Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy

We review HB stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters (GCs) in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the second-parameter problem is presented. A technique is proposed to estimate the HB types of extragalactic GCs on the basis of integrated far-UV photometry. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyrae, is also revisited, giving a distance modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are studied. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and Space Scienc

White light generation through Zn(PO3)2 glass activated with Eu3+ and Dy3+

A spectroscopic investigation of Zn(PO3)2 glass activated with Eu3+ and Dy3+ ions is carried out from photoluminescence data. White light emission is obtained in the glass phosphor, and it is due mainly to europium 5D0\u21927F2 and dysprosium 4F9/2\u21926H15/2,13/2 emissions, being Eu3+ sensitized by Dy3+ through a non-radiative energy transfer. Non-radiative energy transfer from Eu3+ to Dy3+ is also observed upon Eu3+ excitation at 414 nm. An electric quadrupole\u2013quadrupole interaction might be the dominant mechanism in the energy transfer among Dy3+ and Eu3+ ions, as it was revealed from decay time data. The tonality of the white phosphor can be shifted from neutral white (0.364, 0.387) of 4522 K, upon 348 nm excitation, to warm white (0.393, 0.394) of 3811 K, upon 445 nm excitation. Excitations at 348\u2013445 nm range can be attained with InGaN based LED chips, so that Eu3+ and Dy3+ codoped Zn(PO3)2 glasses excited by these LED chips could be appropriated for solid state lighting technology as neutral and warm white light source

Orange and reddish-orange light emitting phosphors: Sm3+ and Sm3+/Eu3+ doped zinc phosphate glasses

A spectroscopy study of Sm3+ and Sm3+/Eu3+ doped zinc phosphate glasses is performed through photoluminescence spectra and decay time profile measurements. Under Sm3+ excitation at 344 nm, the Sm3+ singly doped glass shows an orange global emission with x=0.579 and y=0.414 CIE1931 chromaticity coordinates, whereas the Sm3+/Eu3+ co-doped sample exhibits orange overall emissions (x=0.581 and y=0398, and x=0.595 and y=0.387) and reddish-orange overall emission (x=0.634 and y=0.355) upon excitations at 344, 360 and 393 nm, respectively. Such luminescence from the co-doped sample is originated by the simultaneous emission of Sm3+ and Eu3+. Under Sm3+ excitation at 344 and 360 nm, the Eu3+ emission is sensitized and enhanced by Sm3+ through a non-radiative energy transfer process. The non-radiative nature was inferred from the shortening of the Sm3+ lifetime observed in the Sm3+/Eu3+ co-doped sample. An analysis of the Sm3+ emission decay time profiles using the Inokuti-Hirayama model suggests that an electric quadrupole-quadrupole interaction into Sm-Eu clusters might dominate the energy transfer process, with an efficiency of 0.17

Blue and white light emission in Tm3+ and Tm3+/Dy3+ doped zinc phosphate glasses upon UV light excitation

A spectroscopic study based on photoluminescence spectra and decay time profiles in Tm3+ and Tm3+/Dy3+ doped Zn(PO3)2 glasses is reported. The Tm3+ doped Zn(PO3)2 glass, upon 357 nm excitation, exhibits blue emission with CIE1931 chromaticity coordinates, x = 0.157 and y = 0.030, and color purity of about 96%. Under excitations at 348, 352 and 363 nm, which match with the emissions of AlGaN and GaN based LEDs, the Tm3+/Dy3+ co-doped Zn(PO3)2 glass displays natural white, bluish white and cool white overall emissions, with correlated color temperature values of 4523, 10700 and 7788 K, respectively, depending strongly on the excitation wavelength. The shortening of the Dy3+ emission decay time in presence of Tm3+ suggests that Dy3+\u2192Tm3+ non-radiative energy transfer occurs. By using the Inokuti-Hirayama model, it is inferred that an electric quadrupole-quadrupole interaction might be the dominant mechanism involved in the energy transfer. The efficiency and probability of this energy transfer are 0.12 and 126.70 s-1, respectively

