5d 4f Luminescence Research Articles

Investigation of Electronic Excitations of Pure and Ce Doped 12CaO•7Al2O3 Using Luminescence Spectroscopy

Pure and Ce doped nanoporous 12CaO•7Al2O3 (C12A7) oxides were synthesized using self-propagating combustion method and solid state reaction. Time-resolved photoluminescence method at low temperatures was applied in studies of powder samples and ceramics, which were obtained in a melt-solidification process partly replacing encaged oxygen ions with electrons. In pure C12A7 with the encaged electrons broad luminescence bands peaked at 3.3 and 5.0 eV were revealed. The first one corresponds to the luminescence due to the encaged oxygen ions and latter one is assigned to the recombination of electrons and holes localized on the framework oxygen ions. Ce doping introduces emission band at 3.0 eV due to 5d-4f transitions with several decay components in nanosecond range. The Ce 5d-4f luminescence is efficiently excited through Ce intra-center absorption as well as through energy transfer processes in the intrinsic absorption region of C12A7 above energy gap at 7.0 eV.

Growth and optical properties of Lu 3(Ga,Al) 5O 12 single crystals for scintillator application

Effect of Ga substitution in a (Ce,Lu) 3Al 5O 12 scintillator was examined at the crystals grown by the micro-pulling down(μ-PD) method. Strong suppression of unwanted host luminescence due to an exciton localized around Lu–Al antisite defect was observed even at the Ga concentration of 10 mol%. Less-intense slower components in scintillation decay were obtained upon increasing the amount of Ga. While the radioluminescence intensities of the 5d–4f luminescence of Ce 3+ were not strongly changed, light yield was increased by Ga substitution. The 20% Ga-substituted sample showed even higher light yield than typical Czochralski-grown Ga-free LuAG:Ce.

VUV Luminescence Due to 5d - 4f Transitions in Gd3+ and Lu3+ Ions Doped into Fluoride Crystals

High-resolution (< 1 Aå) VUV emission and excitation spectra as well as luminescence decay kinetics have been studied for several fluoride crystals containing Gd3+ and Lu3+ ions under excitation by synchrotron radiation. The obtained results evidently show that VUV luminescence (~ 10 eV) observed from these crystals originates from 5d-4f transitions in the Gd3+ and Lu3+ ions. Only fast spin-allowed 5d-4f luminescence is observed from Gd compounds, whereas both spin-forbidden and spin-allowed 5d-4f luminescence has been detected from Lu compounds, the latter being observed only at high enough (T > 100 K) temperatures.

Interplay of spin-allowed and spin-forbidden 5d–4f luminescence from rare earth ions

The conditions for co-existence of emissions from spin-allowed and spin-forbidden 5d–4f transitions in rare earth ions and for thermal quenching of these emissions have been analyzed by taking Tm 3+ 5d–4f luminescence in LiYF 4:Tm 3+ and Lu 3+ 5d–4f luminescence in LuF 3 as examples. It is shown that temperature behavior of 5d–4f luminescence agrees with the common trends in decreasing energy splitting between the lowest high-spin and low-spin 5d levels as well as decreasing energy gap between the lowest 5d level and the bottom of the conduction band of the host crystal towards heavier rare earth ions (from Er 3+ to Lu 3+).

Growth and Luminescence Properties of Pr-doped Lu3(Ga,Al)5O12 Single Crystals

Single crystals with the nominal composition of (Pr0.01,Lu0.99)3(Gax,Al1-x)5O12 (Pr:LuGAG) were grown by the micro-pulling-down (µ-PD) method with RF heating and their luminescence properties were characterized. Transparent and crack free crystals that were slightly yellowish green were obtained. Lattice constants and densities systematically increased with increasing of Ga concentration. The luminescence properties of Pr:LuGAG were characterized by photoluminescence spectra and decays, radioluminescence spectra and light yield measurements. 5d–4f luminescence was observed for the x≤0.6 samples. It was found that the 5d–4f emission of Pr3+ is quenched only slightly up to x≤0.4 whereas density systematically increased in the range of 0≤x≤1.0.

