Low-temperature luminescence and thermally stimulated luminescence of BeO: Mg single crystals

Luminescence and thermally stimulated luminescence (TL) of BeO: Mg crystals are studied at T = 6–380 K. The TL glow curves and the spectra of luminescence (1.2–6.5 eV), luminescence excitation, and reflection (3.7–20 eV) are obtained. It is found that the introduction of an isovalent magnesium impurity into BeO leads to the appearance of three new broad luminescence bands at 6.2–6.3, 4.3–4.4, and 1.9–2.6 eV. The first two are attributed to the radiative annihilation of a relaxed near-impurity (Mg) exciton, the excited state of which is formed as a result of energy transfer by free excitons. The impurity VUV and UV bands are compared with those for the intrinsic luminescence of BeO caused by the radiative annihilation of self-trapped excitons (STE) of two kinds: the band at 6.2–6.3 eV of BeO: Mg is compared with the band at 6.7 eV (STE₁) of BeO, and the band at 4.3–4.4 eV is compared with the band at 4.9 eV (STE²) of BeO. In the visible region, the luminescence spectrum is due to a superposition of intracenter transitions in an impurity complex including a magnesium ion. The manifestation of X-ray-induced luminescence bands at T = 6 K in BeO: Mg indicates their excitation during band-to-band transitions and in recombination processes. The energy characteristics of the impurity states in BeO: Mg are determined; the effect of the isovalent impurity on the fluctuation rearrangement of the BeO: Mg structure in the thermal transformation region of STE₁ → STE₂ is revealed.     

Luminescence and electronic excitations in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>: Ce<Superscript>3+</Superscript> crystals

This paper reports on a study of the luminescence emitted by Li₆Gd(BO₃)₃: Ce³⁺ crystals under selective photoexcitation to lower excited states of the host ion Gd³⁺ and impurity ion Ce³⁺ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd³⁺ ions and vibrational energy relaxation between the ⁶ I _j and ⁶ P _j levels. It has been shown that radiative transitions in Ce³⁺ ions from the lower excited state 5d ¹ to ² F _5/2 and ² F _7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.     

Luminescence and relaxed excited state origin in CsI:Pb crystals

Emission and excitation spectra, luminescence polarization and decay kinetics have been studied for CsI:Pb crystals in the 0.36-300 K temperature range. The origin of the excited states responsible for the optical characteristics has been discussed. It has been concluded that the doublet ≈3.70 eV absorption (excitation) band is caused by the electronic transitions into the Pb²⁺ triplet state split due to the presence of a cation vacancy near a Pb²⁺ ion, while the higher-energy bands are of the charge-transfer origin. Like in CsI:Tl, four emission bands of CsI:Pb have been found to belong to the main luminescence centres. Two emission bands, peaking at 3.1 and 2.6 eV, are suggested to arise from the triplet relaxed excited state of a Pb²⁺ ion. Two visible emission bands, peaking at 2.58 and 2.23 eV, are interpreted as the luminescence of an exciton localized near the Pb²⁺ ion.     

Europium luminescence in fluorite upon high-energy excitation

The reflection and luminescence excitation spectra of CaF₂ crystals containing europium ions in divalent (Eu²⁺) and trivalent (Eu³⁺) states were measured in the range from 4 to 16 eV. It was established that, in CaF₂ : Eu³⁺ crystals, luminescence of Eu³⁺ ions (the f-f transitions) is effectively excited both in the charge-transfer band (at ～8 eV) and in the region of the 4f–5d transitions (at ～10 eV) but is virtually not excited in the fundamental region of the crystal (at an energy higher than 10.5 eV). Luminescence of Eu²⁺ ions (the 427-nm band) in CaF₂ : Eu³⁺ is effectively excited in the fundamental region of the crystal; i.e., luminescence of divalent europium ions occurs through the trapping mechanism. Emission of Eu²⁺ ions in CaF₂ : Eu²⁺ crystals is characterized by the excitation band at an energy of 5.6 eV (the 4f → 5d,t _2g transitions), as well as by the exciton and interband luminescence excitations. The results obtained and data available in the literature are used to construct the energy level diagram with the basic electron transitions in the CaF₂ : Eu crystals.     

Excitation of lowest electronic states of thymine by slow electrons

Excitation of lowest electronic states of the thymine molecules in the gas phase is studied by elec- tron energy loss spectroscopy. In addition to dipole-allowed transitions to singlet states, transitions to the lowest triplet states were observed. The low-energy features of the spectrum at 3.66 and 4.61 eV are identified with the excitation of the first triplet states 1₃ A′ (π → π*) and 1₃ A″ (n → π*). The higher-lying features at 4.96, 5.75, 6.17, and 7.35 eV are assigned mainly to the excitation of the π → π* transitions to the singlet states of the molecule. The excitation dynamics of the lowest states is studied. It is found that the first triplet state 1³ A′(π → π*) is most efficiently excited at a residual energy close to zero, while the singlet 2₁ A′(π → π*) state is excited with almost identical efficiency at different residual energies.     

