Positronium Atom in Solids -Peculiarities of Formation and Interconnection with Free Volume Nanostructure-

Abstract

l. Introduction to new atoms One feature of the vehement growth of physics in the 20th century is, undoubtedly, the correlation of this science with other fields of human endeavor, their mutual enrichment. The recent several decades have seen the spectacular advances, for example, in a new area at the junction of elementary-particle physics and chemistry, the so-called chemistry of new atoms, in which either an electron is replaced by another negative particle or a proton by another positive particle. This area of research does not confine itself to chemical studies alone. External influences on the properties of new atoms, and also those of mesons and positrons, provide useful, sometimes unique, information about the structure of solids. Therefore, unlike other reviews on positron annihilation, this paper includes a special part, describing intrinsic properties of positronium in comparison with properties and composition of other new atoms, so that this part can be considered as a general introduction into the field of new atoms. On example of positronium, the main part of this paper demonstrates external influences of the surrounding matter on properties and characteristics of one of the new atoms and the use of this effect for structural studies in solids. Correspondingly, a newcomer will find in this paper a little more than one of the special applications of the positron annihilation method. Atoms with an electron replaced by a negative meson are called mesoatoms. When chemically bound to conventional molecules and radicals they form mesomolecules. Typical examples are µ- and π-mesoatoms. Also possible in principle are E-mesoatoms, in which an electron is replaced by a negative hyperon, antiproton, or antihyperon. Atoms with a proton replaced by a positive particle are called by the name of that particle with an ending ‘ium’. Since a penetration of any positive particle into a nucleus is hindered by Coulomb repulsion (which can be compensated for by the nuclear forces alone), for light positive particles not involved in nuclear interactions, such as positrons e + and µ + -mesons, the atoms of this type cannot have nuclei consisting of many nucleons. These atoms only form when a positive lepton replaces a single nucleon, namely, a proton. In that case, an atom of a positron and an electron is called positronium (Ps). A bound system of µ + -meson (positive muon) and an electron is known as muonium (Mu). Both atoms can be treated as light isotopes of hydrogen. What is more, positronium is the lightest of all known new atoms, the only atom without a nucleus. In fact, since both constituent particles have the same mass, the centre of mass does not coincide with any of them (otherwise such a particle could be regarded as a nucleus), but lies strictly in between them. A nuclear proton can also be replaced by a positively charged hyperon in much the same way as a neutron by a neutral hyperon. The result is the so-called hypernuclei, and the corresponding atoms are referred to as hyperatoms. The structure of their electron shells and their chemical properties are, however, essentially the same as in conventional atoms with the same nuclear charge. Therefore, hyperatoms are generally not included into the list of new atoms. Interest in new atoms chiefly comes from the fact that their characteristics are appreciably influenced by the chemical properties and the structure of their environment. We will be looking at them in more detail in the next section. Examination of the characteristics of new atoms can supply unique evidence concerning the properties of the environment. One important characteristic of new atoms is their lifetime (all of them unstable). The decay characteristics of new atoms are studied using the apparatus of nuclear physics. The first stage of new atom research is the study of the laws by which the environment affects the characteristics and fate of new atoms. The second stage, more important for other research fields, consists in obtaining more complete and new structural and chemical data or data on the kinetics of certain processes based on observations of characteristics of formation, transformation, and disintegration of various new atoms. At the present time both the investigations into annihilation (death) of positrons and positronium and the mesochemical studies of µ- and π-mesoatoms and muonium have attained the second stage and started to yield information of general chemical significance. The field is very extensively promising. The aim of this review is not to embrace the whole problem of “new atoms” or the most of directions of positron annihilation studies. Instead, after general introduction into physics and methodology, detailed discussion is given of so called positronium pick-off annihilation and application of this effect for the studies of the free volume nanostructure in solids, in polymers particularly.