This review is concerned with the utilization of the positron-electron annihilation phenomenon in studies of the physics of condensed matter. The theory of the annihilation process is outlined, the importance of the two-photon mode of decay is established, and it is described how the observable results depend on the initial positron and electron states of the system. A brief account of the principal experimental techniques is included. Discussion of the complex nature of positron behavior in molecular substances suggests the particular value of investigations directed at the two photon pick-off mode of decay of the orthopositronium atoms that are formed in these materials. The possibility of positronium formation in ionic and metallic materials is also considered. The many important electron aspects of positron annihilation in metals are dealt with. The independent particle approach to the analysis of two photon angular distributions is discussed and illustrated. A survey of electronic structure investigations includes studies of polycrystalline and single crystal specimens. The relevance of angular correlation measurements in investigations of the Fermi surfaces of metals and alloys is discussed. More recently developed applications to the study of defected and disordered systems are dealt with. A preliminary account of the interpretation of multicomponentmore » lifetime spectra in terms of varying numbers of distinguishable positron states provides the basis for a discussion of studies of positron trapping by defects, voids. and surfaces in ionic and metallic solids, liquids and powders. (auth)« less

Positron studies of condensed matter

The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.

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Experimental progress in positronium laser physics

The elastic scattering of positrons from noble gases has been formulated using a polarized-orbital approximation. In principle all multipole moments of the positron-atom interaction are included and the polarized orbital is calculated by a perturbed Hartree-Fock scheme. The scattered wave is readily obtained from a potential scattering problem and is used to calculate Zeff and the angular correlation as well as the scattering cross sections of interest. Numerical results are presented for positron-helium scattering in the elastic region.

https://iopscience.iop.org/article/10.1088/0022-3700/10/4/018/pdf

Positron scattering from noble gases

The current status of certain aspects of positron scattering in gases is reviewed. A brief resume of the experimental techniques used in this field is also given. Results for total scattering cross sections in a number of gases are presented along with a detailed discussion of potential systematic errors which can affect such measurements. Important features of the cross sections are pointed out and comparisons are made with electron scattering data. Results from experiments which go beyond total cross section determinations are discussed. Emphasis is placed upon recent measurements of positronium formation cross sections. Recent positron lifetime studies in dense gases are reviewed. The subjects covered include positron annihilation in clusters, ortho-positronium annihilation in bubbles and positronium formation in spurs.

http://iopscience.iop.org/article/10.1088/0034-4885/48/6/001/pdf

Experimental studies of positrons scattering in gases

The effect of spatial and other external confinements on the ground and excited state energy levels of many electron atoms, ions and exotic systems is discussed. Special emphasis is given to analyzing and estimating the changes in the spectral properties of plasma embedded systems in which, apart from changes in the free particle potential due to atom plasma interaction, spatial confinement enters through the introduction of boundary conditions. Effects of weak as well as strong plasma on the dipole polarizabilities, ionization potentials, singly and doubly excited state energy levels, oscillator strengths and transition probabilities have been discussed using simple plasma models but adopting rigorous quantum chemical methods. The spectral line shifts under strongly coupled plasma have been compared with data available from laser plasma experiments. Specific attention has been given to extremely accurate estimates of the energies of different three-body systems under plasma environments. The importance of the use of finite boundary conditions originating from spatial confinement of the plasma has been demonstrated and the effect of electron correlation in estimating various confined atomic properties is shown. Attempt has been made to interpret the changes in the spectral properties of atoms trapped in cavities inside liquid helium environments, by comparing the results estimated on the basis of current quantum chemical methodologies with the data available from laser induced fluorescence experiments.

C. V. Briscoe

Papers

Zero-Point Bubbles in Liquids