Non-exponential Decay Profiles Research Articles

Spectroscopic assessments on Sm3+ ions doped phospho-borate glasses mixed with fluoride modifiers for the applications of reddish-orange visible light

Sm3+ions doped Phospho-Borate glasses were synthesized and their physical and spectroscopic parameters were studied to evaluate its potential reddish-orange emission for photonic applications. Structural investigation made through XRD analysis confirms the amorphous nature. The evaluated bonding parameters from the absorption spectral analysis confirm the ionic bonding of the Sm–O network in the prepared glasses. Four emission bands were observed from the luminescence spectra, and the HT 4G5/2 → 6H7/2 is observed at 601 nm. The oscillator strength values elucidate the intensity of the absorption bands, and the PBKZnF:Sm sample exhibits a higher oscillator strength value. The Judd-Ofelt intensity parameters were observed to trail the trend Ω4 > Ω6. > Ω2 for the majority of the samples. The CIE 1931 color chromaticity investigation confirms that the present glass samples are suitable for reddish-orange media. Barium and strontium-incorporated glasses exhibit outstanding lasing potential, which was confirmed through the efficiency of the quantum yield and some of the radiative parameters like effective bandwidth, transition probability and stimulated emission cross-section. Radiative parameters have been calculated from the luminescence spectra. Amid all transitions, 4G5/2 →6H7/2 transition has higher transition probability and higher stimulated emission cross-section values for all the prepared glass samples. Barium-incorporated glass exhibits a higher emission cross-section of 30.55 × 10−22 cm2 and a transition probability of 30.89 s−1 compared to all other glasses. The non-exponential decay profiles of the fabricated samples were plotted by examining the excitation wavelength at 402 nm and emission wavelength at 600 nm. Of all the prepared glasses, the quantum efficiency is found to be higher for the glass sample PBKSrF:Sm (65 %).

Reddish-orange emission and Judd-Ofelt investigation of Sm3+ ions doped in zince-bismuth-phospho-tellurite glasses for solid lighting application

In this paper, physical, structural and optical properties of zinc bismuth phospho tellurite glass doped with various concentration of Sm3+ ions (x = 0.01, 0.1, 0.3, 0.5 and 1 mol%) were prepared by melt quenching technique. With keen interest in luminescence properties of samarium, spectroscopic properties are investigated using optical absorption spectra, excitation spectra, emission spectra and decay profiles. Applying Judd-Ofelt theoretical approach an effort is made to understand the asymmetry of samarium in phosphate host glass matrix. Using this theory three important parameters such as Ω2, Ω4, Ω6 are determined to calculate emission properties for certain Sm3+ transitions. Luminescence parameters such as stimulated emission cross section (σp) and branching ratios (βexp) have been studied through photoluminescence spectra. The PL spectra for all glasses show four emission bands centered between 561, 601, 646 and 708 owing to the 4G5/2 → 6H5/2 (561), 6H7/2 (601), 6H9/2(646) and 6H11/2(708) transitions of Sm3+ doped PZBTS glass system. Decay time data reveals that single exponential behavior of Sm3+ ions up to 0.5mol%, while non-exponential decay profile for 1 mol% Sm3+ content. CIE1931 chromaticity diagram for the PZBTS glasses were evaluated. CCT values were obtained and shows around 2510 K–2560 K for the prepared glasses. From the results we say that these materials prove to be very good materials for reddish orange lighting applications.

