Dependence of Positron Lifetimes on the Temperature and Phase of Some Organic Media

Abstract

Measurements of the value t2 and the intensity I , of the long component in the lifetime spectra of positrons annihilating in some simple unassociated, polar, and associated organic liquids are reported as a function of temperature. A semi-empirical free volume model for the formation and quenching of ortho-positronium atoms is developed for simple molecular materials to explain the correlation between viscosity 7, density e, and re observed in unassociated and polar liquids. Such a correlation is not observed for associated liquids in which the presence of hydrogen bonds between molecules apparently influences the annihilation process. The data of positron lifetimes during the solid-liquid phase transition in some organic systems are discussed in terms of the free volume model. Es werden Messungen des

Local Free‐Volume Distribution from PALS and Dynamics of Polymers

“…The same has frequently been observed for viscosity and dielectric α-relaxation in small-molecule liquids [Thosar et al, 1973;Malhotra and Pethrick, 1983;Higuchi et al, 1996;Pethrick, 1997;Kanaya et al, 1999;Bartoš et al, , 2001Bartoš et al, , 2002Bartoš et al, , 2009]. However, this is usually not the case for structural mobility (segmental relaxations, viscosity) in polymers [Dlubek et al, 2005b[Dlubek et al, , 2007bKilburn et al, 2006a].…”

“…The research of this early period has been reviewed by several authors [e.g., Ferrell, 1956;Wallace, 1960;Green and Lee, 1964;Steward and Roelling, 1967;Goldanski, 1968]. The pioneering work of Brandt and co-workers in the 1960s [Brandt et al, 1960;Brandt and Spirn, 1966], Tao 1972, the Risø group [Hamielec et al, 1973;Eldrup et al, 1981], and other [Thosar et al, 1973] in the 1970s decisively deepened our knowledge of the interrelation between the free volume in polymers and the annihilation characteristics of Ps and opened the way for a real application of the method. The comprehensive research of the group of Jean [Jean, 1990[Jean, , 1995[Jean, , 1996Jean and others 1992Jean and others , 1994Jean and others , 1996Jean and others , 2003, Pethrick [1997], and many other groups demonstrated the wide potential of this method and broadened its application distinctly.…”

Positron Annihilation in Some Antiferromagnetic Materials

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It is known that the lifetimes and intensities in the lifetime spectra of positrons annihilating in molecular materials a r e sensitive to the state of aggregations (1,2) and exhibit sharp changes around solid-liquid phase transformations. In the present investigation, positron lifetimes and intensities have been studied a s a function of temperature in antiferromagnetic materials with a view to investigate the behaviour of these parameters with the magnetic phase transition from paramagnetic to antiferromagnetic state, which occurs around the Nee1 temperature (T ).

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Temperature and Phase Dependence of Positronium Lifetimes in Halogen Substituted Benzenes

“…Their dipole moments ( t l ) a r e given in Table 1. 2 The liquids were degassed by vacuum freeze-thaw technique. This technique and the slow-fast coincidence set up for the lifetime spectra have been reported earlier (2).…”

“…2 The liquids were degassed by vacuum freeze-thaw technique. This technique and the slow-fast coincidence set up for the lifetime spectra have been reported earlier (2). Table 1 presents the data of z2 and I at different temperatures for halogen 2 substituted benzenes.…”