Optical and gain properties of series of crystals LiF–YF 3–LuF 3 doped with Ce 3+ and Yb 3+ ions

Here, we present first results of systematic studies of host cation variation impact on spectral-kinetic, photochemical and gain properties of Ce 3+-doped LiYF 4 (YLF), LiLuF 4 (LLF) and LiY 1− x Lu x F 4 family crystals. 5d–4f luminescence decay of Ce 3+ ions studies, together with pump–probe experiments, indicate that previously reported twice higher luminescence quantum yield in LLF compared with that of YLF crystals is provided by more efficient upper lasing level feeding due to recombination and higher color center destruction rate in LLF against YLF crystals. Namely, it is responsible for higher energetic characteristics of laser based on Ce 3+:LLF crystals. Strong and wide pump-induced absorption band centered at 310 nm is observed in Ce 3+:YLF. This band is shifted to blue and its intensity goes down with Lu content. We have evaluated free charges recombination rate, excited state absorption cross-section for Ce 3+ ions and some other photodynamic processes related microparameters. Fitting results indicate that pump-induced color centers lifetime decreases with Lu-content in LiYF 4–LiLuF 4 mixture and it can be associated with more efficient color center bleaching by Ce 3+ ions 5d–4f fluorescence.

Crystal growth, optical and luminescence properties of Pr-doped Y2SiO5 single crystals

Using the micro-pulling-down method, Pr3+-doped Y2SiO5 single crystals have been grown and their optical and luminescence properties have been investigated. The position of the lowest 5d absorption level was found at about 246nm and an intense and fast 5d–4f luminescence peaking round 275nm with a shoulder round 315nm has been observed at room temperature. Photoluminescence decay time at room temperature was obtained to be of about 17ns in the leading decay component.

Absolute location of lanthanide energy levels and the performance of phosphors

Methods to construct the absolute position of the energy levels of divalent and trivalent lanthanides with respect to the valence band and conduction band of a host compound are available and can be applied to many different compounds. With the diagrams of lanthanide levels in SrAl 2O 4, LiYSiO 4, and CaGa 2S 4 it will be demonstrated how they can be used to understand phosphor performance. The quenching of Ce 3+ and Eu 2+ 5d–4f luminescence, the electron and hole trapping ability of the lanthanides, and the valence stability of lanthanide ions are discussed.

Study on crystal growth and luminescence properties of Pr-doped RE 2SiO 5 (RE=Y, Lu)

Using the micro-pulling-down method, Pr 3+-doped Y 2SiO 5 and Lu 2SiO 5 single crystals have been grown and their optical and luminescence properties have been investigated. The position of the lowest 5d absorption level was found at about 246 nm and in both materials an intense and fast 5d–4f luminescence peaking round 275 nm with a shoulder round 315 nm has been observed at room temperature. Photoluminescence decay time at room temperature was obtained to be of about 17 and 6 ns in the leading decay component in Pr 3+-doped Y 2SiO 5 and Lu 2SiO 5, respectively. Shorter decay time of Pr 3+-doped Lu 2SiO 5 was explained by the ionization of the lowest 5d 1 state of Pr 3+.

Thermal quenching of Eu2+ 5d–4f luminescence in inorganic compounds

The thermal quenching of Eu2+ 5d–4f emission on Ba, Sr, or Ca sites in compounds is often attributed to a largedisplacement between the ground state and excited state in the configuration coordinatediagram. This work will demonstrate that the ionization of the 5d electron to conductionband states is the genuine quenching mechanism. A model is proposed to explain why insome types of compounds the quenching temperature decreases when going from theBa variant via the Sr variant to the Ca variant and in other types of compounds the reversebehaviour occurs. The nature of the bottom of the conduction band plays an importantrole in this.

Growth and luminescent properties of Pr:KY 3F 10 single crystal

Pr-doped KY 3F 10 (Pr:KYF) single crystals were grown by the micro-pulling-down ( μ-PD) method. The crystals were transparent with a slightly greenish color, 2.5–3.0 mm in diameter and 35–45 mm in length. Neither visible inclusions nor cracks were observed. Photoluminescence spectra and decay kinetics of Pr:KYF were measured. Emission due to Pr 3+ 5d–4f transition, peaking around 260 nm and of the decay time of 24 ns was observed. The short decay time suggests such a Pr-doped material for a scintillator application. To our knowledge, this is the first report of the Pr 3+ 5d–4f luminescence transition accomplished in a fluoride host.

Spectroscopy of cubic elpasolite Cs2NaYF6 crystals singly doped with Er3+ and Tm3+ under selective VUV excitation

