Green Line Emission Research Articles

Tunable and white photoluminescence from Tb3+–Eu3+ activated Ca0.3Sr0.7Al2O4 phosphors and analysis of chemical states by X-ray photoelectron spectroscopy

Tb3+ and Eu3+ activated Ca0.3Sr0.7Al2O4 phosphors with tunable emissions were synthesized via urea assisted combustion method. The phosphors were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and photoluminescence (PL) spectroscopy. The XRD pattern of the Ca0.3Sr0.7Al2O4:Tb3+, Eu3+ powder showed was composed of a single phase associated with monoclinic SrAl2O4, suggesting that SrAl2O4 was the main structure in the Ca0.3Sr0.7Al2O4 mixed aluminate host and Ca2+ substituted the Sr2+ ions. Consistent with the three dimensional structure of SrAl2O4, the XPS data indicated that there were two different lattice sites occupied by Sr2+ in the lattice and resultantly Ca2+ could also be found at two different lattice sites. Tunable and white PL was generated when the phosphors were optically excited at different wavelengths using a monochromatized xenon lamp. The white PL with the Commission International de L’Eclairage (CIE) coordinates (x=0.343, y=0.325) was a result of the combination of the red line emission from Eu3+ and the blue and green line emissions from Tb3+.

On the relationship between atomic oxygen and vertical shifts between OH Meinel bands originating from different vibrational levels

OH Meinel emissions from different vibrational levels are known to occur at slightly different altitudes in the terrestrial airglow. Earlier model studies suggested quenching by atomic oxygen to be a principal cause of these vertical shifts. Here we employ the tropical mesopause region—characterized by pronounced semiannual variations—as a natural laboratory to test the hypothesis that vertical shifts between different OH Meinel bands are a consequence of quenching by atomic oxygen. Nighttime satellite measurements of OH(3–1) and OH(6–2) volume emission rate profiles and atomic oxygen with Scanning Imaging Absorption Spectrometer for Atmospheric Chartography on Envisat are used for this purpose. Upper mesospheric atomic oxygen profiles are retrieved from measurements of the O(1S–1D) green line emission. The results demonstrate that vertical shifts between the OH bands investigated are indeed correlated with the amount of atomic oxygen in the upper mesosphere, corroborating the hypothesis.

Absorption and luminescence properties of terbium ions in heavy metal glasses

Heavy metal glasses doped with Tb3+ ions have been investigated. Influence of glass-former oxides on the absorption and luminescence properties of terbium ions in inorganic glasses containing lead are discussed. Green emission line located at 543 nm due to 5D4 → 7F5 transition of Tb3+ was observed as a most intensive line. Green-to-blue luminescence ratio related to the integrated emission intensity of the 5D4 → 7F5 transition to that of the 5D4 → 7F6 transition was calculated and examined as a function of glass composition. Luminescence lifetimes for the 5D4 excited state of Tb3+ ions in heavy metal glasses were also determined. Correlation between green-to-blue luminescence ratios, measured lifetimes and the energy phonon of the glass hosts was proposed.

Realization of color hue tuning via efficient Tb3+–Mn2+ energy transfer in Sr3Tb(PO4)3:Mn2+, a potential near-UV excited phosphor for white LEDs

A n-UV convertible phosphor Sr3Tb(PO4)3:Mn(2+) with tunable-emitting color has been synthesized by solid state reaction. The GSAS refinement shows that the obtained powder crystallizes as a cubic unit cell with space group I43d, and Mn(2+) ions occupy the Sr(2+) crystallographic sites. Under (7)F6→(5)D3 of Tb(3+) excitation at 381 nm, Sr3Tb(PO4)3:Mn(2+) not only exhibits (5)D4→(7)F6-3 of Tb(3+) green emission lines but also (4)T1→(6)A1 of the Mn(2+) orange emission band. In addition, the intensity ratio of the orange/green emission bands can be enhanced through the increase of Mn(2+) content. The intense orange emission band of the Mn(2+) ions is attributed to the efficient energy transfer from the Tb(3+) to Mn(2+) ions, which has been justified through the luminescence spectra and fluorescence decay dynamics. The energy transfer mechanism was demonstrated to be the electric dipole-dipole interaction based on the Inokuti-Hirayama theoretical model, and the energy transfer efficiency was calculated. Optimal-composition samples show high external quantum efficiency, up to 59.3%, indicating the potential of the powder as a n-UV convertible phosphor for white LEDs.

