Charge Transfer Luminescence in Ce-Doped Fluorides

New emission bands were found in Ce^3? doped SrF₂ and BaF₂ crystals under excitation into the charge transfer region. The bands at 4.85 eV in BaF₂ and 4.35 eV SrF₂ were found in all crystals with Ce-concentration from 0.001 to 1 mol.%, most prominent in 0.01% samples. Decay of luminescence was simple exponential in BaF₂, decay time decreased from 1.3 ms at 65 K to 0.41 μs at 523 K. The decay curve in SrF₂ shows t^?0.3 dependence at room temperature. All experimental results were naturally explained by assumption that new emission belongs to radiative charge transfer recombination in Ce^2?—F_i^o pairs with different distances between them.     

X-ray stimulated luminescence in MgO

Studies have been made of the emission spectrum of MgO crystals induced by X-irradiation at 90 K. Two bands (half-widths ～0.8 eV) were observed to peak at 4.95 and 3.2 eV, respectively, in high purity crystals. Doping with 100 ppm or greater of Fe, Co, Cr, Cu, Mn, and Ni suppressed the luminescence, though in the MgO:Ni crystal the 2.3 eV Ni²⁺ band due to the ¹T_2g→³A_2g transition was observed. In deuterium-doped crystals the ratio of the intensity of 3.2–4.95 eV emission was found to be 1.2 as compared to 8 for the undoped crystals. Prior exposure of the pure crystals to ionizing radiation enhances the 4.95 eV band by a factor of three while not affecting the 3.2 eV band. This enhancement of intensity decays in several stages upon standing at room temperature in a way that reflects the thermal stability of the various components of the composite V-band absorption. These facts together with the observation that the 210 K thermoluminescence peak is composed entirely of 4.95 eV emission indicate that this luminescence band is associated with the recombination of an electron with a hole located in a V-type center, i.e. O?□ + e → (O²?□)1 → O²?□ + 4.95 eV, where the square indicates that the perturbing positive ion vacancy is adjacent to the oxugen ion which has captured the hole. In MgO:Li⁺, which exhibits no V-type centers upon irradiation, the 4.95 eV band was absent and a 2.9 eV emission which may be associated with recombination at the [Li]⁰ center was observe.     

Decay Time of 2.09 eV Luminescence of S2--Vacancy Centers and Their Optical and Thermal Stability in NaCl:S2 Crystals

An investigation was carried out on decay time of the 2.09 eV emission of S²–vacancy pair centers in NaCl crystals. When the crystal was excited by a pulsed 337 nm light from an N₂ laser, the 2.09 eV yellow luminescence principally decayed with the lifetime of 14.2 μs at low temperatures, and exhibited a weak thermal quenching with an activation energy of 51.3 meV above about 150 K. Such S^2--vacancy pair centers responsible for the 2.09 eV luminescence were thermally stable at room temperature, and not bleached even when exposed to ultraviolet lights below about 5 eV. Thus, the 2.09 eV emission center possibly can be used as a solid-state laser active center working at room temperature.     

Spectroscopic study of double sodium-yttrium fluoride crystals doped with erbium Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Er<Superscript>3+</Superscript>: I. Intensities of luminescence spectra and kinetics of luminescence