Abstract Luminescence of cubic Cs2NaYF6 crystals singly doped with 0.1, 1.0 and 10.0 at.% of Tm3+ or 10.0 at.% of Er3+ as well as of stoichiometric crystals Cs2NaErF6 and Cs2NaTmF6 have been studied in the VUV, UV and visible spectral ranges. It has been found that both Er3+- and Tm3+-containing crystals show intense VUV luminescence due to the interconfigurational 5d–4f transitions in Er3+ or Tm3+, the main band peaking at 173 and 176 nm, respectively. The Tm3+ 5d–4f luminescence does not show any concentration quenching even in Cs2NaTmF6 and decay time is ∼2.5 μs for all concentrations of Tm3+ up to 100 at.%. On the other hand, the Tm3+ 4f–4f luminescence is considerably quenched already at 10 at.% concentration of Tm3+ and is practically absent in Cs2NaTmF6. The Er3+ 5d–4f luminescence is remarkably quenched in Cs2NaErF6 as compared to the 10.0 at.% Er3+:Cs2NaYF6 crystal, but some Er3+ 4f–4f emission is still observed in Cs2NaErF6. The 5d–4f luminescence of Er3+ and Tm3+ is quite efficiently excited under band-to-band absorption in the host Cs2NaYF6 crystal. Thus the light yield of VUV luminescence from these crystals is expected to be high enough under high-energy excitation. The above properties make the Cs2NaYF6 crystals heavily doped with Er3+ and especially with Tm3+ rather attractive materials as VUV scintillators.

Radiation defects in Ce3+-activated fluorobromozirconate glass-ceramic X-ray storage phosphors

We report on a photostimulated luminescence (PSL) effect in Ce-doped fluorobromozirconate glasses. Small hexagonal or orthorhombic BaBr2 crystallites are formed in the glass upon annealing. The PSL shows a (381,415)nm doublet which is due to the 5d–4f luminescence of Ce3+ in the orthorhombic BaBr2 crystallites embedded in the glass matrix. The stimulation band of the PSL peaking at 580nm is probably due to perturbed F centres in the crystallites. After X-irradiation the magnetic circular dichroism of the optical absorption showed a broad band in the spectral range between 440 and 800nm, having a derivative-like line shape centred at about 600nm. The spectrum consists of two overlapping bands: one, derivative-like, belongs probably to F-centres, whereas the second band, superimposed on the low-energy region of the F-centre band, is due to a hole centre.

Upconverted VUV luminescence of Nd 3+ and Er 3+ doped into LiYF 4 crystals under XeF-laser excitation

The upconverted VUV (185 nm) and UV (230 and 260 nm) luminescence due to 5d–4f radiative transitions in Nd 3+ ions doped into a LiYF 4 crystal has been obtained under excitation by 351/353 nm radiation from a XeF excimer laser. The maximum upconversion efficiency, defined as the ratio of intensity for 5d–4f luminescence to overall intensity for 5d–4f and 4f–4f luminescence from the 4D 3/2 Nd 3+ level, has been estimated to be about 70% under optimal focusing conditions for XeF laser radiation. A redistribution of intensity between three main components of 5d–4f Nd 3+ luminescence is observed under changing the excitation power density, which favors the most long-wavelength band (260 nm) at higher excitation density level. The effect is interpreted as being due to excited state absorption of radiation emitted. The upconverted VUV and UV luminescence from the high-lying 2F(2) 7/2 4f level of Er 3+ doped into a LiYF 4 crystal has also been obtained under XeF-laser excitation the most intense line being at 280 nm from the spin-allowed transition to the 2H(2) 11/2 4f level of Er 3+, but the efficiency of upconversion for Er 3+ emission is low, less than 5%.

Spectroscopic properties of Pr3+ luminescence in complex fluoride crystals

Spectroscopic properties of several complex fluoride crystals K2YF5, KYF4, LiKYF5, CsY2F7 and CsGd2F7 doped with Pr3+ have been studied under synchrotron radiation and F2 excimer laser excitation. The Pr3+ doped K2YF5 and LiKYF5 crystals with only a single crystallographic site for Pr3+ ions show typical properties as in the case of spectroscopic systems where the low edge of the 4f5d Pr3+ configuration lies slightly below the 1S0 level of the 4f2 Pr3+ configuration. The Pr3+:KYF4 crystal has the same properties caused with only the 5d–4f luminescence of Pr3+ in spite of the fact that in this crystal there are six sites suitable for Pr3+ ions. The luminescence spectrum of the Pr3+:CsY2F7 crystal is determined by both the 5d–4f luminescence and the 4f–4f emission originating from the 1S0 Pr3+ level, which is due to two groups of optical Pr3+ centers with the energy levels of the 1S0 state just between the lowest 5d levels of Pr3+ for these two kinds of centers.

Fast 20 ns 5d–4f luminescence and radiation trapping in BaF 2:Ce

Parity-allowed UV 5d–4f transitions characterized by a high oscillator strength make trivalent cerium an interesting activator for fast and efficient scintillator crystals. Indeed, several materials are known, in which the radiative lifetime of the Ce 3+ ions is shorter than 20 ns (e.g. LuAlO 3:Ce and YAlO 3:Ce). For the case of BaF 2:Ce, however, the value of 27 ns is known from literature. In this communication we show that the 5d–4f luminescence in BaF 2:Ce is in fact faster. Having analyzed several series of time profiles recorded at various temperatures between 25 and 350 K, we conclude that the true radiative lifetime of the Ce 3+ ions in BaF 2 is close to 20 ns and it is the radiation trapping caused by the temperature-dependent reabsorption of cerium emission which prolongs the effective decay time.

