Multiplication Of Electronic Excitations Research Articles

Time-resolved luminescence and excitation spectroscopy of co-doped Gd3Ga3Al2O12 scintillating crystals

Cerium doped Gd3Ga3Al2O12 (GGAG) single crystals as well as GGAG:Ce single crystals co-doped by divalent (Mg2+, Ca2+) and tetravalent (Zr4+, Ti4+) ions have been studied by means of time-resolved luminescence as well as the excitation luminescence spectroscopy in vacuum ultraviolet (VUV) and soft X-ray (XUV) spectral range. Tunable laser excitation was applied for time-resolved experiments in order to obtain luminescence decay curves under excitations in Ce3+, Gd3+ and excitonic absorption bands. The influence of the co-dopant ions on the Ce3+ luminescence decay kinetics is elucidated. The fastest luminescence decay was observed for the Mg2+ co-doped crystals under any excitation below bandgap energy indicating the perturbation of the 5d states of Ce3+ by Mg2+ ions. Synchrotron radiation was utilized for the luminescence excitation in the energy range from 4.5 to 800 eV. Special attention was paid to the analysis of Ce3+ excitation spectra in VUV and XUV spectral range where multiplication of electronic excitation (MEE) processes occur. Our results demonstrated that GGAG:Ce single crystals co-doped by Mg2+ ions as well as the GGAG:Ce crystal annealed in vacuum reveal the most efficient excitation of Ce3+ emission in VUV-XUV excitation range. The role of intrinsic defects in MEE processes in the co-doped as well as in the annealed GGAG:Ce single crystals is discussed.

Luminescence and vacuum ultraviolet excitation spectroscopy of cerium doped Gd3Ga3Al2O12 single crystalline scintillators under synchrotron radiation excitations

Cerium doped Gd3Ga3Al2O12 (GGAG) single crystals as well as GGAG:Ce single crystals co-doped by divalent (Mg2+, Ca2+), trivalent (Sc3+) or tetravalent (Zr4+, Ti4+) ions have been studied by means of the excitation luminescence spectroscopy in vacuum ultraviolet spectral range. Synchrotron radiation from the undulator beam was utilized for the luminescence excitation in the energy range from 4.5 to 800 eV. The influence of the co-dopant ions on the excitonic transitions as well as on the intrinsic defects in GGAG was revealed examining the luminescence emission and excitation spectra of both Gd3+ and Ce3+ ions in all single crystals studied. Special attention was paid to the analysis of Ce3+ excitation spectra in VUV spectral range (4.5–45 eV) where multiplication of electronic excitation (MEE) processes occur. It was obtained that GGAG:Ce single crystals having different co-dopant ions reveal distinguished efficiency of MEE. The role of intrinsic defects in MEE processes in the co-doped GGAG:Ce single crystals was elucidated.

Study of the luminescence of Eu2+ and Eu3+ states in Ca3Ga2Ge3O12:Eu garnet using synchrotron radiation excitation

The work is dedicated to the investigation of Eu2+ and Eu3+ luminescence centers in the Ca3Ga2Ge3O12 garnet using synchrotron radiation excitation. The luminescence of Eu2+ and Eu3+ states was observed in the emission spectra of Ca3Ga2Ge3O12:Eu ceramics under excitation with energies in the exciton range and the range above band gap of this garnet. The luminescence of Eu2+ ions Ca3Ga2Ge3O12 host is observed in the band peaked at 450–460 nm with decay time in the ten ns range and excited in the two wide bands peaked at 5.04 and 6.05 eV, related to the allowed 4f-5d transitions of Eu2+ ions. The Eu3+ luminescence can be excited via the Eu2+ luminescence. We have also found the energies of formation of excitons bound with Eu2+ and Eu3+ ions in Ca3Ga2Ge3O12, being equal to 7.64 and 6.95 eV, respectively, and the onset of interband transitions in this garnet which is equal to 7.83 eV at 8 K. Strong increase of the luminescence intensity of Eu2+ and Eu3+ centers is observed under excitation above 21 eV due to multiplication of electronic excitation in Ca3Ga2Ge3O12 host.

Evolution of Anion and Cation Excitons in Alkali Halide Crystals

The manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.

Эволюция анионных и катионных экситонов в щелочно-галоидных кристаллах (О б з о р)

AbstractThe manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.

