Magnetic quenching of positronium in organic solutions

The magnetic quenching of ortho-positronium (o-Ps) in some pure nonpolar liquids (n-hexane, cyclohexane and benzene) and solutions (nitrobenzene in n-hexane, cyclohexane and benzene; and carbon tetrachloride and biphenyl in n-hexane) is examined for steady magnetic fields up to 14 kG by the positron annihilation lifetime technique. The long lifetime of o-Ps is very sensitive to the strength of the external magnetic field, decreasing as the field strength increases. This effect follows from a well-known principle of atomic physics, the quadratic Zeeman effect. With one exception, all the liquids studied here appear to be normal in this regard. The exception is nitrobenzene in n-hexane, which shows a very marked enhancement of quenching beyond the Zeeman effect at low fields.     

若干有机合成反应的构象效应

通过分析竞争反应的过渡态在构象上能量的高低,探讨构象效应(立体电子效应和位阻效应)对一些有机反应的立体性的影响;而立体电子效应和位阻效应与反应机制,反应物的结构-构象和反应条件等都有关系,最终能影响反应产物的立体结构。阐明其中的缘由可有助于正确预测产物的结构及选择可行的合成线路。     

The chemistry of positronium. IV. Quests on the inhibition of positronium in aqueous solutions

The inhibition of positronium in aqueous solutions of Cl^? and NO₃^? alone or mixed is discussed. An attempt is made to fit the experimental results using test functions, in terms of the spur model and of the kinetic theory of hot atoms.     

Low-temperature formation of positronium in nonpolymer organic systems

The phenomenon of the low-temperature formation of positronium (bound electron-positron system) observed during positron irradiation of a test substance is discussed. The effect is additional relative to the formation of positronium under normal conditions and, like the principal phenomenon, is radiation-chemical in nature. This effect has been discovered and studied mainly for polymers, in which the role of weakly bound electrons and positron localization (trapping) in the intensity of manifestation of this phenomenon was revealed. This review focuses on studies concerning barely investigated nonpolymer (polar and nonpolar) organic systems, in which a relatively high mobility of molecules as compared to polymers can open an opportunity for a more pictorial manifestation of positron and electron trapping by polar centers, both belonging to separate molecules and resulting from collective orientation of the molecules.     

Radiation effect on positronium formation in low-temperature polyethylene

The irradiation effect of γ-rays on polyethylene (PE) has been studied by positron annihilation lifetime spectroscopy (PALS). In the case of non-irradiated PE samples, at a low temperature below the glass-transition temperature, the intensity of the long-lived component of positronium, I₃, increases due to an increase in the concentration of trapped electrons. However, the increase in I₃ obtained in a few MGy γ-irradiated samples became very small due to the effect of induced radicals. It has been observed that the trapped electrons were affected by the intensity of the positron sources used for a PALS experiment.     

Inhibition of positronium formation in an irradiated organic solid

Positron annihilation lifetime spectra and two-gamma angular distributions have been measured for DL-leucine before and after γ-irradiation. The free radicals produced inhibit the formation of positronium rather than effect its ortho-to-para conversion.     

Study of irradiation effect on positronium formation in polypropylene

⁶⁰Co γ-rays and positron irradiation effect on positronium (Ps) formation in the bulk and near surface region of polypropylene (PP) was studied. It is revealed that the reduction of Ps formation probability in PP bulk as a function of elapsed time may not be due to enhanced concentration of oxygen groups, and should not be ascribed to a simple accumulation of free radicals. Results indicate that Ps in PP is not formed through only one branch. That branch sensitive to the irradiation as well as the existence of sample surface seems related to relatively mobile positrons.     

Carbonium ion and positronium quenching by diamagnetic organic molecules

Positronium was found to be highly sensitive to the presence of carbonium ions in the structures of diamagnetic organic molecules. This sensitivity is due to strong positronium quenching by the carbonium ion.     

Spin conversion of positronium in paramagnetic solutions studied by age-momentum-correlation method

Correlated measurement of the lifetime and of the lineshape of the 511 keV annihilation radiation of positrons (age-momentum correlation, AMOC) has become a powerful tool for investigating reactions of positron or positronium in condensed matter as a function of time. The beam-based ⁺ AMOC method installed at the Stuttgart relativistic positron beam facility (E _kin ^e+ 4MeV) offers substantial advantages over the conventional E coincidence technique resulting in a fast data taking due to the unity detection efficiency of the ⁺ scintillator which provides the start signal for the lifetime measurement. In this paper the application of AMOC to positronium chemistry is illustrated by the study of spin conversion of positronium in the system methanol/HTEMPO as a function of HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, a paramagnetic solute) concentration. The same technique can be applied to other systems and other reactions in positronium chemistry, e. g., oxidation or complex formation.     