Na_0.4Y_0.6F_2.2:Er³⁺ (NYF:Er) crystals with an erbium concentration as high as 100 at. % (Na_0.4Er_0.6F_2.2) were grown by the Bridgman-Stockbarger method. The optical spectra were investigated at low (6 K) and room temperatures. It is shown that the absorption spectrum of NYF:Er crystals contains wide bands (790–801 and 965–980 nm) corresponding to the emission range of laser diodes. The peak absorption cross section σ_a for the band peaked at λ=970.4 nm is 0.15×10^?20 cm². On the basis of the analysis of the absorption and luminescence spectra at low (6 and 12 K) temperatures, the structure of the Stark splitting of erbium levels was determined as a structure of quasi-centers for which Stark components are inhomogeneously broadened. The oscillator strengths of the transitions from the ground state of erbium to excited multiplets were calculated from the absorption spectra measured at T=300 K, and the intensity parameters Ω_t were determined by the Judd-Ofelt method: Ω₂=1.65×10^?20 cm², Ω₄=0.56× 10^?20 cm², and Ω₆=1.01×10^?20 cm². These values of the intensity parameters were used to calculate the probabilities of radiative transitions and the branching ratios. The rates of multiphonon nonradiative transitions in NYF: Er were estimated. The luminescence decay kinetics for radiative levels of erbium ions upon their selective excitation by nanosecond laser pulses was studied. The intracenter lifetimes of radiative levels of erbium ions were determined from the luminescence kinetics upon selective ion excitation by low-intensity light in a sample with a low erbium concentration (0.5%). It is demonstrated that, with an increase in temperature from 6 to 300 K, luminescence from the ⁴ G _11/2, ² G(H)_9/2, and ⁴ F _9/2 levels is quenched as a result of multiphonon nonradiative transitions. Luminescence from the ⁴ I _9/2 level is quenched only insignificantly with increasing temperature, and no quenching of luminescence from the ⁴ I _11/2 and ⁴ I _13/2 levels is observed. The spectra of steady-state luminescence of NYF:Er(0.5–15%) crystals were investigated upon broadband excitation by UV and UV-visible lamp light and selective time-resolved laser excitation. It is shown that low-lying levels of erbium ions separated by an energy gap smaller than 2500 cm^?1 are populated via cascade mechanisms. On the basis of the results obtained, it is concluded that NYF:Er ³⁺ crystals are promising candidates for active media of tunable diode-pumped lasers.     

Multiphoton excitation of visible and VUV luminescence in MgO crystals

Irradiation of pure MgO crystals with intense XeCl laser radiation yields VUV emission associated with the decay of bi-excitons. This emission exhibits a 4-photon dependence on incident laser intensity. Collisional excitation of luminescence from Cr³⁺ and Ni²⁺ impurities has been detected in doped samples. This emission also shows a multiphoton dependence on incident laser intensity.     

Time-resolved optical spectroscopy of CsI(Tl) crystals by pulsed electron beam irradiation

Properties of the color and emission centers induced with an electron pulse beam at temperature within 80-300 K have been studied in CsI(Tl) crystals. It has been established by optical spectrometry with time resolution that initial color centers in this crystal are only Tl⁰ and V_k centers, which spontaneously recombine emitting visible light at 2.25 and 2.55 eV. It has been shown that the emission decay kinetics at 80 K include two fast exponential components with decay constants 3 and 14 μs as well as slow hyperbolic component with the power index depending on the wavelength of the emitting light. The temperature effect on the emission kinetics has been studied and it has been directly proved that the emission rise stage at the temperature above 170 K is caused by the recombination of electrons, which are thermally released from single Tl⁰ centers, with V_kA centers. The origin of scintillations in CsI(Tl) crystal is discussed in terms of the tunnel electron transitions from ground state of Tl⁰ centers to ground state of V_k centers at different distances from each other.     

Radio and thermoluminescence studies in CsI doped with F centers

Radioluminescence, thermoluminescence and u.v. excitation measurements in CsI additively colored crystals show a new luminescent component located at 2.5 eV. This is attributed to the radiative recombination of V_k centers with F centers. Another new component at 2.72 eV is observed only under u.v. excitation of 5.14 eV. This is related to the radiative recombination of localised excitons at F⁺ center sites (α band).     

