Observed Decay Time Research Articles

Photochemically induced energy-transfer effects on the decay times of ruthenium complexes in polymers

The effects of photobleaching on absorption properties and decay times of ruthenium complexes adsorbed on silica gel or dissolved in polystyrene or PVC are reported. While complexes with bipyridyl ligands adsorbed on silica gel did not show any decay time decrease at all, complexes having phenanthroline ligands exhibit a significant photodegradation which is manifested not only by a decrease in luminescence intensity but also by decay time decrease and specific absorption spectral changes. The effects are shown to depend strongly on the oxygen and dye concentrations present at the bleaching process. The absorption bands of the photoproducts of ruthenium-phenanthroline complexes overlap with the emission spectra of the intact molecules. Forster resonance energy transfer from intact ruthenium complexes to their own photoproducts, generated by singlet oxygen attack of the phenanthroline bridges of the ligands, is suggested to be responsible for the observed decay time effects.

Formation of C2 Radicals in High-Density C4F8 Plasmas Studied by Laser-Induced Fluorescence

Spatial and temporal variations of C2 radical density in high-density C4F8 plasmas have been investigated by laser-induced fluorescence (LIF) spectroscopy. Hollow-shaped spatial distributions have been observed for the C2 density, indicating that C2 radicals are produced from fluorocarbon film on the chamber wall and are lost in the plasma column by electron impact processes. The C2 density depends largely on the seasoning condition of the vacuum chamber, which supports the surface production of C2 radicals. Bombardment of positive ions onto fluorocarbon film is necessary for such surface production. The mechanism of this surface production is not the physical sputtering of fluorocarbon film but ion-enhanced chemical reactions with fluorocarbon film. Lifetime measurements of C2 radicals have also been carried out in the afterglow. The observed decay time constants (less than 0.8 ms) are shorter for high gas pressures, indicating the loss of C2 due to gas-phase reactions.

An investigation of Q-switched induced quenching of the resonant nonlinearity in neodymium doped fibers

A method of reducing the decay time of the population-inversion-dependent refractive index change in rare-earth doped fibers is investigated, and its suitability for use in all-optical switching is considered. Normally, the temporal response of this type of nonlinearity is dominated by the slow fluorescent decay from the metastable level present in the doped fiber. However, in this instance, the nonlinearity is rapidly quenched by the internally generated lasing signal produced by Q-switching a lasing cavity containing the doped fiber. Using this technique the observed decay time was reduced from 370 /spl mu/s to /spl sim/1 /spl mu/s and a phase modulation of /spl pi/-radians was observed, demonstrating that the effect could be used to achieve complete optical switching in an interferometric device. A model of the temporal behavior of this effect is developed using a rate equation analysis. The induced index change is modeled by summing the effects of individual transitions in the region of the signal wavelength, and by including the effects of assumed transitions in the VUV. An extension to this method is discussed in which the depopulating signal is generated using gain-switched techniques, obviating the need for intracavity modulation, thus making the depopulation mechanism more suitable for practical purposes.

Temperature dependence of the minority carrier lifetime in GaAs/AlGaAs double heterostructures

We have studied the photoluminescence decay time in a series of high quality GaAs/AlGaAs double heterostructure samples, grown by liquid phase epitaxy, with different p-type doping. We have compared the experimentally observed decay time as a function of temperature from 100 to 700 K, with a complete calculation of the radiative recombination rate, including the temperature dependence of the reabsorption factor. We conclude that the observed decay time is well explained by a dominating radiative recombination up to temperatures of about 500 K. The internal quantum efficiency is hence close to unity. At higher temperatures we observe a deviation from the expected values for the radiative recombination, attributed to a nonradiative recombination channel.

Optical properties of the Pb2+-based aggregated phase in a CsCl host crystal: Quantum-confinement effects.

The quantum-confinement effect is demonstrated in the luminescence of the ${\mathrm{CsPbCl}}_{3}$-like aggregated phase in ${\mathrm{Pb}}^{2+}$-doped CsCl single crystals. Namely, microscopic excitonic superradiance is considered to explain the observed picosecond decay kinetics of the aggregated-phase emission band at 420 mn. The low- and high-temperature limits of the temperature dependence of the radiative decay time in the quantum dot are derived and compared with experimental data. Different processes that might influence the observed wavelength and temperature dependence of the observed decay time are discussed.

