Photoluminescence and luminescence excitation spectra in ZnSiP2

Summary Measurements of photoluminescence and luminescence excitation spectra of ZnSiP₂ have been performed at 4.2K and two results were obtained. One is the observation of a new sharp emission line at 1.980 eV, due to the bound exciton associated with the pseudodirect gap. The other is the observation of another new series of absorption lines in the luminescence excitation spectrum of an emission line, at 1.984 eV, in addition to those reported previously. These results indicate that in ZnSiP₂ radiative transitions occur at both the indirect and the pseudodirect gaps. Paper presented at the ?V International Conference on Ternary and Multinary Compound?, held in Cagliari, September 14–16, 1982.     

The temperature dependence of the edge emission of GaSe single crystals at high excitation levels

The temperature dependence of five edge emission lines of GaSe at high excitation levels has been investigated by using the second harmonic of a neodymium-glass laser. The following lines were indentified at 77.3K:hv_B = 2.102 ± 0.002 eV, annihilation of free excitons; hv_C = 2.082 ± 0.003 eV, Auger recombination of free excitons; hv_D = 2.072 ± 0.003 eV and hv_E = 2.055 ± 0.004 eV, annihilation of free excitons with emission of optical phonons (LO₁ = 31 meVandLO₂ = 45 meV, respectively); hv_G = 2.036 ± 0.004 eV, radiative recombination at indirect transition with emission of LO₁ phonons.     

Luminescence of mercuric iodide crystals

Low-temperature (4.2–130 K) photoluminescence spectra of HgI₂ crystals have been measured in the 540–700 nm region. An analysis of the characteristics (intensity vs temperature and excitation power relations, afterglow times, excitation spectra) of the 560, 620, and 635 nm emission bands suggests the following assignments: the 560 nm band is due to radiative annihilation of excitons bound to mercury vacancies, and the “red” emission originates from recombination of free (620 nm) and donor-localized (635 nm) electrons with a hole-filled acceptor level. The energies of the corresponding donor and acceptor levels have been estimated. New emission bands at 540, 545, and 575 nm have been discovered, and their origin discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 67–73 (January 1997)     

Temperature dependence of the fine structure of the C and E absorption bands in RbMnF3 below the Néel temperature

The variation in the parameters (width, position, intensity) of the fine structure lines in the C[⁶ A _1g → ⁴ A _1g , ⁴ E _g(⁴ G)] and E[⁶ A _1g → ⁴ E _g(⁴ D)] bands in RbMnF₃ with temperature is studied in the temperature range 10–70 K. In the C band, two narrow (<6 cm^?1) lines are are distinguished at distances of 77 and 80 cm^?1 from the exciton line at T = 10 K. The other lines in the C band and all lines in the E band are more than 20 cm^?1 wide. It is demonstrated that the narrow lines become allowed because of the spin-exchange interaction within a long-range magnetic order model and originate from the excitation of exciton-magnon bound states and that the other lines are made allowed by the exchange-vibronic mechanism within a short-range magnetic order model and originate from the excitation of bound states composed of an exciton, magnon, and oddparity phonon. The vibrational replicas of the main exciton-magnon-phonon lines are due to the quadratic vibronic interaction with odd-parity vibrations. Variations of the intensities and widths of the absorption lines with temperature indicate that these parameters are affected by relaxation and delocalization of the bound states.     

Luminescence study of the 4f5d configuration of Nd in LiYF4, LiLuF4 and BaY2F8 crystals

Ultraviolet fluorescence of Nd³⁺ ions induced by triphotonic excitation process was studied in Nd-doped LiYF₄, LiLuF₄ and BaY₂F₈ crystals using a technique of time-resolved spectroscopy. The observed ultraviolet luminescence was due to transitions between the bottom of 4f²5d configuration and 4f³ states of Nd³⁺ ions. Narrow emission lines superposed to the broadband emissions were observed. A detailed analysis of luminescence spectrum revealed that the narrow emissions are due to parity and spin allowed radiative transitions from the Stark levels of ⁴K_11/2(5d) state created by the electrostatic interaction between the 5d electron and the two electrons of the 4f² configuration. The narrow emissions are related to the high spin state (S=3/2) which gives f-f characteristics to the f-d broadband emissions. The narrow emissions superposed to the wide emission correspond to 18%, 34% and 43% of the integrated broadband emission at 262 nm observed in LiYF₄, LiLuF₄ and BaY₂F₈ crystals, respectively. Although the 5d-4f² interaction is observed to be weaker than 5d-crystal field interaction, it is stronger enough to select only the radiative transitions from 4f²5d configuration to 4f³ states that preserves the total spin S=3/2.     

Electron excitations in LiB3O5 crystals with defects: Low-temperature time-resolved luminescence VUV spectroscopy

The dynamics of electron excitations and luminescence of LiB₃O₅ (LBO) single crystals was studied using low-temperature luminescence vacuum ultraviolet spectroscopy with a subnanosecond time resolution under photoexcitation with synchrotron radiation. The kinetics of the photoluminescence (PL) decay, the time-resolved PL emission spectra, and the time-resolved PL excitation spectra of LBO were measured at 7 and 290 K, respectively. The PL emission bands peaking at 2.7 eV and 3.3 eV were attributed to the radiative transitions of electronic excitations connected with lattice defects of LBO. The intrinsic PL emission bands at 3.6 and 4.2 eV were associated with the radiative annihilation of two kinds of self-trapped electron excitations in LBO. The processes responsible for the formation of localized electron excitations in LBO were discussed and compared with those taking place in wide-gap oxides.     

