Photoacoustic method of determination of quantum efficiency of luminescence in Mn<Superscript>2+</Superscript> ions in Zn<Subscript>1−x−y</Subscript>Be<Subscript>x</Subscript>Mn<Subscript>y</Subscript>Se crystals

This paper presents step by step the procedure of determination of the quantum efficiency of luminescence of Mn²⁺ ions in the Zn_1−x−yBe_xMn_ySe crystals. The method is based on the photoacoustic spectroscopy approach. In the paper, the experimental spectra of absorbance, transmission, absorption and photoacoustic spectra of the samples are presented and analyzed from the point of view of the possibility of determination of the quantum efficiency of Mn²⁺ ion luminescence at room temperature. It was determined experimentally that in the investigated crystals the quantum efficiency of luminescence in the Mn²⁺ ions is about 35%, 40%, 32% for the absorption peaks at 430 nm, 470 nm, and 510 nm, respectively, for Zn_0.75Be_0.2Mn_0.05Se crystal.     

Mn2+ and Tl+ luminescence in β-aluminas

The compounds AGa₁₁O₁₇ (A = K, Rb, Cs) and AAl₁₁O₁₇ (A = Na, K, Rb) have the β-alumina structure. The Mn²⁺-activated gallates show efficient luminescence under 254 nm excitation with an emission peaking at 498 nm and a quantum efficiency up to 70%. The Tl⁺-activated aluminates show efficient luminescence under 254 nm excitation with an emission peaking at 380–390 nm and a quantum efficiency up to 70%. In AAl₁₁O₁₇: Tl⁺ energy is transferred from Tl⁺ to Mn²⁺, resulting in an emission peaking at 512 nm with a quantum efficiency up to 55%.     

Red Mn2+-luminescence in hexagonal aluminates

On UV excitation, a red luminescence band peaking at 690 nm is observed in Eu, Mn doped aluminate lattices containing large trivalent cations and high Mn²⁺-concentrations. The green Mn²⁺ luminescence band occuring at lower Mn²⁺ contents is quenched by the appearance of the red band. The excitation spectra of the red luminescence show that energy transfer takes place from the green Mn²⁺ centres which are located at tetrahedrally surrounded sites to the red emitting Mn²⁺ centres. The last-named are most probably Mn²⁺ ions surrounded octahedrally by oxygen ions. One of these oxygen ions is located at the crystallographic position normally occupied by the large trivalent cation. The dependence of the intensity of the red luminescence on the Mn²⁺ concentration in La-aluminate samples can be explained if it is assumed that the Mn²⁺ ions in tetrahedral sites (the green emitting ones) are all located near the oxygen ions at the large cation sites.     

Excitonic excitation spectra in ZnS: Cl crystal under pressure

The 3363 Å peak in the excitation spectrum of the S-A luminescence in the ZnS:Mn Cl cubic crystal at R.T. is ascribed to the A exciton. Its pressure coefficient is found as 6.4±0.2 meV/kbar, close to ρ = 6.3±0.2 meV/kbar reported for the gap in ZnS cubic crystal from the reflectivity measurements. This peak decreases by cooling and has not been observed at 85 K. The uv-excitation of the Mn-luminescence at R.T. is due to the energy transfer from the S-A to the Mn²⁺ centers, whereas the excitation peak at the exciton energy at 85 K comes from the excitation of the Mn²⁺ centers by the recombination energy of the Mn²⁺ bound excitons.     

Luminescence spectroscopy and energy transfer in Eu2+/Mn2+-doped KCaPO4 phosphors

Eu²⁺/Mn²⁺-doped KCaPO₄ phosphors were prepared by conventional solid-state reaction. X-ray powder diffraction (XRD), SEM, photoluminescence excitation, and emission spectra, and the luminescence decay curves were measured. Mn²⁺ singly doped KCaPO₄ shows the weak origin-red luminescence band peaked at about 590 nm. The Eu²⁺/Mn²⁺ co-doped phosphors emit two distinctive luminescence bands: a blue one centered at 480 nm originating from Eu²⁺ ions and a broad red-emitting one peaked at 590 nm from Mn²⁺ ions. The luminescence intensity from Mn²⁺ ions can be greatly enhanced with the co-doping of Eu²⁺ ions. The efficient energy transfer from Eu²⁺ to Mn²⁺ was verified by the photoluminescence spectra together with the luminescence decay curves. The resonance-type energy transfer via a dipole–quadrupole interaction mechanism was supported by the decay lifetimes. The emission colors could be tuned by changing the Mn²⁺-doping concentration.     

