Cooperative emission of photons by weakly coupled chromium ions in YAG and LaAlO3

In YAG:Cr³⁺ and LaAlO₃:Cr³⁺ we have found cooperative emissions of one photon by two excited Cr³⁺ ions analogous to those recently observed by us in Al₂O₃:Cr³⁺. The vibronic sideband of this fluorescence confirms our earlier identification of the photons involved with the polar longitudinal optical modes of the host lattice. Fluorescence decay measurements again indicate radiationless decay processes of the doubly excited pair states leading to non-equilibrium R₂ emission. A fluorescence ²A₁→⁴A₂ of chromium ions in YAG has been detected for the first time.     

Electron paramagnetic resonance and photoluminescence properties of α-Al2O3:Cr3+ phosphors

The red emitting Cr³⁺ activated α-Al₂O₃ powder phosphor has been prepared by easy combustion reactions from mixed metal nitrate reactants and urea with ignition temperatures of 500 °C. The as-synthesized powder was characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared techniques. The X-ray diffraction pattern reveals that the phosphor crystallized in the hexagonal α-Al₂O₃ phase directly from the combustion reaction. The EPR spectrum exhibits an intense resonance signal with effective g value at g=3.33 along with a few weak resonance signals with effective g values at g=13.7, 2.34, 1.95, 1.49, and 1.26. The spin concentration (N) and its paramagnetic susceptibility (χ) have been evaluated. The excitation spectrum consists of two broad intense bands at 415 nm and 555 nm and are assigned to ⁴A_2g (F)→⁴T_1g (F) and ⁴A_2g (F)→⁴T_2g (F) transitions, respectively. The intense fluorescence peak around 691 nm is attributed to ²E _g →⁴A_2g transition of Cr³⁺ ion. By correlating EPR and optical data, the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) have been evaluated and discussed. The EPR and optical studies reveal that Cr³⁺ ions are occupying in Al³⁺ sites in octahedral coordination.     

Optical spectroscopy of Cr3+ ions in LiF single crystals

This paper reports on the luminescence spectrum, its excitation-wavelength dependence and decaytime of Cr³⁺:LiF crystals. In contrast to Al₂O₃ and MgO, where the Cr³⁺ luminescence at low temperature is due to the R-line and its vibronic sideband, luminescence from Cr³⁺:LiF is assigned to the broad-band ⁴ T ₂⁴ A ₂ transition. A periodic structure in the vibronic sidebands of this transition occurs with energy separations of ca. 60 meV. The linear polarisation of the vibronic sidebands shows that their radiative transition takes place from the lowest excited level of the ⁴ T ₂ state, i.e. ⁴ B ₁, into the ⁴ A ₂ ground state of Cr³⁺ ions in orthorhombic symmetry sites. The crystal-field parameters calculated from the experimental data, and luminescence decay-time of the two polarized configurations of the ⁴ B ₁(⁴ T ₂)⁴ A ₂ transition are also reported.     

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).     

Influence of spin-orbit coupling on vibronic spectra of metalocomplexes porphin: Manifestation of out-of-plane vibrations

In fine-structure phosphorescence spectra of metallocomplexes of porphin with ions of the Pd(II) and Pt(II) and their meso-deuterated derivatives additional lines have been detected which have no analogs in fluorescence and resonance Raman spectra of metalloporphyrins and in phosphorescence spectra of metallocomplexes of porphin with light ions of the Mg(II) and Zn(II). For Zn-porphin, quantum-chemical calculations of frequencies and forms of in-plane and out-of-plane vibrations have been performed. Based on experimental data and calculation results it has been found, that in vibronic phosphorescence spectra of metallocomplexes of porphin, out-of-plane gerade modes of the E _g symmetry (D _4h symmetry group) are manifested. The activity of out-of-plane vibrations increases with enhancing spin-orbital coupling upon changing to heavier chelated metal ions. Vibronic transitions with participation of out-of-plane gerade E _g vibrations manifest in the T ₁ → S ₀ transition through the vibronic intensity borrowing from the triplet-triplet ³ E _u -³ E _g transition.     