Luminescence study of self-trapped excitons in CdMoO4

Luminescence properties of CdMoO₄ crystals have been investigated in a wide temperature range of T=5–300 K. The luminescence-excitation spectra are examined by using synchrotron radiation as a light source. A broad structureless emission band appears with a maximum at nearly 550 nm when excited with photons in the fundamental absorption region (<350 nm) at T=5 K. This luminescence is ascribed to a radiative transition from the triplet state of a self-trapped exciton (STE) located on a (MoO₄)^2? complex anion. Time-resolved luminescence spectra are also measured under the excitation with 266 nm light from a Nd:YAG laser. It is confirmed that triplet luminescence consists of three emission bands with different decay times. Such composite nature is explained in terms of a Jahn–Teller splitting of the triplet STE state. The triplet luminescence at 550 nm is found to be greatly polarized in the direction along the crystallographic c axis at low temperatures, but change the degree of polarization from positive to negative at T>180 K. This remarkable polarization is accounted for by introducing further symmetry lowering of tetrahedral (MoO₄)^2? ions due to a uniaxial crystal field, in addition to the Jahn–Teller distortion. Furthermore, weak luminescence from a singlet state locating above the triplet state is time-resolved just after the pulse excitation, with a polarization parallel to the c axis. The excited sublevels of STEs responsible for CdMoO₄ luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.     

Synchrotron and laser excitation of luminescence in PbWO<Subscript>4</Subscript>:Tb crystals at different temperatures

The effect of temperature on the spectral luminescence characteristics of PbWO₄:Tb³⁺ crystals with synchrotron and laser excitation is studied. If PbWO₄:Tb³⁺ is excited by synchrotron radiation with λ = 88 nm at 300 K, a faint recombination luminescence of the impurity terbium is observed against the matrix luminescence. When the temperature is reduced to 8 K, the luminescence intensity of PbWO₄:Tb³⁺ increases by roughly an order of magnitude and the characteristic luminescence of the unactivated crystal is observed. Excitation of PbWO₄:Tb³⁺ by a nitrogen laser at 300 K leads to the appearance of emission from Tb³⁺ ions. At 90 K, a faint matrix luminescence is observed in addition to the activator emission. The formation of the luminescence excitation spectra for wavelengths of 60–320 nm is analyzed and the nature of the emission bands is discussed.     

Thermally stimulated conductivity induced in SrF2:Tb by u.v. light

Thermally-stimulated-conductivity was excited in SrF₂:Tb crystals by non-ionizing u.v. light. The electrical glow curves were studied in the range 80°–300°K, and thermal activation energies were computed by various methods. A composed glow peak, with maxima at 157° and 169°K, is attributed to the two stages of thermal decay of V_K centers.     

Spontaneous emission of photons and electrons during chemisorption of chlorine on sodium

Photons and electrons are emitted when Cl₂ molecules react on a Na surface prepared by UHV evaporation. The emission yield per reacting molecule is 10^?7-10^?6 for photons and approximately 10^?5 for electrons. The dominating light emission band has a maximum at hv = 2.15 eV (width 0.6 eV). A less intense u.v. band has a maximum at about 4.7 eV. A drastic decrease in the photon and electron emission at a Cl₂ exposure of about 5.10^?3 torr·sec, is attributed to the formation of a continuous NaCl film on the Na surface.     

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn_1?x Mg _x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 10¹⁸?5.19 × 10²⁰ cm^?3). The luminescent properties of the Zn_1?x Mg _x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn_1?x Mg _x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X _A), neutral donor-bound excitons (D⁰X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn_1?x Mg _x O composites is intimately connected to the film composition and surface morphology.     