VUV SPECTROSCOPY OF CRYSTALLINE EMITTERS BASED ON 5d–4f TRANSITIONS IN RARE EARTH IONS

The spectroscopic properties of several fluoride crystals ( LiYF 4, KYF 4, K 2 YF 5, KLiYF 5, CsY 2 F 7, SrF 2) doped with Nd 3+, Er 3+ or Tm 3+ are analyzed from the viewpoint of their possible applications as active media for VUV solid state lasers. It has been found that in many crystals there exists very efficient nonradiative relaxation from higher-lying 5d states to the lowest 5d level responsible for the spin-forbidden luminescence. In fact, the energy level diagram of 5d–4f transitions for these two ions represents a typical four-level laser scheme with a considerably larger Stokes shift of 5d–4f luminescence from the edge of the strong spin-allowed 4f–5d absorption than for Nd 3+, which causes smaller reabsorption of emitted VUV radiation in the crystals doped with Er 3+ or Tm 3+. Two crystals LiYF 4: Nd 3+ and SrF 2: Er 3+ were tested for laser action under pumping by a pulsed F 2 laser operating at 157 nm. However, no indications of the appearance of stimulated emission have been detected for both crystals. The possible reasons for the absence of lasing are discussed.

Luminescence spectroscopy from the vacuum ultra-violet to the visible for Er3+ and Tm3+ in complex fluoride crystals

Abstract Spectra and decay kinetics of luminescence from several complex fluorides of alkali elements and yttrium or gadolinium doped with Er 3+ or Tm 3+ have been studied in the vacuum ultra-violet (VUV), ultra-violet (UV) and visible spectral ranges under excitation by the 157 nm radiation from a pulsed molecular F 2 -laser. It has been found that yttrium crystals have intense VUV luminescence due to interconfiguration 5d–4f transitions in the rare earth ions. On the other hand, the 5d–4f luminescence from Er 3+ or Tm 3+ in gadolinium crystals is completely absent and the nonradiative decay as a result of cross-relaxation is the dominant decay channel from the 5d level of the doping rare earth ion in these crystals. This process leads to complete conversion of absorbed VUV radiation into the lower energy part of the spectrum. These gadolinium fluorides appear to be promising materials for VUV-excited phosphors with high-quantum efficiency.

VUV spectroscopy of a new fluoride system NaF–(Er,Y)F3

Abstract Emission and excitation spectra as well as luminescence decay kinetics of a new complex fluoride system Na 0.4 (Y 1− x Er x ) 0.6 F 2.2 ( x =0.01, 0.1, 1) have been studied in the vacuum ultraviolet (VUV) spectral range. It has been shown that these crystals have intense VUV luminescence due to the interconfiguration 5d–4f transitions in Er 3+ ion. The spin-allowed 5d–4f luminescence of Er 3+ in this system is very weak, i.e., in these crystals there exists an efficient non-radiative relaxation from higher-lying 5d states of Er 3+ to the lowest 5d level responsible for spin-forbidden luminescence. This makes the studied new fluoride system a promising active medium for the production of VUV solid state laser with optical pumping. Due to rather large bandwidth of Er 3+ 5d–4f luminescence in this system there is a possibility for the construction of tuneable VUV laser.

VUV spectroscopy of KYF 4 crystals doped with Nd 3+, Er 3+ and Tm 3+

Emission, excitation and absorption spectra as well as luminescence decay kinetics of KYF 4 crystals doped with Nd 3+, Er 3+ and Tm 3+ were studied in the VUV spectral range. It is shown that all the crystals have intense VUV luminescence due to interconfiguration transitions originating from the 5d states of the rare earth (RE) ions and terminating on the 4f states. The UV luminescence caused by intraconfiguration 4f–4f transitions in the RE ions is also observed from the crystals doped with the Nd 3+ or the Er 3+ ions under 4f–5d excitation. The spin-allowed 5d–4f luminescence from the crystals doped with Er 3+ or Tm 3+ is extremely weak, i.e. there exists very efficient nonradiative relaxation from higher lying 5d states to the lowest 5d level responsible for spin-forbidden luminescence. This peculiarity of the studied crystals doped with Er 3+ or Tm 3+ is very attractive for developing VUV solid-state lasers based on the crystals of such a type.