Average Decay Time Research Articles

Photoluminescence yield and decay time of proton irradiated and thermally annealed a-Si0.35C0.65:H alloys: A phenomenological model

We have studied the yield and the time evolution of pulsed laser induced photoluminescence in proton irradiated and thermally annealed amorphous hydrogenated silicon carbon alloys prepared by plasma enhanced chemical vapor deposition. Three major fluorescence decay channels have been observed with decay rates independent from proton irradiation and thermal annealing. Lack of correlation between yield and average decay time suggests a very simple phenomenological model which allows evaluation of the nonradiative time constant which is found to be linearly correlated with the photoluminescence yield. Our model suggests that radiative recombination occurs via exciton decay while the nonradiative recombination is driven by the trapping of carriers in defects states.

Steady-State and Time-Resolved Spectroscopy of a Self-Assembled Cyanine Dye Multilayer

Ultrathin organic multilayers, consisting of polyelectrolytes and J-aggregates of a cyanine dye, were obtained using the layer by layer alternate adsorption technique. Buildup of the multilayer is followed by UV−vis absorption spectroscopy. The fluorescence properties of the J-aggregates are studied by steady-state fluorescence spectroscopy and picosecond fluorescence decay measurements. Fluorescence of the cyanine dye, when adsorbed on the substrate, results in a narrow emission band with a small Stokes shift which is characteristic for J-aggregate emission. Time-resolved fluorescence measurements yield a very short average decay time, which can be attributed to the J-aggregates. These results describing the fluorescence properties of multilayer assemblies containing J-aggregates allow one to compare them to those of J-aggregates of the cyanine dye in solution.

Fluorescence Lifetime Microscopy of the Sodium Indicator Sodium-Binding Benzofuran Isophthalate in HeLa Cells

The behavior of the sodium indicator sodium-binding benzofuran isophthalate (SBFI) is investigated in HeLa cells by time-resolved fluorescence microscopy. The fluorescence relaxation of SBFI in HeLa cells can be described by a triexponential for intracellular sodium concentration ([Na+]i) between 0 and 90 mM. Changes in [Na+]i affect neither the fluorescence relaxation times (0.21, 0.60, and 2.7 ns) nor the average decay time (2.2 ns). The preexponential factor of the shortest decay time is negative. However, the ratio of the fluorescence excitation signal at 340 nm to that at 380 nm increases with [Na+]i. To elucidate the behavior of SBFI in cells, experiments are performed on SBFI in buffer at various concentrations of sodium, potassium, and bovine serum albumin (BSA) and at various viscosities. The fluorescence decay is triexponential only in the presence of BSA. The relaxation times are independent of [Na+] and [BSA]. The preexponential factor of the shortest decay time is negative from a certain [BSA] on, which depends on [Na+]. The data indicate that interactions with intracellular components rather than microviscosity influence the SBFI behavior in cells. A model is suggested in which the fluorescence intensities are mainly determined by the signals from the Na+⊂SBFI and SBFI⊂protein complexes.

Photophysics of a phenyl-pyridinium cation with hindered rotation

The influence of the internal rotation on the photophysical parameters of the pyridinium cation N-methyl-9-phenyl-1,2,3,4,5,6,7,8-octahydroacridinium 9PA was studied by absorption and fluorescence spectroscopy as well as by semiempirical calculations. A weak long wavelength absorption shoulder at 340 nm appears in the red edge of longest wavelength absorption band and is caused by a forbidden transition of charge transfer nature involving the phenyl group as the electron donor. The excitation of 9PA at any point of the absorption spectrum leads to a weak (Φ f = 0.03) fluorescence at 420 nm possessing a relatively large Stokes shift with respect to the weak long wavelength absorption shoulder (Δ ν a-f = 5600 cm −1) and a long fluorescence lifetime (average decay time 5.4 ns). The fluorescence corresponds to a forbidden transition from the twisted S 1 state equilibrium geometry generated from the more twisted S 1-Frank-Codon geometry.

