Search

The luminescence of Ce 3+ in perovskite (ABO 3 ) hosts with nd 0 B-site cations, specifically Ca(Hf,Zr)O 3 and (La,Gd)ScO 3 , is investigated in this report. The energy position of the Ce 3+ excitation and emission bands in these perovskites is compared to those of typical Al 3+ perovskites; we find a Ce 3+ 5d 1 centroid shift and Stokes shift that are larger versus the corresponding values for the Al 3+ perovskites. It is also shown that Ce 3+ luminescence quenching is due to Ce 3+ photoionization. The comparison between these perovskites shows reasonable correlations between Ce 3+ luminescence quenching, the energy position of the Ce 3+ 5d 1 excited state with respect to the host conduction band, and the host composition.

Ce3+-doped aluminate garnets are the main phosphors used in solid-state lighting based on blue light emitting diodes (LEDs). Recently, different compositional modifications of garnet hosts have been shown to significantly redshift the yellow-green Ce3+ emission in the garnets. We investigate the high temperature quenching of these phosphors to determine any limitations towards red emission in Ce3+-doped garnets.

In this paper the excitation and emission spectra of Eu2+ in Cs2M2+P2O7, where M2+ = Ca2+ and Sr2+, are presented. The remarkable features of the Eu2+ luminescence in these double phosphate materials include a very large Stokes shift of emission (~1 eV), a high luminescence quenching temperature and a very low energy of the emitted photons, which is unusual for the luminescence of the Eu2+ ion in phosphate based materials. It is shown that the optical spectra can be attributed to an extreme case of the standard Eu2+ 4f65d1→ 4f7[8S7/2] transition.

The luminescence of Ce3+ in scandate perovskite hosts is investigated in this report. In general, the energy position of the Ce3+ excitation and emission bands are at lower energy versus comparable Al3+ perovskites, which is primarily attributed to a larger Ce3+ 5d1 centroid shift. The efficiency of Ce3+ luminescence is low at room temperature (

We report further development of an EPICS-compatible bunch length monitor based on the autocorrelation of coherent transition radiation (CTR). In this case the monitor was used to optimize the beam from the S-band thermionic RF gun on the Advanced Photon Source (APS) linac. Bunch lengths of 400–500 fs (FWHM) were measured in the core of the beam, which corresponded to about 100-A peak current in each micropulse. The dependence of the CTR signal on the square of the beam charge for the beam core was demonstrated. We also report the first use of the beam accelerated to 217 MeV for successful visible wavelength SASE FEL experiments.

The light induced ionization of impurity centers and defects in insulating crystals results in many phenomena such as charge transfer optical transitions in absorption spectra, photoconductivity, the photovoltaic effect, optical bleaching, and photochemical persistent spectral hole burning. This paper considers specific effects in photoionization related phenomena which are closely connected with inversion symmetry of impurity centers. These recently observed effects include (a) the drastic dependence of the photoionization energy on impurity sites lacking inversion symmetry (Li2Ge7O15:Cr3+), (b) the direct dependence of the sign of the photovoltaic effect on the polarity of impurity sites in doped pyroelectric crystals (LiNbO3:Cr3+), and (c) the anomalous broadening of photochemical persistent spectral holes in zero-phonon spectra of inversionless impurity centers (CaS:Eu).

The Advanced Photon Source (APS) injector linac has been reconfigured with a low-emittance RF thermionic gun and a photocathode (PC) RF gun to support self-amplified spontaneous emission (SASE) free-electron laser (FEL) experiments. One of the most critical parameters for optimizing SASE performance (gain length) is the electron beam peak current, which requires a charge measurement and a bunch length measurement capability. We report here initial measurements of the latter using both incoherent optical transition radiation (OTR) and coherent transition radiation (CTR). A visible light Hamamatsu C5680 synchroscan streak camera was used to measure the thermionic RF gun beam's bunch length (σ∼2–3 ps) via OTR generated by the beam at 220 MeV and 200 mA macropulse average current. In addition, a CTR monitor (Michelson Interferometer) based on a Golay cell as the far-infrared (FIR) detector has been installed at the 40-MeV station in the beamline. Initial observations of CTR signal strength variation with gun α-magnet current and interferograms have been obtained. Progress in characterizing the beam at these locations and a comparison to other bunch length determinations will be presented.

