Exponential Lifetime Research Articles

Electron Irradiation Induced Vacancy Defects Detected by Positron Annihilation in 6H-SiC

This paper presents positron lifetime results which give information on the nature of vacancy defects induced by electron irradiation in bulk nitrogen doped (nD-nA= 2.3x1017 cm-3) Cree 6H-SiC. The electron irradiations have been performed at different energies and with different fluences from 51017 e-cm-2 to 31018 e-cm-2. Positron lifetime have been measured with a 22NaCl source as a function of temperature between 15 and 300 K. The lifetime spectra were analyzed as sums of two exponential lifetime components i weighted by the intensities Ii, convoluted with the resolution function. From the temperature dependence of the lifetime spectra we can infer that several vacancy defects exist in the electron irradiated n-type 6H-SiC. The nature of detected vacancy defects depends on the electron energy.

Synthesis, characterization, and optical properties of a cyano-functionalized 2,3,7,8-tetraaryl-1,6-dioxapyrene

2,7-Di(4-cyanophenyl)-3,8-di(4-methylphenyl)-1,6-dioxapyrene ( CN-diox), a symmetrically substituted 2,3,7,8-tetraaryldioxapyrene, was synthesized in seven steps from 1,5-dihydroxynaphthalene. The synthetic methodology incorporated a base-catalyzed ring closure process followed by dehydration to introduce the first tetraaryl-1,6-dioxapyrene. Crystal structure and electrochemical analysis were performed to directly compare the properties of CN-diox to previously reported dioxapyrene derivatives, specifically 1,6-dioxapyrene ( Diox) and 3,8-diethyl-5,10-dimethyl-1,6-dioxapyrene ( Alkyl-diox). Optical spectroscopy studies were performed to evaluate the potential of the 1,6-dioxapyrenes as fluorescent probes. CN-diox revealed a broad absorption centered near 450 nm ( ɛ = 31,900 M −1 cm −1) in THF with a corresponding fluorescence at 619 nm ( Φ f = 0.011). This was in sharp contrast to both Diox and Alkyl-diox which displayed broad absorption bands near 400 nm ( ɛ ∼ 5000–10,000 M −1 cm −1) in THF with corresponding fluorescence near 500 nm ( Φ f = 0.059 and 0.082 for Diox and Alkyl-diox, respectively). The luminescence of CN-diox was found to be solvatochromic ( λ max = 619–644 nm) with single exponential lifetimes of less than 1.3 ns and an excited state dipole moment of ∼22.81 D. Neither Diox nor Alkyl-diox showed solvatochromic properties.

Similar luminescent behavior in an established rhenacarborane complex, [3,3-(CO) 2-3-NO- closo-3,1,2-ReC 2B 9H 11] and a new complex anion, [3,3,3-(CO) 3-8-I- closo-3,1,2-ReC 2B 9H 10] –

The compounds [3,3-(CO) 2-3-NO- closo-3,1,2-ReC 2B 9H 11] and [NEt 4][3,3,3-(CO) 3-8-I- closo-3,1,2-ReC 2B 9H 10] have been shown to be emissive in MeTHF at 77 K, with λ max in the blue region of the visible spectrum. Emission from [3,3,3-(CO) 3-8-I- closo-3,1,2-ReC 2B 9H 10] –, which has been structurally characterized, is phosphorescent with a single exponential decay lifetime, τ=1.65 ms. The complex [3,3-(CO) 2-3-NO- closo-3,1,2-ReC 2B 9H 11] also emits in the solid state at 298 K and has been shown by diffuse-reflectance UV–vis measurement to have a band gap of 2.66 eV.

Exact Bayesian Prediction of Exponential Lifetime Based on Fixed and Random Sample Sizes

We consider the problem of predictive intervals for future observations from an exponential distribution. We consider the following two cases: (i) fixed sample size (FSS), and (ii) random sample size (RSS). Further, we derive the predictive function for both FSS and RSS in closed forms. Next, the upper and lower 1%, 2.5%, 5% and 10% critical points for the predictive functions are calculated. To show the usefulness of our results, we present some simulation examples. Finally, we apply our results to some real data sets in life testing given in Lawless [16].