Excitation process of Ce3+ and Eu2+ ions doped in SrGa2S4 crystals under the condition of multiplication of electronic excitations

Excitation spectra for 5d–4f photoluminescence (PL) bands of Ce3+ and Eu2+ ions doped in SrGa2S4 crystals have been measured at room temperature in the energy region up to 25eV. These spectra allow us to investigate the excitation process of luminescent ions under host excitation with vacuum ultraviolet (VUV) photons. A gentle increase in intensity is commonly observed at around 16.5eV in the excitation spectra. Optical constant spectra were obtained from measurements of reflectivity and absorption edge spectra of undoped SrGa2S4 crystals. These data were compared with the results of a relativistic DV-Xα calculation, to clarify the electronic structure of SrGa2S4. Ultraviolet photoelectron spectroscopy (UPS) has been carried out under the excitation with photons in the 12–20eV range. The detailed analysis of UPS spectra reveals that the multiplication of electronic excitations (MEEs) occurs above 16.5eV. The excitation process of Ce3+ and Eu2+ ions is discussed together with the processes of MEEs.

Optical functions and time-resolved luminescence of lithium hydride single crystals upon far-ultraviolet excitation

Low-temperature reflection spectra of lithium hydride (LiH) single crystals cleaved in ultrahigh vacuum (3×10−10Torr, T=10K), were recorded using synchrotron radiation in vacuum ultraviolet spectral region. Based on the obtained experimental data, the optical functions of LiH in the energy range from 3.7 to 35eV were analyzed using the Kramers–Krönig relations. Time-resolved photoluminescence excitation spectra were studied in detail for the near edge free exciton-phonon luminescence at 4.67eV and photoluminescence at 2.4eV due to the Bi3+ impurity centers. The effect of multiplication of electronic excitations due to inelastic scattering of hot photoelectrons and hot photoholes was revealed at photon energies above 15eV (more than 3Eg). It was found that the radiative lifetime for free excitons in LiH at 4.67eV is less than 1ns as low temperatures as at 10K. The interpretation of the electronic band structure of lithium hydride in the ultraviolet and vacuum ultraviolet spectral regions were carried out on the basis of the present experimental results with the involvement of the available band structure calculations.

Luminescence of LaBr3: Ce,Hf crystals under photon excitation in the ultraviolet, vacuum ultraviolet, and X-ray ranges

This study has been carried out using synchrotron radiation, time-resolved luminescence ultraviolet and vacuum ultraviolet spectroscopy, optical absorption spectroscopy, and thermal activation spectroscopy. It has been found that, in scintillation spectrometric crystals LaBr3: Ce,Hf characterized by a low hygroscopicity, along with Ce3+ centers in regular lattice sites, there are Ce3+ centers located in the vicinity of the defects of the crystal structure. It has also been found that the studied crystals exhibit photoluminescence (PL) of new point defects responsible for a broad band at wavelengths of 500–600 nm in the PL spectra. The minimum energy of interband transitions in LaBr3 is estimated as E g ∼ 6.2 eV. The effect of multiplication of electronic excitations has been observed in the range of PL excitation energies higher than 13 eV (more than 2E g ). Thermal activation studies have revealed channels of electronic excitation energy transfer to Ce3+ impurity centers.

Recombination luminescence of LaPO4-Eu and LaPO4-Pr nanoparticles

The study of the spectral-luminescence parameters of LaPO4-Eu and LaPO4-Pr nanoparticles upon excitation by the synchrotron radiation with photon energies 4–40 eV was performed. The differences of the luminescence intensity dependence on the size for LaPO4-Eu and LaPO4-Pr nanoparticles excited at the range of matrix transparency, the range of band-to-band transitions, and the range of electronic excitation multiplication were revealed. The observed regularities are explained in terms of the electron-phonon and electron-electron scattering, surface losses, and exciton diffusion. The ratio between the length of thermalization and electron mean free path and the size of nanoparticle is determinative for the luminescence intensity upon excitation in the range of fundamental absorption of matrix and X-ray excitation.

Relaxation of electronic excitations in CaF2 nanoparticles

The luminescence properties of CaF2 nanoparticles with various sizes (20–140 nm) are studied upon the excitation by VUV and x-ray quanta in order to reveal the influence of ratio of mean free path and thermalization length of charge carriers and nanoparticle size on the self-trapped exciton luminescence. The luminescence intensity for exciting quantum energies corresponding to optical creation of exciton and to the range of electronic excitation multiplication is not so sensitive to nanoparticle size as for quanta with energy of Eg < hν < 2Eg. The dependences of luminescence intensity on nanoparticle size at the excitation by quanta of various energies are discussed in terms of electron-phonon and electron-electron scattering lengths and energy losses on surface defects.