The bunch length of an electron beam derived from the UCLA Saturnus photoinjector has been measured using a 45° CTR foil. The sudden change of electrons boundary conditions cause them to radiate (transition radiation) with the spectral power entirely dependent upon the degree of coherency, which strongly relates to the beam size. A polarizing Michelson interferometer allowed measurement of the auto-correlation of the coherent transition radiation signal. An analysis method was developed to compensate for undetected low-frequency radiation and systematically extract the bunch length information for a specific beam model. This analysis allowed observation of pulse lengthening due to the space charge, as well as compression with the variation of the RF injection phase. The hypothesis of a satellite beam has been also tested using this analysis.

We have investigated the non-radiative relaxation of the substitutional H − local mode ( v = 965 cm −1 ) in CaF 2 . This system is attractive due to its simplicity, the only possible decay mechanism, aside from radiative relaxation, is non-radiative multiphonon decay into host phonons. Saturation experiments were performed at the Stanford Picosecond Free Electron Laser Center, using a pump-probe technique. We find an energy relaxation time T 1 of 45 ps at low temperature. We compare this result with the relaxation of the local mode of an interstitial H − ion next to a Lanthanum impurity ion, and find a much more rapid relaxation time T 1 of 6 ps.

The energy relaxation times of the fundamental stretching modes in the electronic ground state of OH−, OD−, SH−, and TeH− in a variety of alkali halides are measured by incoherent laser saturation and found to vary from 0.3 to 3 ns at 1.7 K. These vibrational lifetimes are between 4 and 8 orders of magnitude smaller than those of other heteronuclear diatomics diluted in crystals, including the ionic systems of CN− in salts and the neutral deuterides, DCl and ND, and oxides, CO and NO, in rare-gas matrices. Unlike these other systems, the chalcogen-hydride-doped alkali halides have a librational mode at frequencies well above the top of the host phonon band. This makes the librational decay channel a lower order process than relaxation into phonons. An energy gap law can be fit to the data, in which the relaxation times vary exponentially with the number of accepting reorientational modes. This model can explain the fact that OH− and OD− in KCl have nearly the same lifetimes, since the vibrational and libr...

The vibrational relaxation of polyatomic molecular impurities in solids is controlled by a decay scheme that involves both internal modes and vibrational quanta of the host material. Here the relaxation of the asymmetric stretch mode of D 2 O in vitreous As 2 S 3 is reported. Applying a pump-probe technique using infrared laser pulses of the Stanford Picosecond Free Electron Laser, a relaxation time on the order of 200 ps at low temperature is found. From the temperature dependence of the relaxation time, it is concluded that the asymmetric stretch mode relaxes in a process involving two bending modes of the D 2 O molecule and two vibrational quanta of the host.

We report a measurement of the quantum efficiency of a niobium photocathode for the wavelengths λ = 193 nm and λ = 248 nm. Charge yields for a 20 ns excimer laser pulse were measured as a function of DC accelerating anode voltage. Our experimental results for the quantum efficiency are e(248) = 3.2±0.6 × 10−6 and e(193) = 4.5±0.5 × 10−4. We conclude that niobium is a promising photocathode material for a superconducting rf electron gun if the laser wavelength is RE200 nm. Laser pulse energy requirements are estimated for such a gun.

We have investigated the vibrational relaxation of the OH impurity stretch mode in amorphous SiO2. At low temperatures, a short relaxation time of ≈10−10 s is found that varies strongly across the inhomogeneously broadened absorption line. The inhomogeneous broadenining is caused by a site-dependent frequency shift proportional to the strength of the hydrogen bond of the OH to neighboring atoms. Thus the observed lifetime decrease across the absorption line can be attributed to increasing hydrogen bond strength. A simple relation between the site-dependent vibrational lifetime and the frequency shift is presented.

The fundamental stretching modes near 4.5 μm of SeH− and TeH− doped in KCl and KBr are measured at high resolution at 1.7 K. For the KCl-doped samples, prepared by adding elemental Se (Te) to the melt and growing under H2, the concentrations of atomic Se (Te) are determined by neutron activation analysis. An IR/Raman/UV spectroscopic survey of impurities in these crystals indicates that Se (Te) is present mainly in the form of SeH−(TeH−). Thus, ratioing the vibrational absorption strengths to the atomic concentrations gives the integrated cross-sections of SeH−(TeH−), equivalent to an effective charge q ∗ of ~0.14e and a radiative lifetime of ~30 ms. This value of q ∗ is much smaller than theoretical predictions for the free ions, implying a matrix-induced screening of the dipole moment. The lowest-order mechanical anharmonic correction to the vibrational frequencies is found to be ~2% by measuring the first overtones and is used to fit the isotopic shifts in the spectral peaks. These peaks are homogeneously broadened at 1.7 K; if lifetime limited, this implies a vibrational relaxation of the order of 100 ps, nine orders of magnitude faster than the radiative lifetime.