Fluorescence quenching of auramine in fluid solution: a femtosecond spectroscopic study.

The quenching of Auramine fluorescence in ethanol is studied by two ultrafast spectroscopy techniques. The gain band, probed by transient absorption spectroscopy, vanishes in a few picoseconds, while a transient absorption band rises and the ground-state repopulation is delayed. In up-conversion experiments, nonexponential wavelength-dependent fluorescence decays are observed. The average decay times increase with the wavelength and the reconstructed instantaneous spectrum exhibits a few hundred-wavenumber red shift and a broadening while its intensity drops. The previously proposed relaxation model, involving a barrierless internal twisting motion toward a transient dark state, is further examined. In particular, the extinction coefficients of the transient state are extracted from the differential absorption spectra. The band is found to lie in the same wavelength range as the dimethylaniline cation radical. This result is discussed as a possible support for an internal twisting process involving a charge shift.

Molecular radiative transport. III. Experimental intensity decays

A critical experimental test of a previously developed theory of molecular radiative transport is described. It is concluded that the theory gives an accurate description of the effect of radiative transport on fluorescence observables. The numerical coefficients of the fluorescence decay are computed from a Monte Carlo integration procedure that mimics the photon trajectories inside a realistic sample cell, and is carried out only using known molecular and geometrical parameters. The predicted parameters are confronted with the experimental observables accessible in a typical single-photon timing experiment, rhodamine 101 in ethanol being the system studied. The theoretical predictions quantitatively describe the effects of concentration and excitation and emission wavelengths experimentally observed in optical dense nondiffusing media for the two most common geometric arrangements: front-face and right-angle detection. It is shown that radiative transport leads to spatially heterogeneous fluorescence kinetics, as a direct consequence of the existence of a spatial distribution function of electronic excitation inside the sample cell. The agreement between theory and experimental results is good, with the average decay times predicted within ≃3% accuracy for front-face detection.

Changes in cochlear mechanics during vocalization: evidence for a phasic medial efferent effect

The mustached bat, Pteronotus p. parnellii, has a finely tuned cochlea that rings at its resonant frequency in response to an acoustic tone pip. The decay time (DT) and frequency of these damped oscillations can be measured from the cochlear microphonic potential (CM) to study changes in cochlear mechanics. In this report, we describe phasic changes that occur in synchrony with communication sound vocalizations of the bat. Three animals with chronically implanted electrodes were studied. During the experiments, 1–2 ms tone pips were emitted from a speaker every 200 ms. This triggered a computer analysis of the resulting CM to determine the DT and cochlear resonance frequency (CRF) of the ringing. The time relative to vocalizations was determined by monitoring the output of a microphone placed near a bat's mouth. Similar results were obtained from all three bats tested. In a representative case, the average DT was 2.33±0.25 ms while the bat was quiet, but it decreased by 46% to 1.26±0.75 during vocalizations, which indicates a greater damping of the cochlear partition. Sometimes, DT started decreasing immediately before the bat vocalized. After the end of a vocalization, the return to baseline values varied from rapid (milliseconds) to gradual (1–2 seconds). The CRF also changed from baseline values during vocalization, although the amount and direction of change were not predictable. When gentamicin was administered to block the action of medial olivocochlear (MOC) efferents, DT reduction was still evident during vocalization but less pronounced. We conclude that phasic changes in damping occur in synchrony with vocalization, and that the MOC system plays a role in causing suppression. Since suppression can begin prior to vocalization, this may be a synkinetic effect, mediated by neural outflow to the ear in synchrony with neural outflow to the middle ear muscles and the muscles used for vocalization.