A luminescence spectroscopy study of scintillation crystals SrI2 doped with Eu2+

We report experimental study of luminescent properties of modern scintillation material SrI2:Eu2+ carried out over the temperature range from 9 to 450K by the means of the ultraviolet and vacuum ultraviolet spectroscopy with a time resolution. Photoluminescence of the Eu2+ ions at 2.85eV was studied under both the intracenter and interband excitations, including an X-ray excitation. The bandgap of the SrI2 crystal has been estimated on the basis of the obtained results. In the temperature range below 100K the intrinsic luminescence at 3.4eV was revealed and this emission band was assigned by its properties to the luminescence of self-trapped anionic excitons in SrI2. The pronounced manifestation of the effect of multiplication of electronic excitations was revealed in the energy range above 2Eg.

Multiplication of electronic excitations in nanophosphors Lu 2O 3:Eu 3+ and Lu 2O 3:Tb 3+

Luminescence properties of Lu 2O 3:Eu 3+ and Lu 2O 3:Tb 3+ nanocrystalline powders with the particle size varying from 46 to 6 nm were studied under excitation by synchrotron radiation in the photon energy range (up to ∼22.5 eV) covering the region where the processes of multiplication of electronic excitation occur. It was found that the excitation spectra of Tb 3+ emission from all Lu 2O 3:Tb 3+ nanopowders have similar behavior, whereas the shape of the excitation spectra of Eu 3+ emission from Lu 2O 3:Eu 3+ nanopowders strongly depends on the particle size. The difference in the behavior of Lu 2O 3:Eu 3+ and Lu 2O 3:Tb 3+ nanophosphor systems was explained by different mechanisms of the energy transfer from the host to Eu 3+ or Tb 3+ ions (either the hole or electron recombination mechanism, respectively), which are differently influenced by losses of electronic excitations near the particle surface.

Multiplication of electronic excitations and prospects for increasing scintillation efficiency in wide-gap crystals

A comparative analysis of the electron–hole (e–h) excitation mechanism of impurity luminescence has been performed for wide-gap oxides (WGO) and alkali halides crystals (AHC). A low quantum yield of scintillation in WGO with respect to that in AHC is connected with the high value of the average energy needed for the creation of an e–h pair in WGO. In AHC and many WGO, the width of a valence band Ev is smaller than the energy gap Eg and hot valence holes are not able to create any secondary e–h pairs. The Ev/Eg ratio that describes “hole losses” in scintillators is 1.5–2.0 times higher in WGO than in CsI. The scenario of the two-step transformation of an e–h pair or an anion exciton into two emitted photons in α-Al2O3:Mg2+,Ti4+ has been considered.

Multiplication of electronic excitations in AgCl crystals

The kinetics of pulse conduction in silver chloride single crystals is investigated in the temperature range 12–300 K upon picosecond excitation with x-ray bremsstrahlung. It is found that the experimentally observed concentration of conduction electrons is nearly twice as high as the concentration of electron-hole pairs generated by an exciting pulse. This effect is associated with the chain multiplication of band charge carriers.

Multiplication Processes of Electronic Excitations in PbCl2Crystals Excited by Vacuum Ultraviolet Radiation

The excitation spectra for two intrinsic luminescence bands at 3.76 eV (UV) and 2.62 eV (BG) in PbCl 2 have been measured using synchrotron radiation in the energy range up to 25 eV at 10 K. Since the UV and BG bands are connected to excitonic luminescence and electron–hole ( e – h ) recombination luminescence, respectively, an analysis of the difference in their excitation spectra allows us to separate excitonic and e – h mechanisms in the multiplication of electronic excitations. It is found that the intensity of the UV emission is sharply increased at 9.8 eV, which is higher than twice the band gap energy. Secondary excitons are formed by inelastic scattering of hot photoelectrons in the region above 9.8 eV. On the other hand, the BG emission is gradually enhanced in the range of 9–12.5 eV, and the intensity doubles at around 12.5 eV. This result indicates that the creation of two e – h pairs per photon predominantly causes in the region above 12.5 eV. The e – h multiplication originates not only in in...

Creation of electronic excitations and defects by vuv radiation (6-40 eV) in wide-gap solids

The processes of multiplication of electronic excitations (MEE), connected with the creation of secondary excitons or electron-hole (e-h) pairs by hot conduction electrons, are realized in wide-gap metal halides and oxides. In oxides, secondary e-h pairs can be also formed by 27-40 v eV photons due to L 1 VV Auger transitions (with the participation of 2s oxygen holes). The excitation spectra of luminescence and the creation spectra of electron F centres or hole V centres have been measured for Na 6 Al 6 Si 6 O 24 (NaI) 2x sodalites and MgO:Be, respectively, at 8-80 v K. A high local density of excitations has been revealed under MEE conditions in KBr and Br sodalites with self-trapping excitons and holes.

