Three photon annihilations of positrons and positronium in solids with two detectors in coincidence

The two-detector coincidence positron annihilation system at Washington State University was used to observe three-photon annihilation events of positrons. The decay of ortho-positronium (o-Ps) can be distinguished for para-Ps decay and positron two photon annihilations by the unique difference energy distribution. The apparatus was used to detect rare three-photon annihilations of positrons in metals.     

Near-Edge Soft X-ray Absorption Mass Spectrometry of Protonated Melittin

We have investigated the photoionization and photofragmentation yields of gas-phase multiply protonated melittin cations for photon energies at the K-shell absorption edges of carbon, nitrogen, and oxygen. Two similar experimental approaches were employed. In both experiments, mass selected [melittin+qH]^q+ (q=2–4) ions were accumulated in radiofrequency ion traps. The trap content was exposed to intense beams of monochromatic soft X-ray photons from synchrotron beamlines and photoproducts were analyzed by means of time-of-flight mass spectrometry. Mass spectra were recorded for fixed photon energies, and partial ion yield spectra were recorded as a function of photon energy. The combination of mass spectrometry and soft X-ray spectroscopy allows for a direct correlation of protein electronic structure with various photoionization channels. Non-dissociative single and double ionization are used as a reference. The contribution of both channels to various backbone scission channels is quantified and related to activation energies and protonation sites. Soft X-ray absorption mass spectrometry combines fast energy deposition with single and double ionization and could complement established activation techniques.

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On the use of5 5Co in nuclear medicine —Its disadvantage and possible substitute

The use of^{5 5}Co for positron emission tomography (PET) is criticized on the grounds that its 477 keV -rays would interfere with the positron annihilation measurements. This interference will lead to distorded pictures, lower resolution and lower tumor-non tumor ratios for^{5 5}Co-bleomycin. It is suggested that^{5 7}Co-bleomycin used in single photon imaging is better replaced by^{5 7}Ni-bleomycin rather than by^{5 5}Co-bleomycin for PET studies.     

Retinal phosphenes and discrete dark noises in rods: A new biophysical framework

Spontaneous rhodopsin activation produces discrete noises indistinguishable from single-photon responses. However, there is a serious discrepancy between the apparent energy barrier of thermal events compared with that of the photon-driven process. Current estimates of the activation energies of discrete dark noises in vertebrate rod and cone pigments are 40–50 kcal/mol for activation by photon and 20–25 kcal/mol for activation by heat. To reconcile this discrepancy, it was assumed that thermal activation and photon activation of rhodopsin follow different molecular mechanisms. The most convincing hypothesis for a separate low-energy thermal pathway is that the discrete dark noises of rods arise in a small subpopulation of rhodopsins, where the Schiff base linking the chromophore to the protein part has been deprotonated.According to Narici et al.’ experiments (2009, Radiation Measurements), phosphene perception in space travel is due to the ionizing radiation-induced free radicals that generate chemiluminescent photons from lipid peroxidation. These photons are absorbed by the photoreceptors chromophores, which modify the rhodopsin molecules (bleaching) and start the photo-transduction cascade resulting in the perception of phosphenes.Here, we point out that not only retinal phosphenes but also the discrete dark noise of rods can be due to the natural redox related (free radical) bioluminescent photons in the retina. In other words, under regulated conditions, lipid peroxidation is a natural process in cells and also in retinal membranes. Since the natural lipid peroxidation is one of the main sources of bioluminescent photons and the photoreceptors have the highest oxygen demand and polyunsaturated fatty acid (PUFA) concentration, there is a continuous, low level bioluminescent photon emission in the retina without any external photonic stimulation. During photopic or scotopic vision, evanescent bioluminescent photon emission is negligible. In contrast, in dark-adapted retinal cells this evanescent bioluminescent photon emission is not negligible. Therefore, our hypothesis is that the discrete dark noise of rods can be due to these bioluminescent photons.     