Optical properties of Tm-doped GaSe single crystals

Optical properties of Tm-doped GaSe single crystals were investigated by measurements of optical absorption and photoluminescence. The single crystals were grown by the Bridgman technique. The X-ray diffraction analysis revealed that the single crystals were in the ε-type GaSe phase. The optical absorption spectra showed a sharp absorption peak at 582 nm near the band edge, which is due to direct free exciton. The temperature dependence of the energy of the exciton absorption peak was well fitted by the Varshni relation. In the photoluminescence spectrum at 10 K, we observed a very weak emission peak at 586 nm, a relatively strong emission peak centered at 613 nm, and several sharp and narrow emission peaks in the 790-840 nm region. The two emission peaks at 586 and 613 nm were associated with intrinsic emission lines due to direct free exciton and indirect bound exciton. The emission peaks in the 790-840 nm region, which were related to extrinsic emission, were assigned as due to the ³F₄→³H₆ transition of Tm³⁺ ions with a low symmetry of D₃ in the host lattice.     

Resonant exciton trapping at impurity states in silver bromide

Excitation spectra near the indirect exciton edge of AgBr at 1.8K are reported for several luminescence lines from weakly localized excitons. Excitation below the exciton absorption threshold reveals several excited bound exciton states the energetic positions of which are determined. For excitation above the threshold, strong energy dependent structure is observed. It is interpreted in terms of resonant trapping of free excitons in both ground and excited bound exciton states associated with emission of LO(Γ), long wavelength acoustic and intervalley TA(X) and LA(X) phonons as well as combinations and overtones of these. From measurements in doped crystals two bound exciton systems are found to be correlated with Cd²⁺ and Ca²⁺, respectively.     

Spectroscopic investigation of thallium 6<Emphasis Type="Italic">s</Emphasis>6<Emphasis Type="Italic">p</Emphasis><Superscript>2</Superscript> configuration at electron-atom collisions: I. Spectra

Thallium emission spectra in the 115–300 nm range excited by electron-atom collisions at electron energies of 12–300 eV are investigated. A number of weak lines that cannot be unidentified using spectroscopic tables are found in the 140–170 nm range. Two of them (144.9 and 148.0 nm) are attributed to the radiative decay of levels that belong to the 6s6p ² configuration and that lie above the ionization potential. A weak emission with an excitation threshold of about 9 eV is revealed in the vicinity of the Tl II resonance line at 132.2 nm. The excitation function of the emission is measured for electron energies below 15.5 eV. It is found that the emission consists of two lines, which also appear due to the excitation of the 6s6p ² configuration and correspond to the radiative decay of levels that are common with the lines at 144.9 and 148.0 nm mentioned above. Their calculated wavelengths are 130.2 and 132.7 nm, respectively.     

Radiative properties of Eu in BaGa2S4

A photoluminescence study of the blue-green emitting BaGa₂S₄:Eu²⁺ phosphor is reported. Diffuse reflectance, excitation and emission spectra were examined with the aim to enlarge the fundamental knowledge about the emission of the Eu²⁺ rare earth ion in this lattice. The thermal dependence of the radiative properties and the influence of the Eu²⁺ concentration were investigated. The Stokes shift, the crystal field splitting and the activation energy of the thermal quenching were determined. By combining these results with data available in literature, we discussed the radiative properties of the BaAl₂S₄:Eu²⁺ blue phosphor in relation with those determined in this study for the isostructural BaGa₂S₄:Eu²⁺ phosphor.     

The location of the lowest conduction band minima in gallium phosphide from bound exciton luminescence

Two weak satellite series with acceptor-independent displacement energies of 4.2₃ meV and 7.8₃ meV have been observed in the luminescence of excitons bound to neutral acceptors in GaP. These satellites contain broad background luminescence. Rather well defined superimposed peaks show relative strength which increases dramatically with decrease in exciton binding energy. The most plausible mechanism for these satellites involves bound exciton recombination with emission of one phonon to conserve momentum in the indirect transition and one or two further phonons to conserve momentum in g-type inter-valley scattering processes. This model is consistent with all known properties of the satellites and is strongly supported by a quantum-mechanical line width calculation. The narrow components arise from a diffuse tail on the bound exciton wave-function which is enhanced by the electron-hole correlation. The existence of this g-scattering process shows that the conduction band minima in GaP lie at 0.953 K^max_〈100〉, not exactly at K^max_〈100〉 as believed hitherto. This revision has important consequences for several properties of n-type GaP.     