White light generation in Tb3+/Eu3+/Dy3+ triply-doped Zn(PO3)2 glass

A spectroscopic investigation of Tb3+/Eu3+/Dy3+ triply-doped Zn(PO3)2 glass focused on generation of white light is performed through photoluminescence spectra and decay time measurements. The white light emission obtained in the glass phosphor shows excitation wavelength dependent tunable tonality: neutral white (0.385, 0.441) of 4250 K and warm white (0.417, 0.412) of 3429 K, upon 445 and 322 nm excitations, respectively. A quantum yield of 26.1 \ub1 1.2% is attained upon Dy3+ excitation at 445 nm. The white luminescence is due mainly to terbium 5D4 \u2192 7F5, dysprosium 4F9/2 \u2192 6H15/2,13/2 and europium 5D0 \u2192 7F2 transitions. It is demonstrated that non-radiative energy transfers Dy3+ to Tb3+ and Eu3+, and Tb3+ to Eu3+, take place in the glass phosphor excited at 445 or 322 nm. Tb3+/Eu3+/Dy3+ triply-doped Zn(PO3)2 glass, excited by AlGaN (322 nm) or InGaN (445 nm) LEDs, could then be appropriated for solid state lighting technology as neutral or warm white light phosphor

Neutral and warm white light emission in Tb3+/Sm3+ zinc phosphate glasses

A spectroscopy analysis of Tb3+ and Tb3+/Sm3+ doped zinc phosphate glasses based on emission spectra and decay time profiles was performed. The Tb3+ singly doped glass shows a green overall emission with x = 0.258 and y = 0.429 CIE1931 chromaticity coordinates, upon Tb3+ excitation at 318 nm. Under co-excitations of Tb3+ and Sm3+ at 344, 361 and 374 nm, the Tb3+/Sm3+ co-doped glasses display neutral and warm white overall emissions with CIE1931 chromaticity coordinates in ranges of x = 0.407-0.487 and y = 0.437-0.485, color temperature in the range of 2447-4024 K and quantum yield up to 12.38%, depending on the excitation wavelength and relative amount of Tb3+ and Sm3+. In all cases, it was observed that the Sm3+ emission is enhanced by the addition of Tb3+, which is correlated with a quenching of the Tb3+ emission as consequence of a non-radiative Tb3+ -> Sm3+ energy transfer process. The non-radiative nature of the energy transfer process was inferred by the shortening of the Tb3+ emission decay time observed in the Tb3+/Sm3+ co-doped zinc phosphate glasses. An analysis of the Tb3+ emission decay time profiles by the Inokuti-Hirayama model suggests that an interaction electric dipole-dipole into the Tb3+-Sm3+ clusters might dominate in the energy transfer process with efficiency and probability of 0.23-0.25 and 96.22-111.35 s(-1), respectively

White light generation in Dy3+-and Ce3+/Dy3+-doped zinc-sodium-aluminosilicate glasses

A spectroscopic investigation of 1% Dy2O3-singly doped and 0.5% Ce2O3-1.0% Dy2O3-codoped zinc-sodium-aluminosilicate glasses was performed by analyzing their absorption and photoluminescence spectra, and decay times. Warm white yellow light emission, with (0.419, 0.440) CIE1931 chromaticity coordinates and 3579 K color temperature, is obtained in the Dy3+-singly doped glass excited at 399 nm, which fits to the requirements of GaN LEDs. A quantum efficiency of 74% and a very high optical gain (38.7 710-25 cm2 s) were estimated for the dysprosium 4F9/2 level luminescence, which might also make the Dy3+-doped glass a promising gain medium for solid state yellow laser pumped by GaN LEDs. In the Ce3+/Dy3+-codoped glass a radiative energy transfer from Ce3+ to Dy3+ is observed upon UV excitation (310-365 nm), with a Ce3+ to Dy3+ interaction distance that could be greater than 6-12 \uc5. The emission color from the codoped glass can be tuned with the excitation wavelength from blue light (0.247, 0.245), upon 310 nm excitation, to cold white light (0.284, 0.300), with a 9052 K color temperature, upon 365 nm excitation

Dependence of the up-conversion emission of Li+ co-doped Y2O3:Er3+ films with dopant concentration

The effect of dopant concentration on the up-conversion emission, and in particular on the Er3+ related green and red emissions of spray pyrolysis deposited films of Y2O3:Er3+ co-doped with Li+, is reported. Er3+ concentrations in the films in the range of 1.1-5.6 at% (1.5-14 at% Er3+ in the spraying solution) were studied, as well as the effect of co-doping them with Li+. Large concentrations of Er3+ favor the red emission, especially for contents higher than 10 at% in the spraying solution. Li+ co-doping improves the green and red emissions up to 365 and 171 times, respectively, depending on the Er3+ and Li+ concentrations