Decreasing Excitation Wavelength Research Articles

Tb3+ activated Al2Y4O9 green emitting phosphors under ultraviolet and vacuum ultraviolet excitation

Al2Y4−xO9:xTb3+ luminescent powder phosphors were synthesized by the sol-gel method. The prepared samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, photoluminescence (PL), and CIE techniques. The microstructural and structural analyses of samples confirmed the formation of phosphors. In the PL studies, an intense peak was observed in the green region and its intensity increased with increasing Tb3+ ions concentration. Concentration quenching was observed for 0.11 mol of Tb3+ ions concentration. These results were confirmed using CIE analysis. UV-VUV studies at two excitation wavelengths confirmed the suppression of the green peak transitions with decreasing excitation wavelength (or increasing radiation energy). The results support the potential for this phosphor to be used as a single source for green light emission in display devices.

Silver Nanocubes Coated in Ceria: Core/Shell Size Effects on Light-Induced Charge Transfer.

Plasmonic sensitization of semiconductors is an attractive approach to increase light-induced photocatalytic performance; one method is to use plasmonic nanostructures in core@shell geometry. The occurrence and mechanism of synergetic effects in photocatalysis of such geometries are under intense debate and proposed to occur either through light-induced charge transfer (CT) or through thermal effects. This study focuses on the relation between the dimensions of Ag@CeO2 nanocubes, the wavelength-dependent efficiency, and the mechanism of light-induced direct CT. A 4-mercaptobenzoic acid (4-MBA) linker between core and shell acts as a Raman probe for CT. For all Ag@CeO2 nanocubes, CT increases with decreasing excitation wavelength, with notable increase at and below 514 nm. This is fully explainable by CT from silver to the 4-MBA LUMO, with the increase for excitation wavelengths that exceed the Ag/4-MBA LUMO gap of 2.28 eV (543 nm). A second general trend observed is an increase in CT yield with ceria shell thickness, which is assigned to relaxation of the excited electron further into the ceria conduction band, potentially producing defects.

Photoinduced electron transfer in WO3/BiVO4 heterojunction photoanodes: effects of the WO3 layer thickness

The PEC performance of WO3/BiVO4 heterojunction photoanodes with a fixed BiVO4 thick top layer and different WO3 layer thicknesses was investigated under backside irradiation, in comparison with the performance of the same electrodes without a top BiVO4 layer. While the performance of these latter increase with increasing WO3 thickness, the presence of a BiVO4 layer, besides leading to an effective sensitization up to 520 nm, leads to a decrease of incident photon to current efficiency in the short wavelength’s range. After having excluded major WO3 filter effects, this has been attributed to charge carrier recombination effects occurring when both oxides get excited and becoming more relevant with increasing WO3 thickness and decreasing excitation wavelength.

Photoaquation Mechanism of Hexacyanoferrate(II) Ions: Ultrafast 2D UV and Transient Visible and IR Spectroscopies.

Ferrous iron(II) hexacyanide in aqueous solutions is known to undergo photoionization and photoaquation reactions depending on the excitation wavelength. To investigate this wavelength dependence, we implemented ultrafast two-dimensional UV transient absorption spectroscopy, covering a range from 280 to 370 nm in both excitation and probing, along with UV pump/visible probe or time-resolved infrared (TRIR) transient absorption spectroscopy and density functional theory (DFT) calculations. As far as photoaquation is concerned, we find that excitation of the molecule leads to ultrafast intramolecular relaxation to the lowest triplet state of the [Fe(CN)6]4- complex, followed by its dissociation into CN- and [Fe(CN)5]3- fragments and partial geminate recombination, all within <0.5 ps. The subsequent time evolution is associated with the [Fe(CN)5]3- fragment going from a triplet square pyramidal geometry, to the lowest triplet trigonal bipyramidal state in 3-4 ps. This is the precursor to aquation, which occurs in ∼20 ps in H2O and D2O solutions, forming the [Fe(CN)5(H2O/D2O)]3- species, although some aquation also occurs during the 3-4 ps time scale. The aquated complex is observed to be stable up to the microsecond time scale. For excitation below 310 nm, the dominant channel is photooxidation with a minor aquation channel. The photoaquation reaction shows no excitation wavelength dependence up to 310 nm, that is, it reflects a Kasha Rule behavior. In contrast, the photooxidation yield increases with decreasing excitation wavelength. The various intermediates that appear in the TRIR experiments are identified with the help of DFT calculations. These results provide a clear example of the energy dependence of various reactive pathways and of the role of spin-states in the reactivity of metal complexes.