Complete valence double photoionization of SF6

Single photon double ionization of SF(6) has been investigated at the photon energies 38.71, 40.814, and 48.372 eV by using a recently developed time-of-flight photoelectron-photoelectron coincidence spectroscopy technique which gives complete two-dimensional e(-)-e(-) spectra. The first complete single photon double ionization electron spectrum of SF(6) up to a binding energy of approximately 48 eV is presented and accurately interpreted with the aid of Green's function ADC(2) calculations. Spectra which reflect either mainly direct or mainly indirect (via interatomic coulombic decay of F 2s holes) double ionization of SF(6) are extracted from the coincidence map and discussed. A previous, very low value for the onset of double ionization of SF(6) is found to energetically coincide with a peak structure related to secondary inelastic scattering events.     

Determination of carbon impurity in gallium arsenide crystals by photon activation analysis

Determination of carbon impurity in GaAs crystals has been investigated by photon activation analysis using the¹²C(, n)¹¹C reaction. Chemical isolation of the radiocarbon as CO₂ was carried out by the combustion method using Pb₃O₄ as a fusing agent and/or oxidizing accelerator in a high flow rate of oxygen stream. The CO₂ was then collected by passing through a double trap containing NaOH solution, and precipitated finally as BaCO₃. By measuring positron annihilation gamma-rays due to the isolated radiocarbon, it was proved that determination of the above carbon impurity can be achieved easily and favourably. On the basis of the results obtained, a conversion factor for the LVM absorption of carbon at room temperature in Fourier-transform infrared spectroscopy was found to be 1.4·10¹⁶ atoms · cm^–1.     

Optical and microstructural studies on electron irradiated PMMA: A positron annihilation study

The effect of electron irradiation on the free volume related microstructural and optical properties of Poly(methyl methacrylate) have been studied using Positron Annihilation and other techniques. The FTIR spectral study on the irradiated films suggests the existence of CC group and is understood by invoking the carbonaceous clusters as a consequence of chain scission in PMMA. Using UV-Visible absorption spectra the optical parameters like optical energy bandgap and activation energy were determined and the variation of these parameters suggests the existence of defects within the irradiated sample. Following Robertson’s theory, the carbonaceous cluster size is estimated and it increases with increase in electron dose. The XRD study indicates the enhancement of amorphous nature of the film due to chain scission by irradiation. The Positron annihilation result shows that electron irradiation affects the free volume related microstructure and the carbonaceous clusters may act as positron scattering centers.     

Pressure dependence of positron trapping rate,lifetime and defect formation in metals and alloys

In order to determine the pressure dependence of positron annihilation parameters like bulk lifetime and positron trapping rate, positron lifetime and Doppler broadening experiments have been carried out. In aluminium a slight decrease in the bulk lifetime of 3 ps at 530 MPa has been found. Vacancies have been quenched in Fe25at%Al and Fe40at%Al to study the effect of high hydrostatic pressure on the positron trapping rate of vacancies. We like to emphasize that there is no significant pressure dependence of the trapping rate up to 570 MPa shown by these alloys. Measurements of positron annihilation at various temperatures at constant pressure give information about the formation of thermal defects and their enthalpy in metals and alloys. Additionally, the formation volume of the defects can be determined from measurements varying the pressure at constant temperature. The defect formation volumeV _v ^f has been measured in pure Al and in intermetallic phases of the FeAl system at different temperatures. In the ordered B2 structure a effective defect formation volume of 1.42 was observed, in A2 of 0.88 , in DO₃of 0.96 and in pure Al of 0.67 , respectively (=mean atomic volume). While the latter values reflects single vacancy defects, the large formation volume in B2 points to the prevalence of double vacancies and/or, more likely at low temperatures, triple defects.     

An experimental and theoretical study of double photoionization of CF4 using time-of-flight photoelectron-photoelectron (photoion-photoion) coincidence spectroscopy

Single photon double ionization of CF4 has been studied by means of a time-of-flight photoelectron-photoelectron coincidence technique, which has very recently been extended towards ion detection, with energy analysis for the electrons and mass analysis for the ions. The complete single photon double ionization electron spectrum of CF4 up to a binding energy of approximately 51 eV is presented and discussed, also with the aid of accurate ab initio Green's function calculations. From ion detection in coincidence with the ejected electrons, we derive fragmentation pathway-selected double ionization electron spectra of CF4. From the same data we extract the yield of each doubly charged ion or ion pair as a function of the double ionization energy.     

