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The aim of this work is to clarify the scintillation-yield enhancement in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ scintillators obtained by $\mathrm{Li}$ codoping via integrated study of the valence state of activators, the preferential site occupancy of $\mathrm{Li}$ codopants, and defect structures from experimental and theoretical insights. With $\mathrm{Li}$ codoping, the light yield and energy resolution of $10\ifmmode\times\else\texttimes\fi{}10\ifmmode\times\else\texttimes\fi{}10\phantom{\rule{0.1em}{0ex}}{\mathrm{mm}}^{3}$ $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ samples are improved from 15 600 to 24 800 photons/MeV, and 5.3 to 4.3% at 662 keV, respectively. The optical absorption spectra indicate that $\mathrm{Li}$ codoping does not induce conversion of stable ${\mathrm{Pr}}^{3+}$ to ${\mathrm{Pr}}^{4+}$ in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ single crystals. Based on the formation energies of substitutional and interstitial $\mathrm{Li}$ sites using density-functional-theory (DFT) calculations and the ${}^{7}\mathrm{Li}$ nuclear magnetic resonance results, it is shown that the $\mathrm{Li}$ ions prefer to dominantly occupy the fourfold coordinated interstitial sites and fourfold coordinated $\mathrm{Al}$ sites. The systematic analysis of thermoluminescence glow curves, positron annihilation lifetime spectroscopies, and defect formation energies derived from DFT calculations reveals that the concentration of isolated $\mathrm{Lu}$ and $\mathrm{Al}$ vacancies as dominant acceptor defects is reduced by $\mathrm{Li}$ codoping, whilst the shallow ${\mathrm{Li}}_{i}$ interstitial defects and the deep ${V}_{O}$ oxygen vacancies are introduced simultaneously. We propose that the lowering of hole trapping at defects resulting from $\mathrm{Li}$ codoping contributes to the scintillation-yield enhancement.

We show that $\mathrm{N}$-polar $\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ heterostructures exhibit significant $\mathrm{N}$ deficiency at the bottom $(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ interface, and that these $\mathrm{N}$ vacancies are responsible for the trapping of holes observed in unoptimized $\mathrm{N}$-polar $\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ high electron mobility transistors. We arrive at this conclusion by performing positron annihilation experiments on $\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ heterostructures of both $\mathrm{N}$ and $\mathrm{Ga}$ polarity, as well as state-of-the-art theoretical calculations of the positron states and positron-electron annihilation signals. We suggest that the occurrence of high interfacial $\mathrm{N}$ vacancy concentrations is a universal property of nitride semiconductor heterostructures at net negative polarization interfaces.

Background: The $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$ reaction plays a vital role in various astrophysical scenarios. Its reaction rate must be accurately known in the present era of high precision astrophysics. The cross section of the reaction is often measured relative to a low energy resonance, the strength of which must therefore be determined precisely.Purpose: The activation method, based on the measurement of $^{15}\mathrm{O}$ decay, has not been used in modern measurements of the $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$ reaction. The aim of the present work is to provide strength data for two resonances in the $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$ reaction using the activation method. The obtained values are largely independent from previous data measured by in-beam $\ensuremath{\gamma}$ spectroscopy and are free from some of their systematic uncertainties.Method: Solid state TiN targets were irradiated with a proton beam provided by the Tandetron accelerator of Atomki using a cyclic activation. The decay of the produced $^{15}\mathrm{O}$ isotopes was measured by detecting the 511 keV positron annihilation $\ensuremath{\gamma}$ rays.Results: The strength of the ${\mathrm{E}}_{p}=278\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ resonance was measured to be $\ensuremath{\omega}{\ensuremath{\gamma}}_{278}=(13.4\ifmmode\pm\else\textpm\fi{}0.8)\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ while for the ${\mathrm{E}}_{p}=1058\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ resonance $\ensuremath{\omega}{\ensuremath{\gamma}}_{1058}=(442\ifmmode\pm\else\textpm\fi{}27)\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$.Conclusions: The obtained ${E}_{p}=278$ keV resonance strength is in fair agreement with the values recommended by two recent works. However, the ${E}_{p}=1058\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ resonance strength is about 20% higher than the previous value. The discrepancy may be caused in part by a previously neglected finite target thickness correction. As only the low energy resonance is used as a normalization point for cross section measurements, the calculated astrophysical reaction rate of the $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$ reaction and therefore the astrophysical consequences are not changed by the present results.

This work is supported by Eusko Jaurlaritza under Grants No. IT-1005-16 and No. IT-756-13 and by Spanish Ministry of Economy and Competitiveness under Grant No. MAT2015- 65165-C2-R (MINECO/FEDER). I. Unzueta also wants to acknowledge Eusko Jaurlaritza for Grant No. PRE-2014-214.

$\gamma$ spectra for positron annihilation in noble-gas atoms are calculated using many-body theory for positron momenta up to the positronium-formation threshold. This data is used, together with time-evolving positron-momentum distributions determined in [arXiv:1706.01434 (2017)], to calculate the time-varying $\gamma$ spectra produced during positron cooling in noble gases. The $\gamma$-spectra and their $\bar{S}$ and $\bar{W}$ shape parameters are shown to be sensitive probes of the time evolution of the positron momentum distribution, and thus provide a means of studying positron cooling that is complementary to positron lifetime spectroscopy., Comment: This version contains minor update to references, otherwise unchanged

We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of Be_{Ga} in the positron annihilation signals in Be-doped GaN, while the emergence of V_{Ga} at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.