Enhancement of green long lasting phosphorescence in CaSnO3:Tb3+ by addition of alkali ions

Long lasting phosphors of CaSnO3:Tb3+ added alkali ions (Li+, Na+, K+) were prepared by solid-state reaction. The phosphorescence of samples consists of a group of green emission lines originating from 5D4→7FJ transitions of Tb3+. The afterglow spectra and concentration quenching behaviors of fluorescence were investigated in the Tb3+ mono-doped sample. The result shows the optimal doping concentration of Tb3+ is 0.3mol%. In the co-doped samples, the doping concentrations of Tb3+ and alkali ions are both at 0.3mol%. It is found from the afterglow decay curves that the introduction of alkali ions can prolong the phosphorescent lasting time and the sample of incorporating Na+ shows the best result. Tb3+ and alkali ions can substitute Ca2+ ions, acting as hole and electron traps, respectively. The thermoluminescence (TL) spectra are also investigated. The depths of traps for the mono- and co-doped samples are calculated to be 0.622, 0.541, 0.529 and 0.538eV, respectively. Moreover, the possible mechanism of the green long lasting phosphorescence is proposed based on the experiment results.

Variation of Emission Line Width in Mid- and High-Latitude Corona

Spectroscopic studies of the solar corona, using the high spatial and spectral resolution 25-cm coronagraph at the Norikura observatory for equatorial off-limb observations, indicated that the variation of radiance and line width with height is different for different temperature lines. The line width of the forbidden red emission line [\FeX] 6374 \AA was found to increase with height and that of the green emission line [\FeXIV] 5303 \AA decrease with height. This had been interpreted in terms of the interaction between different temperature plasma but needed to be confirmed. Further observations were made on several days during 2004, in two emission lines simultaneously covering the mid-latitude and polar regions to investigate the existence of the observed variation in other parts of the solar corona. In this study, we have analysed several raster scans that cover mid- and high-latitude regions of the off-limb corona in all four bright emission lines [\FeX] 6374 \AA, [\FeXI] 7892 \AA, [\FeXIII] 10747 \AA, and [\FeXIV] 5303 \AA. We find that the FWHM of the red line increases with height and that of the green line decreases with height similar to that observed in equatorial regions. The line widths are found to be higher in polar regions for all of the observed emission lines except for the green line. Higher values of FWHM in polar regions may imply higher non-thermal velocities which could be further linked to a non-thermal source powering the solar-wind acceleration, but the reason for the behaviour of the green emission line remains to be explored.

Interionic cross relaxation and tunable color luminescence in KY3F10:Tb3+ nano/microcrystals synthesized by hydrothermal approach

Color tunable phosphors of KY3(1−x)Tb3xF10 (KY3F10:xTb3+) were synthesized by the hydrothermal (HT) approach. The luminescence consists of a group of blue and green emission lines originating from 5D3→7FJ and 5D4→7FJ transitions of Tb3+, respectively. The emitting color tunes from blue to green by gradually increasing Tb3+ concentrations, which is attributed to the enhanced 5D3–5D4 cross-relaxation (CR) between two Tb3+ ions, as described by (5D3, 7F6)–(5D4, 7F0). The CR process is analyzed based on Inokuti–Hirayama model and ascribed to the electric dipole–dipole interaction. The energy transfer critical distance and critical concentration between Tb3+ ions are evaluated to be 8.03Å and 4.61×1020cm−3, respectively. Moreover, the initial transfer rate on acceptor concentration is also presented with an averaged CR coefficient.