Theoretical studies on the S-N interactions in sulfoximine

The potential energy surface of sulfoximines has been searched using ab initio MO and Density Functional Calculations. The electronic structures of the isomers of sulfoximine have been studied using HF/6-31+G*, MP2(full)/6-31+G* and B3LYP/6-31+G* levels. Final energies of these molecules have been calculated at the high accuracy G2 and CBS-Q levels. Though a formal SN double bond is generally considered between sulfur and nitrogen in these systems, theoretical studies do not show any π interaction between them. S-N rotational barriers, bond dissociation energies, atomic charge analysis, and NBO analysis all indicate only a single bond across S-N with a very strong ionic interaction.     

Sensitization-initiated electron transfer via upconversion: mechanism and photocatalytic applications

Sensitization-initiated electron transfer (SenI-ET) describes a recently discovered photoredox strategy that relies on two consecutive light absorption events, triggering a sequence of energy and electron transfer steps. The cumulative energy input from two visible photons gives access to thermodynamically demanding reactions, which would be unattainable by single excitation with visible light. For this reason, SenI-ET has become a very useful strategy in synthetic photochemistry, but the mechanism has been difficult to clarify due to its complexity. We demonstrate that SenI-ET can operate via sensitized triplet–triplet annihilation upconversion, and we provide the first direct spectroscopic evidence for the catalytically active species. In our system comprised of fac-[Ir(ppy)₃] as a light absorber, 2,7-di-tert-butylpyrene as an annihilator, and N,N-dimethylaniline as a sacrificial reductant, all photochemical reaction steps proceed with remarkable rates and efficiencies, and this system is furthermore suitable for photocatalytic aryl dehalogenations, pinacol couplings and detosylation reactions. The insights presented here are relevant for the further rational development of photoredox processes based on multi-photon excitation, and they could have important implications in the greater contexts of synthetic photochemistry and solar energy conversion.

A full picture of a new multi-photon excitation mechanism relying on sTTA upconversion is provided, together with selected photocatalytic applications. All mechanistic steps are investigated and the catalytically active species is observed directly.     

Semiclassical model for the absorption or emission of a photon by a charged particle in an intense static electric field

Quantum simulations made using Floquet methods show that a charged particle can exchange energy with an oscillating potential barrier in discrete quanta , where is the frequency of oscillation. However, this exchange is classically forbidden because no other mass is included in the model, so that energy and momentum could not both be conserved in the absorption or emission of a photon. We define a semiclassical mechanism for these inelastic processes in which a photon may be absorbed by a charged particle moving against an intense static electric field, or emitted when the particle moves with this field. In this model, the particle has an energy loss Q in photon absorption, and an energy gain Q in photon emission. Then the particle travels a short distance at constant momentum until the energy increment Q is made up by the interaction with the static electric field, after which the particle resumes classical motion with the initial energy plus or minus exactly one quantum. We use the energy–time uncertainty relation to determine the minimum value for the static electric field that is required for this process, and this value is typical of the experimental conditions for laser-assisted scanning tunneling microscopy and laser-assisted field emission where the exchange of quanta is found to occur.     

Raman bandshape analysis and DFT study of CO and CH stretching modes of NN-Dibutyl formamide in DMSO solvent`

The study reports the inter-molecular interactions and dynamics of CO and CH stretching modes of NN- Dibutyl Formamide (DBF) in DMSO solvent using Raman spectroscopic technique. The Raman band of CO and CH stretching modes have been deconvoluted into two distinct bands for neat as well as in DMSO solvent. Peak wavenumbers of CO stretching modes show red shift while CH stretching modes shows blue shift with the increase in solvent concentrations. The optimized geometric parameters, vibrational wavenumbers, Mulliken atomic charges and natural bond orbitals of the molecule has been computed using Density Functional Theory (DFT) method with basis set 6-31 +G (d, p). In addition, the same basis set has been carried out with counterpoise keyword accounting BSSE calculation on monomer and dimer states with DMSO solvent to present an appropriate interacting environment. IEF-PCM solvation model has also been computed using the same basis set and compares the geometrical parameters and vibrational wavenumbers of the molecules and in their complexes. In order to get a complete study of the DBF + DMSO complexes, explicit solvation model has also been calculated for Monomer DBF in two solvent molecules. Theoretical calculations of frequencies have been compared with the experimental findings and the results are found in good agreement.     