Dependence of Adenine Raman Spectrum on Excitation Laser Wavelength: Comparison between Experiment and Theoretical Simulations.

We acquired the Raman spectra of adenine in powder and aqueous phase using excitation lasers with 532, 633, and 785 nm wavelengths for the region between 300 and 1500 cm-1. In comparison to the most distinct peak at 722 cm-1, the peaks between 1200 and 1500 cm-1 exhibited a characteristic increase in cross-section with decreasing excitation wavelength in both phases. This trend can be reproduced by different density functional theory (DFT) calculations for the adenine molecule in the gas phase as well as in the aqueous phase. Furthermore, from the calculation on the π-stacked dimer, hydrogen-bonded dimer, and trimer, we find that this trend toward excitation laser wavelength is not sensitive to the packing. When comparing the Raman spectra given by different excitation wavelength, one should take care in analyzing the cross-section, and present day DFT calculations are able to capture general trends in the excitation laser wavelength dependence of the Raman activity.

Micro-Raman investigation of pigments and carbonate phases in corals and molluscan shells

We report the results of a micro-Raman investigation, in resonance conditions, on the pigments and on the nature of the biogenic carbonate in different species of corals and marine shells of a wide variety of colours. Corallium rubrum (Anthozoa) and Stylaster roseus (Hydrozoa) show calcite and aragonite skeletons, respectively. The shells are mostly aragonitic, with the exception of bivalves of Pectinidae family which reveal a calcite/aragonite distribution. The main Raman features of the pigments (carotenoids or polyenes) show Raman dispersion (upshift of the C=C (ν 1 ) and C–C (ν 2 ) stretching vibration frequencies of the polyenic chain with decreasing excitation wavelength). Partially or completely demethylated polyenes with different conjugation lengths may be responsible for the colours of Corallium rubrum and of most of the marine shells. Stylaster roseus and the dark brown colours of Gibbula sp (Gastropod class) show typical bands of carotenoids, similar to those found in canthaxanthin. Overtones and combination bands are described in terms of the four fundamental vibrations of polyenes. A constant ratio between ν 1 and ν 2 frequencies, nearly independent of the Raman excitation wavelength, has been determined from a large set of data for corals and shells. A simple relationship between the main Raman frequencies and the effective conjugation length N of the polyenic chains is suggested.

Tunable white luminescence and energy transfer in novel Cu^+, Sm^3+ co-doped borosilicate glasses for W-LEDs

The luminescent properties of novel Cu⁺, Sm³⁺ single- and co-doped borosilicate glasses were systematically investigated by absorption, excitation, emission spectra and decay curves. Cu⁺ single-doped glasses emit broad luminescence band covering all the visible range. And their peaks shift to blue with decreasing excitation wavelength from 330 to 280 nm. Cu⁺, Sm³⁺ co-doped samples generate the varied hues from blue white to pure white and eventually to yellow white due to an efficient energy transfer from Cu⁺ to Sm³⁺. Our research indicates the potential application of Cu⁺, Sm³⁺ co-doped borosilicate glasses as converting phosphors for white LEDs pumped by UV LED chips.

Electroabsorption and Electrophotoluminescence of Poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene)

Electric-field-induced changes of the absorption, photoluminescence, and photoluminescence excitation spectra and the photoluminescence decay profile have been measured for poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene). The magnitudes of the change of electric dipole moment and molecular polarizability of this compound have been determined for excitation into the first excited state having a strong absorption, and a large charge-separated character, which is relevant for photovoltaic devices, is confirmed. The photoluminescence of this compound is quenched by application of an electric field, and the quenching efficiency increases with decreasing excitation wavelength. Field-assisted dissociation is considered to occur from the photoexcited Franck–Condon state, resulting in the field-induced decrease in the population of the emitting state, which induces the quenching of photoluminescence.

Photo-fragmentation cross-section of gaseous 2,4,6-trinitrotoluene at different ultraviolet wavelengths

The photo-fragmentation cross-section of 2,4,6-trinitrotoluene (TNT) vapor at room temperature was determined at different ultraviolet wavelengths (254, 300, 340, and 400 nm) by measuring the concentration of NO molecule with cavity ring down spectroscopy and correcting for the photo-fragmentation cross-section of NO 2. Nitric oxide (NO) molecules are produced by the TNT photo-fragmentation processes via an intermediate production of NO 2. Our results reveal that the photo-fragmentation cross-section of TNT changes appreciably with change in wavelength with xenon arc lamp illumination, increasing with decreasing excitation wavelength. The maximum value of cross-section was observed at the shortest photo-fragmentation wavelength studied (254 nm), which is closest to the wavelength of an absorption peak of TNT near 220 nm.

