Lifetime Τ3 Research Articles

Reaction between NiO and Al2O3 in NiO/γ-Al2O3 catalysts probed by positronium atom

NiO/γ-Al2O3 catalysts with NiO content of 9wt% and 24wt% were prepared by solid state reaction method. They are annealed in air at temperatures from 100°C to 1000°C. Positron lifetime spectra were measured to study the microstructure variation during annealing process. Four positron lifetime components were resolved with two long lifetime τ3 and τ4, which can be attributed to the ortho-positronium lifetime in microvoids and large pores, respectively. It was found that the longest lifetime τ4 is rather sensitive to the chemical environment of the large pores. The NiO active centers in the catalysts cause decrease of both τ4 and its intensity I4, which is due to the spin-conversion of positronium induced by NiO. However, after heating the catalysts above 600°C, abnormal increase of the lifetime τ4 is observed. This is due to the formation of NiAl2O4 spinel from the reaction of NiO and γ-Al2O3. The generated NiAl2O4 weakens the spin-conversion effect of positronium, thus leads to the increase of o-Ps lifetime τ4. Formation of NiAl2O4 is further confirmed by both X-ray diffraction and X-ray photoelectron spectroscopy measurements.

Long-lived electron spins in a modulation doped (100) GaAs quantum well

We have measured T1 spin lifetimes of a 14 nm modulation-doped (100) GaAs quantum well using a time-resolved pump-probe Kerr rotation technique. The quantum well was selected by tuning the wavelength of the probe laser. T1 lifetimes in excess of 1 μs were measured at 1.5 K and 5.5 T, exceeding the typical T2* lifetimes that have been measured in GaAs and II-VI quantum wells by orders of magnitude. We observed effects from nuclear polarization, which were largely removable by simultaneous nuclear magnetic resonance, along with two distinct lifetimes under some conditions that likely result from probing two differently localized subsets of electrons.

A new 3D photoluminescent Cu(I) coordination polymer built from 5-[4-(imidazol-1-yl)phenyl]tetrazolide with 4-connected SrAl2(sra) topology

Abstract The solvothermal reaction of CuCl2·2H2O with 5-[4-(imidazol-1-yl)phenyl]tetrazolide (HL) generates a new three-dimensional (3D) coordination polymer, [Cu2(μ4-L)2]n. In title compound, tetrahedral Cu(I) ions are connected by μ4-L bridges into the 3D framework with a 4-connected SrAl2(sra) topology. The result of thermal analysis indicates that title compound is quite stable to heat. Excitation and luminescence data observed at room temperature show that title compound emits bright green fluorescence and possesses moderate fluorescence lifetimes (τ1 = 3.25 μs, τ2 = 8.22 μs at 556 nm).

Study on defect properties of nanocrystalline TiO2 during phase transition by positron annihilation lifetime

The defect properties of nanocrystalline TiO2 were investigated by positron annihilation lifetime spectroscopy (PALS) and X-ray diffraction (XRD) as a function of annealed temperature that ranged from 300 to 850°C. Below 500°C, the measured positron lifetimes of τ1 (200–206ps) and τ2 (378–402ps) revealed the existence of mono-vacancy and vacancy-clusters at grain surface and in the micro-void of intergranular region. Between 500 and 750°C, the phase transition from anatase to rutile was probed by the variations of positron lifetime and XRD pattern. With the increasing temperature from 500 to 850°C, the positron lifetime τ1, τ2 and its intensity I2 sharply decreased from 200ps, 378ps, and 60% to 135ps, 274ps, and 33%, respectively. The results clearly indicate that the mono-vacancy or vacancy-clusters at grain surface and micro-voids between the grains were annealed out during the phase transition.

Abstracts of the 17th International Symposium on Bioluminescence and Chemiluminescence ‐ (ISBC 2012)

WARNING : The light-emitting molecular structures responsible for the chemiluminescence and fluorescence phenomena are not necessarily the same!