Some detailed investigations of optically detected Ē(2 E) →4 A 2 cross relaxation in ruby

Cross relaxation betweenē(² E) and⁴ A ₂ states of Cr³⁺ in ruby at an applied external magnetic field ofH=5336 Oe was measured by monitoring the ¦? 1/2〉_ex→ ¦? 3/2〉_g optical transition in the temperature region of 1.6 to 4.2 °K. The chromium concentration varied from 2.9· 10^?4 to 4 · 10^?6 Cr³⁺/Al³⁺. With a concentration greater than 2 · 10^?5, the light intensity of the observed transition increases when cross relaxation takes place, while below this value it decreases. By measurement of the fluorescent intensity of one transition and simultaneously inducing EPR ground state transitions, we monitored the effect of trapping. Taking the value for trapping from fluorescence decay time measurements, we have used rate equations for calculating the actual change of excited state population when cross relaxation occurs. With this phenomenological model we are able to explain our experimental data. Finally some calculations for the effective spin temperature in theē(² E) state as a function of Cr³⁺ concentration as well as for various applied magnetic fields have been done.     

Luminescence of impurity 3d and 4f metal ions in different crystalline forms of Al2O3

The ² E-⁴ A ₂ luminescence spectra of Cr³⁺ ions in Al₂O₃ are investigated in the course of transitions between the structural forms γ-δ-θ-α. The spectral lines observed are assigned to Cr³⁺ ions in these structural forms, which are identified by an X-ray powder diffraction analysis. The lifetimes of the Cr³⁺ excited states in transient forms of Al₂O₃ are measured. Investigations of the luminescence spectra of Al₂O₃: Eu³⁺ demonstrate that the Eu³⁺ ions can form regular centers only in α-Al₂O₃ and, unlike the Cr³⁺ ions, give no rise to similar centers in moderately ordered θ-Al₂O₃.     

Spectrum and energy levels of Cr3+ ions and exchange-coupled Cr3+ pairs in lanthanum aluminate

From the optical spectra the energy levels of the Cr³⁺ ion in LaAlO₃ were determined. In the cubic field approximation we obtained with the parametersDq=1750cm^-1,B=661 cm^-1, andC=2908 cm^-1 a good agreement between the calculated and measured energies. For higher Cr³⁺ concentrations the fluorescence spectrum of the exchange-coupled first nearest (1N) Cr³⁺ pairs was separated from the vibronic fluorescence spectrum of the single ions by selective excitation. From the experimental data the energy level scheme of the 1N pairs is constructed for the states ¦⁴ A ₂·⁴ A ₂〉 and ¦⁴ A ₂·² E〉. In the ground state the pairs are ordered antiferromagnetically due to an isotropic exchange interaction with an exchange integralJ=(?68 ± 1) cm^-1. From the fluorescence decay the intrinsic lifetimes of the fluorescent states of the single ions, the 1N and the 2N pairs were determined to be equal to (86± 4) msec, (1.75± 0.25) msec, and (31 ± 5) msec respectively. The fluorescence decay shows an energy transfer between the single ions and the pairs.     

Combustion synthesized MgAl2O4:Cr phosphors—An EPR and optical study

Magnesium aluminate (MgAl₂O₄) doped with trivalent chromium (Cr³⁺) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer-Emmet-Teller (BET) adsorption isotherms and diffuse-reflectance UV-vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110 K. The EPR spectrum exhibit resonance signals at g=5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr³⁺ ions. The luminescence of Cr³⁺-activated MgAl₂O₄ exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper ²E_g→⁴A_2g ground state of Cr³⁺ ions. By correlating EPR and optical data the crystal field splitting parameter (D_q), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr³⁺ ions with the ligands and the Cr³⁺ ions are in distorted octrahedral environment.     

Spectral studies on Cr ions doped in sodium-lead borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption and emission spectra of Cr³⁺ ions doped in (30−x) (NaPO₃)₆+30PbO+40B₂O₃+xCr₂O₃ (x=0.5, 2.0, 3.0, 4.0 and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g values at g≈4.55 and g≈1.97. The EPR spectra of x=3.0 mol% of Cr₂O₃ in sodium-lead borophosphate glass sample were studied at various temperatures (295-123 K). The intensity of the resonance signals increases with decrease in temperature. The optical absorption spectrum exhibits four bands characteristic of Cr³⁺ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibit one broad band characteristic of Cr³⁺ ions in octahedral symmetry. This band has been assigned to the transition ⁴T_2g (F)→⁴A_2g (F). Correlating EPR and optical data, the molecular bonding coefficient (α) has been evaluated.     

