tb3+ ion luminescence in Y3 (Alx, Gay)5O12 host lattices

Abstract VUV excitation spectra of the visible luminescence of Tb3+ activated Y3(Alx, Gay)5O12 compounds and their diffuse reflectance spectra are presented. The crystal field strength, the absorption threshold of the matrix and the external quantum efficiency (Qext) associated with the different excitation mechanisms (4f8 → 4f7 5d transitions in the activator or interband transitions in the host) decrease when the gallium concentration increases. Such variations are ascribed to an increase of the oxidizing character of the host matrix along with the gallium concentration. The decrease of the external quantum efficiency (Qext) is discussed in terms of a trivalent rare-earth ion (RE3+) configurational coordinate model and related to the intervention of the photoionized state [RE3+ → RE4+ + e−]. The non radiative relaxation rate increases when this state is stabilized.affd

VUV spectroscopy of wide band-gap crystals doped with rare earth ions

Abstract Spectral and kinetic properties of vacuum ultraviolet (VUV) luminescence from different fluoride crystals doped with Nd 3+ , Er 3+ or Tm 3+ are discussed. Concentration quenching of VUV luminescence, the impact mechanism of the energy transfer from the matrix to the emission centers as well as nonradiative relaxation processes in the rare earth ions have been detected and studied. The crystal properties are analyzed from the viewpoint of their possible applications as materials for fast VUV scintillators and VUV solid state lasers. Only some of Nd 3+ doped crystals possess simultaneously both high light output and relatively good timing properties of VUV luminescence and such crystals can be considered as promising materials for fast VUV scintillators. On the other hand, many of Er 3+ and Tm 3+ doped fluoride crystals are very attractive candidates as active media for tunable VUV solid state lasers with optical pumping.

Spectroscopic properties of Eu3+ ions in X1-Y2SiO5 at nanometric scale

Abstract The influence of concentration and temperature on the luminescence properties of the Eu 3+ activated X 1 –Y 2 SiO 5 are investigated for the first time. For high Eu 3+ concentration, a cross relaxation process between Eu 3+ ions results in the quenching of 5 D 1 emission. At low temperature and high concentration, the excitation transfer between Eu 3+ ions in the two sites enables the fluorescence of site 2 disappears entirely. In addition, the samples showed some interesting phenomenon related to size effect.

Optical excitations in lanthanide and actinide compounds

Abstract Systematic analysis of optical spectra of lanthanide and actinide ions in the solid state has led to general trends in the behaviour of 4f and 5f elements. In this respect, the 5f electrons in compounds keep their character of an internal shell relatively well, as do 4f; nevertheless, the greater radial extension of 5f orbitals makes the 5f ions in the light actinides look more like 3d transition elements than lanthanides. Since the 5f states in some actinide compounds are at the frontier between localization and delocalization, the nature of the chemical bonding will push the 5f states on the appropriate side of the border. Trends in the values of semiempirical parameters describing the interactions experienced by the f electron are presented including trends in charge transfer ad f-d transitions.