Luminescence properties of ultrasmall amorphous Si nanoparticles with sizes smaller than 2 nm

Using electrochemical etching of amorphous Si films deposited on metal molybdenum and subsequent ultrasonic treatment in water and chloroform solutions, we have fabricated the suspensions of amorphous Si nanoparticles with a size range of 1–3 nm (the probable size is at 1.7 nm with a Gaussian fitting), which are uniformly dispersed in the solvents. We examined the photoluminescence (PL) spectra of the two kinds of suspensions and found that for the chloroform suspension, the PL spectrum lasts to blueshift as the excitation wavelength decreases. For the water suspension, the PL spectrum initially shows blueshift with decreasing excitation wavelength and then remains unchanged in peak position when the excitation wavelength is below 320 nm. We attribute this difference to different surface bonding structures of amorphous Si nanoparticles, induced by the solvents. This work indicates that the surface structure has a large influence on the PL properties of Si nanoparticles with sizes smaller than 2 nm.

Rayleigh-calibrated fluorescence quantum yield measurements of acetone and 3-pentanone

We measured fluorescence quantum yields of acetone and 3-pentanone as a pure gas and with nitrogen diluent at room temperature at 20, 507, and 1013 mbar using 248, 266, and 308 nm excitation by calibrating the optical collection system with Rayleigh scattering from nitrogen. At 20 mbar with 308-nm excitation, the fluorescence quantum yields for acetone and 3-pentanone are 7 +/- 1 x 10(-4) and 1.1 +/- 0.2 x 10(-3), respectively, and each decreases with decreasing excitation wavelength. These directly measured values are significantly lower than earlier ones that were based on a chain of relative measurements. The observed pressure and excitation wavelength dependence is in qualitative agreement with a previously developed fluorescence quantum yield model, but the absolute numbers disagree. Changing acetone's fluorescence rate constant to 3 x 10(5) s(-1) from its previous value of 8 x 10(5) s(-1) resulted in good agreement between our measurements and the model.

Polarized Fluorescence of Diphenylhexatriene-Incorporated Poly(Styrene) Film at Room Temperature

Poly(styrene) is a highly viscous, and cross-linked polymer at room temperature. This makes it ideal to use as a molecular fixer. The polarized fluorescence of a diphenylhexatriene (DPH)-incorporated poly(styrene) film has been studied. The excitation and emission wavelength dependence of the anisotropy of fluorescence of a luminophore-incorporated poly(styrene) film reveals that with decreasing excitation wavelength the anisotropy changes remarkably but is independent of emission wavelength. The investigation estimates an angle of no more than 7.4° between the absorption and the emission transition dipole moment for DPH, suggesting poly(styrene) as a suitable medium to evaluate the mutual orientation of absorption and the emission transition dipole moments at room temperature.

The Susceptibility of Non-UV Fluorescent Membrane Dyes to Dynamical Properties of Lipid Membranes

Fluorescence spectroscopy and microscopy are powerful techniques to detect dynamic properties in artificial and natural lipid membrane systems. Unfortunately, most fluorescent dyes that sense dynamically relevant membrane parameters are UV sensitive. Their major disadvantage is a high susceptibility to fluorescence bleaching. Additionally, the risk for hazardous damages in biological components generally increases with decreasing excitation wavelength. Therefore the use of non-UV–sensitive membrane dyes would provide significant advantage, particularly for applications in fluorescence microscopy, which usually implies high local excitation intensities. We applied steady-state fluorescence spectroscopy techniques to several UV and non-UV membrane dyes to detect and compare dynamically relevant excitation and emission characteristics. Small unilamellar liposomes (composed of egg yolk phosphatidylcholine) served as a model system for biological membranes. The dynamic properties of the membranes were varied by two independent parameters: the intrinsic cholesterol content (0–50 mol%) and temperature (10–50°C). We tested four non-UV–sensitive membrane dyes: 9-diethylamino-5H-benzophenoxazine-5-one (Nile Red), 4-(dicyanovinyl)julolidine (DCVJ), N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide (FM 4-64), and 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiIC18). We also tested three derivatives of DiIC18: DiIC16 and DiIC12 differ in acyl chain length and Fast-DiIC18 provides double bonds between hydrocarbon atoms. The spectral results were compared to established fluorescence characteristics of four UV membrane dyes: the anisotropy of 1-6-phenyl-1,3,5,-hexatrien (DPH), two derivatives of DPH (TMA-DPH and COO−-DHP), and the generalized polarization of 6-dodecanoyl-2-dimethyl-aminonaphthalene (Laurdan). Our results indicate that the tested non-UV dyes do not reveal dynamically relevant membrane parameters in a direct manner. However, spectral characteristics make DiIC18, Nile Red, and DCVJ promising probes for the microscopic detection of lateral lipid organization, an indirect indicator of membrane dynamics. In particular, DiIC18 showed very selective shifts in the emission spectra at defined temperatures and cholesterol contents that have not been reported elsewhere.