Optical response and luminescence characteristics of Sm3+ and Tb3+/sm3+ co-doped potassium-fluoro-phosphate glasses for reddish-orange lighting applications

Potassium-fluoro-phosphate (KFP) glasses singly doped with different concentrations of samarium (0.1, 0.3, 0.5, 1.0, 1.5 and 2.0 mol%) and co-doped with Tb3+(0.1, 0.5, 1.0 mol%) were prepared, and their luminescence properties were investigated. Optical transition properties of Sm3+ ions were evaluated by using Judd–Ofelt theory. Various emission properties were studied from luminescence spectra. Reddish-orange emission of Sm3+ exhibits mainly by 4G5/2 level to 6H7/2 located at 600nm. Concentration quenching and energy transfer were observed from fluorescence spectra and decay curves respectively. The non-exponential decay profiles were analyzed Inokuti–Hirayama model (I–H). From this model, various energy transfer kinetic parameters were also calculated and analyzed. By co-doping with Tb3+, energy transfer from Tb3+ to Sm3+ takes place and contributes to enhancement in emission intensity and intense reddish-orange emission was obtained. The optimized properties of the Tb3+/Sm3+ co-doped KFP glasses indicate that these glasses can be used as luminescent materials.

Room‐temperature luminescence decay of colloidal semiconductor quantum dots: Nonexponentiality revisited

While time‐resolved luminescence spectroscopy is commonly used as a quantitative tool for the analysis of the dynamics of photoexcitation in colloidal semiconductor quantum dots, the interpretation of the virtually ubiquitous nonexponential decay profiles is frequently ambiguous, because the assumption of multiple discrete exponential components with distinct lifetimes for resolving the decays is often arbitrary. Here, an interpretation of the room‐temperature luminescence decay of CdSe/ZnS semiconductor quantum dots in colloidal solutions is presented based on the Kohlrausch relaxation function. It is proposed that the decay can be understood by using the concept of Förster resonance energy transfer (FRET) assuming that the role of acceptors of photoexcitation energy is played by high‐frequency anharmonic molecular vibrations in the environment of the quantum dots. The term EVFRET (Electronic ‐ Vibrational Förster Resonance Energy Transfer) is introduced in order to unequivocally refer to this energy transfer process. image

Reactions of the CN Radical with Benzene and Toluene: Product Detection and Low-Temperature Kinetics

Low-temperature rate coefficients are measured for the CN + benzene and CN + toluene reactions using the pulsed Laval nozzle expansion technique coupled with laser-induced fluorescence detection. The CN + benzene reaction rate coefficient at 105, 165, and 295 K is found to be relatively constant over this temperature range, (3.9-4.9) x 10(-10) cm(3) molecule(-1) s(-1). These rapid kinetics, along with the observed negligible temperature dependence, are consistent with a barrierless reaction entrance channel and reaction efficiencies approaching unity. The CN + toluene reaction is measured to have a rate coefficient of 1.3 x 10(-10) cm(3) molecule(-1) s(-1) at 105 K. At room temperature, nonexponential decay profiles are observed for this reaction that may suggest significant back-dissociation of intermediate complexes. In separate experiments, the products of these reactions are probed at room temperature using synchrotron VUV photoionization mass spectrometry. For CN + benzene, cyanobenzene (C(6)H(5)CN) is the only product recorded with no detectable evidence for a C(6)H(5) + HCN product channel. In the case of CN + toluene, cyanotoluene (NCC(6)H(4)CH(3)) constitutes the only detected product. It is not possible to differentiate among the ortho, meta, and para isomers of cyanotoluene because of their similar ionization energies and the approximately 40 meV photon energy resolution of the experiment. There is no significant detection of benzyl radicals (C(6)H(5)CH(2)) that would suggest a H-abstraction or a HCN elimination channel is prominent at these conditions. As both reactions are measured to be rapid at 105 K, appearing to have barrierless entrance channels, it follows that they will proceed efficiently at the temperatures of Saturn's moon Titan ( approximately 100 K) and are also likely to proceed at the temperature of interstellar clouds (10-20 K).

