near-UV Range Research Articles

Magnetic and optical properties of nanocorrugated Co films

Nanostructured Co films were prepared on the top of a polymethyl methacrylate (PMMA) colloidal crystals by magnetron sputtering. Optical reflectance spectra were studied in the range of near UV, IR, and visible light for p- and s-polarizations. Valleys were observed in the spectra and their positions scaled with the PMMA sphere diameter. Both the surface plasmon resonance and the dipole resonance of single Co nanocaps should be considered to explain the obtained results. Magneto-optic measurements showed the qualitative change of the magnetization curve and the enhancement of magneto-optic rotation at wavelength λ=632 nm in comparison with the control Co film.

Ultrafast excited-state dynamics of tryptophan in water observed by transient absorption spectroscopy

The excited-state dynamics of tryptophan in water is resolved by femtosecond transient absorption (TA) experiments covering the near-UV and Vis range of wavelengths. After a 1 ps solvent relaxation time, the excited-state decays non-radiatively, while an absorption band rises simultaneously at 425 nm on the nanosecond time scale. The 425 nm induced absorption appears to be the signature of the primary photoproduct resulting from the quenching of Trp excited-state in water. While time-resolved Trp fluorescence spectroscopy has characterized Trp’s excited-state lifetime in detail, the present TA experiments give first evidence for the spectroscopic signatures of the quenching-induced photoproduct.

Formation dynamics and nature of tryptophan's primary photoproduct in aqueous solution

The excited state quenching and photoproduct formation of tryptophan in water is studied by femtosecond transient absorption experiments covering the near-UV and Vis range of wavelengths. The quenching of the excited state absorption occurs simultaneously with the rise of a photoproduct characterized by two absorption bands at 350 nm and 425 nm. Both processes are characterized by the same biexponential kinetics, and the time constants found are in excellent agreement with previous time-resolved fluorescence measurements. By varying the pH and comparing with the transient spectra of Trp incorporated in a peptide where electron transfer is the dominant quenching mechanism, we suggest that the photoproduct is a zwitterionic form of Trp with the indole moiety protonated, formed via excited state proton transfer from the side chain amine group, in agreement with conclusions drawn from nanosecond experiments. The present work thus fills the gap between ultrafast fluorescence decay and nanosecond flash photolysis experiments, and pinpoints Trp's fluorescence quenching mechanism at acidic and neutral pH.

A red europium (III) ternary complex for InGaN-based light-emitting diode

One kind of europium (III) ternary complex was synthesized, and its photoluminescence properties were investigated. This complex exhibits broad excitation band in near-UV range, and strong red emission which is due to the 5D 0– 7F j transitions of Eu 3+ ions. The luminescence quantum yield for the Eu 3+ complex is 0.17. Thermogravimetric analysis confirms a high thermal stability of the complex with a decomposition temperature of 344 °C. All the characteristics indicate that the Eu 3+ complex is a highly efficient red phosphor suitable to be excited by near UV light. An intense red light-emitting diode was fabricated by combining the europium (III) ternary complex with a ~395 nm-emitting InGaN chip.

Synthesis and luminescent properties of a new red-emitting phosphor for solid-state lighting: Eu 0.1Gd xLa 1.9−xTeO 6 (0.02⩽ x⩽0.1)

The photoluminescence (PL) emission and excitation behavior of red-emitting Eu 0.1Gd x La 1.9− x TeO 6 (0.02⩽ x⩽0.1) powder phosphors is reported. Three dominant bands centered at 395, 466 and 534 nm characterized the excitation spectrum. Under the excitation of 395 nm UV light, the emission spectrum exhibits an intense peak centered at 616 nm corresponding to the 5D 0→ 7F 2 transition of Eu 3+. Because the f→f transitions are located in the wavelength range of blue or near-UV range, optimized phosphor, Eu 0.10Gd 0.08La 1.82TeO 6, is a promising material for solid-state lighting based on GaN LEDs applications.

Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area

Abstract. Data taken from the MCMA-2003 and the 2006 MILAGRO field campaigns are used to examine the absorption of solar radiation by the organic component of aerosols. Using irradiance data from a Multi-Filter Rotating Shadowband Radiometer (MFRSR) and an actinic flux spectroradiometer (SR), we derive aerosol single scattering albedo, ϖ0, λ, as a function of wavelength, λ. We find that in the near-UV spectral range (250 to 400 nm) ϖ0, λ is much lower compared to ϖ0, λ at 500 nm indicating enhanced absorption in the near-UV range. Absorption by elemental carbon, dust, or gas cannot account for this enhanced absorption leaving the organic carbon component of the aerosol (OA) as the most likely absorber. We use data from a surface deployed Aerodyne Aerosol Mass Spectrometer (AMS) along with the inferred ϖ0, λ to estimate the Mass Absorption Cross section (MAC) for the organic aerosol. We find that the MAC is about 10.5 m2/g at 300 nm and falls close to zero at about 500 nm; values that are roughly consistent with other estimates of organic aerosol MAC. These MAC values can be considered as "radiatively correct", because when used in radiative transfer calculations, the calculated irradiances/actinic fluxes match those measured at the wavelengths considered here. For an illustrative case study described here, we estimate that the light absorption by the "brown" (organic) carbonaceous aerosol can add about 40% to the light absorption of black carbon in Mexico City. This contribution will vary depending on the relative abundance of organic aerosol relative to black carbon. Furthermore, our analysis indicates that organic aerosol would slow down photochemistry by selectively scavenging the light reaching the ground at those wavelengths that drive photochemical reactions. Finally, satellite retrievals of trace gases that are used to infer emissions currently assume that the MAC of organic carbon is zero. For trace gases that are retrieved using wavelengths shorter then 420 nm (i.e. SO2, HCHO, halogenoxides, NO2), the assumption of non-zero MAC values will induce an upward correction to the inferred emissions. This assumption will be particularly relevant in polluted urban atmospheres and areas of biomass burning where organic aerosols are particularly abundant.

Fluorescence spectroscopy of synthetic melanin in solution

We report a detailed investigation of fluorescence properties of synthetic eumelanin pigment in solution. A complete set of fluorescence spectra in the near-UV and visible range is analysed. Excitation spectra at a few selected emission energies are also investigated. Our measurements support the hypothesis that fluorescence in eumelanin is related to chemically distinct oligomeric units that can be selectively excited. Fluorescence due to large oligomer systems is spectrally differentiated from that due to monomers and small oligomer systems. Fluorescence excitation measurements show the contribution of 5,6-dihydroxyndole-2-carboxylic acid and 5,6-dihydroxyndole monomers to the emission of small-size oligomers.

Investigation of the Microwave Iron-Production Process with Multipoint Pyrometric and Spectroscopic Measurements

Carbothermic reduction of magnetite in a 2.45-GHz microwave multimode furnace was investigated with multipoint pyrometric and spectroscopic measurements. Both experimental results emphasize the importance of surface heating of the specimen by microwave-generated plasma for reducing iron oxide. The pyrometric observation shows a shift of the heating mode from the direct volumetric heating by microwave to the surface heating by microwave-generated plasma, when the temperature of the material suddenly rises from ∼800 to ∼1000 °C accompanied by light emission from plasma. The emission spectrum in the near-UV range (240-310 nm) changes drastically from a continuous spectrum to a line emission spectrum of iron, representing progress of carbothermic reduction of iron oxide. The multipoint spectroscopic observation indicates extensive carbothermic reduction occurring on the entire upper surface of the specimen.

Photochemical synthesis of gold nanoparticles in elastomer films of poly(butyl acrylate) latex

Gold particles are formed by photochemical reduction of HAuCl4 in the bulk of poly(butyl acrylate) elastomer films under the action of monochromatic light with the wavelengths of 254 and 365 nm with direct participation of the ester chromophore group of the polymer having intrinsic photosensitivity in the near-UV range (254 nm). On irradiation at the wavelength of 365 nm, the reduction of HAuCl4 is ineffective and stops at the stage of Au(I) formation. The physicomechanical properties of elastomer films modified with gold particles (tensile strength and relative elongation) were studied.

