B. Lévay

Solid–solid phase transitions and molecular motions in long‐chain paraffins studied by positron annihilation techniques

Positron annihilation lifetime and Doppler‐broadening measurements were performed on n‐dotriacontane (n‐C32H66) and n‐tritriacontane (n‐C33H68) between 80 K and their melting points. The annihilation parameters plotted as a function of temperature show irregularities above the room temperature, which reflect solid–solid phase transitions in these materials. The transition temperatures derived from the positron annihilation measurements are in excellent agreement with literature values determined by other techniques. In the case of n‐C32H66 additional phase transitions not reported previously in the literature were observed. The lifetime vs temperature curves could be reasonably well interpreted within the framework of the free volume model taking into account the effect of special types of molecular motions and structural defects. On the basis of our measurements, linear dependence of ortho‐positronium (o‐Ps) lifetime on the size of the cavity around o‐Ps could be expected. The proposed empirical relation...

Studies on the atomic capture of stopped negative pions in binary mixtures of 3He with other gases

Abstract Systematic experimental study has been carried out on the atomic capture of negative pions by 3He in binary gas mixtures of 3He + Z, where Z is Ne, Ar, Kr, Xe, N2, O2, CO2 and sf6. The results are analysed in the framework of a phenomenological model. It is shown that there is no pion transfer from the 3 H eπ − mesic atoms to the heavier Z-atoms. The probabilities of pion capture in the various atoms of the mixtures are found to be proportional to the atomic concentrations, thereby excluding the possibility of a concentration dependence in the atomic capture ratio A(Z/3He). In contradiction to previous assumptions the probability of pion capture into an atomic orbit is not proportional to the stopping power of the components of the mixture. The atomic capture ratio of pions in a 3He + 4He mixture is A( 4 He / 3 He ) = 0.75 ± 0.13 , which might be the indication of an isotopic effect. The branching ratio for the charge-exchange reaction at rest (π − + 3 He → π 0 + 3 H ) is found to be 0.128 ± 0.012.

Temperature breaking of hydrogen bonds in ammonia studied by π−-meson capture in hydrogen

Abstract The capture probability of stopped π − mesons by hydrogen atoms of ammonia increases with temperature in the liquid phase but in the supercritical phase it is temperature-independent. This can be attributed to the temperature breaking of hydrogen bonds. Rough estimates are given for the fraction of broken hydrogen bonds at various temperatures.

Reactivity of some condensed heterocyclic cations towards ortho-positronium atoms in acetonitrile solution

Abstract The effect of changes in chemical structure and charge distribution of molecules on their reactivity towards an ortho-positronium ( o -Ps) atom was investigated by positron annihilation lifetime spectroscopy. Molecules with very similar structural frameworks were selected as model systems. Bridgehead nitrogen-containing condensed heterocyclic pyridoazinium salts with 1–3 nitrogen atoms and their methyl-substituted analogues were studied in acetonitrile solutions. In both groups the reactivity towards o -Ps atoms increased drastically with the increasing number of N atoms, while only a slight decrease of reactivity was observed due to methyl substitution. The strong electron-withdrawing effect of the N atoms and the weak electron-donating character of the methyl group accounted for these observations. In contrast to the reaction with o -Ps atom, methyl substitution affected drastically the chemical reactivity of these molecules in ring opening reactions with secondary amines, where the methylated compounds did not react even under forcing conditions. In the light of the o -Ps lifetime measurements a possible explanation for this fact is that the methyl group sterically hinders the attack of the secondary amines.

A new phenomenon to be taken into consideration when investigating phosphorescence by liquid scintillation method

Abstract Investigating the phosphorescence radiation in the measurement of low-energy β-memitters by liquid scintillation method a new group of phenomena was observed. In the presence of scintillator molecules the phosphorescence of the bottle, of the solvent and that of the additives increased. This phenomenon—called “the introduction of phosphopescence”—may influence the choice of the optical filter and the scintillator decisively, and necessitates the phosphorescence ce investigation of every individual components in the presence of the scintillator molecule.

Isomer effect on the reactivity and inhibition capacity of nitrophenols and nitrophenolates towards ortho-positronium in methanol solutions

Abstract The effect of dissociation on the reactivity and inhibition capacity of nitrophenol isomers towards ortho-positronium atom has been investigated in methanol and methanol containing 0.5 M NaOH solutions. It has been found that as a result of dissociation the rate constants (KPs) and the inhibition coefficients (σ) decreased significantly in all cases. A comparison of the rate constants of the scavenging of spur electrons with the inhibition coefficients also suggests that the scavenging of positrons plays an important role in the inhibition processes. In the case of the ortho isomer our results strongly support the idea of the stabilizing effect of the intramolecular hydrogen bond between the OH and NO2 groups of the neutral molecule.

Effect of dissociation on the reactivity and inhibition capacity of p-nitro-phenol towards ortho-positronium in methanol solutions

The effect of dissociation on the reactivity and inhibition capacity of p-nitro-phenol /p-NPH/ towards orthopositronium atom has been investigated by performing positron lifetime measurements in methanol and methanol containing 0.5M NaOH solutions, respectively. In the latter case the solute exists in anionic form /p-NP−/. It has been found that as a result of dissociation, both the rate constant /k/ and the inhibition coefficient /σ/ decrease significantly. Their values are k=/7.5±0.4/×108 s−1 M−1 and σ=6.8±0.2 M−1 for p-NPH and kd=/0.6±0.2/×108 s−1 M−1 and σd=4.0±0.2 M−1 for p-NP−. The possible reasons for these effects are discussed.

Atomic capture and transfer of negative pions stopped in binary mixtures of hydrogen with polyatomic gases

Abstract The atomic capture and transfer of stopped negative pions have been studied in binary gas mixtures of H 2 + M , where M is CCl 2 F 2 , CClF 3 , CBrF 3 or SF 6 . The π 0 yield, versus relative atomic concentration C A of M , goes through a maximum at C A ∼ 0.1 and levels off at zero at high concentrations. This phenomenon together with other observed characteristics of the atomic capture and transfer of pions in these systems is interpreted in the frame of a phenomenological model. The average transfer coefficients \ gL Z exhibit a weak concentration dependence. The estimated average atomic capture ratios A ( Z H ) are lower than those found for noble gases, probably because of the mutual screening of the constituent atoms in the molecules. The probability of pion capture in an atomic orbit is not proportional to the stopping power of the components of the mixture.

Positron annihilation in solid charge-transfer complexes

Abstract Positron lifetime and angular correlation measurements have been carried out on 1:1 charge-transfer complexes, on their pure donor and acceptor components and on the 1:1 M mechanical mixtures of these components. Complex formation reduced the intensity of the long-lifetime component of the donor compounds nearly to the low level of the acceptors. The angular correlation curves obtained for the pure acceptor and the complex were practically identical and were substantially broadened as compared to that of the donor.