Creation of F centres and multiplication of electronic excitations in Na6Al6Si6O24(NaBr)2x optical ceramics under VUV irradiation

The reflection spectra, the excitation spectra of various emissions (2.4–4.5 eV) and the creation spectra of F centres have been measured in Na6Al6Si6O24(NaBr)2x optical sodalite ceramics (x = 0.94 and x = 0.81) using VUV radiation of 6–35 eV. An analysis of the spectra allowed us to separate several groups of electronic excitations: the photons of 6.7–8.3 eV excite or ionize Br− centres in the β-cages, 8.5–32 eV photons generate electronic excitations of the aluminosilicate carcass, while 33 eV photons excite Na+ ions up to the 2p53s state. The creation mechanism of F centres, connected with the trapping of conduction electrons by pre-irradiation bromine vacancies and localization of holes at Br− centres in β-cages, has been revealed in sodalites at 80 K. The efficiency of F centre creation triples in the multiplication region of electronic excitations of the aluminosilicate carcass (20–30 eV) in a sample with x = 0.94. Using methods of thermoactivation spectroscopy, the creation of thermally stable F centres (up to 480 K) has been detected in sodalites irradiated by x-rays or 27 eV photons. However, such F centres cannot be created by 7.6–11 eV photons, i.e. when there is no multiplication of excitations and an exciting photon forms only one electronic excitation.

Vacuum ultraviolet spectroscopy of U:LiF, Cu, and U:NaF, Cu crystals

Luminescence and excitation spectra of doped LiF and NaF crystals are studied by time-resolved optical and luminescent vacuum ultraviolet (VUV) spectroscopy (2–40 eV energy range, T=10–295 K) with the use of synchrotron radiation of the X-ray and the VUV ranges and pulsed electron beams. Spectral kinetic parameters of luminescence and energies of excited states of U6+ ions are determined. The dominant role of the electron-hole mechanism for energy transfer to impurity centers is established. The effect of multiplication of electronic excitations is clearly manifested for E > 25 eV in NaF:U, Cu crystals and determines their high scintillation yield (137% relative to Tl:CsI when detected in the current regime).

Excitonic and electron-hole processes in NaCl and NaCl:Ag crystals under conditions of multiplication of electronic excitations

The excitation spectra of and emissions of self-trapped excitons (3.4 eV and 5.4 eV, respectively) as well as the excitation spectra of 5.17 eV luminescence of Ag+ impurity centres were measured in NaCl and NaCl:Ag crystals using synchrotron radiation of 5 - 38 eV. Fast and slow components of these emissions were detected. An analysis of the differences in the excitation spectra measured at 8 and 295 K allowed us to separate the excitonic and electron-hole (e-h) mechanisms of the multiplication of electronic excitations. A photon of 17 - 19 eV forms an e-h pair and a secondary exciton, while the absorption of a 21 - 27 eV photon causes the creation of two e-h pairs. Using luminescent and photoelectric methods, it was shown that a 2p3s Na+ cation exciton, formed at the absorption of a 33.4 eV photon, decays with the creation of an anion exciton with a 3p hole component and two e-h pairs. Three e-h pairs are formed after the absorption of a 31 eV photon by a chlorine ion.

Optical and luminescent VUV spectroscopy of La2Be2O5 crystals

Electronic excitations and the processes of their radiative relaxation are studied in pure and Ce3+ ion-doped crystals of lanthanum beryllate excited by synchrotron radiation in the x-ray and VUV ranges by methods of optical and luminescent vacuum ultraviolet time-resolved spectroscopy. Manifestations of excitons of the valence band are absent in the reflection spectra. However, a fast (τ=1.7 ns) and a slow (microsecond range) channel corresponding to two possible types of self-trapped excitons (STE) are found in radiative relaxation of intrinsic electronic excitations at T=10 K. The slow channel corresponds to emission of STE formed through recombination, the fast channel corresponds to emission of relaxed metastable excitons from the STE state. In the energy region higher than 14 eV (E>2Eg), the effect of multiplication of electronic excitations due to generation of secondary electron-hole pairs resulting from inelastic scattering of both hot photoelectrons and hot photoholes is exhibited.