Spectroscopy of multisites Cr3+ and Nd3+ in yttrium aluminium garnet

Cr³⁺ and Nd³⁺ non-equivalent crystal field sites were observed. By use of a high-resolution dye laser,²E and⁴A₂ splittings, zero-phonon⁴A₂ →²E,²T₁,⁴T₂ transitions and vibronic levels of²E doublets of Cr³⁺ non-equivalent centres were recorded. Energy transfers from Cr³⁺ multisites to Nd³⁺ were studied by site-selective spectroscopy and fluorescence decays.     

On the theory of lasing at vibronic transitions in impurity crystals

Some aspects of lasing at vibronic transitions in impurity crystals are theoretically studied. The threshold conditions for a vibronic laser are shown to be dependent on the strength of the interaction of optical centers with a local vibration, which forms the vibronic spectrum, and the crystal lattice temperature. The theory can easily be generalized to the spectrum containing a structureless phonon sideband and well agrees with the experimental temperature dependence of the output power of a Mg₂SiO₄:Cr⁴⁺ forsterite laser.     

YGG:Cr3+晶体的光谱特性

本文通过实验研究了YGG:Cr³⁺晶体的光谱特性,报道了室温下的吸收谱,10,133,300K的荧光谱,以及荧光寿命、无辐射跃迁几率、辐射量子效率与温度之间的依赖关系。从吸收谱及荧光谱中确定在C_3i(S₆)低对称场微扰下,Cr³⁺离子在基质YGG中²T₁能级分裂的子能级及基态⁺A₂→²E零声子跃迁R线的位置。 关键词：     

Vibrational Analysis of the Elpasolites Cs2NaAlF6 and Cs2NaGaF6 Doped with Cr3+ Ions by Fluorescence Spectroscopy

Interest in 3d transition metal impurities in ionic crystals has increased due to their important role in the laser activity of these materials. Moreover, recent advances in tunable solid-state lasers and high-power semiconductor laser diode arrays have generated a strong interest in investigating new compounds that emit in the visible and near-infrared spectral regions. In particular, many optical studies have been devoted to Cr³⁺-doped fluoride crystals as a consequence of the high quality of some Cr³⁺-based laser materials. In the present investigation, the low-temperature emission spectra of Cr³⁺ ions in the hexagonal elpasolites Cs₂NaAlF₆ and Cs₂NaGaF₆ have been measured. Each compound has two crystallographically inequivalent octahedral sites for the Al³⁺ and Ga³⁺ ions that can be occupied by Cr³⁺ ions. For both materials, the luminescence spectrum presents two zero-phonon lines accompanied by a well-defined vibrational structure. The different peaks of the emission broad band are described in terms of phonons of the lattice and normal modes of the octahedral complex [CrF₆]^3–. A detailed analysis of the vibrational structure observed leads to the conclusion that the ² E and ⁴ T ₂ excited states of the [CrF₆]^3– ion are displaced along the e _g and a _1g and probably the t _2g coordinates.     

Electron paramagnetic resonance and optical absorption studies of manganese ions doped in polyvinyl(alcohol) complexed with polyethylene glycol polymer films

Electron paramagnetic resonance (EPR), optical absorption, and infrared spectral studies have been carried out on Mn²⁺ ions doped in poly(vinyl alcohol) (PVA) complexed with polyethylene glycol (PEG) films prepared by solution cast technique. The EPR spectra of 0.25?mol% Mn²⁺ ions doped polymer complex (PVA+PEG) at room temperature exhibit sextet hyperfine structure (hfs), centered at g????1.99. The spin?CHamiltonian parameter values indicate that the ground state of Mn²⁺ ion is d⁵ and the site symmetry around Mn²⁺ ions in tetragonally distorted octahedral site. The spin concentration participating in the resonance is measured as a function of temperature and it is observed that it obeys Boltzmann??s law. The paramagnetic susceptibility (??) is calculated from the EPR data at various temperatures (93?C333?K) and it obeys the Curie?CWeiss law. The optical absorption spectra exhibits two bands which are assigned to ⁶A_1g (S)??⁴A_1g (G) or ⁴E_g (G) and ⁶A_1g (S)??⁴T_2g (G) transitions. The infrared spectrum exhibits few bands due to the presence of O?CH, C?CH, and C=C groups.     