Electric field effect on positronium formation in gamma-irradiated polypropylene and polyethylene

The effect of an external electric field (up to 48 kV cm⁻¹) on positronium (Ps) formation in γ-irradiated polypropylene and polyethylene has been studied by means of positron lifetime spectroscopy. The application of an electric field to unirradiated polypropylene and polyethylene considerably decreased the Ps formation. For example, the intensity of the longest-lived component (I₃) for one of the polypropylene samples was reduced from 26% to 18% by a field of 48 kV cm⁻¹. It was found that the electric field effect for the irradiated samples becomes weaker when increasing the irradiation dose. This result is discussed on the basis of the spur reaction model of Ps formation.     

Electron delocalization of metal electrons in Fe complexes by positronium spin conversion reactions

The rate constants, k_CR, of conversion reactions, CR, of ortho- into para-positronium atoms promoted by 3d paramagnetic complexes of V^II, Cr^II, Cr^III, Mn^II, Co^II and Ni^II ions are linearly correlated with the electron delocalization β caused by the ligands. β's may be evaluated by means of the empirical equation β=1−th, where t and h are constants characteristic of the ion and ligands, respectively. The determination of the unknown t(Fe^II) value is here described. It was derived by means of the rate constants of the CR caused by some Fe^II complexes. It was found that t(Fe^II)=0.14±0.01. A relationship between t and the oxidation number of the metal ions is suggested.     

On the role of free volume in pick-off annihilation and positronium chemical reactions

Free volume effects are important for positronium formation, pick-off annihilation and chemical reactions. The relation between the long-lived component intensity and the number of defects is considered for the case of selective localization of positrons and positronium in ordered and disordered sites of plymers. Positronium lifetimes for the large pores (r1 nm) are discussed. In positronium chemical reactions recent results show crucial role of the bubble parameters in liquids.     

The kinetics of radical reactions in the radiolysis of aqueous solutions of organic nitro compounds

Pulsed radiolysis and computer simulation of gamma radiolytic decomposition of organic nitrates in aqueous solutions were performed to determine the rate constants for reactions with the participation of intermediates determining the mechanism of the process. 2,4,6-Trinitrotoluene, 2,4-dinitrotoluene, and cyclic nitramine, cyclotrimethylene-trinitramine, were used as substrates. The bimolecular rate constants for the reactions of hydrated electrons (e _aq ^? ) and hydroxyl radicals (^?OH) with the substrates and constants for the recombination of electron adducts and carbon-centered radicals (the products of the detachment of the H atom from the nitro compound molecule by the OH radical) were determined by direct measurements with the use of high-speed spectrophotometry. Computer simulation of the reaction scheme was used to estimate the rate constant for significant reactions, monomolecular forward and back reactions of electron adducts and electron transfer to molecular oxygen, and refine the rate constant for the reaction of e _aq ^? with tert-butanol.     

On the positronium spin conversion reactions caused by some macrocyclic Co complexes

The rate constants, k_CR, of ortho- into para-positronium (o-Ps→p-Ps) spin conversion reactions, CR, caused by the high-spin [Co^IIsep]²⁺, [Co^IIdinosar]²⁺ and [Co^IIdiamsar]²⁺ macrocyclic complexes and also by high-spin [Co^II sen]²⁺ tripod complex were measured at several temperatures. The delocalizations, β, of Co^II unpaired electrons, promoted by the mentioned ligands, were determined by using the previously established correlations between k_CR and the electron delocalization β of unpaired metal electrons. β is given by the ratio between the Racah inter-electronic repulsion parameters of complexes, B, and that of the free ions, B₀. The β values are compared with those of the Co^II complexes with en (1,2-ethanediamine), pn (1,2 propanediamine) and dien (2,2′ diamino diethylamine) ligands. The k_CR rate constants are also compared with those of the Ps oxidation reactions, OR, promoted by the corresponding Co^III complexes. It is concluded that, unlike OR's, the CR's do not occur by formation of hepta-coordinate adducts with Ps atoms.     

Pressure effect in hydrogen isotope recombination reactions in liquid helium at cryogenic temperature

The behavior of recombinations of H+H→H₂, H+T→HT and T+T→T₂ in liquid helium at cryogenic temperature of 1.6 K was investigated under pressurized condition using radiochemical methods. It was found that the isotope ratios of H₂, HT and T₂ were changed with the pressure. The changes will contribute to the bubble formation, chemical potential and the effective masses of H and T. This work is a preliminary information to carry out chemical reactions in liquid helium.