Anomalous wavelength dependent effects in photoacoustic measurements of Nd3+ luminescence quantum efficiency in various laser materials

Values for the luminescence quantum efficiency of Nd³⁺ in phosphate glass, silicate glass and YAG have been obtained from measurement of the photoacoustic and optical absorption spectra. For the phosphate glass a quantum efficiency value of 0.31 ±0.10 was obtained for excitation in the wavelength range 350–750 nm, silicate glass gave a value of 0.72 ±0.10 for the range 350–600 nm. In both cases, anomalously high photoacoustic signals were obtained in the IR, around 800 nm, suggesting lower quantum efficiencies in this region. YAG gave a value of 0.95 ±0.10 for the excitation range 450–800 nm with smaller than expected PA signals in the UV (355 and 334 nm bands) suggesting the existence of an alternative decay mechanism following excitation in this region.     

Luminescence channels of manganese-doped spinel

Two independent luminescence channels are observed from manganese-doped spinel Mn:MgAl₂O₄. The luminescence around 520 nm is assigned to transition from the lowest electronic excited state ⁴T₁ to the ground state ⁶A₁ of Mn²⁺ (3d)⁵ ion by analyzing the excitation spectrum and electron spin resonance measurement. The emission at 650 nm is triggered by the band edge excitation and is assigned similarly to the charge-transfer process associated with the manganese ion.     

Exciton and intracenter luminescence in Cd<Subscript>0.6</Subscript>Mn<Subscript>0.4</Subscript>Te/Cd<Subscript>0.5</Subscript>Mg<Subscript>0.5</Subscript>Te quantum-well structures

Exciton luminescence and intracenter luminescence (IL) of Mn²⁺ ions in Cd_0.6Mn_0.4Te/Cd_0.5Mg_0.5Te structures with quantum wells (QWs) 7, 13, and 26 monolayers thick were studied. It was established that in QWs the intensity of exciton luminescence with respect to that of IL is a few orders of magnitude higher than that in bulk crystals. The spectral position of manganese IL profile changes noticeably in going from a bulk crystal to a QW of the same composition. The nonexponential parts of the IL decay curves are determined by excitation migration and the cooperative upconversion process, whose contribution is high under strong excitation and efficient migration. At 77 K, the IL decay constant τ within the exponential region increases with decreasing QW thickness. The decay constant τ in a QW, unlike in a bulk Cd_0.5Mn_0.5Te crystal, decreases substantially under cooling from 77 to 4 K.     

Feasibility Study of VUV Sensitization Effect of Tb3+

The possibility to use Tb³⁺ as luminescence sensitizer for enhancement of the conversion efficiency of vacuum-ultraviolet (VUV) radiation into visible light was examined. We studied the luminescence properties of K₃Tb(PO₄)₂ and Ba₃Tb(PO₄)₃ activated by Eu³⁺, and of SrAl₁₂O₁₉ co-doped with Mn²⁺ and Tb³⁺ at excitation over the 120 to 300 nm wavelength range. It is shown that Tb³⁺ ions, exhibiting a strong absorption band in the VUV, can provide efficient sensitization of Eu³⁺ and Mn²⁺ emissions for excitation in this spectral range, giving rise to intense red and green luminescence, respectively. This study provides a proof for the concept of VUV sensitization, which enables the engineering of luminescence materials with improved efficiency for excitation from a noble gas discharge.     

Spodumene and garnet luminescence excited by subnanosecond electron beams

Pulsed cathodoluminescence of spodumene and yttrium-aluminum garnet crystals activated by Mn²⁺ and Nd³⁺ ions, respectively, is investigated. The luminescence was excited upon crystal irradiation by electron beams with current densities of 35 and 100 A/cm² and average electron energy of ∼ 50 keV for 0.1, 0.25, and 0.65 ns. It is demonstrated that the electron beam duration decreased to several tenth of a nanosecond does not lead to essential changes of the mechanisms of pulsed cathodoluminescence excitation and character of its spectrum, but in this case, the intensity of luminescence of the hole centers increases compared with the intracenter luminescence.     