Phase dependent photoluminescence and energy transfer in Ca 2P 2O 7: Eu 2+, Mn 2+ phosphors for white LEDs

α- and β-Ca 2P 2O 7: Eu 2+, Mn 2+ phosphors were prepared by solid-state reaction. Phase transition from tetragonal (β-phase) to monoclinic (α-phase) is performed. A strong orange emission of Mn 2+ is observed in both α-and β-Ca 2P 2O 7: Eu 2+, Mn 2+ upon near ultraviolet (UV) excitation through energy transfer from Eu 2+ to Mn 2+. The transfer efficiencies for various Mn 2+ concentrations are estimated based on lifetime measurements of the fluorescence of Eu 2+ in the two phases. The photoluminescence excitation spectra of α-Ca 2P 2O 7: Eu 2+, Mn 2+ can cover 400 nm of the near-UV range, denoting its potential use as a phosphor with intense orange component for white light emitting diodes (LEDs).

Two-Photon Uncaging with Fluorescence Reporting: Evaluation of the o-Hydroxycinnamic Platform

This paper evaluates the o-hydroxycinnamic platform for designing efficient caging groups with fluorescence reporting upon one- and two-photon excitation. The model cinnamates are easily prepared in one step by coupling commercial or readily available synthons. They exhibit a large one-photon absorption that can be tuned in the near-UV range. Uncaging after one-photon excitation was investigated by 1H NMR, UV-vis absorption, and steady-state fluorescence emission. In the whole investigated series, the caged substrate is quantitatively released upon photolysis. At the same time, uncaging releases a strongly fluorescent coproduct that can be used as a reporter for quantitative substrate delivery. The quantum yield of double bond photoisomerization leading to uncaging after one-photon absorption mostly lies in the 10% range. Taking advantage of the favorable photophysical properties of the uncaging coproduct, we use a series of techniques based on fluorescence emission to measure the action uncaging cross sections with two-photon excitation of the present cinnamates. Exhibiting values in the 1-10 GM range at 750 nm, they satisfactorily compare with the most efficient caging groups reported to date. Noticeably, the uncaging behavior with two-photon excitation is retained in vivo as suggested by the results observed in living zebrafish embryos. Reliable structure property relationships were extracted from analysis of the present collected data. In particular, the careful kinetic analysis allows us to discuss the relevance of the o-hydroxycinnamic platform for diverse caging applications with one- and two-photon excitation.

Luminescence of the semimagnetic semiconductor CdMnTe in the wavelength range 370–400 nm at high excitation levels

Luminescence of the semimagnetic semiconductor CdMnTe was studied. It was revealed that, in the near-UV range, the luminescence spectrum of CdMnTe contains a band whose position does not depend on the Mn concentration in the solid solution. The temperature and lux-watt characteristics of the band were investigated, and time-resolved measurements were performed. The totality of the data obtained permits the conclusion that the revealed band can be attributed to the luminescence of clusters of a metastable cubic modification of manganese telluride.

Comparative evaluation of Fe(III) reducing power-based antioxidant capacity assays in the presence of phenanthroline, batho-phenanthroline, tripyridyltriazine (FRAP), and ferricyanide reagents

The chemical diversity of antioxidants in complex matrices such as plant extracts makes it difficult to separate and quantify antioxidants from these solutions. Therefore it is desirable to establish methods that can measure the total antioxidant capacity (TAC) levels directly from plant extracts. Iron(III)-based TAC assays, especially the most widely used FRAP (ferric-reducing antioxidant power), play an important role in this regard. However, many problems have been reported in the application of the FRAP assay, the most serious one being the incomplete oxidation of a number of antioxidants during the time protocol of the assay. Thus, six different ferric ion-based total antioxidant capacity (TAC) assays have been comparatively tested, modified, and improved so as to obtain more sensitive and precise results for complex mixtures, namely: 1,10-phenanthroline ( o-phen) method (with incubation), batho-phenanthroline method (with incubation), original FRAP method, modified FRAP method (with incubation), original ferricyanide method, and modified ferricyanide method (with incubation). Two new assays in this regard (i.e., o-phen and batho-phen) have been established, and the existing assays (FRAP and ferricyanide) have been modified so as to let the oxidation reactions of antioxidants reach completion. The molar absorptivity for a variety of antioxidants was highest for modified FRAP, batho-phen, and original FRAP methods. The absorption maximum wavelength shifted batochromically to a higher extent for modified ferricyanide, FRAP, and batho-phen procedures, decreasing the possibility of interferences due to organics absorbing in the near-UV range of the visible spectrum where most antioxidant assays are performed. The linear concentration ranges were shown to be further extended and linear correlation coefficients improved with respect to the most widely used ferric-based assay, FRAP. Of the six assays tested and developed, only the modified ferricyanide procedure gave high intercept values and low addivitity of TAC values of constituents in complex mixtures, requiring further attention of method optimization. Thus, it was shown that the most widely used FRAP could be effectively modified, and o-phen, batho-phen, and ferricyanide methods constitute cheaper alternatives to FRAP under certain conditions, with partly improved molar absorptivity (and thus sensitivity) for antioxidants, lower intercept values (and higher precision), broader linear range (and higher flexibility), and better additivity of TAC values of antioxidant constituents in mixtures.