The non-radiative relaxation of the SH vibrational stretch mode in hydrogen doped As2S3 glass has been measured using infrared picosecond laser saturation spectroscopy. A lifetime on the order of 10−10 s is found at low temperature that varies by a factor of 6 across the broad SH absorption line, demonstrating a site-dependent relaxation rate. A temperature dependence study indicates that the stretch mode relaxes through a multi-vibrational decay into modes of the glassy host.

The vibrational relaxation times of SH- and OH- in potassium halides, measured by incoherent laser saturation, vary from 0.8 to 7 ns at 1.7 K. A librational side band at +300 cm-1 provides a possible decay channel. The strength of this mode is strongly temperature dependent, possibly owing to barrier hopping.

In this paper we present the calculated electronic structures and associated properties of a green phosphor, Gd 2 O 2 S : Tb 3+ and compare with experiment. Calculations are performed using the spin polarized relativistic version of the self-consistent field X α scattered wave molecular orbital cluster method and the augmented spherical wave band structure supercell method. The band structures and the associated energy level diagrams of this phosphor will be used to study the luminescence mechanism of Gd 2 O 2 S : Tb 3+ and to interpret its optical, photoconductivity and XPS spectra.

The behavior of the far infrared radiation produced by the passage of a mm long bunch of electrons through a vertical 0.44 T magnetic field is found to

The longitudinal charge distribution of electron bunches in the Fermilab/NICADD photoinjector was determined using the coherent transition radiation produced by electrons passing through a thin metallic foil. The autocorrelation of the transition radiation signal was measured with a Michelson-type interferometer. The response function of the interferometer was determined from measured and simulated intensity spectra for low electron bunch charge and maximum longitudinal compression. Both pyroelectric and Golay detectors were used for these measurements. A Kramers-Kronig technique was used to determine longitudinal charge distribution. Measurements were performed for electron bunch lengths in the range from 0.3 to 2 ps (rms). To test the accuracy of this interferometric method, the longitudinal charge distribution was measured for double-peaked electron bunches with known distance between the two pulses. The agreement between measured bunch length and simulation is within 30%.

Using infrared and optical absorption spectroscopy in combination with photoconductivity measurements on uranium-doped calcium fluorite crystals, we clarify a controversy in the assignment of U2+ and U4+ ion valences in CaF2 crystals which exhibit different coloration. The energy thresholds of donor-like processes (photoionization) follow the sequence of ionization potentials for uranium ions of different valences and indicate the green and yellow colors to be due to U4+ of different charge compensation.

We report on the characterization of sol-gel derived lithium niobate via trivalent chromium probe ions, a study that is motivated by recent reports on the synthesis of high quality sol-gel lithium niobate (LiNbO$_3$). In order to assess the quality of sol-gel derived LiNbO$_3$, we incorporate Cr$^{3+}$ during the hydrolysis stage of the sol-gel process. A comparison of the Cr$^{3+}$ emission and photo-excitation data on both sol-gel and melt-grown LiNbO$_3$ shows that the sol-gel derived material is highly stoichiometric.

The University of Georgia, in collaboration with GE Global Research, is investigating the relevant quenching mechanism of phosphor coatings used in white light devices based on UV LEDs in a focused eighteen month program. The final goal is the design of high-efficacy white UV-LEDs through improved and optimized phosphor coatings. At the end of the first year, we have reached a fundamental understanding of quenching processes in UV-LED phosphors and have observed severe quenching in standard devices under extreme operating conditions. Relationships are being established that describe the performance of the phosphor as a function of photon flux, temperature, and phosphor composition. These relationships will provide a road map for the design of efficient white light LEDs during the final six months of the project.