Synthesis and characterization of white light emitting Ca_xSr_1-xAl_2O_4:Tb^3+,Eu^3+ phosphor for solid state lighting

A white light emitting CaxSr1-xAl2O4:Tb3+;Eu3+ phosphor was synthesized by a combustion method using metal nitrates as precursors and urea as a fuel. The X-ray diffraction patterns from the samples showed phases associated with monoclinic structures of CaAl2O4 and SrAl2O4. White photoluminescence with the CIE coordinates (x = 0.343, y = 0.325) was observed when the phosphor was optically-excited at 227 nm using a monochromatized xenon lamp. The white photoluminescence was a result of the combination of blue and green line emissions from Tb3+, and red line emission from Eu3+. The structure and photoluminescence properties of this phosphor are reported.

Simulations of imaging Fabry–Perot interferometers for measuring upper-atmospheric temperatures and winds

We apply an onion-peeling inversion to simulated imaging Fabry-Perot interferometer (FPI) observations for retrieving altitude profiles of neutral temperatures and winds in the thermosphere in order to investigate the effects of realistic gradients in thermospheric parameters on the retrieved parameters. A forward model is developed that simulates the interference spectrum observed by an FPI using the redline (630.0 nm) and greenline (557.7 nm) emissions simulated based on climatological models. This forward model is applicable to both ground- and satellite-based simulations and generates a two-dimensional interference ring pattern. We simulate both a single-band ground-based FPI and a dual-band satellite-based FPI in order to study the effects of realistic vertical and horizontal gradients on the retrieved parameters.

THE EMPIRICAL MODE DECOMPOSITION TO STUDY THE QUASI-BIENNIAL MODULATION OF SOLAR MAGNETIC ACTIVITY AND SOLAR NEUTRINO FLUX

The time variability of solar activity indices such as sunspot areas (SAs) and green-line coronal emission, fluxes of solar energetic protons and galactic cosmic rays (CRs) in the period 1974–2001 has been investigated through the empirical mode decomposition (EMD). We found that the quasi-biennial periodicity is a prominent time scale of solar variability, having the energetic particle indices an amplitude comparable with the 11 years one. Moreover, we provide evidence for the quasi-biennial modulation of the solar neutrino flux, which results to be also significantly correlated with the fluxes of solar energetic protons and galactic CRs. The significance of all the correlation has been tested by applying both bootstrap and Monte Carlo methods.

A COUPLED CHEMISTRY-EMISSION MODEL FOR ATOMIC OXYGEN GREEN AND RED-DOUBLET EMISSIONS IN THE COMET C/1996 B2 HYAKUTAKE

The green (5577 \AA) and red-doublet (6300, 6364 \AA) lines are prompt emissions of metastable oxygen atoms in the $^1$S and $^1$D states, respectively, that have been observed in several comets. The value of intensity ratio of green to red-doublet (G/R ratio) of 0.1 has been used as a benchmark to identify the parent molecule of oxygen lines as H$_2$O. A coupled chemistry-emission model is developed to study the production and loss mechanisms of O($^1$S) and O($^1$D) atoms and the generation of red and green lines in the coma of C/1996 B2 Hyakutake. The G/R ratio depends not only on photochemistry, but also on the projected area observed for cometary coma, which is a function of the dimension of the slit used and geocentric distance of the comet. Calculations show that the contribution of photodissociation of H$_2$O to the green (red) line emission is 30 to 70% (60 to 90%), while CO$_2$ and CO are the next potential sources contributing 25 to 50% ($<$5%). The ratio of the photo-production rate of O($^1$S) to O($^1$D) would be around 0.03 ($\pm$ 0.01) if H$_2$O is the main source of oxygen lines, whereas it is $\sim$0.6 if the parent is CO$_2$. Our calculations suggest that the yield of O($^1$S) production in the photodissociation of H$_2$O cannot be larger than 1%. The model calculated radial brightness profiles of the red and green lines and G/R ratios are in good agreement with the observations made on comet Hyakutake in March 1996.