Ultrafast Twisting Dynamics of Photoexcited Auramine in Solution

Subpicosecond fluorescence up-conversion and transient absorption spectroscopy is applied to study the excited-state dynamics of auramine, a diphenylmethane dye, in liquid solutions. The fluorescence decays, on a time scale of a few picoseconds to a few tens of picoseconds, are found to be nonexponential and solvent viscosity dependent. They can be fitted as a sum of two exponentials in ethanol and three exponentials in decanol with a larger average lifetime in the more viscous solvent. The decays exhibit wavelength-dependent time constants, whereas the fluorescence rise time is instrument limited (150 fs) at all wavelengths. The average decay time increases with the wavelength across the steady-state emission spectrum. The spectral reconstruction indicates a few hundred wavenumbers dynamic Stokes shift accompanied by a drop in the intensity in both solvents. From transient absorption experiments, the fluorescent state population is shown to decay to an intermediate dark state and then to the ground state, with a viscosity-dependent rate. A barrierless or quasi-barrierless photoreaction involving the rotational diffusion of the phenyl rings, with a change in the radiative transition rate along the reaction path, is proposed to explain the wavelength-dependent nonexponential fluorescence decays. Both fluorescence and transient absorption data are discussed in support of an adiabatic photoreaction involving internal twisting and charge shift.

Recombination dynamics of free and localized excitons inGaN/Ga0.93Al0.07Nquantum wells

Time-resolved photoluminescence studies have been performed on ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{G}\mathrm{a}}_{0.93}{\mathrm{Al}}_{0.07}\mathrm{N}$ multiple quantum wells grown by molecular-beam epitaxy on a sapphire substrate. Radiative recombinations of excitons in both the wells and barriers exhibit average decay times of $\ensuremath{\sim}330\mathrm{ps},$ at $T=8\mathrm{K},$ with a significant spectral distribution of times. This distribution is interpreted in terms of localization of carriers by potential fluctuations due to alloy disorder and to well width and depth variations. The temperature dependence of the radiative lifetime ${\ensuremath{\tau}}_{r}$ of excitons in the wells is deduced from the measurement of both the photoluminescence decay time and intensity. We find that ${\ensuremath{\tau}}_{r}$ increases linearly with temperature dependence $\ensuremath{\partial}{\ensuremath{\tau}}_{r}/\ensuremath{\partial}T=20.5\ifmmode\pm\else\textpm\fi{}0.7{\mathrm{ps}\mathrm{}\mathrm{K}}^{\mathrm{\ensuremath{-}}1},$ for $T<60\mathrm{K}.$ From this result, we deduce the radiative lifetime of free excitons in the low-temperature limit (related to the oscillator strength) to be of 2.4 ps, i.e., half one order of magnitude smaller than for GaAs/Ga-Al-As quantum wells.

A nanosecond fluorescence study of the simultaneous influx of Ca 2+ and Cd 2+ into liposomes

Nanosecond fluorescence decay characteristics of the calcium-binding probe Quin2 and two of its cation complexes were examined by time-resolved fluorescence spectroscopy. Binding of Ca 2+ and Cd 2+ resulted in fluorescence lifetime enhancements as compared to that of free Quin2 (〈 τ〉=0.9 ns). The Quin2–Ca 2+ complex displays a monoexponential decay of τ=7.4 ns, while the cadmium complex gives an average decay time of ca. 4 ns. Lifetime measurements made on heterogeneous cationic solutions demonstrate that decay times for individual complexes can be retrieved. Time-resolved measurements were used to monitor the kinetics of ionomycin-mediated calcium and cadmium transport across artificial membranes. Fluorescence decays, collected on the time-scale of seconds, were sufficient to measure individual ion fluxes or those of mixtures into liposomes. The combination of steady-state and time-resolved fluorescence techniques offers the unique advantage of simultaneously detecting other cations in the presence of calcium.