Characterization of phosphor materials for use in plasma display panel by time-resolved vacuum-ultraviolet laser spectrometry.

Phosphor materials that were manufactured for use in a plasma display panel (PDP) were investigated by employing a newly designed time-resolved vacuum-ultraviolet (VUV) spectrometer, which consists of a pulsed VUV laser and a fast photodetector. The VUV spectrometer was used to collect quantum efficiency data as well as the rise and decay times for the PDP phosphor luminescence. Both the rise and decay times increased with decreasing excitation wavelength in the VUV region. This result can be explained by a change in the mechanisms of photoexcitation and luminescence, that is, from charge-transfer excitation to host-lattice excitation below 200 nm. The present instrument was also used for an evaluation of the phosphor materials (Ba(1 - x)MgAl10O17:Eu2+(x)) by changing the Eu2+ concentration. The obtained data suggest that the impurities and defects are located inside the host crystal. Thus, the VUV spectrometer constructed in this study has considerable potential for use in investigating the nature of PDP phosphor materials.

Spectroscopy and photophysics of tropolone in condensed media

A systematic study of the spectroscopy and photophysics of tropolone in condensed media has been undertaken by measuring its UV–visible absorption, emission and emission–excitation spectra in a number of solvents of varying structure, at temperatures between 77 K and 295 K, and by measuring its quantum yields of emission, ϕem, and time-resolved emission decays as a function of temperature, T, and excitation wavelength, λex, in each solvent. In weakly interacting solvents, such as 3-methylpentane and perfluoro-2-n-butyltetrahydrofuran, the only species yielding significant fluorescence is the S1, 1(π,π*) state of the intramolecularly hydrogen-bonded neutral molecule. In acidic aqueous solution at pH <3, the emitting state is also 1(π,π*) S1, but the intramolecular hydrogen bond has been disrupted by the solvent. In basic aqueous solution at pH >8 the emitting species is the corresponding excited anion. In some solvents, emission from photochemical products prevents reliable measurements from being made at room temperature. Contrary to previous reports, no emission from solvent-stabilised 1(n,π*) excited states is observed. In glass-forming media the fluorescence quantum yields and the lifetimes of the emitting species increase in a sigmoidal fashion as the temperaure decreases, and are a factor of more than 100 greater at 77 K than at room temperature. The fluorescence quantum yields also decrease with increasing excitation energy in the UV, an effect which is most pronounced in more weakly interacting solvents at temperatures near room temperature. A mechanism is proposed. Both the spectra and the excited state temporal decays indicate that emission from short-lived, vibrationally unrelaxed S1 constitutes the majority of the fluorescence at room temperature, a small fraction of the total emission at 77 K, and an increasing fraction of the total emission with decreasing excitation wavelength. The variations of ϕem and lifetime with temperature are attributed to the effects of temperature-dependent solvent relaxation dynamics on the non-radiative decay, based on qualitative correlations between the observed parameters describing the effects in different media and known characteristics of the media, including their glass transition temperatures and polarity.

Absolute emission spectra from Bacillus subtilis and Escherichia coli vegetative cells in solution

Spectrally resolved emission (270-560 nm) from dilute suspensions of washed Bacillus subtilis and Escherichia coli were measured by use of tunable laser excitation between 270 and 300 nm. Integrated absolute emission cross sections increase with decreasing excitation wavelength and range from 1.8 x 10(-12) to 6.0 x 10(-11) cm(2)/(particle sr). An emission band near 340 nm dominates all observed spectra. At each excitation wavelength spectrally resolved emissions from the E. coli and B. subtilis suspensions are indistinguishable.