Intrinsic photoluminescence of pure β-ThBr 4 single crystal

The investigation of the intense blue photoluminescence of β-ThBr 4 single crystal has been carried out by optical studies. Absorption and emission spectra versus temperature are presented. The thermal quenching of the blue photoluminescence has been performed by measuring the temperature dependence of the fluorescence lifetime. The analysis of these data yields activation energy value of 3622 cm -1. Using the time-dependent theory, the experimental low temperature emission profile has been simulated. The data are interpreted in terms of a charge transfer process between the bromine 4p levels and the thorium 5f-6d levels. A single configuration-coordinate, quadratic coupling model is proposed to explain the observed decay time thermal dependence.

Dynamics of the nitrogen-bound excitons in 6H SiC.

We have measured the photoluminescence decay time of the P, R, and S bound excitons at the neutral nitrogen donors in 6H SiC using picosecond pulsed excitation. At 2 K the decay times are 8.0, 1.8, and 1.5 ns, respectively, which are significantly faster than previously reported values for shallow donors in other indirect-band-gap materials such as Si or GaP. Each of the observed decay times is found to be independent of the doping level in the sample, and also temperature independent at low temperatures, but decreases when the bound excitons are thermally ionized. The decay time related to different donors exhibits a strong dependence on the binding energy of the donor level. We suggest that the dominating mechanism responsible for the observed decay time is a phononless Auger process. In high-purity samples we have also measured the free-exciton decay time at low temperatures to be 12 ns.

Radiative and non-radiative decay processes of the excited state of the colored form of photochromic furylfulgide

The time evolution of the luminescence of the colored form of a furylfulgide dispersed at various concentrations in a poly(methyl methacrylate) film was measured as a function of the luminescence photon energy. The observed decay time of the luminescence is about 1–2 ns and one order of magnitude shorter than the radiative lifetime (14 ns) estimated from the absorption intensity. The decay time is independent of temperature below 77 K. These results suggest that the non-radiative tunneling process from the excited state to the ground state is responsible for the decay.

Capture and recombination of acceptor bound excitons in the transition region from a two-dimensional to a quasi-three-dimensional GaAs/AlGaAs system

The capture and recombination processes for acceptor bound excitons (BEs) have been studied for GaAs/AlGaAs multiple quantum well structures, by varying the barrier thickness for a constant 100 Å well width. The observed decay time for the acceptor BE increases rapidly with decreasing barrier width, and is determined by the relative confinement of the electron and hole wave functions. The capture rate of free excitons to the BE state of the neutral acceptor increases by about a factor 7 from a two-dimensional to a three-dimensionlike system.

Dynamics of the Nitrogen Bound Excitons in 6H and 3C SiC

AbstractWe have measured the photoluminescence decay time of the bound excitons at the neutral nitrogen donors in the 6H and 3C polytypes of SiC. At 2K the decay times are 8.0 ns, 1.8 ns and 1.5 ns, for the P, R and S bound excitons in 6H SiC. For the nitrogen exciton in 3C, we find a decay time of 160 ns. These values are faster than previously reported for shallow donors in other indirect bandgap materials such as Si or GaP. Each of the observed decay times is found to be independent of the doping level in the sample, is temperature independent at low temperatures but decrease when the bound excitons are thermally ionised. The decay time related to different donor levels in 6H exhibits a strong dependence on the donor binding energy. We suggest that the dominant mechanism responsible for the observed decay time is a phonon-less Auger process. In high-purity 6H samples we have also measured the free exciton decay time at low temperatures to be 12 ns.

Growth of carbon clusters. The simplest process, 2C1? C2, observed via spectrometry and chemical reaction

A graphite sample has been irradiated with a YAG laser under an argon atmosphere. The emission due to the C2(d 3Πg→ a 3Πu) Swan band was observed with a decay time 20 times the intrinsic lifetime of C2(d 3Πg). In the presence of benzene, which reacts with C1, the observed decay time of the C2 Swan band decreased with increasing benzene around the graphite. Furthermore, the reaction products between benzene and C1 or C2 were identified and their distribution varied remarkably with benzene concentration. From these results, the simplest growth process of carbon clusters, 2C1→ C2, was confirmed experimentally. Comparing the emission spectra from the irradiated graphite surface under an argon atmosphere and under vacuum, the collisional relaxation of C1 with argon was found to be indispensable to the growth process.