Photoluminescence of Mn doped epitaxial GaAs

We have measured the near band edge photoluminescence of Mn doped liquid phase epitaxially grown GaAs. The photoluminescence spectra at 2°K shows, at low excitation intensities, a structure of up to eight sharp peaks (widths .2 to 1.0 meV) between 1.517 and 1.512 eV, besides the lower energy bands near 1.41 eV due to the deep Mn acceptor level and the usual donor-acceptor bands around 1.47 eV. Attempts to relate the sharp lines to the Mn electronic states, introduced by doping, were unsuccessful. It is our belief that the presence of this particular impurity in our samples allows for whatever states are responsible for the sharp line structure, to reveal themselves in the emission spectrum. A most unespected result is that near band edge sharp line luminescence is observed for impurity concentration as high as 10¹⁸cm^-3.     

Electron energy loss spectra and X-ray photoelectron spectra of Ca2V2O7

We have measured the electron energy loss spectra of Ca₂V₂O₇ in the reflexion mode, at incident energies between 200 and 2400 eV, and the X-ray photoelectron spectra excited by Al K α radiation. The abundant loss structures observed can be correlated with the possible interband transitions, collective oscillations, and excitation of O2s and V3p electrons within the V₂O₇^4- ion. The gap width and molecular orbital (MO) spread (or splitting) is about l eV larger in the V₂O₇^4- ion than in its component VO₄^3- ion. Excitation of O2s states, which may occur together with some MO over-gap transitions, displaces the collective oscillations about 7 eV towards lower energies. Deeper V3p electrons are excited with a maximum energy loss some 7 eV above their binding energy. Cross transitions from Ca3p levels into some empty states of the V₂O₇^4- ion, or direct transitions to available states of the Ca²⁺ ion could not be unambiguously identified. The energy dependence of the excitation cross section and of the electron penetration depth results in a significant variation of the relative intensity of various losses over the investigated energy range.     

An ab initio molecular orbital study of the electronically excited and cationic states of the ozone molecule and a comparison with spectral data

A number of valence and Rydberg, singlet and triplet excited states for ozone in the excitation energy range 1–12eV have been calculated by large scale CI methods preceded by MCSCF studies. A comparison of the theoretical intensity envelope with the VUV + EELS spectrum has been made. The present work supports the assignments for the Huggins + Hartley bands as having two electronic origins, 2 ¹A₁ and 1 ¹B₂. The experimental ～ 9.3eV and ～ 10.2eV bands of the VUV spectrum must have adventitious superposition of valence states on Rydberg transitions, because the high oscillator strengths of the valence states cannot be attributed to the 8.8eV broad band. A number of new valence and Rydberg states have been calculated, and these lead to the conclusion that the experimental 9–11 eV VUV spectral range in particular must yield more experimental states than the few so far identified. This suggests a major need for more sophisticated methods of experimental study for the excited state manifolds. The use of various MCSCF/CI studies of the vertical cationic states, supports the IP order as ²A₁ < ²B₂ < ²A₂. A re-analysis of the 12–13.4eV range of the UV-photoelectron band has been performed, with a view to determining the adiabatic IPs more accurately. The present work suggests that the adiabatic IP₂ lies at 12.86eV, slightly lower than has been assumed, with consequential effect on the analysis of the VUV spectrum near 9.4eV.     

Temperature dependence of the luminescence of Li6Gd x Y1 ? x (BO3)3:Eu crystals

The luminescence and recombination processes in crystals of lithium borates Li₆Gd _x Y_{1 ? x} (BO₃)₃:Eu (LGYBO:Eu) have been studied. The photoluminescence (PL) spectra upon selective photoexcitation to lower excited states of Gd³⁺ and Eu³⁺ ions and the temperature dependences of the photoluminescence intensity at different energies of excitation photons have been measured in a wide temperature range: 10?C500 K. We observed a photoluminescence band at 3.97 eV, which is due to the ⁶ P _J ?? ⁸ S _7/2 transitions in Gd³⁺ ions, and a characteristic line spectrum in the range of 1.6?C2.2 eV, which is related to the radiative f-f transitions in impurity Eu³⁺ ions, occurring mainly from the lower excited ⁵ D ₀ level to the ⁷ F _J states (J = 0, 1, ??, 6). The influence of the O-Eu charge transfer states and the vibrational relaxation between the ⁶ I _J and ⁶ P _J levels of the Gd³⁺ ion on the temperature dependences of the intrinsic and impurity luminescence intensities is discussed.     

Anti-stokes luminescence of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn_0.6Cd_0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 10¹⁴–10¹⁵ photons/cm²·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn_0.6Cd_0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn_0.6Cd_0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 617–621, September–October, 2007.     