Raman and photoluminescence studies on intrinsic and Cr-doped ZnSe single crystals

Intrinsic and Cr-doped single crystals of ZnSe, grown by the seeded physical vapor transport technique, are studied using Raman and photoluminescence spectroscopic techniques. The coupling between the longitudinal–optical (LO) phonon and the hole plasmons, in Cr-doped ZnSe, results in a downward shift of the LO peak with increasing dopant concentration. The ratio of integrated intensities of LO and transverse–optical (TO) modes in ZnSe:Cr shows a systematic increase, with increasing temperature and decreasing excitation wavelength. This occurs due to the interaction of the LO phonon field with the surface electric field in the depletion layer as an enhancement of LO mode intensity. The localized Raman modes of CrZn–Se are seen in Cr-doped samples, which corroborate the photoluminescence findings of the presence of Cr2+ and Cr1+ deep levels. The LO and TO modes of intrinsic ZnSe show upward renormalization in their peak positions with decreasing excitation wavelength. The interaction of discrete phonons with the electronic continuum of the conduction band in ZnSe is attributed to the renormalization of Raman peaks. The large electron capture cross section of deep-level Cr2+ and Cr1+ impurities prevents the observation of band-to-band photoluminescence transitions at ∼2.67 eV in ZnSe:Cr; the charge transfer process Cr2+↔Cr1+ results in two emission bands at 2.25 and 1.40 eV.

Optical and Thermal Electron Transfer Activation of Dioxygen by Viologen Dithiolene Metalates

Photoinduced electron transfer activation of dioxygen by redoxactive charge-transfer ion pairs of the type {A2+[Pt(mnt)2]2-} (A2+ = 2,2‘-,4,4‘-bipyridinium or cycloalkylated biimidazolium dication; mnt2- = maleonitriledithiolate) occurs through an optical electron transfer within an ion pair. This affords the primary redox products A•+ and [Pt(mnt)2]- as indicated by laser flash photolysis. Under argon the transients recombine by fast second-order kinetics. Under dioxygen a different behavior is observed. In the case of acceptors with a first reduction potential more positive than −0.6 V back electron transfer prevails. When the potential is more negative however, A•+ reduces O2 by pseudo-first-order kinetics to generate O2•-, while [Pt(mnt)2]- accumulates in the solution. Quantum yields increase with decreasing excitation wavelength. This suggests that internal conversion of the initially populated excited state to the photoreactive ion pair charge-transfer state is more efficient upon excitation to the ...

Photoluminescence mechanism for blue-light-emitting porous silicon

A comparative study of photoluminescence (PL) and photoluminescence excitation (PLE) spectra of porous silicon (PS) and Si oxide, both emitting blue light, is reported. The PLE spectrum of Si oxide monitored at 460 nm displays a three-peak structure with peak wavelengths around 265, 300, and 360 nm. The PLE intensity of the PS sample increases with decreasing excitation wavelength, and has three shoulders located at wavelengths near the three PLE peak wavelengths of Si oxide. When the PS sample is oxidized for half an hour at 1150 \ifmmode^\circ\else\textdegree\fi{}C [the temperature at which nanometer Si particles (NSP's) inside PS disappear], blue-light PL intensity reduces greatly, and its PLE spectrum changes into a spectrum very similar to that of Si oxide. The experimental result strongly indicates that for the blue-light emission from PS there are two types of photoexcitation processes: photoexcitation occurs in NSP"s and in the Si oxide layers covering NSP's, and radiative recombination of electron-hole pairs is carried out in luminescence centers located on the interfaces between NSP"s and Si oxide and in those inside Si oxide layers.

Substitutional photochemistry of Ru( η6-mes) 22+and Ru( η6-mes)(CH 3CN) 32+ (mes = 1,3,5-trimethylbenzene)

Irradiation of Ru( η 6-mes) 2 2+ (mes is 1,3,5-trimethylbenzene, or mesitylene) in acetonitrile results in the substitution of one mesitylene ring by solvent to yield the half-sandwich complex, Ru( η 6-mes)(CH 3CN) 3 2+. The quantum efficiency of this process exhibits little dependence upon the excitation wavelength or the presence of oxygen, but varies with solvent in the order acetonitrile > water > acetone. Photolysis of Ru( η 6-mes)(CH 3CN) 3 2+ in acetonitrile occurs via two competing pathways: substitution of mesitylene by solvent to produce Ru(CH 3CN) 6 2+, and exchange of coordinated CH 3CN with bulk solvent. Quantum efficiencies for both pathways increase with decreasing excitation wavelength and, at each wavelength, CH 3CN exchange is more efficient than mesitylene substitution. Photochemical mechanisms for these processes are discussed and comparisons with related complexes are presented.