Synthesis and biological testing of aminoxyls designed for long-term retention by living cells

Owing to recent advances in electron paramagnetic resonance (EPR) imaging methodologies, it is now potentially possible to track and image, in real time in vivo, cells that had been tagged with aminoxyl spin probes. We had previously reported that living cells can accumulate 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl [1] to high (millimolar) intracellular concentrations through passive incubation with the corresponding acetoxymethyl (AM) ester [2]. In the present study, we show that under physiological conditions aminoxyl [1] is rapidly extruded by cells through an organic anion transport mechanism, resulting in an intracellular exponential lifetime (t(1/e) or tau) of just 9.84 min at 37 degree C. Through successive rational structural modifications, we arrived at (2,2,5,5-tetramethylpyrrolidin-1-oxyl-3-ylmethyl)amine-N,N-diacetic acid [10], which can still be accumulated by cells to high intracellular concentrations, but which, with an intracellular exponential lifetime of tau]= 114 min, is well retained by cells for long periods of time, where one expects 14% retention even after 5 h. These results suggest that it should be feasible to use EPR imaging to perform in vivo tracking of populations of cells that have accumulated high intracellular levels of aminoxyls.

One- and two-sample prediction based on doubly censored exponential data and prior information

On the basis of a doubly censored sample from an exponential lifetime distribution, the problem of predicting the lifetimes of the unfailed items (one-sample prediction), as well as a second independent future sample from the same distribution (two-sample prediction), is addressed in a Bayesian setting. A class of conjugate prior distributions, which includes Jeffreys' prior as a special case, is considered. Explicit expressions for predictive densities and survivals are derived. Assuming squared-error loss, Bayes predictive estimators are obtained in closed forms (in particular, the estimator of the number of failures in a specified future time interval is given analytically). Bayes prediction limits and predictive estimators under absolute-error loss can readily be computed using iterative methods. As applications, the total duration time in a life test and the failure time of a {\it k-out-of-n} system may be predicted. As an illustration, a numerical example is also included.

Matrix isolation spectroscopy of H2O, D2O, and HDO in solid parahydrogen

We present infrared (IR) absorption spectra over the 800–7800 cm−1 region of cryogenic parahydrogen (pH2) solids doped with H2O, D2O and HDO molecules. Analysis of the rovibrational spectra of the isolated H2O, D2O and HDO monomers reveals their existence as very slightly hindered rotors, typically showing only 2–5% reductions in rotational constants relative to the gas phase. The nuclear spin conversion (NSC) of metastable J=1 ortho-H2O (oH2O) and para-D2O (pD2O) molecules follow first order kinetics, with single exponential decay lifetimes at T=2.4K of 1900±100 s, and 860±50 s, respectively. We report without discussion some absorptions of water clusters produced during sample annealing. We report and assign a number of absorptions to oH2–water pairs or ‘complexes’. The main features of the oH2–H2O and oH2–D2O spectra are explained qualitatively by assuming a semi-rigid C2v structure with the oH2 acting as a proton donor to the O atom. Surprisingly, NSC of oH2–water complexes proceeds at very nearly the same rate as for the corresponding water monomer. We report unassigned spectra of larger (oH2)n–water clusters, and the even more surprising observation of the prolonged survival of oH2O and pD2O molecules clustered with several oH2 molecules. We report and assign a number of water dopant-induced IR absorption features of the pH2 host, along with cooperative water–pH2 transitions in which the vibrational excitation of the pH2 solid is accompanied by a pure rotational transition of the water dopant.

Combination of Qualitative and Quantitative Sources of Knowledge for Risk Assessment in the Framework of Possibility Theory*

This paper focuses on a representation of system reliability in the framework of possibility theory. Particularly, given a (probabilistic) quantitative knowledge pertaining to the time to failure of a system (risk function) and some qualitative knowledge about the degree of pessimism and optimism of the information supplied by the quantitative knowledge, one constructs a possibilistic reliability function of the system. The latter models the possibility of the system to survive up to the current time t. The proposal is motivated by the observation that the system may fail even if it is considered unlikely by the (probabilistic) risk function. Besides, research from cognitive science shows that probability values, particularly when a human factor is involved, tend to be overestimated or underestimated depending on the level of confidence the user is facing. This methodology involves implicitly the combination of the two pieces of knowledge into a more refined knowledge expressed in the possibility framework. Rational assumptions are put forward in order to guide the construction of the possibilistic model. Several ramifications of the proposal, particularly considering special exponential lifetime distributions will be investigated. Particularly, when reasoning in average, or looking for typical elements, new results on (possibilistic) mean time to failure (PMTF) will be pointed out. On the other hand the absence of any qualitative knowledge makes the proposal as a counterpart of the so-called probability–possibility transformations investigated in fuzzy/possibility literature. Comparisons with the preceding will be investigated for special cases of exponential lifetime distributions.

