Search results for "BAND"
showing 10 items of 2610 documents
A size dependent discontinuous decay rate for the exciton emission in ZnO quantum dots
2014
The time resolved UV-fluorescence in ZnO quantum dots has been investigated using femtosecond laser spectroscopy. The measurements were performed as a function of particle size for particles between 3 and 7 nm in diameter, which are in the quantum confined regime. A red shift in the fluorescence maximum is seen while increasing the particle size, which correlates with the shift in band gap due to quantum confinement. The energy difference between the UV-fluorescence and the band gap does, however, increase for the smaller particles. For 3.7 nm particles the fluorescence energy is 100 meV smaller than the band gap energy, whereas it is only 20 meV smaller for the largest particles. This indi…
High-resolution photoacoustic Raman spectroscopy of gases
1992
A high-resolution photoacoustic Raman spectroscopy experiment is described. The resolution achieved by using two single-mode pulsed lasers is about 0.0054 cm −1 (full width at half maximum intensity). The experiment was tested first on the v 1 /2v 2 bands of CO 2 and gave an increase of at least about one order of magnitude in the signal-to-noise ratio with respect to stimulated Raman spectroscopy at low pressure (ca. 10 Torr ≃ 1.3 kPa). The sensitivity is also demonstrated by the study of the weak hot band v 1 +v 2 −v 2 of CO 2 . In both cases, the experimental line shape is well reproduced by taking into account Doppler and collisional effects. A comparison with CARS spectra was also made
New binary antimonide Hf5Sb3
1999
Abstract Hf5Sb3 can be prepared by arc-melting of hafnium and previously synthesized HfSb2. It crystallizes like the high-temperature modification of Zr5Sb3 in the Y5Bi3 structure type, space group Pnma, Z=4, a=740.75(9) pm, b=871.8(1) pm, c=1073.6(1) pm, V=693.3(1) 106pm3. An antimonide analogous to the low-temperature form of Zr5Sb3 was not obtained during our investigations. In the structure of Hf5Sb3, the Hf atoms form a three-dimensional network with numerous short Hf–Hf contacts, including the Sb atoms in severely distorted two- and three-capped trigonal prismatic voids. Calculations of the electronic structure, performed with the extended Huckel approximation as well as the TB-LMTO-A…
Hf27Si6P10, a novel metal-rich compound with P2 groups
2000
The new ternary metal rich compound Hf27Si6P10 has been synthesized by reduction of HfP with Hf and Si; Hf27Si6P10 crystallizes in a new structure type, a characteristic and unexpected feature of which is the presence of P2 groups; the structural results are interpreted with the aid of high-level band structure calculations.
Ab initio modeling of excitonic and charge-transfer states in organic semiconductors: the PTB1/PCBM low band gap system.
2013
A detailed quantum chemical simulation of the excitonic and charge-transfer (CT) states of a bulk heterojunction model containing poly(thieno[3,4-b]thiophene benzodithiophene) (PTB1)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is reported. The largest molecular model contains two stacked PTB1 trimer chains interacting with C60 positioned on top of and lateral to the (PTB1)3 stack. The calculations were performed using the algebraic diagrammatic construction method to second order (ADC(2)). One main result of the calculations is that the CT states are located below the bright inter-chain excitonic state, directly accessible via internal conversion processes. The other important aspects…
Subphthalocyanines as narrow band red-light emitting materials
2007
A series of new light emitting subphthalocyanines, lower homologues of phthalocyanines, were synthesized having color points covering the red-orange region of the visible spectrum. Additionally, they were found to be of potential use as narrow band emitters for red-light emitting diodes.
Dyes That Bear Thiazolylazo Groups as Chromogenic Chemosensors for Metal Cations
2011
A family of dyes (L 1-L 6) that contain a thiazolylazo group as signalling subunit and several macrocyclic cavities with different ring sizes and type and number of heteroatoms as binding sites has been synthesized and characterized. Solutions of L 1-L 6 in acetonitrile show broad and structureless absorption bands in the 554-577 nm range with typicalmolar absorption coefficients that range from 20000 to 32000 M -1 cm -1. A detailed protonation study was carried out with solutions of L 1, L 2 and L 5 in acetonitrile. Addition of one equivalent of protons to L 1 and L 2 resulted in the development of a new band at 425 and 370 nm, respectively, which was ascribed to protonation in the aniline…
Circular and linear dichroism of aggregates of chlorophyll a and chlorophyll b in 3-methylpentane and paraffin oil.
1996
A circular (CD) and linear dichroism (LD) study of the water adducts of the green plant chlorophylls a (Chl a) and b (Chl b) in hydrocarbon solvents 3-methylpentane and paraffin oil is presented. A strong red shift of the Qy-absorption band from 663 to 746 nm (1678 cm−1) is observed as the water adduct of Chl a is formed. The Chl a-water adduct shows a strong, nonconservative CD signal, which is characterized by a positive peak at 748 nm and two negative peaks at 720 and 771 nm. The maximum CD (AL - AR) is only one order of magnitude smaller than the isotropic absorption maximum. We propose that this exceptionally strong signal is the so-called psi-type CD. The LD spectrum was measured in a…
Modelo empírico para la determinación de clorofila-a en aguas continentales a partir de los futuros Sentinel-2 y 3. Validación con imágenes HICO
2014
[EN] Chlorophyll-a concentration is one of the main indicators of inland waters quality. Using CHRIS/PROBA images and in situ data obtained in four lakes in Colombia and Spain, we obtained empirical models for the estimation of chlorophyll-a concentration, which can be directly applied to future images of MSI Sentinel-2 and OLCI Sentinel-3 sensors. The models, based on spectral band indices, were validated with data from the hyperspectral sensor HICO, onboard of the International Space Station.
Evaluation of Sentinel-2 Red-Edge Bands for Empirical Estimation of Green LAI and Chlorophyll Content
2011
ESA’s upcoming satellite Sentinel-2 will provide Earth images of high spatial, spectral and temporal resolution and aims to ensure continuity for Landsat and SPOT observations. In comparison to the latter sensors, Sentinel-2 incorporates three new spectral bands in the red-edge region, which are centered at 705, 740 and 783 nm. This study addresses the importance of these new bands for the retrieval and monitoring of two important biophysical parameters: green leaf area index (LAI) and chlorophyll content (Ch). With data from several ESA field campaigns over agricultural sites (SPARC, AgriSAR, CEFLES2) we have evaluated the efficacy of two empirical methods that specifically make use of the…