Cation vacancies in perovskites doped with La and Gd

Whereas doping CaTiO3 or SrTiO3 with ∼0·05 formula units (f.u.) of trivalent rare earth ions substituted for Ca can yield majority positron annihilation lifetime τ1 increases of up to 30% due to the presence of cation vacancies, similar doping of BaTiO3 produces increases of only ∼10%. The difference is attributed to hole trapping around cation vacancies in BaTiO3. The absence of significant changes to τ1 for La doped CaTiO3 when oxidised samples are reduced by sintering in a hydrogenous atmosphere is attributed to the formation of oxygen vacancies that co-exist with the original cation vacancies. Solid state nuclear magnetic resonance showed that a significant fraction of the La ions in BaTiO3 containing 0·04 f.u. of La were in cubic coordination, irrespective of the charge compensation scheme employed. Doping of sintered SrTiO3 with La to produce up to 0·2 f.u. of Sr cation vacancies had no significant effect on Sr leaching in deionised water at 90°C.

Thermal stability of grain boundaries in nanocrystalline Zn studied by positron lifetime spectroscopy

Nanocrystalline Zn prepared by compacting nanoparticles with mean grain size about 55nm at 15MPa has been studied by positron lifetime spectroscopy. For the bulk Zn sample, the vacancy defect is annealed out at about 350°C, but for the nanocrystalline Zn sample, the vacancy cluster in grain boundaries is quite difficult to be annealed out even at very high temperature (410°C). In the grain boundaries of nanocrystalline Zn, the small free volume defect (not larger than divacancy) is dominant according to the high relative intensity for the short positron lifetime (τ1). The oxide (ZnO) inside the grain boundaries has been found having an effect to hinder the decrease of average positron lifetime (τav), which probably indicates that the oxide stabilizes the microstructure of the grain boundaries. This stabilization is very important for the nanocrystalline materials using as radiation resistant materials.

Two-photon excitation with pico-second fluorescence lifetime imaging to detect nuclear association of flavanols

Two-photon excitation enabled for the first time the observation and measurement of excited state fluorescence lifetimes from three flavanols in solution, which were ∼1.0ns for catechin and epicatechin, but <45ps for epigallocatechin gallate (EGCG). The shorter lifetime for EGCG is in line with a lower fluorescence quantum yield of 0.003 compared to catechin (0.015) and epicatechin (0.018).In vivo experiments with onion cells demonstrated that tryptophan and quercetin, which tend to be major contributors of background fluorescence in plant cells, have sufficiently low cross sections for two-photon excitation at 630nm and therefore do not interfere with detection of externally added or endogenous flavanols in Allium cepa or Taxus baccata cells. Applying two-photon excitation to flavanols enabled 3-D fluorescence lifetime imaging microscopy and showed that added EGCG penetrated the whole nucleus of onion cells. Interestingly, EGCG and catechin showed different lifetime behaviour when bound to the nucleus: EGCG lifetime increased from <45 to 200ps, whilst catechin lifetime decreased from 1.0ns to 500ps. Semi-quantitative measurements revealed that the relative ratios of EGCG concentrations in nucleoli associated vesicles: nucleus: cytoplasm were ca. 100:10:1.Solution experiments with catechin, epicatechin and histone proteins provided preliminary evidence, via the appearance of a second lifetime (τ2=1.9–3.1ns), that both flavanols may be interacting with histone proteins. We conclude that there is significant nuclear absorption of flavanols. This advanced imaging using two-photon excitation and biophysical techniques described here will prove valuable for probing the intracellular trafficking and functions of flavanols, such as EGCG, which is the major flavanol of green tea.