Optical properties of Ho-doped novel oxyfluoride glasses

Novel oxyfluoride glasses SiO₂-Al₂O₃-Na₂O-ZnF₂ doped with Ho³⁺ and Ho³⁺/Yb³⁺ were fabricated. The optical properties of the synthesized glasses were experimentally and theoretically investigated in detail. The experimental and calculated oscillator strengths of Ho³⁺ were determined by measurement of absorption spectrum of Ho³⁺-singly doped glass. According to the Judd-Ofelt theory, the Judd-Ofelt parameters were calculated, by which the radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were obtained. Visible upconversion luminescence was observed under 980 nm diode laser excitation and the influence of Yb³⁺ concentration on the emission bands were also investigated. The dependence of the upconversion emission intensity upon the excitation power was examined, and the upconversion mechanisms were discussed.     

Luminescence properties of novel NaBa4(BO3)3:Ce, Eu phosphors

Novel Eu³⁺, Ce³⁺ activated NaBa₄(BO₃)₃ phosphors were synthesized by solid-state reactions. The excitation spectrum of NaBa₄(BO₃)₃:Ce³⁺ consists of an intense band peaking at 350 nm and a weak band in the higher energy side, and the emission spectrum exhibits a blue band with a maximum at about 420 nm. The Eu³⁺ emission in NaBa₄(BO₃)₃ consists of the transitions from ⁵D₀ to ⁷F_J, and the excitation spectrum consists of broad excitation band peaking at 270 nm and some intense narrow lines. The optimum doped concentration, the critical distance of the concentration quenching, and the fluorescence lifetime have also been investigated.     

Photoluminescence degradation mechanism of BaMgAl10O17：Eu2＋ phosphor by vacuum ultraviolet irradiation

<正>A real high power vacuum ultraviolet light source is applied to the investigation on the vacuum ultraviolet irradiation degradation of BaMgA₁₀O₁₇:Eu²⁺ phosphor.The degradations of emission intensity and color quality of the sample are clearly observed after irradiation.It reveals that the oxidation of Eu²⁺ during irradiation is partly responsible for the degradations.The excitation and absorption spectra show that some traps generated during irradiation have negative influence on the luminescence of sample and these traps have been identified as positively charged oxygen vacancies by positron annihilation.The investigations on host emission and decay curve further confirm that these oxygen vacancies are involved in the perturbation of energy transfer from the host to Eu²⁺ and finally result in the degradation.     

Radiative transition of two indirect excitons to direct exciton in thallous halides

Weak emission lines are observed at the energies far lower than the energies of edge emissions in TlCl and TlBr. They are explained as due to the radiative decay of two indirect X⁺₆ × R^?₆ excitons, both at the same M-point, by leaving one direct X⁺₆ × X^?₆ excitons through zero-phonon process.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Photoluminescence of Eu in SrGa2S4

A photoluminescence (PL) study of the green-emitting SrGa₂S₄:Eu²⁺ phosphor is reported. Diffuse reflectance, excitation, and emission spectra were examined with the aim to enlarge the fundamental knowledge about the emission of the Eu²⁺ ion in this lattice. The thermal dependence of the radiative properties was investigated. In particular, the Stokes shift, the crystal field splitting and the activation energy of the thermal quenching were determined. By combining these results with the information presented in literature, we discussed the location of the Eu²⁺ levels relative to the valence and conduction bands of SrGa₂S₄.     

Low-temperature X-ray and photoluminescence of lead halide crystals

The emission spectra of PbF₂, PbCl₂ and PbBr₂ monocrystals are measured under optical and X-ray excitation at liquid helium temperature. Certain emission bands are attributed to the radiative transitions in the self-trapped cation exciton (excited Pb²⁺ion). The increase of trapped exciton via electron-hole recombination is discussed.     

Magneto-Luminescence of excitons bound to neutral acceptor in cadmium telluride

The radiative recombination of excitons bound to neutral acceptor (A⁰, X) in high-purity CdTe has been investigated in magnetic fields up to 100 KG. A doublet structure observed in (A⁰, X) emission line is explained by considering the j-j coupling between two holes and an electron. Zeeman splittings in emission lines originate from the transition between J = 1/2 and 3/2 states in bound-exciton complex, and the acceptor-ground state. It is supposed that the acceptor which binds excitons is due to certain complex having C_3υ symmetry which is lower than the host lattice.     

Spectroscopic investigations and luminescence spectra of Nd and Dy doped different phosphate glasses

Spectral properties of Nd³⁺ and Dy³⁺ ions in different phosphate glasses were studied and several spectroscopic parameters were reported. Covalency of rare-earth-oxygen bond was studied in these phosphate glass matrices with the variation of modifier in host glass matrix. Using Judd-Ofelt intensity parameters (Ω₂, Ω₄ and Ω₆), radiative transition probabilities (A) and radiative lifetimes (τ_R) of certain excited states of Nd³⁺ and Dy³⁺ ions are estimated in these glass matrices. From the magnitudes of branching ratios (β_R) and integrated absorption cross-sections (Σ), certain transitions of both the ions are identified for laser excitation. From the emission spectra, peak stimulated emission cross-sections (σ_P) are evaluated for the emission transitions observed in all these phosphate glass matrices for both Nd³⁺ and Dy³⁺ ions.