Conformational studies on Arabidopsis sulfurtransferase AtStr1 with spectroscopic methods

Sulfurtransferases/rhodaneses (Str) are enzymes widely distributed in archaea, prokaryota and eukaryota, and catalyze the transfer of sulfur from a donor molecule to a thiophilic acceptor substrate. In this reaction, Str cycles between the sulfur-free and the sulfur-substituted form. Two-domain Str consist of two globular domains of nearly identical size and conformation connected by a short linker sequence, which is elongated in plant two-domain Str proteins compared to Str in other organisms. The two-domain Arabidopsis thaliana Str1 protein (At1g79230) was expressed in Escherichia coli as a mature protein, as a variant without the elongated linker sequence, and as AtStr1C332S and AtStr1C339V. The persulfuration state of the purified recombinant proteins was investigated in the presence and absence of sulfur donors by fluorescence spectroscopy. The secondary structure was analyzed by circular dichroism (CD) in the far-UV range, while overall changes in tertiary structure were determined by CD in the near-UV range. Finally, protein stability was analyzed by tryptic digestion. The elongated linker sequence is essential for correct conformation and stability, and thereby affects the catalytic activity of AtStr1. Replacement of the catalytic cysteine residue C332 leads to higher rigidity of the molecule, whereas replacement of C339 does not lead to any conformational changes, providing evidence of the direct involvement of C339 in catalysis.

Evolution of the Near-UV Emission Spectrum Associated with the Reduction Process in Microwave Iron Making

The structure of the emission spectrum in the near-UV range (240 nm-310 nm) changes drastically from the continuous spectrum to a discrete line spectrum with increasing sample temperature during the carbothermic reduction of magnetite in a 2.45 GHz microwave multimode furnace. The continuous spectrum can be assigned as a cathodoluminescence of magnetite. The dynamic evolution of the spectrum from continuous to discrete represents the progress of the reduction from magnetite to iron.

Efficient Ultrafast Frequency Conversion Sources for the Visible and Ultraviolet Based on BiB$_{3}$O$_{6}$

Efficient frequency conversion of high-repetition-rate femtosecond and picosecond pulses with wide tunability throughout the visible, blue, and UV is demonstrated in the nonlinear optical crystal BiB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> (BIBO). Using direct single-pass second harmonic generation of Kerr-lens-mode-locked Ti:sapphire laser, average output powers close to 1 W at conversion efficiencies in excess of 50% are demonstrated with broad spectral coverage across 370-450 nm in the near-UV and blue spectral range. Output pulse durations of 220 fs and 2.8 ps have been obtained at a pulse repetition rate of 76 MHz. By implementing a synchronously pumped femtosecond optical parametric oscillator (OPO) based on BIBO, the generated spectral range is further extended to cover the entire visible spectrum, across the green-yellow-orange-red regions. Average output powers of up to 270 mW in pulses of 120 fs at 76 MHz repetition rate have been generated across the 480-710 nm range. Internal frequency doubling of the visible OPO signal pulses in the crystal of beta-BaB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> has permitted high-repetition-rate femtosecond pulse generation with wide tunability across 250-350 nm in the UV at up to 100 mW of average power. The potential of BIBO for efficient frequency conversion of high-intensity pulses is also demonstrated through third harmonic generation of amplified microjoule picosecond pulses at 1064 nm. Output pulse energies of 216 muJ in 29 ps duration at 25 Hz repetition rate have been obtained at 355 nm with overall conversion efficiencies of up to 50%.