Mg2+/Ca2+ Induced Changes in Structure, Dynamics and Stability of Dream Protein

Downstream Regulatory Element Antagonist Modulator (DREAM) is a member of the neuronal calcium sensor family that controls activity of potassium voltage channels and regulates c-fos and prodynorphine gene transcription in a Ca2+ dependent manner. Here, we have investigated the impact of Mg2+ and Ca2+ binding on the structure, stability and dynamics of DREAM and DREAM C-terminal domain (DREAM-C). Mg2+ binding to the apoDREAM does not alter the secondary or tertiary structure as based on CD and Trp emission spectra whereas the association of Mg2+ to either EF-3 or EF-4 in apoDREAM-C results in an increase in protein secondary structure and alteration of the tertiary structure. Ca2+ binding to either apo- or Mg2+DREAM and apo- or Mg2+DREAM-C triggers larger conformational changes as evident from the blue-shift in emission spectra and the decrease of Stern-Volmer constant. Ca2+ triggered changes in DREAM conformational dynamics were characterized by time-resolved fluorescence. In apoDREAM, single tryptophan residue exhibits two lifetimes (τ1=3.38 ns, f1=73% and τ2=7.72 ns, f2=27%). Ca2+ binding to apo- and Mg2+DREAM leads to the shortening of the first lifetime and decrease of the fractional contribution (τ1 = 1.75 ns, f1 = 47% and τ2 = 7. ns, f2 = 53%). Furthermore, the impact of Ca2+/Mg2+ on DREAM stability was determined in equilibrium folding studies. The binding of Ca2+ increase the protein stability by ∼ 7 kcal mol−1 whereas the impact of Mg2+ on DREAM stability is significantly smaller, ∼ 1 kcal mol−1. The stability of the C terminal domain in both apo and Ca2+ bound form is significantly smaller compared to the full length protein, suggesting that the inter-domain interactions significantly contribute to the structural and functional properties of DREAM.

Free volume and phase transitions of 1-butyl-3-methylimidazolium based ionic liquids from positron lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) was used to study a series of ionic liquids (ILs) with the 1-butyl-3-methylimidazolium cation ([C4MIM](+)) but different anions [Cl](-), [BF4](-), [PF6](-), [OTf](-), [NTf2](-), and [B(hfip)4](-) with increasing anion volumes. Changes of the ortho-positronium (o-Ps) lifetime parameters with temperature were observed for crystalline and amorphous (glass, supercooled, and normal liquid) states. Evidence for distinct phase transitions, e.g. melting, crystallization and solid-solid transitions, was observed in several PALS experiments. The o-Ps mean lifetime τ3 showed smaller values in the crystalline phase due to dense packing of the material compared to the amorphous phase. The o-Ps lifetime intensity I3 in the liquid state is clearly smaller than in the crystallized state. This behaviour can be attributed to a solvation of e(+) by the anions, which reduces the Ps formation probability in the normal and supercooled liquid. These phenomena were observed for the first time when applying the PALS technique to ionic liquids by us in one preliminary and in this work. Four of the ionic liquids investigated in this work ([BF4](-), [NTf2](-), [PF6](-) and [Cl](-) ILs) exhibit supercooled phases. The specific hole densities and occupied volumes of those ILs were obtained by comparing the local free volume with the specific volume from pressure-volume-temperature (PVT) experiments. From the o-Ps lifetime, the mean size vh of free volume holes of the four samples was calculated and compared with that calculated according to Fürth's hole theory. The hole volumes from both methods agree well. From the Cohen-Turnbull fitting of viscosity and conductivity against PALS/PVT results, the influence of the free volume on molecular transport properties was investigated.

Investigation of positron annihilation in Fe52Co34Hf7B6Cu1 amorphous alloy treated by intermediate frequency magnetic pulse

The effect of intermediate frequency magnetic pulse treatments on Fe52Co34Hf7B6Cu1 amorphous alloy prepared by melt-spun technique is investigated. The microstructure and structural defects of the treated specimens are investigated by TEM, Mössbauer spectroscopy and position annihilation lifetime spectra. The results show that the treated specimens by intermediate frequency magnetic pulse are partially crystallized, the content of the crystallization phase increases with the increase of the magnetic pulse frequency, and the crystalline volume fraction is 33.1% at a frequency of 2000 Hz. For the as-quenched amorphous alloy, the annihilation lifetime τ1 is 150.5 ps in the monovacancy-like free volume, and the relative intensity I1 is 77.7%. The annihilation lifetime τ2 is 349.7 ps in the microvoid, and the relative intensity I2 is 22.3%. With the increase of the magnetic pulse frequency, the values of τ1 and τ2 of the treated specimens decrease, I1 increases, I2 and τ decrease compared with the as-quenched amorphous alloy.