Localized and self-trapped exciton states in PbI2-PbBr2 mixed crystals

Time-integrated luminescence spectra and time behaviour of the luminescence have been measured in PbI2(1-x)Br2x mixed crystals in order to elucidate the relaxation processes of free, localized and self-trapped excitons. The luminescence spectra for values of the composition ratio x below 0.05 exhibit recombination lines due to free and bound exciton states. Above x = 0.07, the luminescence coming from the localized exciton states which we denote as L states, becomes dominant. The observed decay time of this L band luminescence at the lowest energy is 100 ps. This rather short decay time is explained as due to the transition of the localized exciton state to lower energy excited states including a self-trapped exciton.

Characterization of silicon-on-insulator field-effect structures by transient photoconductivity

Abstract Transient photoconductivity measurements were carried out in thin-film transistors of silicon-on-insulator structures which were formed by oxygen implantation and subsequent lamp annealing. They show that traps arising primarily due to surface states at the top silicon–oxide interface are responsible for the observed dispersive transport in these films. As the gate voltage is increased and more holes are pulled to the surface, the form of transients changes from dispersive to non-dispersive due to saturation of the surface traps. Upon further increase of the gate voltage the form and the magnitude of the photocurrent prior to recombination remains unchanged, but the observed decay time gets longer for higher gate voltages. This occurs due to a reduction in the recombination centre density for the majority carriers corresponding to the movement of the demarcation level.

Electron transport in selenium-based amorphous xerographic photoreceptors

The present paper is the sequel to a previous report. In this paper, electron transport in various selenium-based amorphous xerographic photoreceptors is described. As before, measurements were carried out using xerographic time-of-flight (XTOF) and conventional time-of-flight (TOF) experiments. From the observed flight time the electron drift mobilityμ e was deduced. Transient photocurrent signals were also recorded at low fields to determine the decay time constant. For a given composition, the sample thickness, light intensity, substrate material and top contacts (in TOF) were varied to investigate whether the observed decay time constantτ e is a meaningful bulk parameter as in the case of hole transport. Detailed results will be presented to show that the interpretation ofτ e is complicated. The more plausible explanation for the observed decay is the presence of a uniform distribution of positive space charge in the bulk.μ e andτ e were also measured as a function of composition and applied field. Experimental data suggest that electron transport is shallow trap-controlled. Light doping of selenium with arsenic or tellurium creates shallow traps and hence reducedμ e. The field dependence of electron mobility is in the formμ e ∝E n whereE is the applied field andn is a constant less than unity. No electron response can be detected in chlorine-doped Se-As or Se-Te alloys.

The radiative lifetime of acetone: resolution of an apparent discrepancy

Past determinations of the radiative lifetime of acetone (and other ketones) used two methods. One was based on the integrated absorption coefficient and the relationship between the Einstein A and B coefficients. The other, on the ratio between the measured quantum yield and the measured lifetime. The latter resulted in consistently shorter radiative lifetimes than the former. Based on low-pressure bulk and jet results, it is shown that the short observed decay time used in the second method is due not to external quenching but to an intramolecular process. The Stern-Volmer mechanism is inapplicable in this case since the process cannot be properly represented by a rate constant. The Einstein coefficient method is thus the prefered one.

Effects of Magnetic Dipolar Interaction on Two-Quantum Free Decay in a Solid

Effects of magnetic dipolar interaction between like nuclei on two-quantum (TQ) free decay were examined in a multi-level NMR system, Al in Al 2 O 3 . TQ free decay was observed using a TQ π/2-pulse followed by a probe pulse. The effect of probe pulse was carefully examined to deduce the intrinsic decay time of the TQ coherence. The second moments for two- and three-quantum transitions were calculated. The observed decay time is in good agreement with that obtained from the calculated second moment by assuming Gaussian decay. Basic equations for TQ resonance are also presented.

