Dexter's Formula Research Articles

Structure, photoluminescence, Judd-Ofelt analysis, and thermal stability studies of Eu3+-doped Ba2Tb8(SiO4)6O2 red phosphor with high color purity

In this study, a series of Eu3+-doped Ba2Tb8(SiO4)6O2 red-emitting phosphors were developed by using a conventional solid-state reaction technique. The crystalline structure, surface morphology, photoluminescence properties, energy transfer processes, and thermal behavior of the Eu3+-doped Ba2Tb8(SiO4)6O2 phosphors were studied. The X-ray diffraction patterns confirmed the formation of the phase, and the Rietveld refinement study revealed that the Eu3+-doped Ba2Tb8(SiO4)6O2 phosphors have a hexagonal crystal structure with the space group P63/m. Energy-dispersive X-ray analysis confirmed the homogenous distribution of Eu3+ ions over the Tb3+ sites. Photoluminescence (PL) spectroscopy was used to test the luminescent properties of the synthesized phosphors. The PL spectra showed that upon UV and blue light exposure, the Eu3+-doped Ba2Tb8(SiO4)6O2 phosphors exhibited red emission associated with the 5D0→7FJ transitions. The mechanisms of concentration quenching and energy transfer were analyzed using Blasse and Dexter's formula. Judd-Ofelt parameters (Ω2, Ω4) and other radiative parameters were analyzed using PL emission spectra to investigate the luminescence behavior of Eu3+-doped Ba2Tb8(SiO4)6O2 phosphors. The results of the thermal stability studies show that Eu3+-doped Ba2Tb8(SiO4)6O2 has good thermal properties with a high activation energy of approximately 0.23 eV. The calculated Commission International del'Eclairage coordinates (x = 0.62 and y = 0.36) of the prepared phosphors confirm that the emitted light is in the red portion with a high color purity of about 99.9%. These findings show that Eu3+-doped Ba2Tb8(SiO4)6O2 phosphors are a viable option for solid-state lighting technology.

Energy transfer dynamics in thermally stable Sm3+/ Eu3+ co-doped AEAlBS glasses for near UV triggered photonic device applications

Transparent, Sm3+/Eu3+ co-doped alkaline earth alumino borosilicate (AEAlBS) glasses have been synthesized by employing melt quenching process and explored their down-shifting luminescent properties to understand their utility in visible red photonic devices applications. The amorphous (non-crystalline) nature of the as-prepared glasses was analyzed with help of an X-ray powder diffraction (XRD) pattern, containing a broad peak. Photoluminescence (PL) properties demonstrate that the glasses can be proficiently excited by near-UV with a dominant peak centered at 402 nm. The PL emission spectra exhibit four emission peaks with an intense peak placed at 599 nm under 402 nm excitation. The optimum emission intensity was obtained for 0.5 mol% Sm3+ ions doped in AEAlBS glasses. In the present work, Sm3+ ion is acting as an effective sensitizer for Eu3+ activator in AEAlBS glasses, and part of energy gets transferred from sensitizer (Sm3+) to activator (Eu3+) ions. The PL intensity of Eu3+ ions is increasing at the expense of a decrease in the intensity of Eu3+ ion peaks in the co-doping AEAlBS glasses at λex = 402 nm. The efficient ET from sensitizer to activator ions proved to be dipole-dipole in nature via employing Dexter's formula with Reisfeld's approximation. The experimental lifetime values calculated from the PL decay profiles are decreasing with the surge in Eu3+ ion concentration in the as-prepared glasses. Inokuti Hirayama (I-H) model applied to the PL decay profiles confirms the ET process responsible for a decrease in experimental lifetimes as dipole-dipole in nature. The outcome of the I-H model is in consonance with the result given by Dexter theory. The CIE coordinates for single Sm3+ doped glasses are falling in the orange region and gradually surge into the red region by co-doping with Eu3+ ions in AEAlBS glasses. The temperature-dependent emission analysis reveals that the PL intensity at 150 °C and 200 °C perseveres up to 94.34 and 91.30% of the PL intensity at ambient temperature, respectively. All the obtained results contemplate the superiority of the multifunctional Sm3+/Eu3+ co-doped AEAlBS glasses for near UV triggered photonic device applications.