Electron spin coherence exceeding seconds in high-purity silicon

Silicon is one of the most promising semiconductor materials for spin-based information processing devices. Its advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of a (28)Si form with no magnetic nuclei overcomes a primary source of spin decoherence in many other materials. Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than that achieved in isolated high-vacuum systems such as trapped ions. Here we examine electron spin coherence of donors in pure (28)Si material (residual (29)Si concentration <50 ppm) with donor densities of 10(14)-10(15) cm(-3). We elucidate three mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime T(2) up to 2 s. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by ~200 nm. A magnetic field gradient suppresses such interactions, producing an extrapolated electron spin T(2) of 10 s at 1.8 K. These coherence lifetimes are without peer in the solid state and comparable to high-vacuum qubits, making electron spins of donors in silicon ideal components of quantum computers, or quantum memories for systems such as superconducting qubits.

Probing the Phase Transition in Nanocrystalline TiO&lt;sub&gt;2&lt;/sub&gt; Powders by Positron Lifetime (PAL) Technique

Positron annihilation techniques (PAT) have recently been successfully employed for the characterization of phase transitions in metals and compounds. In the present study, positron annihilation lifetime (PAL) measurements have been carried out on a nanocrystalline titania (TiO2) in the form of powders that had been heat-treated at temperatures ranging from 300 to 1273K. The PAL spectra were analyzed into two lifetime components. The shorter lifetime τ1 (185-300 ps) is attributed to positron annihilation in vacancies and the longer lifetime τ1 (400-580 ps) to positrons in microviods at interfaces. The rutile phase of TiO2 powders was utilized as a reference in order to compare their behavior with the commercially supplied and widely available anatase phase (Degussa P25). The influence of the heat-treatment upon the nanostructure during the transition of the anatase to rutile phase were also investigated by X-ray diffraction (XRD), TEM and BET surface area methods. Understanding of this effect is expected to enhance our knowledge of the morphology and nanocrystallite size of TiO2 powders and their T-dependence, and hence their physical properties.

Polyoligopeptides functionalized zinc(II)porphyrins: Step towards artificial hemes

Two new zinc(II)porphyrin oligopeptide conjugates (zinc(II)-5,10,15,20-bis[4-(peptide)- phenyl]porphyrin (5) and -tetrakis[3,5-di(peptide)phenyl]porphyrin (9; peptide = -CH2(CO)Gly-Phe-Ala-CNH2) were prepared using the click chemistry with azides and ethynyl-containing precursors. The spectroscopic signature (S0→S1 and transient T1→Tn absorption, excitation and emission spectra) are typical for zinc(II)porphyrin and shows no perturbation upon anchoring the oligopeptides, whereas some small decreases in the photophysical parameters (𝜏F and ΦF), and larger decrease in T1 lifetimes are noted, which are attributable to the known "loose bolt" effect. The structure for 9 in solution was addressed qualitatively using computer modeling and the comparison of the bimolecular fluorescence quenching rate constants between 5 and 9 using C60 as a photooxidative agent. While 5 exhibits a totally accessible zinc(II)porphyrin unit for a C60 approach, 9 shows a slower quenching rate constant meaning some steric hindrance must be present.