Eddy turbulence, the double mesopause, and the double layer of atomic oxygen

Abstract. In this study, we consider the impact of eddy turbulence on temperature and atomic oxygen distribution when the peak of the temperature occurs in the upper mesosphere. A previous paper (Vlasov and Kelley, 2010) considered the simultaneous impact of eddy turbulence on temperature and atomic oxygen density and showed that eddy turbulence provides an effective mechanism to explain the cold summer and warm winter mesopause observed at high latitudes. Also, the prevalent role of eddy turbulence in this case removes the strong contradiction between seasonal variations of the O density distribution and the impact of upward/downward motion corresponding to adiabatic cooling/heating of oxygen atoms. Classically, there is a single minimum in the temperature profile marking the location of the mesopause. But often, a local maximum in the temperature is observed in the height range of 85–100 km, creating the appearance of a double mesopause (Bills and Gardner, 1993; Yu and She, 1995; Gusev et al., 2006). Our results show that the relative temperature maximum in the upper mesosphere (and thus the double mesopause) can result from heating by eddy turbulence. According to our model, there is a close connection between the extra temperature peak in the mesosphere and the oxygen atom density distribution. The main feature of the O density height profile produced by eddy turbulence in our model is a double peak instead of a single peak of O density. A rocket experiment called TOMEX confirms these results (Hecht et al., 2004). Applying our model to the results of the TOMEX rocket campaign gives good agreement with both the temperature and oxygen profiles observed. Climatology of the midlatitude mesopause and green line emission shows that the double mesopause and the double layers of the green line emission, corresponding to the double O density height profile, are mainly observed in spring and fall (Yu and She, 1995; Liu and Shepherd, 2006). Further observations of the oxygen atom densities and the double mesopause would improve our understanding of the impact of turbulence on critical mesospheric parameters.

Photoluminescence properties and energy level locations of RE3+ (RE = Pr, Sm, Tb, Tb/Ce) in CaAlSiN3 phosphors

RE3+ (RE = Pr, Sm, Tb, Tb/Ce)-activated CaAlSiN3 samples were prepared by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. An interesting observation is that the 5d bands of Pr3+ and Tb3+ are at rather low energy in CaAlSiN3 compared to oxides. Pr3+-doped CaAlSiN3 shows strong 4f15d1 → 4f2 emission bands (334–480 nm) as well as typical 4f2 → 4f2 emission lines (480–800 nm) of Pr3+ under 4f2 → 4f15d1 excitation. Sm3+-doped CaAlSiN3 exhibits bright red emission originating from 4G5/2 → 6HJ (J = 5/2, 7/2 and 9/2) transitions, and the charge transfer band of Sm3+ was observed at an unusually low energy of 3.91 eV. In Tb3+-doped samples, the direct Tb3+ 4f8 → 4f75d1 excitation leads to 5D3 → 7FJ (J = 6, 5, 4, 3, 2, 1) (blue) and 5D4 → 7FJ (J = 6, 5, 4, 3) (green) line emissions. The bands at about 252 nm in the excitation spectra are attributed to the host lattice absorption. It is observed that there is energy transfer from the host lattice to the luminescent activators (Pr3+, Sm3+, Tb3+). In Tb3+/Ce3+-codoped CaAlSiN3, an effective energy transfer process exists from Tb3+ to Ce3+. A detailed energy level diagram that contains the position of the 4f and 5d levels of all divalent and trivalent lanthanide ions with respect to the valence and conduction bands of CaAlSiN3 has been constructed and explained.

