R.S. Meltzer

Green phosphorescence of CaAl2O4:Tb3+,Ce3+ through persistence energy transfer

Single-crystal CaAl2O4 fibers doped with Ce3+ or Tb3+ or double doped with Tb3+ and Ce3+ were prepared by the laser-heated pedestal-growth method. Ce3+-doped CaAl2O4 was found to have persistent phosphorescence in the blue violet lasting more than 10 h, while Tb3+-doped CaAl2O4 showed only a weak afterglow. However, long persistent green phosphorescence from Tb3+ was observed in the doubly doped fiber, CaAl2O4:Tb3+,Ce3+. It was found that the enhanced green phosphorescence from Tb3+ is due to persistence energy transfer from the thermally released electrons to Ce3+ followed by energy transfer to Tb3+. The finding suggests a promising approach for controlling the color of persistent phosphors.

UV index climatology over the United States and Canada from ground‐based and satellite estimates

[1] Long-term monthly mean UV index values for Canada and the United States were calculated using information from two sources: from noon erythemal UV estimated from Total Ozone Mapping Spectrometer (TOMS) total ozone and reflectivity data and from UV index values derived from observations of global solar radiation, total ozone, dew point, and snow cover. The results are presented as monthly maps of mean noon UV index values. Mean UV index values in summer range from 1.5 in the Arctic to 11.5 over southern Texas. Both climatologies were validated against spectral UV irradiance measurements made by Brewer spectrophotometers. With snow on the ground the TOMSbased data underestimate UV by up to 60% with respect to Brewer measurements and UV derived from global solar radiation and other parameters. In summer, TOMS UV index climatology values are from 10 to 30% higher than those derived from global solar radiation and other parameters. The difference is probably related to aerosol absorption and pollution effects in the lower troposphere that are not currently detected from space. For 21 of 28 midlatitude Brewer sites, long-term mean summer UV measured values and UV derived from global solar radiation and other parameters agree to within +5 to 7%. The remaining seven sites are located in ‘‘clean’’ environments where TOMS estimates agree with Brewer measurements while UV derived from global solar radiation and other parameters is 10–13% lower. Brewer data also demonstrate that clean and ‘‘typical’’ sites can be as little as 70–120 km apart. INDEX TERMS: 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); KEYWORDS: UV index, Brewer, TOMS, pyranometer, climatology, ozone

Effect of the matrix on the radiative lifetimes of rare earth doped nanoparticles embedded in matrices

The radiative lifetimes, τR, of the excited states of rare earth (RE) ions contained in nanocrystalline insulators are different compared to their values in crystallographically equivalent bulk crystals. Their lifetimes depend on the effective index of refraction of the media consisting of nanoparticles and the substance filling the space between them. Here the radiative rates were studied as a function of particle size and the amorphous matrices containing the nanoparticles. Effects due to the effective index of refraction and site distortions associated with the increased inhomogeneous broadening were observed.

Electron-phonon interaction in rare earth doped nanocrystals

Nanoscale materials exhibit properties which differ considerably from their bulk counterparts due to modifications of their phonon density of states due to finite size effects. Rare earth doped insulating nanoparticles provide an ideal model system for studying the fundamental interactions between electronic states and phonons since the narrow lines due to rare earth 4fn→4fn transitions provide a high-resolution probe, while the insulating host eliminates the additional complications of collective electronic interactions. In this paper we investigate how the size restricted geometry of the nanoparticle influences (i) electronic relaxation by the one-phonon emission process and (ii) optical dephasing by the two-phonon Raman process.

Studies of the spectroscopic properties of Pr3+ doped LaF3 nanocrystals/glass

Abstract Emission and excitation spectra are presented for praseodymium (Pr 3+ , 0.01 at%) doped oxyfluoride glass host containing LaF 3 nanocrystals. Two types of Pr 3+ ions, those in LaF 3 nanocrystals and those in the glass host, are characterized by spectroscopic and dynamical studies. Pr 3+ ions in the glass are selectively excited by pumping the 4f5d band and UV emission is observed. For Pr 3+ ions in the glass, the relaxation of 3 P 0 is dominated by multi-phonon processes to 1 D 2 .