The kW‐class infrared (IR) Free Electron Laser (FEL) at Jefferson Lab had the capability of producing intracavity Thomson scattering of the IR off the electron beam thus producing high average flux, sub‐picosecond x‐rays. We have measured these x‐rays and demonstrated the energy tuneability range from 3.5 keV to 18 keV. The corresponding flux and brightness has been estimated and will be discussed. This year, 2002, the FEL was disassembled and has been reconfigured to produce 10 kW average power IR. We present the estimated x‐ray capabilities for the new FEL and discuss potential applications.

In alkali halide crystals containing CN - ions, pulsed UV radiation below the band gap is observed to produce F centers and then IR vibrational fluorescence, in that order. The stretch mode excitations are driven by inelastic collisions from conduction band electrons produced by the UV ionized F centers.

The frequency dependent relaxation properties of the SH vibrational stretch mode in hydrogen doped As 2 S 3 glass have been measured using IR picosecond laser saturation spectroscopy. A low temperature lifetime in the order of 10 -10 s is found that varies by a factor 6 across the broad SH absorption line

High-frequency phonons in mixed crystals exhibiting glass-like properties show a frequency dependence of the decay rate that differs from the v5 dependence of the anharmonic phonon decay in crystalline insulators. We present a scaling law that describes the transient frequency distribution of non-equilibrium phonons in the glass-like system.

A test stand for the 10 MeV, 5 mA average current injector for the CEBAF FEL is currently under construction. The injector tests will progress through two phases. The first phase will be devoted to characterizing the gun transverse and longitudinal emittance performance as a function of bunch charge, beam size, and energy. The goal of the second phase is to achieve the nominal requirements of the 10 MeV injector, including bunch length, emittance, charge per bunch, and energy stability. This paper summarizes the diagnostics planned to be used in these experiments.

not Available.

not Available.

Coherent transition radiation, generated by the passage of mm-long bunches of electrons through a thin metal foil, has been observed in the far infrared. The intensity was compared to the smaller amount of Cherenkov radiation produced when ${\mathrm{SF}}_{6}$ gas was introduced into the electron path. Wake-field radiation was ruled out by varying the geometry of the metal chamber near the electron beam. The intensity, the polarization, and the spectral and angular distribution of the transition radiation were measured and its coherent nature was identified.

not Available.

This report describes experiments that elucidate the phosphorescence mechanism in Sr2MgSi2O7:Eu2+, RE3+ phosphors. The first step of phosphorescence, the storage of carriers in traps, is traced to thermally assisted photoionization of Eu2+ ions, supported by thermal quenching studies that give the thermal ionization threshold of Eu2+. The trapping of these delocalized electrons has been detected using the spectroscopic signature of Sm2+ ions created by electron capture by Sm3+ in samples doped with Eu2+ and Sm3+ ions. The final steps for phosphorescence, liberation of trapped carriers and recombination with the luminescence center, have also been studied by monitoring the Eu2+ luminescence induced by optical excitation into the Sm2+ 4f6→4f55d1 absorption band. These results show that the phosphorescence in this system starts with the promotion of electrons into the conduction band and subsequent reduction of RE3+ ions, not from hole formation and RE3+ oxidation, as has been reported for similar systems. F...

This letter reports on phosphor quenching by saturation, a sublinear phosphor response with excitation intensity, in light emitting diode (LED) packages using 405nm LEDs and Ca5(PO4)3Cl:Eu2+,Mn2+ phosphors. This saturation is due to the high light flux incident on the phosphors in these LED packages and the slow radiative relaxation rate of Mn2+. Apart from known saturation processes of Mn2+ ground state depletion and energy transfer between excited Mn2+ ions, an additional quenching pathway, Eu2+→Mn2+ (excited) energy transfer, is taken into account to quantitatively fit both the efficiency under pulsed operation and the time-resolved luminescence of Ca5(PO4)3Cl:Eu2+,Mn2+.

Pulse compression of submillimeter long electron bunches at the Vanderbilt FEL has been observed. The bunch length is determined by analyzing the coherent transition radiation spectrum produced by the charge distribution, and we show that bunch length changes as small as 15 μm (corresponding to 50 fs) can be detected.