Sensitivized photoluminescence of manganese in calcium sulfate phosphors

Although the sensitivization mechanism of the excitation of manganese in calcium sulfate phosphors is considered to be the most probable [1] it has not been sufficiently confirmed by direct luminescence measurements, and in certain cases, the results of these measurements contradict each other. In [4] the sensitivizing action of the WO ₄ ^?2 group on the luminescence of the Mn²⁺ ion was established for mixed phosphors based on calcium sulfate and tungstenate, but this effect was not studied in detail. Previously, we observed lattice-defect sensitivization of the radioluminescence of Mn²⁺ in radioactive CaSO₄·Mn phosphors [5]. On the other hand, the work of Rudolph and Ruffer [2] on the cerium sensitivization of Mn²⁺ luminescence is not confirmed by other researchers [3]. In the same work it was shown that Pb, Sn, and Bi also sensitivize Mn²⁺ luminescence in CaSO₄.     

Influence of Mg on luminescence efficiency and charge compensating mechanism in phosphor CaAl12O19:Mn

Synthesized by a modified solid state method in air and mixed with MgO, the red phosphor of CaAl₁₂O₁₉:yMn⁴⁺ (y=0.001-1.5%) enhanced its photoluminescence efficiency by 3.5 times. The influence of MgO on crystal phases, luminescence intensity and spectral characteristics of the composition modified phosphor has been investigated by spectroscopic experiments and luminescence decay measurements. It is observed that the decay time of Mn⁴⁺ luminescence prolongs linearly with increase of MgO in the composition, indicating that the excitation energy transfer and non-radiative relaxation between Mn⁴⁺-Mn⁴⁺ pairs decrease. The presence of Mg²⁺ leads to a transformation of Mn⁴⁺-Mn⁴⁺ pairs connected with interstitial O²⁻ to isolated Mn⁴⁺ ions and therefore eliminates energy transfer and provides charge compensation as well.     

Optical spectroscopy of Mn2+ ions in CaCl2 single crystals

Abstract

In the present investigation the excitation and fluorescence spectra and lifetimes of Mn²⁺ ions in calcium chloride, for various manganese concentrations and sample temperatures have been studied for the first time. The fluorescence spectrum consists of an asymmetric broad band, which upon lowering the sample temperature, shifts its maximum from 580 nm at 300 K to 596 nm at 11 K. This luminescence band was associated with the ⁴T_lg(⁴G)→⁶A_lg(⁶S) spin-forbidden transition in the manganese ions occupying Ca-sites in the lattice of CaCl₂. The excitation spectrum of the Mn²⁺ fluorescence revealed the features of manganese ions in octahedral coordination and consisted of nine excitation peaks which were associated with Mn²⁺-crystal-field-sensitive transitions. A crystal field analysis of the wavelength positions of these transitions by means of the model developed by Curie et al. allowed us to determine the magnitude of the cubic field splitting 10Dq, the reduced Racah parameter B', the Koide-Pryce covalency parameter ε and the spin transfer coefficients f _[sgrave] and f _σ. From the measurement of the temperature dependence of the Mn²⁺ fluorescence lifetime, we have also obtained information about the different mechanisms which are involved in the relaxation of excited Mn²⁺ ions in this host crystal in the temperature range (11–300 K).     

Luminescence of YAG doped with Eu,Yb, and Mn ions under VUV excitation

Hydrothermal synthesis has been successfully used to obtain fine-crystalline powders of yttrium aluminum garnet (YAG) doped with manganese ions and codoped with cerium and manganese ions. Using the method of high-temperature solid-state synthesis, ceramic specimens of YAG that contain europium and ytterbium ions have been obtained. In synthesized YAG:Eu and YAG:Yb ceramics, no luminescence that can be attributed to 5d-4f transitions in Eu²⁺ or Yb²⁺ ions has been detected, even though the scheme of energy levels of these ions constructed with respect to YAG energy bands indicates that there is a potential possibility of the occurrence of 5d-4f luminescence for Eu²⁺ ions in YAG. At room temperature, the luminescence spectrum of hydrothermally synthesized YAG doped with manganese ions consists of two broad bands with maxima at ～600 and ～750 nm and does not contain any narrow bands in the red or IR range. Therefore, the spectrum contradicts to the properties of the luminescence of Mn²⁺, Mn³⁺, or Mn⁴⁺ ions in YAG described in the literature, even though the obtained hydrothermal specimens can contain noticeable concentrations only of Mn³⁺ ions.     