ITAL]ROSAT[/ITAL] Observations of a New X-Ray Transient in the Small Magellanic Cloud

We describe ROSAT observations of a new X-ray transient and its probable optical companion in the Small Magellanic Cloud. The transient, designated RX J0117.6-7330, appeared at a position approximately 5 min southeast of the X-ray pulsar, SMC X-1, in 1992 October 1-2 PSPC observations centered on the pulsar. It was detected again in a similar observation 246 days later at a counting rate diminished by a factor of 270, which corresponds to an average epsilon-folding decay time of 44 days. No periodic pulsations have been detected. The average 1992 flux level would be produced by a source radiating isotropically with a luminosity of 1.6 X 10(exp 37)(D/50 kpc)(exp 2) ergs/s in the energy range 0.2-2.5 keV. The 5 sec radius positional error circle includes the probable optical counterpart in the form of a magnitude V = 14.2 star with the spectral characteristics of type Be.

Optical Properties of Ingan/GaN Multi Quantum Well Structures

AbstractA set of GaN/InGaN multiple quantum wells (QWs) with well thickness 30 Å and barrier thickness 60 Å were grown by MOCVD on sapphire substrates. The n-type Si doping of the InGaN QWs was varied, in order to produce a different electron concentration in the QWs for the different samples. Optical spectra were obtained by time resolved photoluminescence spectroscopy. The data show weak excitonic spectra from the QWs as well as a broad deeper emission with a much stronger intensity. The spectral shape becomes narrower and the energy position shifts to higher energies with increasing doping. The two different emissions are not easily separated in CW or time integrated spectra, but are clearly observed in a time resolved spectral measurement due to their different recombination rates. The deeper emission has a long and non-exponential decay, with an average decay time in the order of several hundred nanoseconds. The higher energy exciton emission has a much faster decay of about 1 ns. The lower energy band is tentatively explained as due to separately localized electron-hole (e-h) pairs in the QW.

Picosecond fluorescence decay studies on water-stressed pea leaves: energy transfer and quenching processes in photosystem 2

Detached leaves of pea (Pisum sativum) were submitted to water stress at different relative air humidities. The photosynthetic activity of photosystem 2 (PS2) was monitored by time-resolved picosecond chlorophyll (Chl) fluorescence spectroscopy. In the first days the well-known fast Chl fluorescence decay was observed which indicated high PS2 activity. After a few days the average fluorescence decay time τm reached a maximum, depending on the wilting conditions, but always at a relative loss of leaf mass of 80%. After this maximum, τm decreased within a few hours, the fluorescence decay became similar to that one of an intact leaf, but an additional fluorescence decay component with a lifetime of 3.6 ns appeared. At first the primary quinone QA was reduced due to inhibition of the electron transfer to the secondary quinone QB. Simultaneously, water deficiency caused an electron lack at the oxidizing site of PS2. This disabled the primary electron donor of PS2, tyrosine Z, from reducing the oxidized reaction centre of PS2 (P680+). Thus a recombination of P680+-pheophytin-QA- took place, and the energy was lost as heat. With further water stress, QA was decoupled from PS2. The new fluorescence decay component could therefore be assigned to energetically decoupled antenna complexes.

Investigations on the electron transfer processes in Photosystem II: New insights by chlorophyll fluorescence decay measurements with additional saturating light pulses

Electron transfer processes in leaves were investigated by chlorophyll fluorescence decay measurements. A fast chlorophyll fluorescence decay was observed in the intact state, reflecting normal electron transfer in Photosystem II. After treatment with DCMU a slow chlorophyll fluorescence decay was measured due to blocked electron transfer after the primary quinone QA. Additional saturating light pulses, one between each two measuring pulses, were used to completely reduce QA of the intact leaf: the chlorophyll fluorescence decay became similar to that of a DCMU treated leaf. A decreased electron donation rate to the reaction centre P680 was obtained after treatment with hydroxylamine. The intensity of the additional saturating light pulses was not sufficient to reduce all QA under this condition and only a small increase of the average chlorophyll fluorescence decay time occurred. Following our previous paper [Berg et al. (1997) Photosynthetica 34, in press], we investigated the effects of water stress with the additional saturating light pulses. An almost complete reduction of QA was possible after water stress started. A small, but systematic shortening of the slow chlorophyll fluorescence decay followed, up to a relative loss of leaf mass of 80%. At this time a rapid shortening of the chlorophyll fluorescence decay occurred, caused by an electron deficiency at the donor site of PS II. Additional saturating light pulses had no effects on the chlorophyll fluorescence decay any more, revealing a radiationless recombination between the reduced primary quinone Q\(_A^ - \) and the oxidized reaction centre P680+.