Luminescence of Ca(NbO3)2:Pr3+: Pr3+ and self-trapped exciton emission

Photoluminescence and time resolved photoluminescence spectra of Ca(NbO₃)₂ doped with Pr³⁺, excited under 37,000 cm^?1 (270 nm), obtained at high hydrostatic pressure up to 20 kbar applied in a sapphire anvil cells, are presented. At ambient conditions, the emission spectrum obtained in the time interval 0–1 μs is dominated by spin allowed transitions from the ³P₀ state. The luminescence related to transitions from ¹D₂, characterized by a decay time equal to 33 μs, is observed when one excites directly the Pr³⁺ ion with 30,770 cm^?1 (325 nm) wavelength. The introduction of Pr³⁺ impurities in Ca(NbO₃)₂ does not quench the self-trapped exciton (STE) luminescence. This luminescence, peaking at 20,000 cm^?1 (500 nm), having a decay time of 61 ± 1 μs, still occurs when the crystal is excited with a wavelength of 37,000 cm^?1 (270 nm) or shorter. Under such excitation a fraction of the STE luminescence is reabsorbed by Pr³⁺ ions; in this case the emission lifetime of the ¹D₂ → ³H₄ transition of Pr³⁺ is 64 ± 3 μs. This effect is stable also at high pressure.     

Effect of Br− ions on the F-center formation in KCl crystals under u.v. light irradiation

The relative efficiency of the F-center formation, Y_F in KCl crystals has been studied as a function of photon energy of u.v. light between 5.7 and 10.0 eV. Y_F is maximum at the peak of the absorption band due to the localized excitons at Br^? ions, and increases with Br^? concentration. Results suggest an important role of the localized excitons in the F-center formation.     

Effects of neon matrix environment on three vibronic emission systems of PbS

Laser sources have been used to explore three emission band systems of the PbS molecule in solid neon matrices. The D → X emission (origin near 29 630 cm^?1), excited biphotonically, consists of broad bands originating from V′ = 0. With laser excitation tuned into the region of the band system origin near 21 860 cm^?1, the B → X system shows emission narrowing of inhomogeneously broadened absorption. Hot luminescence from the B state is also reported. In the a → X system near 14 625 cm^?1 the intensities of phonon sidebands on the high-frequency side of the zero-phonon lines are found to be very sensitive to laser power. Lifetimes of the a, A, and B states of PbS in solid argon have been measured as 260, 0.95, and 1.8 μsec.     

Dy-doped Lu2SiO5 single crystal: spectroscopic characteristics and luminescence dynamics

Spectroscopic and kinetics properties of Lu₂SiO₅:Dy³⁺ (LSO:Dy) single crystal with 1 and 5 at.% of activator were investigated. The polarised absorption and unpolarised emission spectra were measured at 10–300 K. Parameters characterising radiative relaxations of LSO:Dy were estimated by the Judd–Ofelt model. The crystal-field energy structure was derived from low-temperature optical spectra exhibiting the presence of two non-equivalent Dy³⁺ sites. It was found that dysprosium ions in site 1 and in site 2 do not form isolated subsystems; these subsystems are coupled by an effective spectral energy migration process. The LSO:Dy crystal exhibits a strong luminescence in the visible. Strong ion–ion interactions were observed for LSO:Dy (5 at.%); luminescence decays are non-exponential and the macro-parameter of donor–acceptor interaction C _da amounts to 5.3 (10^?52 m⁶?s^?1) and 7.8 (10^?52 m⁶?s^?1) at 10 and 300 K, respectively. Laser potential related to the ⁴F_9/2→⁶H_13/2 yellow luminescence in Dy:LSO was assessed based on evaluation of the emission cross section values. It was concluded that the crystal is a promising material for visible laser operation.     

Thermoluminescence and exo-electron emission from alkali azides

Thermoluminescence and emission of exo-electrons have been studied from uv-irradiated and non-irradiated samples of potassium, rubidium, cesium and sodium azides. The experimental data led to the conclusion that the luminescence phenomena observed at low temperatures are due to reactions involving “V” centers as well as the decomposition intermediates N ₂ ^? and N ₄ ^? . Above room temperature, the coincidence of luminescence and exo-electron emission suggests true thermoluminescence; however, excited nitrogen species are also believed to be responsible for part of the emission. At temperatures near the melting point luminescence and electron emission are associated with the thermal decomposition of the specimen.     