Matrix isolation spectroscopy of H 2O, D 2O, and HDO in solid parahydrogen

We present infrared (IR) absorption spectra over the 800–7800 cm −1 region of cryogenic parahydrogen ( pH 2) solids doped with H 2O, D 2O and HDO molecules. Analysis of the rovibrational spectra of the isolated H 2O, D 2O and HDO monomers reveals their existence as very slightly hindered rotors, typically showing only 2–5% reductions in rotational constants relative to the gas phase. The nuclear spin conversion (NSC) of metastable J=1 ortho-H 2O ( oH 2O) and para-D 2O ( pD 2O) molecules follow first order kinetics, with single exponential decay lifetimes at T=2.4 K of 1900±100 s, and 860±50 s, respectively. We report without discussion some absorptions of water clusters produced during sample annealing. We report and assign a number of absorptions to oH 2–water pairs or ‘complexes’. The main features of the oH 2–H 2O and oH 2–D 2O spectra are explained qualitatively by assuming a semi-rigid C 2 v structure with the oH 2 acting as a proton donor to the O atom. Surprisingly, NSC of oH 2–water complexes proceeds at very nearly the same rate as for the corresponding water monomer. We report unassigned spectra of larger ( oH 2) n –water clusters, and the even more surprising observation of the prolonged survival of oH 2O and pD 2O molecules clustered with several oH 2 molecules. We report and assign a number of water dopant-induced IR absorption features of the pH 2 host, along with cooperative water– pH 2 transitions in which the vibrational excitation of the pH 2 solid is accompanied by a pure rotational transition of the water dopant.

Spectral properties of cationic water-soluble metallo-porphyrins immobilized in a perfluorosulfonated ion-exchange membrane

The behavior of palladium(II), platinum(II) and rhodium(III) complexes with two cationic water-soluble porphyrins: meso-tetrakis(4-N-methylpyridyl)porphyrin and meso-tetrakis(4-N,N,N-trimethylaminophenyl)porphyrin in Nafion® membrane and in Nafion® solution have been studied by UV-visible absorption and emission spectroscopy. All metalloporphyrins are localized in the interfacial region of Nafion®. The six-coordinate Rh(III) complexes are monomeric inside Nafion®, but immobilized four-coordinate Pd(II) and Pt(II) complexes form face-to-face dimers with a characteristic blue shifted Soret band. All immobilized complexes exhibit a strong red phosphorescence at room temperature in the absence of oxygen. Phosphorescence decays for all metalloporphyrins were found to be a single exponential but phosphorescence lifetimes noticeably increased in the Nafion® membrane as compared to lifetimes in aqueous solution.

Radiation damping in classical systems: The role of nonintegrability

The interaction between matter and fields is a classical problem. Still there are difficulties. Time symmetry is broken. So the radiation damping belongs to a class of phenomena, which includes transport properties. We propose a radically different approach based on the extension of the dynamics of integrable systems. We consider a simple model, a harmonic oscillator interacting with a field. For integrable systems, it is well known that there exists a unitary transformation U. However, in the radiation damping we have resonances between the action of the particle and the actions of the field. This makes the system an example of Poincar\'e nonintegrable systems. We extend the unitary operator to a new star-unitary operator $\ensuremath{\Lambda}.$ This changes the dynamical description of radiation damping. Once we know the Hamiltonian, we can of course write the Hamilton equations. But we have the possibility to go to new descriptions. The invertible $\ensuremath{\Lambda}$ transformation gives many new aspects, which are hidden in the initial description. For example, we show that there are fluctuations, and that there exists an irreducible probability description. The transformation to $\ensuremath{\Lambda}$ representation corresponds to a transformation to Markovian probability equations. We can always come back to the initial representation by the inverse transformation. We have verified this remarkable prediction by detailed numerical calculations. We need the $\ensuremath{\Lambda}$ transformation to obtain the definition of a dressed unstable mode, which has a well-defined lifetime. In the initial representation there are various time scales and there is no strictly exponential lifetime. The situation is the same as the one we have studied in the quantum case in recent papers.