Intra-center energy transfer dynamics in Cd1−xMnxTe semi magnetic semiconductors

Intra-center luminescence of Cd_1?xMn_xTe semi magnetic semiconductors under low excitation density was investigated both experimentally and by Monte-Carlo simulation. Experimental time-resolved spectra of 2 eV-band under different photon energy for excitation were used. The approach revealed that Mn²⁺–Mn²⁺excitation energy transfers take place by means of resonant dipole–dipole interaction. Besides energy transfer dynamics is strongly influenced by hopping-assisted quenching. Having been intra-center excitation selective-, mixed- and non-selective types of excitation are proved to occur if photon energy for excitation is increased. This is originated from overlapping of ⁴T₁- and ⁴T₂-states. Under inter-band excitation it was established that Mn²⁺-ion excitation takes place with the aid of excitonic energy transfer, with excitation energy being centered at exciton energy. Under temperature rise the transfer rate vigorously enhances due to great increase of overlap integral of Mn²⁺- ions' side-bands. The quenching is proved to be limited in accordance with existing theory. Inhomogeneous broadening diminishes as a result of fast fluctuation rate of excited ions' energy.     

New aspects of the luminescence of magnesiumtitanate part II: Activation with manganese

Measurements of the emission and excitation spectra of powdered Mg₂TiO₄:Mn⁴⁺ phosphors reveal that single Mn⁴⁺ ions are not responsible for the luminescence — as assumed till now — but so-called N centres. These centres are excited either by direct UV absorption or by radiationless energy transfer from the excited ²E state of the Mn⁴⁺ ions to the N centres. The Mn⁴⁺ ions absorb the energy corresponding to the transition ⁴A_2g→ ²E which is used for the excitation of the N centres, but do not emit the corresponding lines. The observed different luminescence spectra (sharp lines or broad bands) depend on the annealing conditions and indicate that the N centres consist of Mn⁴⁺ ions associated with unknown lattice defects. The annealing process does not only form the tetravalent state of the manganese, but creates above all the N₁ centres, which emit sharp lines. Two more lines observed at 697.8 nm and 699.4 nm, called R^m lines, are due to a certain amount of MgTiO₃:Mn⁴⁺ in the phosphor.     

Thermal dependence of impurity induced fluorescence in the system RbMgxMn1-xF3

The absorption spectrum of RbMnF₃ and the excitation spectra of the system RbMg_xMn_1-xF₃ at 10 K as well as the fluorescence spectra and lifetimes of Mn²⁺ in the systems RbMg_xMn_1-xF₃ and KMg_xMn_1-xF₃ in the region 10–300 K were measured. The lifetime and fluorescence temperature dependence suggest that the origin of the fluorescence occurs at Mn²⁺ sites slightly perturbed by impurity ions and that a non-radiative energy transfer mechanism is responsible for the observed thermal quenching. By using different Mn²⁺ concentrations in the above systems the dependence of the energy transfer on the Mn²⁺ concentration is shown. Finally, a preliminary observation on laser stimulated Mn²⁺ luminescence in the system RbMg_xMn_1-xF₃ is reported.     

长波紫外激发下的三基色发光玻璃

报道了一种新的可用长波紫外有效激发的三基色发光玻璃.这种三基色荧光玻璃样品是在相同硅硼酸盐基质中掺杂Ce³⁺/Mn²⁺, Ce³⁺/Tb³⁺以及Eu²⁺并采用熔融法制备出的.其中Mn²⁺,Tb³⁺,Eu²⁺作为激活离子,Ce³⁺作为敏化剂向激活离子提供能量.由于敏化剂的加入使这种三基色发光玻璃在长波紫外激发下获 关键词： 三基色发光 能量传递 长波紫外 发光玻璃     

Concentration quenching and migration of excitations in a bulk Cd<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>Te crystal

The curves of intracenter luminescence decay for Mn²⁺ ions in the Cd_0.5Mn_0.5Te semiconductor solid solution, obtained in a low-temperature experiment, have been simulated by the Monte Carlo method. The features of the kinetics of the 2-eV band in the time interval where significant nonexponentiality of relaxation at different points of the emission band profile manifests itself, as well the integral kinetics and energy relaxation, have been considered. Migration of ion excitations and concentration quenching (which was previously disregarded) are considered to be the main mechanisms determining the kinetic curve formation. It was established that excitation by 2.34-eV photons leads to both selective (intracenter) and band excitation of Mn²⁺ ions. Comparison of the results of numerical simulation and experiment showed that the characteristic values of the migration and quenching rates (W _m and W _q , respectively) are close in magnitude and W _{q, m} ≈ 0.1/τ, where τ is the lifetime at the long-wavelength band wing with the exponential kinetics. The estimated quantum yield (0.56) indicates significant influence of the concentration quenching on the 2-eV luminescence quantum yield in Cd_{1 ? x} Mn _x Te and Zn_{1 ? x} Mn _x S crystals with a high concentration of Mn²⁺ ions.