The electronic structures of Mg,Al and Si in octahedral coordination with oxygen from SCF Xα MO calculations

Molecular orbital calculations performed using the SCF Xα Scattered Wave Cluster method are presented for the octahedral oxyanions MgO₆^?10, AlO₆^?9 and SiO₆^?8. The AlO₆^?9 results are used to assign and interpret the X-ray photoelectron spectra (XPS), X-ray emission (XES) and u.v. spectra of Al₂O₃. Agreement between calculation and experiment is good for valence band and fair for conduction band orbitais. The SCF Xα results for MgO₆^?10 are also in good agreement with observed valence band energies in MgO, but in this case the lowest energy features in the u.v. spectrum are not assignable in terms of either the calculations or the X-ray spectral results. The substantial increase in covalency expected between the Mg and Si oxides is evidenced in the calculations by an increase in valence region width from 2.6 to 5.3 eV and an increase in valence-conduction band separation from 5.2 to 10.0 eV. The calculated trends are in reasonable agreement with u.v. spectral data and with absolute valence orbital binding energies derived from X-ray spectra. A comparison of the SiO₆^?8 calculation with the analogous tetrahedral SiO₄^?4 calculation shows the valence band in the octahedral oxyanion to be much simpler in structure and somewhat narrower than that in the tetrahedral oxyanion. Using the orbital structure calculated for the valence bands of tetrahedral and octahedral oxides, a method is presented for calculating atomization energies directly from X-ray spectral data for SiO₂, Al₂O₃ and MgO. Results are in good agreement with experiment but the method involves an empirical parameter which is not presently understood in detail. Studies of trends in p-type bonding orbital binding energies derived from experimental data provide a qualitative explanation for the preferred coordination numbers in the Mg, Al and Si oxides.     

Optical and theoretical investigations of V ions in CdZnTe crystal

High-resistivity CdZnTe:V crystals are investigated by photoluminescence (PL) and by time-resolved PL in the infrared spectral range. A double peaked emission band is detected around 0.8 eV and it is related to vanadium doping. No-phonon lines of the internal transitions were detected. This emission is interpreted as a balance between the ⁴T₁(⁴P)→⁴T₁(⁴F) internal transition and an electronic transition from the conduction band to the ⁴T₁(⁴F) ground state of V²⁺. The corresponding decay time after laser excitation gives evidence to the contribution of two different recombination processes. These two emission bands are separated by time-resolved luminescence. Crystal-field calculations of the detected transition energies based on Tanabe-Sugano scheme are presented and the Racah parameter B and crystal-field intensity D_q were determined.In addition, a model is developed in terms of one-electron orbital, to explain the characteristics of the PL excitation processes of V²⁺. Excitations with above and below band edge energy confirm the proposed schemes.     

Properties of erbium luminescence in bulk crystals of silicon carbide

The infrared luminescence of Er³⁺ ions has been studied in bulk crystals of silicon carbide 6H-SiC doped with erbium in the process of their growth. The erbium centers of different symmetry in the crystals are revealed by the EPR technique. A number of intense luminescence bands of erbium ions are observed at a wavelength of about 1.54 μm. The luminescence can be excited by the light with quantum energies above and below the band gap of SiC. It is found that the luminescence exhibits unusual temperature behavior: as the temperature increases, the luminescence intensity abruptly rises starting with 77 K, passes through a maximum at ∼240 K, and, in the vicinity of ∼400 K, decreases down to the values observed at 77 K. The activation energies for the flare-up and quenching of the Er³⁺ luminescence are estimated at E _A ≈130 and ≈350 meV, respectively. The mechanisms of the flare-up and quenching of the Er³⁺ luminescence in SiC are discussed. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 5, 2000, pp. 809–815. Original Russian Text Copyright ? 2000 by Babunts, Vetrov, Il’in, Mokhov, Romanov, Khramtsov, Baranov.