Chemical quenching and inhibition of positronium in Cr2O3/Al2O3 catalysts

The pore structure of Cr2O3/Al2O3 catalysts and the surface chemical properties of these pores were characterized by positron lifetime and coincidence Doppler broadening (CDB) measurements. Four lifetime components could be resolved from the positron lifetime spectrum, with two long lifetime components and two short lifetime components. The two long lifetimes τ4 and τ3 are attributed to ortho-positronium (o-Ps) annihilation in large pores and microvoids, respectively. With increasing Cr2O3 content, both τ4 and its intensity I4 show sharp decrease, while τ3 and its intensity I3 keep nearly unchanged. The Doppler broadening S parameters also show sharp decrease with increasing Cr2O3 content. Detailed analysis of the CDB spectrum reveals that the parapositronium (p-Ps) intensity also decreases with increasing Cr2O3 content. This indicates that the change of o-Ps lifetime τ4 is due to the chemical quenching by Cr2O3 but not spin-conversion of positronium. The decrease of o-Ps intensity I4 indicates that Cr2O3 also inhibits positronium formation.

Effect of TiO2 Formation on the Free Volume Properties of Electrospun PMMA Nanohybrids

Positron annihilation lifetime spectroscopy (PALS) has been performed on a series of PMMA nanohybrids containing nanometric TiO2, which were produced by means of different preparation methods, i.e., melt mixing, electrospinning combined with solution mixing, or in-situ sol–gel growth methods, to study the effect of filler content and constituents on the free volume properties. The PMMA nanocomposites containing titania precursor or in-situ-formed TiO2 additives exhibit altered free volume properties compared to adding commercial TiO2 P25 fillers. The orthopositronium (o-Ps) lifetime (τ3) (free volume cavity size) was constant with composition in P25/PMMA nanohybrids due to the absence of interfacial interaction. However, in TiO2 precursor/PMMA composite fibers the free volume cavity size decreased substantially with hydroxyl group concentration and recovered after hydrothermal treatment. Additionally, a strong correlation between the glass transition temperature and the o-Ps lifetime in the nanohybrids wa...

Laser spectroscopy of the (1s22s2p) 3P0–3P1 level splitting in Be-like krypton

Heavy few-electron ions, such as He-, Li- and Be-like ions, are ideal atomic systems to study the effects of correlation, relativity and quantum electrodynamics. Very recently, theoretical and experimental studies of these species achieved a considerable improvement in accuracy. Be-like ions are interesting because their first excited state, i.e. (1s22s2p)3P0, has an almost infinite lifetime (τ0) in the absence of nuclear spin (I), as it can only decay by a two-photon E1M1 transition to the (1s22s2)1S0 ground state. In addition, the energy difference between the 3P0 and the next higher-lying 3P1 state is expected to remain almost completely unaffected by QED effects, and should thus be dominated by the effects of correlation and relativity. Therefore, we want to determine the (1s22s2p) 3P0–3P1 level splitting in Be-like krypton (84Kr32+), which has I=0, by means of laser spectroscopy at the experimental storage ring at GSI. In such an experiment, the energy splitting can be obtained with very good accuracy and can be compared with recent calculations.

Positron annihilation and relaxation dynamics from dielectric spectroscopy and nuclear magnetic resonance: Cis–trans-1,4-poly(butadiene)

We report a joint analysis of positron annihilation lifetime spectroscopy (PALS), dielectric spectroscopy (BDS), and nuclear magnetic resonance (NMR) on cis-trans-1,4-poly(butadiene) (c-t-1,4-PBD). Phenomenological analysis of the orthopositronium lifetime τ(3)-T dependence by linear fitting reveals four characteristic PALS temperatures: T(b1)(G)=0.63T(g)(PALS), T(g)(PALS), T(b1)(L)=1.22T(g)(PALS), and T(b2)(L)=1.52T(g)(PALS). Slight bend effects in the glassy and supercooled liquid states are related to the fast or slow secondary β process, from neutron scattering, respectively, the latter being connected with the trans-isomers. In addition, the first bend effect in the supercooled liquid coincides with a deviation of the slow effective secondary β(eff) relaxation related to the cis-isomers from low-T Arrhenius behavior to non-Arrhenius one and correlates with the onset of the primary α process from BDS. The second plateau effect in the liquid state occurs when τ(3) becomes commensurable with the structural relaxation time τ(α)(T(b2)). It is also approximately related to its crossover from non-Arrhenius to Arrhenius regime in the combined BDS and NMR data. Finally, the combined BDS and NMR structural relaxation data, when analyzed in terms of the two-order parameter (TOP) model, suggest the influence of solidlike domains on both the annihilation behavior and the local and segmental chain mobility in the supercooled liquid. All these findings indicate the influence of the dynamic heterogeneity in both the primary and secondary relaxations due to the cis-trans isomerism in c-t-1,4-PBD and their impact into the PALS response.