Determination of the Förster Distance in Polymer Films by Fluorescence Decay for Donor Dyes with a Nonexponential Decay Profile

Fluorescence resonance energy transfer (FRET) experiments were carried out on three pairs of donor-acceptor dyes in polymer films in which the donor dyes had absorption maxima in the range of 350-450 nm. Two of the donors, a coumarin dye and a naphthalimide dye covalently bound to polystyrene, gave nonexponential decays in the absence of acceptors. The decay profiles could be fitted to a stretched exponential form with a beta value on the order of 0.9. We developed equations for analyzing donor fluorescence intensity decay profiles for donor-acceptor mixtures in rigid matrices for the case of donors showing relatively small deviations from exponentiality. To test these equations, we calculate values of the Förster radius (R0(FR)) from the decay profile data and compare these values to the Förster radius R0(SO) determined by the traditional spectral overlap method. Agreement between these values validates the methodology developed here for the use of such donor dyes in FRET studies of more complex polymer systems.

Optical sensing of NO2 in tin oxide nanowires at sub-ppm level

Tin oxide nanostructures are currently considered as promising materials for gas sensing. Some of their physical properties, such as visible light photoluminescence, are still not fully understood and require further investigations. In this work we report on the quenching of tin oxide nanowires photoluminescence induced by nitrogen dioxide adsorption at the ppm and sub-ppm level. The recombination dynamics was investigated by means of time-resolved photoluminescence, finding non-exponential decay profiles. Neither the photoluminescence spectral shape nor the decay rates are found to depend significantly on the nitrogen dioxide concentration, suggesting that the gas-nanostructure interaction resides in a net reduction of the density of states which are responsible for the radiative transitions.

Temporal behavior of biexciton luminescence under two-photon excitation in CuCl quantum dots

Temporal behavior of luminescence of biexcitons under two-photon resonant excitation has been measured in CuCl quantum dots. The non-exponential decay profile was observed, and its fast decay component becomes dominant with shortening the decay time for higher pump intensity. This characteristic feature of the biexciton emission is observable only for the two-photon resonant excitation with realization of population inversion in biexciton–exciton system at the initial stage and indicates the possibility of Dicke's superradiance, which is originated from the coherent interaction of diploes among many quantum dots.

Photoluminescence Spectra and Dynamics of Single CdS Nanocrystals Studied by Scanning Near-Field Optical Microscopy

Photoluminescence (PL) spectra and dynamics of hexagonal CdS nanocrystals embedded in Al 2 O 3 matrices have been studied by means of a scanning near-field optical microscope at 8 K. The PL spectra of individual CdS nanocrystals show narrow lines with single-exponential decay profiles, although the macroscopic PL spectra measured by conventional optics show a broad band with non-exponential decay profiles. The PL peaks are observed at energies below the lowest free-exciton energy of CdS nanocrystals. The luminescence mechanism of CdS nanocrystals is discussed.

Ultrafast relaxation dynamics of neutral soliton pairs in a quasi-one-dimensional halogen-bridged mixed-valence platinum complex [Pt(en)2][Pt(en)2Br2](ClO4)4

Ultrafast optical response of photoexcited states is studied by femtosecond pump and probe spectroscopy in a quasi-one-dimensional halogen-bridged mixed-valence platinum complex, [Pt(en)2][Pt(en)2Br2](ClO4)4 (en=ethylenediamine). We observed two kinds of photo-induced absorption (PA) signals, and both of the PA signal intensities follow non-exponential decay profiles. The signals are ascribed to two kinds of neutral soliton pairs with different configurations, that is, a spatially confined soliton and antisoliton pair and a noncorrelated soliton and antisoliton pair. Their formations are found to be closely related with both the excitation energy and the excitation density.

Decay Profiles of Mn2+Photoluminescence in CdMnTe

Decay profiles of Mn 2+ photoluminescence in Cd 1- x Mn x Te ( x =0.6) have been investigated under various excitation and observation energies. From the analysis of time resolved spectra, we find that the excited states of d-d transition relax to lower energy by the amount of 5 meV to 40 meV on the time scale of ∼10 µs and that the non-exponential decay profiles of Mn 2+ PL are understood as a result of the energy relaxation. At 2 K, the relaxation time under the lower energy excitation than the peak of the excitation spectrum (∼2.2 eV) is ∼40 µs and that of the high energy excitation is ∼15 µs. With increasing temperature, the relaxation time decreases. These behaviors explain well previous results of site selected and non-site selected photoluminescence obtained by CW measurements.