The study of phosphor materials generally focuses on systems with a high quantum efficiency. On the other hand, a thorough understanding why certain systems show very low quantum yield will certainly lead to a better understanding of phosphor materials in general. Here we present studies on cerium-doped lutetium oxide and yttrium oxide crystals that are characterized by a complete quenching of the 4f-5d luminescence. To find the origin of the quenching mechanism, as well as to understand the difference to other Ce 3+ systems with high quantum efficiencies, we have performed extensive absorption, photoexcitation, and photoconductivity studies on single crystals of these systems, prepared by the laser-heated-pedestal growth technique. It is experimentally verified that the quenching mechanism is determined by the location of the first excited level of the cerium ions with respect to the conduction band - a general result that can be applied to a broad range of materials and will allow us to characterize and engineer future phosphor materials.

Y 2 O 3 :Eu 3+ has been extensively investigated as a red phosphor for fluorescent lamps and color displays. In particular, there have recently been several studies examining the effect of impurities on the luminescence efficiency for Y 2 O 3 :Eu 3+ under UV excitation. In this work we investigate the effects of codoping Y 2 O 3 :Eu 3+ with Ca 2+ and Zr 4+ on the emission produced by cathode-ray excitation. Specifically, we have measured the relative efficiency, fluorescent lifetimes, and fluorescent spectra under cathode-ray excitation and compare these results to those previously observed under UV excitation.

The spatial propagation of acoustic-phonon energy in an insulating medium that contains resonance-elastic and inelastic scattering centers is studied theoretically. We find that the phonon pulses arriving at different distances relative to a pointlike source show pseudoballistic character: Arrival time and pulse duration increase linearly with distance.

By means of inalastic neutron scattering we have determined the dispersion relation of the magnetic excitations in CsFeCl3 at different temperatures. The dispersion in c-direction, along the Fe-chains is typically ferromagnetic and in the hexagonal plane antiferromagnetic. Due to the lack of an applicable theory the data were parametrized by the simple heuristic formula:ħω = [2J[1 - cos πqc] [A + 2J(1 - cos π)qc)] + [C + J' (1.5 + γ(q))]2] 1 2 The gap was found to be C = 0.148 THz, the easy plane anisotropy A = 0.308 THz, the ferromagnetic interaction J = 0.148 THz and the antiferromagnetic interaction J' = -0.04 THz. At 1.25 K all excitations had a width smaller than the instrumental resolution ΔE = 0.025 THz. These results can be interpreted as follows: CsFeCl3 is a singlet ground state system with strong ferromagnetic interaction J along the crystallographic c-axis and weak antiferromagnetic interaction J' in the plane perpendicular to c. In addition we have measured the influence of a magnetic field along the hexagonal c-axis. The splitting found agrees with the assumed level scheme yielding g = 2.5 for the first excited level.

In GaAs-IREDs and GaAs0.6P0.4-LEDs a photoconductive and a bolometric FIR response has been observed at low temperatures by applying different pulsed optically pumped FIR lasers. The short circuit current responsivity peaks at λ=400 μm being about 1.5 mA/W. By monitoring the rapid modulation of the FIR radiation due to mode beating of the pump laser, an upper limit of 1 ns was established to the time constant of the photoconductive signal.

An experimental and theoretical study of transient frequency distributions of spontaneously decaying phonons is reported. After pulsed phonon excitation by phonon-combination absorption of infra-red radiation, a decay cascade of acoustic phonons is observed in a CaF2 crystal at low temperature. As long as the phonons do not escape from the crystal, the cascade can be described by a scaling function that is characteristic for spontaneous phonon decay in a crystal containing mode-mixing elastic scattering centres.

In this paper new methods for generation and detection of terahertz-phonons and application of these methods for a study of dynamical properties of high-frequency acoustic phonons in a crystal at low temperature are discussed. Terahertz-phonons are generated via multiphonon absorption by use of pulsed infrared radiation. For phonon detection phonon-induced fluorescence is observed. We have investigated the anharmonic decay of acoustic phonons. The experimental results indicate that the decay of acoustic phonons in the linear part of the dispersion curves can be described by a phonon decay cascade that can be characterized by a shape function. The optical techniques allow also to study for tetrahertz-phonons elastic scattering at impurities and the behaviour at crystal surfaces. We will, finally, discuss the role of the dynamics of tetrahertz-phonons for particle detection via phonons.

A new method for generation of high-frequency phonons is reported. Phonons were generated in CaF2 crystals by multiphonon absorption of infrared radiation of a CO2 laser. The new method was applied to study transient broadband spectra of nonthermal phonons.