Structural, luminescent properties and Judd-Ofelt analysis of CaMgSiO4:Eu3+ phosphor for solid state lighting

A series of Eu3+ doped calcium magnesium silicate (CaMgSiO4) phosphor samples were successfully prepared via solid state synthesis technique. Their structural and morphological studies were investigated by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) measurements. Phase structure and phase purity of sample were investigated by Rietveld refinement method. The vibrational and bonding behavior of silicate groups were confirmed by Fourier transform infrared (FTIR) and Raman spectroscopy. The luminescent properties of prepared samples were studied by photoluminescence (PL) and thermoluminescence (TL) characteristics. The emission spectra recorded under the excitation wavelength λex = 396 nm. The highest emission peak was observed at 595 nm which corresponds to the 5D0 → 7F1 transition of Eu3+ ions with selection rule ΔJ = 1. The concentration quenching and energy transfer mechanisms were elucidated by Blasse and Dexter's formula. The photometric parameters correlated color temperature (CCT), color rendering index (CRI) and color purity (CP) were computed using CIE chromaticity co-ordinate diagram. In addition, Judd-Ofelt (JO) parameters (Ω2, Ω4) and other derived radiative parameters were calculated using Judd-Ofelt theory from the PL emission spectra. Time resolved fluorescence spectra were carried out to determine the decay lifetime of the samples. The prepared samples are expected to be suitable candidates for the applications as solid state devices, lighting and optical displays.

Transfer and back transfer processes in Yb3+–Er3+ codoped fluoroindate glasses

The kinetics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb3+ and Er3+ has been studied. It was possible to excite selectively Yb3+ ions and detect emission due only to these ions or combined with the emission coming from the Er3+ ions. At low concentration of Yb3+ (0.1 mol %) the emission decay of these ions is nonexponential when there is back transfer from Er3+ ions; the dynamics is well described using the fluorescence “transfer function” model. Moreover, the evolution of the emission from the I11/24 (Er3+) level is in good agreement with the behavior predicted by the model. The critical radii obtained from the fitting for forward and backward energy transfer are compared with those calculated with the Dexter formula. When the Yb3+ concentration is increased, the migration among these ions is very important and the emission decay of the Yb3+ ions is nearly exponential with the lifetime of these ions. This behavior is explained considering that, at high Yb3+ concentration and due to the migration, the transfer from Yb3+ to Er3+ ions is restricted to very closed couples of ions with high back transfer probability. The limitations of the fluorescence “transfer function” model associated with the correlation effect in the excitation of Er3+ ions by energy transfer are discussed.

Kinetics of transfer and backtransfer in Yb 3+-Er 3+ codoped fluoroindate glasses

The dynamics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb 3+ and Er 3+ have been studied using the fluorescence ‘transfer function’. From the fitting to the decay curves of the luminescence the critical radii for these processes have been obtained. These values are also calculated with the Dexter formula.

High pressure action on the SA and Mn emission in the cubic, faulted and hexagonal ZnS crystals

The pressure coefficient d E/d P appears to be sensitive to the ZnS lattice symmetry for the SA donor-acceptor pair, but not for the Mn 2+ centres emission. The HP variation of the relative intensities for the SA and Mn emissions suggests a resonant transfer between these centres. The classical Dexter's formula for P dd does not explain the results for the cubic crystal, whereas the preferential transfer from the SA centers to the 4 E( 4 G) level in Mn 2+ follows all reported observations.

Energy transfer between samarium and europium in phosphate glasses

A study of energy transfer from samarium to europium in phosphate glasses was performed for a range of donor and acceptor concentrations corresponding to a donor-acceptor distance of 13–24 Å. The energy transfer probabilities were calculated. The mechanism of transfer was deduced by fitting the experimental decay curves to the theoretical curves obtained by Inokuti and Hiroyama. Theoretical transition probabilities based on Dexter's formula were calculated. It was inferred that the energy is transferred by a dipole-quadrupole mechanism which is assisted by phonons. It was possible to indicate the path by which the transfer takes place.