Photoluminescence studies of nitrogen-doped TiO2 powders prepared by annealing with urea

Photoluminescence and excitation spectra have been investigated for undoped and nitrogen-doped TiO₂ powders at low temperatures. A broad luminescence band peaking at 2.25?eV is observed in the undoped TiO₂ powders. The 2.25?eV luminescence band exhibits a sharp rise from 3.34?eV in the excitation spectrum reflecting the fundamental absorption edge of anatase TiO₂. On the other hand, the N-doped TiO₂ powders obtained by annealing with urea at 350 and 500°C exhibit broad luminescence bands around 2.89 and 2.63?eV, respectively. The excitation spectra for these luminescence bands rise from the lower energy side of the fundamental absorption edge of anatase TiO₂. The origin of the luminescence bands and N-related energy levels formed in the band-gap of TiO₂ are discussed.     

Photoluminescence of concentration series of CaF2: Mn phosphors excited by VUV radiation

Fluorescent characteristics of a series of powder CaF₂: Mn phosphors (from 0.01 to 2.47 wt. % of Mn in the mixture) excited by VUV radiation with quantum energies up to 14 eV at 293 K and up to 12 eV at 85 K are measured. Narrow excitation bands of Mn²⁺ centers found at 7.9 and 8.6 eV (at 293 K) are assigned to partially forbidden transitions of electrons from the ground state ⁶ S split by the crystalline field (10 Dq=0.71 eV from the literature) in two sublevels to the excited level corresponding to the ⁶ D term of a free Mn²⁺ ion (3d ⁵ → 3d ⁴4s transitions). A wide nonelementary excitation band in the region of 9.1–10.3 eV is interpreted as photogeneration of near-activator D-excitations: allowed transitions of electrons from levels that are split from the top of the valence band under the influence of an impurity ion to the free 4s-orbital of a Mn²⁺ ion. Channels of energy transport in the CaF₂: Mn system are briefly analyzed.     

Ultraviolet luminescence of Li6Gd(BO3)3: Ce crystals under selective excitation in the region of 4d → 4f transitions

The subnanosecond time-resolved ultraviolet luminescence of Li₆Gd(BO₃)₃: Ce crystals under selective excitation by ultrasoft X-rays in the region of the 4d??4f core transitions at temperatures of 7 and 293 K has been investigated for the first time. The performed investigation has revealed the following features: an intense fast component of the luminescence decay kinetics in the subnanosecond range due to the high local density of electronic excitations and the processes of Auger relaxation of the core hole; the modulation of the luminescence excitation spectrum by the ??giant resonance?? absorption band of the 4d-4f photoionization in the energy range 135?C160 eV; and a new broad luminescence band at an energy of 4.44 eV due to the direct radiative recombination between the genetically related electron in the states of the conduction band bottom and hole in the 4f ground state of the Ce³⁺ ion.     

Threshold energy electron impact excitation of carbon dioxide and carbon disulphide

The threshold energy electron impact excitation spectra of CO₂ and CS₂ have been studied using the sulfur hexafluoride scavenger technique. The main results are triplet state excitation and autoionisation of negative ions associated with resonant excited states of the molecules. This confirms previous data concerning diatomic molecules. Furthermore, transitions such as ¹Π_g?X¹Σ_g⁺ and ¹Π_u?X ¹Σ_g⁺ are only weakly induced by low energy electrons, while the corresponding triplet excitations are probably more easily produced. Structures at 5.6, 6.1 and 6.6 eV observed in CS₂ are due to negative ions and/or to ³Π_u, ³Π_g excitation.The autoionisation of CO₂^?(X²Π_u) proceeds also by ejection of a thermal energy electron and leads to highly excited vibrational levels (3–5 eV) of the ground electronic state of CO₂.     

The ClO4 radical: a theoretical study on ground and excited states

In agreement with previous studies, the ground state of ClO₄ has been confirmed to be X²B₁. Vertical excitation energies and oscillator strengths were calculated by MRCI methods for doublet and quartet states of ClO₄. The highest oscillator strength was found for 1²A₁ at 2.95 eV. This state has been identified as the upper state seen by Kopitzky and co-workers in the absorption spectrum of ClO₄. Two higher states, 2²A₁ and 3²A₁, at 4.19 and 8.12 eV, respectively, also have relatively high oscillator strengths. Rydberg states start at about 9.5 eV. Geometry optimizations were performed by DFT and CCSD(T) methods. After extensive testing, the B3LYP density functional, together with the 6-311 + G(3df) basis set were chosen for calculations. Optimized geometries of seven excited states were obtained. The adiabatic excitation energy of 1²A₁ (2.40 eV) agrees closely with the observed band origin at 2.46 eV. Three excited states have one or two imaginary vibrational modes. Electron affinity and heat of formation of ClO₄ agree with literature values. None of the quartet states was found to be stable.     