Dual Fluorescence and Ultrafast Intramolecular Charge Transfer with 6-N,N-Dialkylaminopurines. A Two-State Model

6-N,N-Dimethyl-9-methyladenine (DMPURM) and 6-N,N-dimethyladenine (DMPURH) show dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state in solvents of different polarity over extended temperature ranges. The fluorescence quantum yields are very small, in particular those of LE. For DMPURM in acetonitrile (MeCN) at 25 °C, for example, Φ'(ICT) = 3.2 × 10(-3) and Φ(LE) = 1.6 × 10(-4). The large value of Φ'(ICT)/Φ(LE) indicates that the forward LE → ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield Φ(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S(0). For DMPURM in MeCN with Φ(ISC) = 0.22, Φ(IC) = 1 - Φ(ISC) - Φ'(ICT) - Φ(LE) = 0.78, whereas in cyclohexane an even larger Φ(IC) of 0.97 is found. The dipole moment gradually increases upon excitation, from 2.5 D (S(0)), via 6 D (LE) to 9 D (ICT) for DMPURM and from 2.3 D (S(0)), via 7 D (LE) to 8 D (ICT) for DMPURH. From the temperature dependence of Φ'(ICT)/Φ(LE), a reaction enthalpy -ΔH of 11 kJ/mol is obtained for DMPURM in n-hexane (ε(25) = 1.88), increasing to 17 kJ/mol in the more polar solvent di-n-butyl ether (ε(25) = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE → ICT reaction (k(a)). The femtosecond excited state absorption spectra at 22 °C undergo an ultrafast decay: 1.0 ps in CHX and 0.63 ps in MeCN for DMPURM, still shorter (0.46 ps) for DMPURH in MeCN. With DMPURM in n-hexane, the LE fluorescence decay time τ(2) increases upon cooling from 2.6 ps at -45 °C to 6.9 ps at -95 °C. The decay involves ICT and IC as the two main pathways: 1/τ(2) ≅ k(a) + k(IC). As a model compound (no ICT) is not available, its lifetime τ(0)(LE) ∼ 1/k(IC) is not known, which prevents a separate determination of k(a). The excited state reactions of DMPURM and DMPURH are treated with a two-state model: S(0) → LE ⇄ ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (<5 × 10(-5)) and dominated by impurities, due to enhanced IC from LE to S(0).

Free volume changes on optical switching in azobenzene‐polymethylmethacrylate blends studied by a pulsed low‐energy positron beam

AbstractPolymers including chromophores, which can be switched by light, have been studied extensively during the last years due to a host of potential applications which arise from the marked changes in physical properties on switching. Even though there is clear evidence that the free volume has a significant influence on the isomerization kinetics, the question of free volume changes on switching was only addressed recently. Using a pulsed low‐energy positron beam the ortho‐positronium lifetime τ3 was taken as a very sensitive free volume probe, and no change in free volume was detected on isomerization in an azobenzene‐polymethylmethacrylate (PMMA) copolymer containing about 8 wt % of the azobenzene moiety. Here, we report for the first time on free volume changes in an azobenzene‐PMMA blend with an azobenzene moiety concentration as high as 40 wt %. Using the same pulsed low‐energy positron beam, small but significant changes of τ3 were observed between the structurally relaxed dark and the UV‐illuminated states suggesting a decrease in free volume of the order of 10%. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011