Dynamics of protein and chromophore structural changes in the photocycle of photoactive yellow protein monitored by time-resolved optical rotatory dispersion.

The dynamics of the PYP photocycle have been studied using time-resolved optical rotatory dispersion (TRORD) spectroscopy in the visible and far-UV spectral regions to probe the changes in the chromophore configuration and the protein secondary structure, respectively. The changes in the secondary structure in PYP upon photoisomerization of the chromophore can be described by two exponential lifetimes of 2 +/- 0.8 and 650 +/- 100 ms that correspond to unfolding and refolding processes, respectively. The TRORD experiments that follow the dynamics of the chromophore report three exponential components, with lifetimes of 10 +/- 3 micros, 1.5 +/- 0.5 ms, and 515 +/- 110 ms. A comparison of the kinetic behaviors of the chromophore and protein shows that during the decay of pR(465) an initial relaxation that is localized to the chromophore hydrophobic pocket precedes the formation of the chromophore and protein structures found in pB(355). In contrast, the protein and chromophore processes occur with similar time constants during inactivation of the signaling state.

Operator semi-group of density evolution equation for a repairable redundant system with two same components

For a repairable redundant system consisting of two same components with exponential lifetime and general repair time distribution, the probability densities of the system in some state at time t were determined by a group of ordinary and partial differential equations, called density evolution equations. It was proved that the time-dependent solution of the density evolution equations uniquely exists and strongly converges to its steady state density solution by a semi-group method. In this proof, it is not necessary to suppose that the repair rate function is bounded. The technique of the proof is valuable for many density evolution equations.

Triplet state energy and electron transfer in coronene-doped polymethylmethacrylate

The phosphorescence yield and decay kinetics of coronene in deoxygenated polymethylmethacrylate (PMMA) have been determined over the temperature range 77–300 K. At temperatures below ∼200 K the decays were close to single exponential (lifetime = 9.3 s at 77 K). However, at higher temperatures the decays were best fitted by the function I(t)=I 0 exp(at+bt n) , where 0.3< n<0.4. This kinetic behaviour is attributed to long range energy transfer via a number of steps following a quasi-one-dimensional random walk from the coronene triplet state to trap sites. Discontinuities were observed in the temperature dependence of the phosphorescence yield of coronene in polymethylmethacrylate at temperatures of 175 and 255 K, corresponding to the onset of rotations within the polymer molecule.

Ultraviolet excitation fine tuning of luminescence bands of oxygen-deficient centers in silica

The photoluminescence (PL) activity of oxygen deficient centers in commercial silica samples has been reconstructed by exciting in the spectral range of their optical absorption band around 5 eV. The fine tuning of the excitation was provided by a tunable pulsed optical parametric oscillator seeded by a Nd:yttrium–aluminum–garnet laser. The experimental results, analyzed in the spectral moment framework, indicate the presence of different centers whose spectral features are modulated by structural disorder typical of the glassy matrix. The photoluminescence excitation spectra at different emission energies evidence the composite structure of the luminescence bands around 3.1 and 4.2 eV, usually ascribed to triplet and singlet transitions of the same oxygen deficient center. Besides the well-known emission bands centered at 3.15 and 4.20 eV, the contribution of a new PL band below 3 eV is reported. Time resolved photoluminescence measurements of the low energy band give an estimate of its spectral characteristics, with peak position around 2.8 eV, full width at half maximum of about 1 eV and exponential lifetime of 840 ns.

Activation of human aortic endothelial cells by LDL from Type 1 diabetic patients: an in vitro study

An altered interaction between circulating LDL and endothelial cells might be at the basis of the increased prevalence of atherosclerosis in diabetes mellitus. The aim of the present work was to investigate the effect of a short incubation period with LDL from Type 1 diabetic patients in good metabolic control on endothelial cells derived from human aorta (HAEC). Cultured HAEC were incubated for 3 h with culture medium alone (control HAEC), with native LDL from healthy subjects (control LDL), or with native LDL from Type 1 diabetic patients (Type 1 LDL). After the incubation the following parameters were evaluated: endothelial cell nitric oxide synthase (NOS) activity, nitric oxide (NO) and peroxynitrite production, Na +/K +-ATPase and Ca 2+-ATPase activities, intracellular Ca 2+ concentration and fluidity of the superficial part of the plasma membrane studied by 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). Moreover, we studied the cellular activation, evaluated by the fluid phase endocytosis of TMA-DPH, and the microetherogeneity of the membrane surface, evaluated by dynamic fluorescence. HAEC incubated with control LDL showed compared with control HAEC: increased anisotropy and exponential lifetime of TMA-DPH, and enhanced TMA-DPH internalization. HAEC incubated with Type 1 LDL showed compared with both control HAEC and HAEC incubated with control LDL: (i) increased Na +/K +-ATPase and Ca 2+-ATPase activities, and intracellular Ca 2+ concentration; (ii) increased NOS activity, NO and peroxynitrite production; (iii) increased anisotropy of TMA-DPH; (iv) enhanced internalization of the probe. The exponential lifetime and the width of distribution of TMA-DPH were significantly increased by Type 1 LDL only in comparison with control HAEC. The results suggest that a short-term interaction with LDL from Type 1 diabetic patients causes alterations of the plasma membrane surface and of cellular functions in endothelial cells in a possibly atherogenic way.

