Positron-electron correlation

In Ref. 122 we raised the question of whether one can use basis functions (49) in non-BO quantum mechanical calculations of molecular diatomic systems containing positrons and whether those functions are capable of providing a proper representation for the positron-nucleus and positron-electron correlation effects in a diatomic system. e+LiH was chosen as a target system. Along with e+LiH, we also performed the calculations of HPs and Li+ because the total energies of these systems are needed for determining the dissotiation energy. 

The positron-electron correlation potential outside the metal or semiconductor surface described by the image potential can be written as... 

These traps, (Fig. 6) and similar effects in the motion of holes and other charges through polymers, would eventually be correlated also with such structural probes as positron lifetimes in macromolecular solids. Extensive recent studies of positron lifetime are based on positronium decay. In this, the lifetime of o-positronium (bound positron-electron pair with total spin one) is reduced from about 140 nanoseconds to a few nanoseconds by "pick-off annihilation" in which some unpaired electron spins in the medium cause conversion quenching of orthopositronium to para-positronium. The speed of the t2 effect is supposed, among other things, to represent by pick-off annihilation the presence of defects in the crystalline lattice. In any case, what amounts to empty space between molecules can then be occupied by orthopositronium.(14,15,16) It is now found in linear polyethylene, by T. T. Wang and his co-workers of Bell Laboratories(17) that there is marked shift in positron lifetimes over the temperature range of 80°K to 300°K. For... 

The experimental techniques involved in measuring the angular correlation and the Doppler broadening of the two annihilation gamma-rays were introduced in section 1.3. These techniques rely on the fact that the motion of the positron-electron pair immediately prior to annihilation causes the two gamma-rays to be emitted in directions differing... 

The two quantities which can be observed when an individual positron annihilates in condensed matter are the positron age r, which is the time interval between implantation and annihilation of the positron, and the momentum p of the annihilating positron-electron pair. Time-resolved information on the evolution of positron states is obtained by correlated measurements of the individual positron lifetime (= positron age) and the momentum of the annihilating positron-electron pair (Age-Momentum Correlation, AMOC). AMOC measurements are an extremely powerful tool for the study of reactions involving positrons. It not only provides the information obtainable from the two constituent measurements but allows us to follow directly, in the time domain, changes in the e+e momentum distribution of a positron state (cf. Sect. 1). 

Since both 511 keV photons resulting from a 2y-annihilation event transmit equivalent information, one photon may be used to determine the age of the annihilating positron and the other for the correlated measurement of the momentum of the annihilating positron-electron pair by measurement of the... 

Cautious readers may feel uneasy about the apparently surreptitious disappearance of the negative-energy states from this theory. The anti-commutation relations between the creation and annihilation operators of electrons and positrons are sufficient to derive how the the negative-energy states enter in electron correlation problems. Labzowsky [15] and Sapirstein [16] have derived... 

Attempts to verify the above volume diffusion mechanism experimentally included X-ray and electron diffraction experiments with electrodes that were corroded at > Ec, as well as investigations by positron annihilation spectroscopy (PAS). In the former case, the occurrence of broadened diffraction lines at Bragg angles between those of the bulk alloy and the pure, noble component was taken as a confirmation of the volume diffusion mechanism [54, 120, 131]. More direct evidence was obtained from the PAS experiments with dezincified brass, where experimental positron Kfetimes correlated well with calculated values in vacancies or vacancy aggregates [78-80]. On the other hand, it has been objected that Eq. (20) predicts a dependence of the current density, which is in contradiction to many experimental results. It has been shown, however, that this particular problem may... 

The procedure sketched above has recently been applied to the ground state of medium-heavy helium-like ions. We have evaluated the effect of QED combined with electron correlation, defined as the interaction with at least two Coulomb interactions. The QED part is here restricted to first order and consists of non-radiative effects (retardation of the electromagnetic interaction and effect of virtual electron-positron pairs) as well as radiative effects (electron self-energy, vacuum polarization and vertex correction). 

The sizes and concentration of the free-volume cells in a polyimide film can be measured by PALS. The positrons injected into polymeric material combine with electrons to form positroniums. The lifetime (nanoseconds) of the trapped positronium in the film is related to the free-volume radius (few angstroms) and the free-volume fraction in the polyimide can be calculated.136 This technique allows a calculation of the dielectric constant in good agreement with the experimental value.137 An interesting correlation was found between the lifetime of the positronium and the diffusion coefficient of gas in polyimide.138,139 High permeabilities are associated with high intensities and long lifetime for positron annihilation. 

Several experimental techniques such as Compton scattering, positron annihilation, angular correlation, etc., are used for measuring momentum densities. One of the most popular techniques involved in measuring momentum densities is termed as electron momentum spectroscopy (EMS) [29]. This involves directing an electron beam at the surface of the metal under study. Hence EMS techniques fall under what is classified as coincidence spectroscopy. 

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