S. Vielhauer

Luminescent properties of Gd2SiO5 powder doped with Eu3+ under VUV?UV excitation

The luminescent properties of Gd2SiO5 powder crystals doped with Eu3+ were investigated using synchrotron radiation and a VUV laser (157.6?nm) as excitation sources. The excitation spectra in the range of 160?330?nm monitoring the red emission from Eu3+ ions reveal bands corresponding to intraconfigurational 4f?4f transitions of Gd3+ and charge transfer states (CTS) of Eu3+?O2?, as well as interband transitions of the Gd2SiO5 host, indicating an efficient energy transfer process from the host or directly from Gd3+ to Eu3+ ions. The inspection of emission lines from Eu3+ ions suggests that there are three inequivalent sites for Eu3+ in this host. The excitation band around 215?nm and the unique emission features associated with this excitation band were attributed to one of the inequivalent Eu3+ sites, which exhibits an unusually weak coupling with host lattice. The energy levels of Eu3+ in the Gd2SiO5 host were tentatively assigned according to the laser excited emission spectra.

Branched relaxation of electronic excitations in rare-gas crystals with traps of different types

Luminescence excitation spectra are used as a probe of competition between exciton traps of two types: exciton localization centres and centres of exciton trapping followed by trans- forming to another electronic state. A strong competition between localization and transforming centres takes place only under excitation above the dielectric gap Eg (with energy E>E g). For E<E g the action of transforming traps is usually suppressed by localization centres. A strong change in excitation spectra near the point Eg is explained by a nontrivial structure of the exciton band formed by the mixing of free excitons with polaronic states—excimer quasi- molecules. The nature and action mechanism of both types of trap are explored. To localize heavy exciton polarons, small lattice defects are sufficient. Transforming traps exist in the form of stable molecular ions, conditioned by large defects capturing light electrons, and impurity centres. The trapping cross sections of the centres of different types are strongly (but in different ways) dependent on the exciton energy.

VUV spectroscopy of a new fluoride system NaF–(Er, Y)F3

Abstract Emission and excitation spectra as well as luminescence decay kinetics of a new complex fluoride system Na 0.4 (Y 1− x Er x ) 0.6 F 2.2 ( x =0.01, 0.1, 1) have been studied in the vacuum ultraviolet (VUV) spectral range. It has been shown that these crystals have intense VUV luminescence due to the interconfiguration 5d–4f transitions in Er 3+ ion. The spin-allowed 5d–4f luminescence of Er 3+ in this system is very weak, i.e., in these crystals there exists an efficient non-radiative relaxation from higher-lying 5d states of Er 3+ to the lowest 5d level responsible for spin-forbidden luminescence. This makes the studied new fluoride system a promising active medium for the production of VUV solid state laser with optical pumping. Due to rather large bandwidth of Er 3+ 5d–4f luminescence in this system there is a possibility for the construction of tuneable VUV laser.

VUV-luminescence and excitation spectra of the heavy trivalent rare-earth ions in fluoride matrices

VUV 4fn → 4fn−15d transitions of Gd3+, Er3+, Tm3+, and Lu3+ in fluoride matrices have been analyzed with high-resolution luminescence and excitation spectroscopy. In trifluorides, strong electron-phonon coupling has been found. In the other matrices, the luminescence spectra clearly yield zero-phonon lines and phonon replica, indicating intermediate coupling. The energies of the zero-phonon lines observed are compared with theoretical predictions. Near the threshold of f → d excitations, some of the excitation spectra yield sharp structures which cannot be explained with phonon replica but will be discussed in terms of the energy levels of the 4fn−15d configuration.

CATION AND ANION ELECTRONIC EXCITATIONS IN MgO AND BaF2 CRYSTALS UNDER EXCITATION BY PHOTONS UP TO 75 eV

The creation of primary and secondary cation excitations was investigated for MgO and BaF2 crystals using the energy-and time-resolved luminescence method. It was shown that the primary cation excitations in MgO created by photons of 50–65 eV decay with the formation of anion free excitons and electron–hole pairs. The formation of secondary cation holes by hot photoelectrons in the Ba2+ 5p shell causes the increase of cross-luminescence intensity whereas the creation of neither primary nor secondary cation excitons causes the appearance of cross-luminescence in BaF2.

VUV luminescence of BaF2, BaF2:Nd and BaY2F8 crystals under inner-shell excitation

The VUV luminescence properties of several Ba containing crystals in the temperature range of 10–300 K were investigated using energy- and time-resolved spectroscopy under excitation by XUV photons. The results are compared with those obtained under excitation by photons up to 40 eV. The revealed new emission bands can originate from the radiative recombination of valence electrons with the outermost Ba 2+ 5p core holes and from the radiative transitions of multiply charged Ba ions. The possible inter-atomic cross-relaxation of holes from the fluorine to the barium ion in BaF2 was revealed as well. r 2002 Elsevier Science B.V. All rights reserved.

Deep VUV Scintillators for Detectors Working in Cryogenic Environment

The properties of various inorganic scintillator materials with different origin of their VUV luminescence have been studied under pulsed excitation by XUV (50-1000 eV) synchrotron radiation. A comparison of spectral and timing characteristics of these crystalline VUV emitters has been performed. The processes of energy transfer from the host crystal to the luminescence centers of different nature were analyzed from the features of excitation spectra in the regions of inner-shell absorption.

Excitation of intrinsic and extrinsic luminescence by synchrotron radiation in a NaF crystal

Abstract The reflection spectrum as well as the excitation spectrum of self-trapped exciton emission at 2.6 eV has been measured in a NaF crystal at 5 K. The width of a forbidden band equals approximately 12.3 eV.

VUV Luminescence Due to 5d - 4f Transitions in Gd3+ and Lu3+ Ions Doped into Fluoride Crystals

High-resolution ( < 1 Å) VUV emission and excitation spectra as well as luminescence decay kinetics have been studied for several fluoride crystals containing Gd and Lu ions under excitation by synchrotron radiation. The obtained results evidently show that VUV luminescence (h ~ 10 eV) observed from these crystals originates from 5d-4f transitions in the Gd and Lu ions. Only fast spin-allowed 5d-4f luminescence is observed from Gd compounds, whereas both spin-forbidden and spin-allowed 5d-4f luminescence has been detected from Lu compounds, the latter being observed only at high enough (T > 100 K) temperatures.

Exciton self-trapping into diatomic and triatomic molecular complexes in xenon cryocrystals

Our recent study of molecular trapped centers in Xe cryocrystals is extended to triatomic self-trapped excitons. Time- and spectrally-resolved molecular luminescence is measured in the temperature range 5–60 K. The processes of intrinsic exciton self-trapping into diatomic and triatomic molecular complexes and extrinsic exciton trapping at lattice imperfections are separated by selective photoexcitation of Xe cryocrystals with synchrotron radiation. The temperature dependences of the triplet lifetimes of molecular exciton subbands are measured for the first time.