Measurement of ion parameters during a forced switching off of current in vacuum

This paper presents the results of ion parameters measured during a forced current commutation of a diffuse vacuum arc, aimed at evaluating the role of ions in discharges occurring rarely after the counterpulse commutation process, in a contact arrangement corresponding to a low-voltage vacuum switch. A sinusoidal half-wave current was interrupted near its amplitude of 400 A with a counterpulse of rise velocities 2.4 and 32 A//spl mu/s. In this way, a forced switching off of dc current was modeled. A retarding field analyzer was used for ion parameter measurements at the forced commutation process. Ion potentials up to 100 V were measured at the current zero moment, and the average decay times of ions after current zero were in the range from a few microseconds up to 30 /spl mu/s as a function of observed ion potential. The measured values of decay times show the rather negligible role of ions in the discharge process after the counterpulse commutation in the examined contact arrangement. The rise time of the countercurrent is proposed as a factor in the successful forced current commutation under these conditions, and values up to a few amperes per microsecond of countercurrent rise velocity can be considered as the limit value.

Time-Resolved Photoluminescence of GaN / Ga0.93Al0 .07N Quantum Wells

AbstractThe recombination dynamics of excitons in GaN / Ga0.93Al0.07N multiple quantum wells is studied versus lattice temperature. The average decay time of photoluminescence measured at 8K is of ∼330 ps, with a substantial variation of times within the emission line. This is interpreted in terms of carrier localization due to alloy disorder and to well width and depth variations. The radiative lifetime τr of excitons in the wells is found to increase linearly with temperature, with ∂τr / ∂T = 20.5 ± 0.7 ps.K−1. The radiative lifetime of free excitons in the low-temperature limit is deduced to be 2.4 ps, consistent with a longitudinal-transverse splitting ћωLT in GaN of 0.6 meV, in excellent agreement with recent estimations. The ratio of the lifetimes of localized and free excitons is found coherent with the picture of electrons and holes independently localized on short-range defects, instead of excitons localized as a whole on long-range potential fluctuations.

Photophysical Studies of Ultrathin Films: Characterization of PS and PMMA on Fused Quartz by Fluorescence Spectroscopy of Pyrene and (4-(1-Pyrenyl)butyl)trimethylammonium Bromide

Ultrathin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) coated on fused quartz plates (or slides) by dip coating are examined by using fluorescence spectroscopy. Cations of the pyrene derivative (4-(1-pyrenyl)butyl)trimethylammonium bromide (PBN) are adsorbed onto the quartz slides from aqueous solutions. The rate of dynamic quenching and prompt quenchingby O 2 of the PBN fluorescence on the slide in the presence of PMMA and PS films at varying degrees of coverage indicates that, at low coverage, PS films are patchy While PMMA films are continuous. These results are in agreement with published data from AFM studies of similar systems. The fluorescence of pyrene dissolved in ultrathin films on quartz and on hydrophobic (surface derivatized) quartz slides is also examined. As revealed by the trend in pyrene III/I and average fluorescence decay time with coverage, a fraction of the pyrene molecules in PS films diffuse to the polar quartz surface, where they become adsorbed. PMMA, on the other hand, blocks access of pyrene to adsorption sites on the quartz surface presumably by an interaction between carbonyl groups and quartz surface hydroxyls. This fluorescence technique thus provides additional information to AFM in the penetration of reactive species, such as O 2 , to the silica-polymer interface and in the polymer-mediated interaction of molecular dopants, such as pyrene, with the silica surface.