Ce3+ energy levels relative to the band structure in CaS: evidence from photoionization and electron trapping

The energy positions relative to the host valence and conduction bands of the 5d excited states and 4f ground states of Ce3+ in CaS were determined from photoluminescence and photoconductivity measurements. The determination was based on the observation that the 5d electrons of Ce3+ are trapped after transport in the conduction band only when they were excited to the upper 5d-state (Eg), which, therefore, must lie above the minimum of the conduction band of CaS.

Spectral hole burning in crystalline Eu2O3 and Y2O3 : Eu3+ nanoparticles

Abstract Optical dephasing of nanoparticles of Eu2O and Y2O3 : Eu3+ is studied, using transient spectral hole burning, as a function of temperature and particle size, d. A strongly enhanced optical dephasing rate is observed whose magnitude depends on 1 d 2 . This is interpreted as an enhancement of the electron-phonon interaction. A calculation for the two-phonon Raman scattering rate, which includes a smoothed phonon density of states for a nanoparticle. describes the nearly exponential temperature dependence of the hole line width, but does not properly describe the particle size dependence. A more complete theory which takes into account the spatial dependence of the amplitude of the phonon surface modes is required.

The mixing of the 4f2 1S0 state with the 4f5d states in Pr3+ doped SrAl12O19

Abstract The 1 S 0 state of Pr 3+ in SrAl 12 O 19 lies near to the 4f5d band. The admixing of the opposite parity 5d components by the crystal field without a symmetry center can be significant, thus affecting the properties of the 1 S 0 state. In a crystal with D 3h symmetry, the major 4f5d components which can be mixed into 1 S 0 are 2 −1/2 (|fdΓ 1 1 F 3 3〉-|fdΓ 1 1 F 3 -3〉) and 2 −1/2 (|fdΓ 1 1 H 5 3〉-|fdΓ 1 1 H 5 -3〉). A calculation of the 4f5d wavefunctions indicates that the states which may be mixed into 1 S 0 lie mainly at the middle and upper parts of the 4f5d configuration. The mixed 4f 2 1 S 0 wavefunction is then calculated and its spectral properties are discussed.

Time-resolved ultranarrow optical hole burning of a crystalline solid: Y 2 O 3 :Eu 3+

Optical hole burning is used to study optical dephasing of the 7F0 ⇒ 5D0 transition in samples of Y2O3:Eu3+ grown by both flame fusion and laser-heated pedestal growth. The samples exhibited a wide variation in their low-temperature hole widths, and the measured hole linewidths were in reasonable agreement with homogeneous linewidths obtained from values of T2 that were determined with two-pulse photon echoes. The sample that had the longest value of T2 (510 μs) from the echo measurements gave a hole width of 3.5 kHz. This is believed to be the narrowest spectral feature observed directly in the frequency domain for a solid. Other samples produced wider holes that were not correlated with the Eu3+ impurity concentrations. Time-resolved hole burning in one of the samples showed a slow and continuous broadening of the spectral hole from 130 kHz at 3 ms to 900 kHz at 10 s after hole burning. It is suggested that the additional broadening is associated with some form of disorder modes in this ordered crystalline solid.

New localized/delocalized emitting state of Eu2+ in orange-emitting hexagonal EuAl2O4

Eu2+-activated phosphors are being widely used in illuminations and displays. Some of these phosphors feature an extremely broad and red-shifted Eu2+ emission band; however, convincing explanation of this phenomenon is lacking. Here we report a new localized/delocalized emitting state of Eu2+ ions in a new hexagonal EuAl2O4 phosphor whose Eu2+ luminescence exhibits a very large bandwidth and an extremely large Stokes shift. At 77 K, two luminescent sites responsible for 550 nm and 645 nm broadband emissions are recognized, while at room temperature only the 645 nm emission band emits. The 645 nm emission exhibits a typical radiative lifetime of 1.27 μs and an unusually large Stokes shift of 0.92 eV. We identify the 645 nm emission as originating from a new type of emitting state whose composition is predominantly that of localized 4f65d character but which also contains a complementary component with delocalized conduction-band-like character. This investigation provides new insights into a unique type of Eu2+ luminescence in solids whose emission exhibits both a very large bandwidth and an extremely large Stokes shift.