Spectroscopy of multisites Cr3+ and Nd3+ in yttrium aluminium garnet

Cr³⁺ and Nd³⁺ non-equivalent crystal field sites were observed. By use of a high-resolution dye laser,²E and⁴A₂ splittings, zero-phonon⁴A₂ ²E,²T₁,⁴T₂ transitions and vibronic levels of²E doublets of Cr³⁺ non-equivalent centres were recorded. Energy transfers from Cr³⁺ multisites to Nd³⁺ were studied by site-selective spectroscopy and fluorescence decays.     

Radiative transitions and spectral-hole burning in MgO: Cr<Superscript>3+</Superscript> nanocrystals

Luminescence spectra of Cr³⁺ ions in nanocrystalline MgO obtained by sol-gel technology were studied. The radiative lifetime of excited ²E states of cubic and tetragonal Cr³⁺ centers was found to be considerably longer than that of bulk single crystals. This effect is caused by a modified effective refractive index of the inhomogeneous medium. Burning of long-lived spectral holes in the ⁴A₂-²E transition profile of cubic centers was observed, which sets nanocrystalline samples strongly apart from bulk crystals, where no hole burning was found.     

Phonon density of states of CdI2 deduced fro,the fine structure of the 3A2g → 1Eg electronic transition of CdI2:Ni2+

The absorption and MCD spectra of the ³A_2g → ¹E_g transition of CdI₂:Ni²⁺ at about 12,500 cm^?1 have been measured. The unusual vibronic fine structure is explained by the coupling of the phonons of the CdI₂ lattice to the electronic states of Ni²⁺. The dispersion curves for the acoustical and optical branches in the Brillouin Zone of CdI₂ are deduced.     

Charge compensation and the luminescence of Cr3+ in KMgF3

Studies of the photoluminescence spectra of Cr³⁺ ions in KMgF₃ crystals co-doped with Cr³⁺ and Ni²⁺ ions are reported. Several crystal field sites are identified by the different R-line spectra due to the ² E⁴ A ₂ transition and broadband luminescences associated with the ⁴ T ₂⁴ A ₂ transitions. Cr³⁺ ions substituting without local charge compensation on the octahedral Mg²⁺ site give rise to a low temperature R line in photoluminescence at =702.3 nm with a radiative decaytime of 3 ms at T=14 K. At T=300 K this isotropic centre gives rise to an unpolarized broadband ⁴ T ₂⁴ A ₂ emission, which results from the thermal occupancy of an excited ⁴ T ₂ state just above the ² E level which, at lower temperature, gives rise to emission in the R-line. Other crystal field sites are due to some Cr³⁺ ions having Mg²⁺ or K⁺ vacancies in nearest-neighbour positions, these vacancies being required to maintain charge neutrality in doped fluoride perovskites. The Cr³⁺–K⁺ vacancy complex results in the centre having trigonal symmetry, and low temperature, photoluminescence via R ₁ and R ₂ lines at 716.8 nm and 716.0 nm, respectively. Finally, Cr³⁺ ions having a nearest neighbour Mg²⁺ vacancy have tetragonal symmetry, experiencing weak crystal fields. In consequence, the ⁴ T ₂ level lies below ² E and the photoluminescence spectrum at low temperature takes the form of a polarized broad ⁴ T ₂⁴ A ₂ band with peak at 760 nm and radiative decaytime of 54 s.     

Intensity and bandwidth of multiphonon vibronic transitions of rare-earth ions in crystals

The theory of multiphonon vibronic coupling to electronic transitions is applied in analysing fluorescence spectra of Eu²⁺ in BaFCI, which consist of the 4f⁷(⁶P_7/2,) → 4f⁷(⁸S_7/2) and 4f⁶5d → 4f⁷ transitions, and the 4f⁷-4f⁶5d excitation spectrum of Ce³⁺ in YPO⁴. The 4f electrons are weakly coupled to lattice vibration modes so that only weak one- and two-phonon sidebands are observable in the 4f-4f optical transitions, whereas the electron-phonon coupling is significantly stronger for a 5d electron. Accordingly, intensive multiphonon vibronic transitions overwhelmingly dominate the 4f⁶5d → 4f⁷ spectrum. It is shown that the extended Judd-Ofelt theory for weak vibronic coupling in the framework of the M-process is equivalent to the Huang-Rhys theory for the δ-process. In the analysis of experimental data, contributions from local ligand modes and lattice acoustic modes are separated, and the coupling strength is evaluated, in terms of the Huang-Rhys parameter S, for the 4f-4f and 5d-4f vibronic transitions.