Interpretation of the temperature dependence of the luminescence intensity, lifetime, and decay profiles in porous Si.

The temperature dependences of the luminescence intensity, lifetime, and decay profiles for porous Si are studied from 5 to 271 K. The radiative decay rates were determined from the tails of the decay curves and found to have an activation-type temperature dependence above 10 K. To describe the nonradiative process we propose a model in which we assume a tunneling and a thermally activated escape of the photoexcited carriers through barriers with a Gaussian distribution in height. The temperature dependence of intensity, lifetime, and nonexponential decay profiles are successfully interpreted in terms of this model.

Luminescence from porous silicon

Predominantly orange-red, but occasionally green, blue or deep red luminescence is observed following electrochemical etching of p-type epitaxial silicon in HF solution. Photoluminescence and cathodoluminescence of the same samples are compared. The dependence of the photoluminescence spectrum on the sample temperature and on the excitation density, considered together with its nonexponential decay profile, suggest that the dominant luminescence process in porous silicon is recombination of quantum-confined carriers in a disordered medium. In addition, cathodoluminescence reveals the presence of 'hot spots' which show bright features of uncertain origin.

Modulation of the time-resolved photoluminescence of cadmium selenide by adduct formation with gaseous amines

The time-resolved, band edge photoluminescence (PL) of cleaved samples of single-crystal n-CdSe has been measured in the presence of N{sub 2}, NH{sub 3}, and CH{sub 3}NH{sub 2} (30% mixtures of the amines in N{sub 2}). Nonexponential decay profiles are modeled. At a low incident average power of {approx} 0.1 mW, both {tau} and {beta} reversibly increase relative to a N{sub 2} ambient upon exposure of CdSe to NH{sub 3} and CH{sub 3}NH{sub 2}, which engage in adduct formation with the semiconductor surface; the magnitude of the enhancement in both decay parameters is larger for the more basic CH{sub 3}NH{sub 2} than for NH{sub 3}. Increases in incident power cause the absolute values of {tau} and {beta} to increase while reducing the dependence of these parameters on gaseous ambient. Interfacial properties that contribute to these effects are discussed and comparisons with steady-state PL properties are presented.

Photochemistry in polymer solids—4. On the intensity dependence of non-exponential decay of benzophenone phosphorescence in poly(methyl methacrylate)

Change in the intensity of the irradiation laser pulse did not influence the non-exponential decay profile of benzophenone phosphorescence in poly(methyl methacrylate) at 20°. This result is incompatible with the biphotonic triplet-triplet annihilation process, but supports the diffusion-controlled dynamic quenching mechanism including non-equilibrium √t term.

Nonexponential decay profile of the2S13state of He-like ions

A mechanism is proposed which explains the nonexponential decay $2^{3}S_{1}\ensuremath{\rightarrow}1^{1}S_{0}+\ensuremath{\gamma}(M1)$ recently observed in beam-foil experiments on He-like atoms. This mechanism suggests that the nonexponential nature of this decay is due to the inclusion of unresolved $M1$ satellite photons from Li-like ions. By using models of excited-state populations which are suggested by recent beam-foil experiments, we are able to calculate a decay profile of heliumlike sulfur which agrees with the experimental results.

FREE-FALL DIFFUSION IN A DISCHARGE AFTERGLOW

The afterglow decay of the number density and electron temperature of a plasma in which collisions between electrons and molecules are rare is studied both theoretically and experimentally. The theoretical approach, using the first three moment equations solved in a one-dimensional geometry, predicts non-exponential decay profiles in time for both density and temperature. Experimental results are obtained using a cylindrical mercury vapor discharge tube at pressures where the mean free path of the electrons is much larger than the transverse dimensions of the tube. The average electron number density is measured by a surface-wave cavity method and the electron temperature by means of Langmuir probes. The theoretical and experimental results are in good agreement, especially for the density decay.