Electronic deactivation in single chains, nano-aggregates and ultrathin films of conjugated oligomers

Fluorescence spectra, quantum yields Φ F, decay curves, and fluorescence anisotropies of oligothiophenes and oligophenylenevinylenes with varying chain lengths are determined in the dissolved state, in nano-aggregates and in ultrathin films. Φ F decreases in the series monomer > aggregate ≈ disordered film > oriented film by four powers of ten, whereas the average decay times remain unchanged. The orientational relaxation time is 35 ps in condensed oligothiophenes. The strong increase of Φ F with disorder is mainly attributed to the increasing density of luminescence centers.

Development of glycine- and GABA-gated currents in rat spinal motoneurons.

1. Developmental changes in glycine- and gamma-aminobutyric acid (GABA)-activated currents were studied in spinal motoneurons of embryonic and neonatal rats with the use of whole cell recording techniques. 2. Pressure ejection of glycine or GABA onto motoneuron somata produced Cl(-)-mediated inward currents and membrane depolarizations. During embryonic development, the average amplitude of GABA-gated currents was threefold larger than that of glycine-gated currents, but as a result of a large eightfold postnatal increase in glycine-activated currents, similar currents were produced by both amino acids after birth. 3. At all ages the decay of glycine- and GABA-gated currents best fit one-exponential curve, and their time constants were similar. The average decay time constant decreased by twofold after birth. 4. The ionic specificity of glycine- and GABA-gated channels was studied to determine whether the large amplitude of GABA-activated currents in embryonic motoneurons resulted from the contribution of an outward HCO-3 movement. Manipulations of Cl- and HCO-3 concentrations produced changes in the reversal potentials of glycine and GABA that were similar to the calculated changes in the equilibrium potentials of Cl-. This suggested that glycine- and GABA-gated currents were Cl- specific, and HCO-3 movement did not contribute more to the current generated by GABA than that produced by glycine. 5. Glycine- and GABA-gated currents were associated with severalfold increases in membrane conductance. The conductance increase generated by GABA in embryonic motoneurons was sevenfold larger than that generated by glycine, but similar conductance changes were produced by both amino acids after birth.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage dependence of conductance changes evoked by glycine release in the zebrafish brain.

1. The kinetics and mechanisms underlying the voltage dependence of inhibitory postsynaptic currents (IPSCs) recorded in the Mauthner cell (M cell) were investigated in the isolated medulla of 52-h-old zebrafish larvae, with the use of whole cell and outside-out patch-clamp recordings. 2. Spontaneous miniature IPSCs (mIPSCs) were recorded in the presence of 10(-6) M tetrodotoxin (TTX), 10 mM MgCl2, and 0.1 mM [CaCl2]o. Depolarizing the cell from -50 to +50 mV did not evoke any significant change in the distribution of mIPSC amplitudes, whereas synaptic currents were prolonged at positive voltages. The average decay time constant was increased twofold at +50 mV. 3. The voltage dependence of the kinetics of glycine-activated channels was first investigated during whole cell recording experiments. Currents evoked by voltage steps in the presence of glycine (50 microM) were compared with those obtained without glycine. The increase in chloride conductance (gCl-) evoked by glycine was time and voltage dependent. Inactivation and reactivation of the chloride current were observed during voltage pulses from 0 to -50 mV and from -50 to 0 mV, respectively, and they occurred with similar time constants (2-3 s). During glycine application, voltage-ramp analysis revealed a shift in the reversal potential (ECl-) occurring at all [Cl-]i tested. 4. The basis of the voltage sensitivity of glycine-evoked gCl- was first analyzed by measuring the relative changes in the total open probability (NPo) of glycine-activated channels with voltage.(ABSTRACT TRUNCATED AT 250 WORDS)