Excited-state laser spectroscopy of the impurities in III–V and II–VI semiconductors

At temperatures where the shallow donors become occupied, weak impurity lines appear on sweeping the field with laser wavelenghts where photon energy is insufficient to cause transitions from the ground state ti any of the excited states if the impurities. With all the materials investigated, the two most prominent of these lines, which are attributed to transitions between excited states, lie on the same two curves on a dimensionless diagram of excitation energy against cyclotron energy. One of these lines is identified as arising from the 2p⁻ to 2s transition. The other probably also originates from the 2p⁻ state. The photon energy for the 311μ laser line is just sufficient to cause transitions from the 1s to 2p⁻ state in n-GaAs. Because of this almost exact coincidence, fine structure due to the central cell corrections for individual impurities is particularly well resolved in magnetic field.     

TiO2∶Mo体系的光子雪崩上转换

在978nm激光二极管的激发下，Mo掺杂的TiO₂材料表现出很强的宽带上转换发光 ，该发光来源于［MoO₄］^2-基团的激发态³T₁, ³T₂能级到基态¹A₁能级的电子跃迁.通过研究发光强度与抽运功率的关系及上转换发光的上升时间曲线，发现TiO₂∶Mo体系的上转换发光中存在着雪崩机制，应用转 关键词： 上转换 光子雪崩 转移函数理论     

Ultraviolet luminescence of single crystals and single-crystal films of YAlO3

The nature of intrinsic emission bands of yttrium orthoaluminate in the UV spectral region at _max=220 nm (5.63 eV) and 330 nm (4.13 eV) is studied on the basis of the luminescence of single crystals and single-crystal films of YAlO₃ and Ce: YAlO₃ excited by synchrotron radiation sources with an energy of 3–25 eV at 9 and 300 K. The single crystals and single-crystal films were obtained, respectively, from solution and solution-melt by liquid-phase epitaxy and are characterized by considerably different concentrations of substitutional and vacancy defects. It is found that only the luminescence band at 300 nm, which has the decay time τ=4.1 ns and is excited in a band shifted from the range of interband transitions by 0.25 eV, has exciton-like character. The luminescence band at 220 nm with τ=0.1 µs at 9K, which is observed only for YAlO₃ single crystals and is absent in the luminescence of single-crystal films, is associated with antisite defects of the Y _Al ³⁺ type, which are a specific type of cationic isoelectronic impurities. It is shown that the phosphors based on single-crystal films of YAlO₃ have a simpler scintillation decay kinetics than their bulk analogues due to the absence of channels of excitation energy dissipation associated with the antisite defects of Y _Al ³⁺ type and vacancy defects.     

The nature of the predominant acceptors in high quality zinc telluride

We report very sharp bound exciton luminescence spectra in high quality melt-grown very lightly compensated ZnTe, p-type with N_A-N_D in the low 10⁺¹⁵ cm^-3. Bound exciton localisation energies at seven shallow neutral acceptors with E_A between ~55 and ~150 meV are very insensitive to E_A. Optical absorption and dye laser luminescence excitation spectroscopy were necessary to obtain a full separation of the transitions due to different acceptors, together with a study of certain ‘two-hole’ luminescence satellites in which the acceptor is left in a series of orbital states after bound exciton decay. Two shallow acceptors are P_Te and As_Te, a third possibly Li_Zn while a fourth, relatively prominent in our best undoped crystals, may be a complex. A deeper, 150 meV acceptor, frequently reported in the ZnTe literature and electrically dominant in most of our undoped crystals has the Zeeman character of a point defect. We present clear evidence from our spectra that this energy does not represent the binding of a single hole at a doubly ionized cation vacancy, a popular attribution since 1963. This acceptor may be covered by another impurity, possibly Cu_Zn. We also report bound phonon effects, lifetime broadening of excited bound exciton states and observe a single unidentified donor with E_D ~18.5 meV. This energy is determined using selective dye laser excitation at the weak neutral donor bound exciton line and from the onset of valence band to ionized donor photo-absorption.     

Luminescence kinetics of an Y0.8Yb0.2F3:Tm3+ solid solution crystal

Upconversion luminescence kinetics of Tm³⁺ doped Y_0.8Yb_0.2F₃ solid solution crystal was studied for various values of pulse excitation parameters: pulse duration, wavelength and excitation power. Analysis of obtained results allowed a conclusion about the presence of transient processes. The transient processes found in upconversion luminescence kinetics are characterized by duration commensurate with lifetime of the excited energy levels of the activator ions. Upon completion of these processes a stable equilibrium state is established between the processes of population and spontaneous decay of the excited energy levels of Tm³⁺ ions. Conditions under which the equilibrium state can be maintained have been considered.