Numerical modeling of an end-plugged theta pinch

Analytical and numerical studies of an end-plugged theta pinch are described. The analytical model treats the ablated plug plasma in the quasi-static limit where radiation losses balance energy flowing from the main plasma. This model is used to calculate the enhancement in energy confinement due to an ablating end plug for various plug species. The numerical model employs a one-dimensional, time-dependent magnetohydro-dynamic code. Results of calculations simulating the Scylla IV-P end-plugged theta pinch experiment are presented. The calculations achieve good agreement with the observed decay time of the energy line density. Moreover, the observed tendency toward longer decay times at lower atomic number is also predicted. However, certain notable discrepancies are found. For Si plugs, the calculations indicate a somewhat longer decay time than observed with SiO2 plugs. In addition, an axial compression wave driven by plug ablation causes the calculated energy line density to rise after 15 to 20 μsec which was not observed in the experiments. This is believed to be a feature of the one radial cell model which forbids axial wave dispersion; such dispersion would tend to mute the appearance of such waves. For fusion reactor scale plasma, the calculations predict that higher atomic number leads to negligible enhancements in confinement time.

Spectroscopy and lasing in K<inf>5</inf>NdLi<inf>2</inf>F<inf>10</inf>(KNLF)

A new stoichiometric compound of Nd, K 5 NdLi 2 F 10 has been synthesized and its spectroscopic and lasing properties are reported. The material is characterized by a room temperature decay time of 300 μs, increasing to 500 μs in diluted samples. This represents the least amount of concentration quenching reported in this class of compounds. The spectroscopically determined peak emission cross section for the 1048 nm lasing line is 12 \times 10^{-20} cm2, about twice the value obtained from laser experiments. A calculated radiative lifetime agrees with observed decay time for samples with less than 10 percent Nd content, indicating a quantum efficiency near unity. The concentration quenching in this material is discussed and found to be considerably lower than predicted from spectroscopically obtained rates of pair cross relaxation.

EUV continua of solar flares 1420-1960 A

view Abstract Citations (38) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS EUV continua of solar flares 1420 - 1960 Å. Cook, J. W. ; Brueckner, G. E. Abstract Measurements of absolute continuum intensities in the wavelength range 1420-1960 A are presented for the SN subflare of 1973 August 9 and the 2B flare of 1973 September 7, observed from the NRL spectrograph aboard Skylab. Continuum measurements are also given for two active-region plages, which are used for background nonflaring levels. Major enhancement of the flare continuum intensity above the plage backgrounds occurs in the Si I 3P and 1D continua. The observed brightness-temperature minimum of the plages is approximately 310 K above the quiet-sun minimum, and occurs at longer wavelength. The earliest flare brightness-temperature minima are approximately 200 K above the plage minimum, and also at longer wavelength. The observed decay time of the flare brightness temperature, which is comparable to the Solrad 9 X-ray decay time, is much longer than the Ulmschneider radiative relaxation time for the 1973 September 7 flare and marginally longer for the 1973 August 9 flare, suggesting continuous excess heating of the plage temperature minimum region throughout the period of our flare observations. Publication: The Astrophysical Journal Pub Date: January 1979 DOI: 10.1086/156775 Bibcode: 1979ApJ...227..645C Keywords: Faculae; Far Ultraviolet Radiation; Solar Flares; Solar Spectra; Brightness Temperature; Skylab Program; Stellar Spectrophotometry; Solar Physics; Extreme UV:Solar Flares; Extreme UV:Solar Plages; Solar Flares:UV Spectra full text sources ADS |

Vibrational excitation of N2 during optical pumping of Hg by 253.7 mμ resonance radiation

The observed decay time and the population density of the metastable 6 3P0 state of mercury in a N2:Hg gas mixture are markedly reduced with increased duration of an intense 253.7 mμ resonance radiation pulse. We show that this phenomenon is due to a vibrational excitation of the nitrogen in the spin–orbit relaxation decay of the 6 3P1 to the 3P0 state of Hg which is followed by the quenching of the 3P0 state. Such quenching takes place by diffusion and deactivation of 3P0 at the walls or by a three-body excimer formation in the gas. Observations are confirmed by a numerical solution of the rate equations for the different excited states in the system. The impact of these results on the performance of the optically pumped Hg laser is discussed.