Dielectric and Ferroelectric Properties of Sol-Gel Derived YMnO3 Films

There has been considerable interest in ferroelectric (FE) films especially for non-volatile memories and ultra high density DRAM applications. Such FE films typically consist of lead zirconate titanate (PZT) with novel oxide contacts, or layered perovskite such as Sr2Bi2TaO9. Recently, there have been reports of sputtered YMnO3 films which exhibit a single polarization axis and do not contain any volatile species of Pb or Bi. Single crystal YMnO3 exhibits satisfactory polarization (6 C/cm2) and low coercive field (<20 kV/cm). Additionally, the dielectric constant of YMnO3 is quite low (<30) which should facilitate ferroelectric switching. In this study, sol-gel derived YMnO3 films were prepared on platinized Si wafers and their dielectric and ferroelectric properties were characterized. Their electrical properties will be discussed with respect to Y/Mn stoichiometry ratio, hexagonal phase development and processing conditions. The potential of YMnO3 as a material in non-volatile memories is evaluated.     

DFT study on the unsaturated germylenoid H2CGeNaF

The unsaturated germylenoid H₂CGeNaF was studied by using the DFT B3LYP method in conjunction with the 6-311+G(d, p) basis set. Geometry optimization calculations indicate that H₂CGeNaF has three equilibrium configurations, in which the p-complex is the lowest in energy and is the most stable structure. Two transition states for isomerization reactions of H₂CGeNaF are located and the energy barriers are calculated. For the most stable one, vibrational frequencies and infrared intensities have been predicted.     

Single Base Pair Mismatch Detection Using Cationic Conjugated Polymers through Fluorescence Resonance Energy Transfer

A simple method using water‐soluble conjugated polymers and a DNA intercalator has been proposed for single base pair mismatch detection with enhanced detection efficiency. Fluorescence resonance energy transfer (FRET) was used as an indicator for unwinding of dsDNA due to base pair mismatch at elevated temperatures. The optical amplification effect of the CPs helps to achieve enhanced detection efficiency.

     

Stability of functionalized C60 paramagnetic dimers and monomers

Density functional theory is used to calculate the bond dissociation energy to cleave the C₆₀C₆₀ bond of the paramagnetic X-C₆₀C₆₀-X and X-C₆₀C₆₀ dimers where X is F, OH, O and H. The results show that these dimers would not be stable much above room temperature and therefore cannot constitute the paramagnetic phase needed to form the observed ferromagnetism which has been shown to be stable up to 800 K. The calculated bond dissociation energies to remove an F, OH or H from a single C₆₀ are large suggesting that they could be the source of the unpaired spin needed for the high temperature ferromagnetism.     

Long-living positron and positronium states in zeolites and microcrystalline oxides

Positron annihilation (PA) investigations were performed on zeolites (X, Y and ZSM-5) and on microcrystalline MgO, Al₂O₃ and SiO₂, providing long lifetime components attributed to o-Ps atoms. In addition to the positron lifetime (LT) measurement, the energy distribution (ED) of the annihilation gamma radiation spectrum was recorded in the 30 keV–1.5 MeV range for different samples and was compared to reference distributions for Si and GaAs samples, where no long-living Ps states are formed. Apart from the strong correlation with the water content in the samples, the positron data collected testify much more pronounced positronium hosting features for powders of the mentioned oxides than for zeolites. Positron LT spectroscopy combined with recording of the ED of the annihilation radiation provides reliable information on the formation of long living 3 states.     

Positron spectroscopy of rehydrated pyrogeneous silica surface

Positron annihilation method has been used for investigating amorphous nonporous pyrogeneous silica of high purity. The influence of water adsorption on the structure of the silanol surface is shown. The effect of swelling, that is an increase of interglobular free volume at intermediate degrees of silica hydration has been observed.