Nitrogen-doped p-type ZnO films prepared from nitrogen gas radio-frequency magnetron sputtering

Wide band gap nitrogen-doped p-type ZnO films are prepared by radio-frequency magnetron sputtering from a 99.99% purity ZnO target. The sputtering gas is Ar mixed with various flow rates of nitrogen gas. Hole concentrations increase from 1.89×1015to2.11×1019cm−3 as the N2 flow rate decreases from 15to6SCCM (SCCM denotes cubic centimeter per minute at STP), i.e., increasing N2 flow rate above 6SCCM decreases the p-type carrier concentration. Microphotoluminescence (PL) spectra peaks are in the near-UV range and change from 384nm(3.23eV)to374nm(3.32eV) with increasing N2 flow rate. The PL peaks agree with the band gap of bulk ZnO, which comes from the recombination of free excitons. Raman spectra show six peaks: 436 (E2 high-frequency phonon mode for undoped ZnO film), 581 [A1 (LO) mode in ZnO:N film], 275, 508, 640, and 854cm−1 (local vibrational modes of Raman features in N-doped ZnO film).

O‐Nitrobenzyl Photolabile Protecting Groups with Red‐Shifted Absorption: Syntheses and Uncaging Cross‐Sections for One‐ and Two‐Photon Excitation

We evaluated the o-nitrobenzyl platform for designing photolabile protecting groups with red-shifted absorption that could be photolyzed upon one- and two-photon excitation. Several synthetic pathways to build different conjugated o-nitrobenzyl backbones, as well as to vary the benzylic position, are reported. Relative to the reference 4,5-dimethoxy-2-nitrobenzyl group, several o-nitrobenzyl derivatives exhibit a large and red-shifted one-photon absorption within the near-UV range. Uncaging after one-photon excitation was studied by measuring UV-visible absorption and steady-state fluorescence emission on model caged ethers and esters. In the whole series investigated, the caged substrates were released cleanly upon photolysis. Quantum yields of uncaging after one-photon absorption lie within the 0.1-1 % range. We observed that these drop as the maximum wavelength absorption of the o-nitrobenzyl protecting group is increased. A new method based on fluorescence correlation spectroscopy (FCS) after two-photon excitation was used to measure the action uncaging cross section for two-photon excitation. The series of o-nitrobenzyl caged fluorescent coumarins investigated exhibit values within the 0.1-0.01 Goeppert-Mayer (GM) range. Such results are in line with the low quantum yields of uncaging associated with cross-sections of 1-50 GM for two-photon absorption. Although the cross-sections for one- and two-photon absorption of o-nitrobenzyl photolabile protecting groups can be readily improved, we emphasize the difficulty in enlarging the corresponding action uncaging cross-sections in view of the observed trend of their quantum yield of uncaging.

CagelikeAu32detected in relativistic density-functional calculations of optical spectroscopy

The optical absorptions of different ${\mathrm{Au}}_{32}$ isomers in the whole frequency range from the far-infrared (FIR) to near-ultraviolet (UV) have been calculated using the relativistic density-functional method in order to identify their geometrical structures. It is found that there exists a distinctive difference between the absorption spectra of the icosahedral cage-like ${\mathrm{Au}}_{32}$ and its amorphous isomers. The former shows significant absorption peaks in the visible and near-UV range, and a characteristic FIR-active mode at $46\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, making it possible to be distinguished from others, which suggests that the optical spectra can thus be used as an efficient experimental tool to detect the ``golden fullerene.''

Photophysics of a Series of Efficient Fluorescent pH Probes for Dual‐Emission‐Wavelength Measurements in Aqueous Solutions

This paper evaluates the 5-aryl-2-pyridyloxazole backbone to engineer donor-acceptor fluorescent pH probes after one- or two-photon absorption. Parent fluorophores, as well as derivatives that can be used to label biomolecules, can be easily obtained in good yields. These molecules exhibit a large one-photon absorption in the near-UV range, and a strong fluorescence emission that covers the whole visible domain. The 5-aryl-2-pyridyloxazole derivatives also possess significant cross sections for two-photon absorption. Upon pyridine protonation, large shifts were observed in the absorption spectra after one- and two-photon excitation, as well as in the emission spectra. This feature was used to measure the pK(a) of the investigated compounds that range between 2 and 8. In most of the investigated derivatives, the pK(a) increased upon light excitation and protonation exchanges took place during the lifetime of the excited state, as shown by phase-modulation fluorometry analysis. Several 5-aryl-2-pyridyloxazole derivatives are suggested as efficient probes to reliably measure the pH of aqueous solutions by means of ratiometric methods that are dependent on fluorescence emission.