Fluorescence lifetime measurements of disulfonated aluminium phthalocyanine in the presence of microbial cells.

The fluorescence lifetimes of disulfonated aluminium phthalocyanine (AlPcS2) in the presence of several different microbial cells are measured using the technique of time correlated single photon counting (TCSPC) employing front-face illumination. The microbes studied are: Escherichia coli, Porphyromonas gingivalis (Gram-negative bacteria), Streptococcus mutans (a Gram-positive bacterium), and the yeast Candida albicans. Complex fluorescence decays are observed when AlPcS2 is in the presence of these microbes; the fluorescence decay data can be fitted to a distribution of exponential lifetimes indicating that the AlPcS2 molecules experience a range of micro-environments The average fluorescence lifetimes of AlPcS2 in the presence of the microbes studied range from 4.85 to 5.95 ns, indicating differences in the cellular localisation of AlPcS2 with each of the microbes studied. These novel data are presented alongside a brief summary of existing AlPcS2 fluorescence lifetime data measured both in solution and in model biological systems.

Estimation of components reliability in a parallel system using masked system life data

In this paper we estimate the reliability of the individual components that belong to a parallel system using masked-system life test data. In particular, we compute the maximum likelihood and Bayes estimates of component reliabilities. The considered system consists of J independent components having non-identical complementary exponential lifetime distributions. It is assumed for Bayes estimates that the unknown parameters are independent random variables having symmetrical triangle probability density functions. The approaches are illustrated with a two-component parallel system. A numerical simulation study is introduced to show how one can use the present approaches to compute these estimations for a practical problem. Also in the numerical simulation study a comparison between Bayes and maximum likelihood estimates is introduced.

Asymptotically optimal component assembly plans in repairable systems and server allocation in parallel multiserver queues

AbstractT identical exponential lifetime components out of which G are initially functioning (and B are not) are to be allocated to N subsystems, which are connected either in parallel or in series. Subsystem i, i = 1,…, N, functions when at least Ki of its components function and the whole system is maintained by a single repairman. Component repair times are identical independent exponentials and repaired components are as good as new. The problem of the determination of the assembly plan that will maximize the system reliability at any (arbitrary) time instant t is solved when the component failure rate is sufficiently small. For the parallel configuration, the optimal assembly plan allocates as many components as possible to the subsystem with the smallest Ki and allocates functioning components to subsystems in increasing order of the Ki's. For the series configuration, the optimal assembly plan allocates both the surplus and the functioning components equally to all subsystems whenever possible, and when not possible it favors subsystems in decreasing order of the Ki's. The solution is interpreted in the context of the optimal allocation of processors and an initial number of jobs in a problem of routing time consuming jobs to parallel multiprocessor queues. © John Wiley &amp; Sons, Inc. Naval Research Logistics 48: 732–746, 2001

One-electron reduction potentials of dietary carotenoid radical cations in aqueous micellar environments

The one-electron reduction potentials of the radical cations of five dietary carotenoids (β-carotene, canthaxanthin, zeaxanthin, astaxanthin and lycopene) in aqueous micellar environments have been obtained from a pulse radiolysis study of electron transfer between the carotenoids and tryptophan radical cations as a function of pH, and lie in the range of 980–1060 mV. These values are consistent with our observation that the carotenoid radical cations oxidise tyrosine and cysteine. The decays of the carotenoid radical cations in the absence of added reactants suggest a distribution of exponential lifetimes. The radicals persist for up to about 1 s, depending on the medium.