Continuous ground-based multiwavelength airglow measurements

[1] A new spectrograph instrument, called the Continuous High-resolution Instrument for Multiwavelength Echelle Spectroscopy (CHIMES), has been designed to make simultaneous and spatially overlapping ground-based measurements of the green line and red line airglow emissions (5577 A and 6300 A) continuously, 24 hours-a-day. The spectrograph uses a 50 mm long, 50 μm wide slit, and varies the exposure time at different times of day (daytime, twilight, and nighttime), from 2 s in daytime to 10 min at night. It utilizes an Echelle grating to achieve dispersion of 0.05 A/pixel at 5577 A. Daytime 6300 A and 5577 A airglow from this instrument are extracted by comparing the measured spectra to direct solar spectra and extracting small increases in flux in the Fraunhofer absorption lines at those wavelengths, after compensating for the Ring effect contribution. We present the first ground-based measurements of the daytime 5577 A airglow, as well as example measurements of daytime, twilight, and nighttime airglow signatures.

Effects of solar cycle variations on oxygen green line emission rate over Kiso, Japan

A sixteen year long dataset of mesospheric OI 557.7 nm green line nightglow emission rate, measured over Kiso (35.79°N, 137.63°E), Japan using ground-based photometers is spectrally investigated using the Hilbert-Huang Transform (HHT). The spectrograms reveal the presence of semi-annual, annual and quasi-biennial oscillations in consonance with the results obtained from wavelet analysis in an earlier study. In addition, due to the use of the HHT, we have been able to investigate the very low frequency solar cycle variation in the emission rate. It is found that there is a significant solar cycle effect on the oxygen green line emission rate. The mean amplitude of variation is approximately 20% and it is also found that it is maximum at midnight. A correlation study between the means of the emission rate and the solar radio flux at 10.7 cm also shows that the effect of solar activity on the oxygen green line emission rate is maximum at midnight.

Thermospheric observations of equatorial wavenumber 4 density perturbations from WINDII data

[1] Previous studies have established that the diurnal eastward-propagating non-migrating tide of wavenumber 3 (DE3) is produced by latent heat release in deep tropospheric convection and therefore is longitude dependent. Longitudinal variations of F-region ionospheric electron density show a zonal wavenumber 4 pattern, expected for satellite observations of DE3 for a fixed local time with respect to a rotating Earth. Satellite observations of winds show wavenumber 4 reaching 180 km, indicating neutral coupling of the troposphere with the thermosphere. In the present work, WINDII measurements of O(1S) green line emission at 250 km excited by photoelectron impact on atomic oxygen are interpreted as neutral density observations; showing wavenumber 4 to be a common density perturbation, with a fractional amplitude variation of about 10%. This result confirms direct neutral coupling of troposphere and thermosphere but also opens the way to other studies of neutral density variations at 250 km.

Modification of NaYF4:Yb,Er@SiO2 Nanoparticles with Gold Nanocrystals for Tunable Green-to-Red Upconversion Emissions

Lanthanide-doped upconversion nanocrystals which can convert near-infrared lights to visible lights have attracted growing interest because of their great potentials in biomedical engineering. However, it remains a grand challenge to maneuver the intensity ratio between different emission lines and enable tunable upconversion functions. Herein, we present a facile method to integrate NaYF4:Yb,Er upconversion nanocrystals with gold nanocrystals for constructing NaYF4:Yb,Er@SiO2@Au hybrid nanostructures, in which a silica layer on NaYF4:Yb,Er nanocrystals serves as an interface for gold decoration. The green-to-red emissions of the upconversion nanocrystals can be conveniently tuned by altering the amount of surrounding gold nanocrystals. We further demonstrate the capability of utilizing both green and red emission lines for spontaneous signaling in an emitter−quencher-based bioassay by implementing this novel hybrid nanostructure. It is anticipated that the controllability in upconversion fluorescence in this hybrid nanostructure may provide a platform for widely exploring applications in biological imaging, detection, and sensing.