Energy Transfer by Resonance between TI+ Ions in KI Single Crystals

Energy transfer by dipole-dipole interaction between Tl + ions in KI single crystals was studied with several kinds of crystals different in Tl + concentration at low temperatures. Since two emission bands (2845 Å and 3044 Å) which appear under excitation at the B absorption band of Tl + ions overlap with the A absorption band of the ions, there is a possibility of energy transfer by resonance from excited Tl + ions to normal Tl + ions. The absorption coefficient of the A band and the emission bands produced by excitation at the B band were measured; and, thereby, the critical transfer distance R 0 was obtained from Dexter's formula. Furthermore, the decay curves of the donor emissions were measured by a pulse method and their shapes were analyzed with Eisenthal and Siegels's theory. The values of R 0 thus obtained for 2845 Å and 3044 Å emissions were 35 Å and 26 Å, respectively. They were in good agreement with those obtained from Dexter's formula, that is, 33 Å and 24 Å respectively.

Resonant Energy Transfer between ExcitedFCenters in KI

Resonant energy transfer is observed between excited states of $F$ centers in KI at 7\ifmmode^\circ\else\textdegree\fi{}K by monitoring the decay in time of the excited-state population in samples of high concentration of excited states, ${F}^{*}$. The large population of excited $F$ centers is obtained with a pulse from a $Q$ switched ruby laser. Nonexponential decay curves---showing an increase over the exponential luminescent decay rate at low concentrations---are found in samples containing more than 2\ifmmode\times\else\texttimes\fi{}${10}^{16}$ (${F}^{*}$ centers)/cc. It is shown by quantitative comparison of the time decay with theory that the mechanism for resonance transfer is dipole-dipole interaction. For this purpose the rate equation and its solution are derived for the case of transfer among the same states of like species. The individual interaction constant for dipole-dipole interaction is calculated using Dexter's formula. It agrees well with the interaction constant obtained from the experimental results.

Some contributions to hemodynamics of systolic (arteriosclerotic) hypertension with special reference of pharmacodynamical effects of Rauwolfia serpentina

Of 64 hypertensive patients treated with Rauwolfia serpentina there were observed 66% whose blood pressures were decreased by 20 mmHg in systolic and by 15 mmHg in diastolic; 38% whose dynamical mean pressures were lowered by 20 mmHg; 30% whose systolic and diastolic pressures were reduced to normal range (150/90mmHg). Of 15 cases medicated with R. s. above 60 years old there were ascertained 67, 40, 7% and of 46 patients administered with R. s. under 59 years old 65, 37, 35% correspondently.It is possibly due to the high incidence of systolic hypertension in advanced aging group that both systolic and diasnolic pressure were reduced at large percentage in cases above 60 years old. In order to further elucidate this point, the elasticity moduli were calculated by means of Dexter's formula, and following results were obtained. Of 15 patients there were observed 14 cases (93%) with diminished elasticity moduli, and 87% after the medication with Rauwolfia. This suggests that the majority of patients above 60 years old observed here is sure to be arteriosclerotic (systolic) hypertensive cases, whereas of total 65 patients there were 52 cases (80%) with elastivity hypertension in the sense of Dexter. Circulatory dynamics were analysed by means of Wezler-Boger's method in 19 cases with hypertension above 60 years old of which 13 patients (69%) showed larger E' than normal range. Dexter's classification agrees with that of Wezler-Boger's in 14 cases of 19, therefore it appears that Dexter's classification is of considerable clinical value.The above mentioned can be summerized as follows:-1. Pros & Cons upon the medication with Rauwolfia serpentina in the hypertensive patients of aged above 60 years old are discussed from the hemodynamic analytical standpoints.2. In the hypertensive cases of aged it is sometimes difficult to estimate the hemod-ynsmics by means of Wezler-Boger's method. In such a case, taking care to avoid danger, we are compelled to make shift with Fick-Cournand's method.3. The percentage of hypertensive potency of Rauwolfia is proved larger in the senil group than in the young.4. The vascular elasticity calculated from Dexter's formula roughly agrees with volume elasticity and elasticity modulus obtained from Wezler Boger's method5. The hypertensive cases of aged are frequently subject to the senil arteriosclerotic high blood pressure. Classifying them according to Wezler Boger's interpretation, most of them can be placed under the category of W+E', M+E' or M+E'+W type. While the circulatory dynamics of M+E' type can be scarcely improved by administration with Rauwolfia, W+E' type can be ameliorated fairly well. In the M form the treatment by the use of Rauwolfia is very often superflous (Authors' Referate).