Sol–gel synthesis and green upconversion luminescence in BaGd2(MoO4)4:Yb3+,Er3+ phosphors

Yb3+/Er3+ codoped BaGd2(MoO4)4 phosphor powders were prepared by the Sol–gel method and the upconversion luminescence properties were investigated in detail. Under 980nm semiconductor laser excitation, BaGd2(MoO4)4:Yb3+,Er3+ phosphor exhibits green upconversion luminescence with peaks at 530 and 550nm, which are due to the transitions of Er3+ (2H11/2)→Er3+ (4I15/2) and Er3+ (4S3/2)→Er3+ (4I15/2), respectively. Both of the two green emission lines are produced by populating Er3+ ions to the excited state through a two-photon process. By monitoring the intensities of the green upconversion luminescence, the optimum conditions for the Sol–gel synthesis were determined when the molar ratio of citric acid to total chelate metal cations was 2:1 and the sintering temperature was at 1073K. The concentration quenching effect for Er3+ was found at the optimum doping concentration of 6mol%, and the critical distance for the neighboring Er3+ was determined to be about 21.5Å.

AUTOMATIC UNSUPERVISED CLASSIFICATION OF ALL SLOAN DIGITAL SKY SURVEY DATA RELEASE 7 GALAXY SPECTRA

Using the 'k-means' cluster analysis algorithm, we carry out an unsupervised classification of all galaxy spectra in the seventh and final Sloan Digital Sky Survey data release (SDSS/DR7). Except for the shift to restframe wavelengths, and the normalization to the g-band flux, no manipulation is applied to the original spectra. The algorithm guarantees that galaxies with similar spectra belong to the same class. We find that 99 % of the galaxies can be assigned to only 17 major classes, with 11 additional minor classes including the remaining 1%. The classification is not unique since many galaxies appear in between classes, however, our rendering of the algorithm overcomes this weakness with a tool to identify borderline galaxies. Each class is characterized by a template spectrum, which is the average of all the spectra of the galaxies in the class. These low noise template spectra vary smoothly and continuously along a sequence labeled from 0 to 27, from the reddest class to the bluest class. Our Automatic Spectroscopic K-means-based (ASK) classification separates galaxies in colors, with classes characteristic of the red sequence, the blue cloud, as well as the green valley. When red sequence galaxies and green valley galaxies present emission lines, they are characteristic of AGN activity. Blue galaxy classes have emission lines corresponding to star formation regions. We find the expected correlation between spectroscopic class and Hubble type, but this relationship exhibits a high intrinsic scatter. Several potential uses of the ASK classification are identified and sketched, including fast determination of physical properties by interpolation, classes as templates in redshift determinations, and target selection in follow-up works (we find classes of Seyfert galaxies, green valley galaxies, as well as a significant number of outliers). The ASK classification is publicly accessible through various websites.

Photoluminescence properties of rare earth doped α-Si 3N 4

The photoluminescence properties of Eu 2+, Ce 3+ and Tb 3+ doped α-Si 3N 4 have been studied and a possible structural model has been proposed on the basis of the Rietveld refinement of X-ray powder diffraction data. Nearly single phase rare earth doped α-Si 3N 4 was synthesized by a solid state reaction at 1600 °C in N 2–H 2 atmosphere starting from amorphous Si 3N 4 and rare earth oxides or nitrides. Because of small crystal field splitting of the 5d levels, the excitation and emission bands of Eu 2+ and Ce 3+ are positioned at higher energies as isolated ions in comparison with that in Ca-α-Sialon. Both Eu 2+- and Ce 3+-doped α-Si 3N 4 show blue band emission peaking at about 470 and 450 nm, respectively, under UV excitation. α-Si 3N 4:Tb 3+ exhibits dominant green line emission mainly arising from 5D 4→ 7F J ( J=6–3) with weak 5D 3→ 7F J ( J=6–3) transitions of Tb 3+ when excited by UV light. The thermal stability of α-Si 3N 4:Eu 2+ is comparable with that of Ca-α-Sialon:Eu 2+ and is much better than that of α-Si 3N 4:Ce 3+.