In the article by Ma~\emph{et al.}~[Phys.~Rev.~A {\bf 94}, 052709 (2016)], $\gamma$-ray spectra for positron annihilation on molecules were calculated in the independent-particle approximation with the positron wavefunction set to unity. Based on comparisons with experimental data they concluded that inner valence electrons play a dominant role in positron annihilation. These conclusions are incorrect and resulted from fallacious analysis that ignored the known effect of the positron wavefunction on the spectra., Comment: Accepted for publication in Physical Review A

The superconducting pairing state of LaPt$_{4}$Ge$_{12}$ is studied by measuring the magnetic penetration depth $\lambda(T,B)$ and the superfluid density $\rho_s(T)$ using a tunnel-diode-oscillator (TDO)-based method and by transverse field muon-spin rotation ($\mu$SR) spectroscopy. $\lambda(T)$ follows an exponential-type temperature dependence at $T\ll T_{c}$, but its zero-temperature value $\lambda(0)$ increases linearly with magnetic field. Detailed analyses demonstrate that both $\lambda(T)$ and the corresponding $\rho_{s}(T)$, measured in the Meissner state by the TDO method are well described by a two-gap $\gamma$ model with gap sizes of $\Delta_1(0)=1.31k_{B}T_c$ and $\Delta_2(0)=1.80k_{B}T_c$ and a very weak interband coupling. In contrast, $\rho_s(T)$, derived from the $\mu \rm{SR}$ data taken in a small field, can be fitted by a single-gap BCS model with a gap close to $\Delta_2(0)$. We conclude that LaPt$_{4}$Ge$_{12}$ is a marginal two-gap superconductor and the small gap $\Delta_1$ seems to be destroyed by a small magnetic field. In comparison, in PrPt$_4$Ge$_{12}$ the $4f$-electrons may enhance the interband coupling and, therefore, give rise to more robust multiband superconductivity., Comment: 5 pages, 4 figures

Several first-principles calculations of positron-annihilation characteristics in solids have added gradient corrections to the local-density approximation within the theory by Arponen and Pajanne [Ann. Phys. (N.Y.) 121, 343 (1979)] since this theory systematically overestimates the annihilation rates. As a further remedy we propose to use gradient corrections for other local density approximation schemes based on perturbed hypernetted-chain and on Quantum Monte Carlo results. Our calculations for various metals and semiconductors show that the proposed schemes generally improve the positron lifetimes when they are confronted with experiment. We also compare the resulting positron affinities in solids with data from slow-positron measurements., Comment: 16 pages, 5 figures, 3 tables

Calculations of the positron binding energies to all atoms in the Periodic Table are presented and atoms where the positron-atom binding actually exists are identified. The results of these calculations and accurate calculations of other authors (which existed for several atoms only) are used to evaluate recommended values of the positron binding energies to the ground states of atoms. We also present the recommended energies of the positron excited bound levels and resonances (due to the binding of positron to excited states of atoms) which cannot emit positronium and have relatively narrow widths. Such resonances in positron annihilation and scattering may be used to measure the positron binding energy.

We carry out measurements of the two-dimensional angular correlation of the positron annihilation radiation (2D-ACAR) to reconstruct the complex multisheet Fermi surface (FS) of the cubic rare-earth (RE) compound TmGa3. We discover a correlation between the antiferromagnetic structures and the nesting of the FS along the [110] directions. Moreover, we propose methods to estimate the density of states at the Fermi energy ( EF) and the electronic contribution to the specific heat [we obtain N(EF) = 13.6 states/Ryd cell and gamma = 2.4 mJ/mole K2].

Experiments have shown that positrons can attach to molecules via vibrational Feshbach resonances. This leads to increased annihilation rates, the magnitudes of which depend upon molecular structure. Presented here is a simplified rate-equation model to describe the competition between annihilation while the positron is attached to the molecule, positron ejection from the entrance state, and diffusion of the vibrational energy to multimode states followed by similar ejection due to vibrational deexcitation. The latter ejection process can involve vibrations more strongly coupled to the positron continuum, producing suppression of the annihilation, or those more weakly coupled to the continuum, resulting in enhanced annihilation rates. This model elucidates the role that mode coupling can play in determining resonant annihilation amplitudes. Simple limits are obtained and compared with experimental results for selected molecules.

We have used a low-energy positron beam to investigate the defects in N-doped and undoped ZnSe layers grown on GaAs:Si or ZnSe substrates. In N-doped ZnSe we observe positron trapping at vacancies, identified as VSeNSe complexes, and at negative ions. Based on positron data, we give estimates for the concentrations of vacancies and negative ions. The results suggest that in addition to forming isolated NSe acceptors or VSeNSe pairs, incorporated nitrogen also forms donor-type defects leading to compensation of holes. In undoped ZnSe layers, the results show presence of Zn vacancies in concentrations 10–10 cm. The Zn vacancy concentration correlates with the dislocation density at the ZnSe/GaAs interface.

Positron lifetime measurements have been carried out in CeNiSn, in the temperature range of 5 to 300 K. In the 300 and 150-K temperature interval the positron lifetime is seen to decrease from $178\ifmmode\pm\else\textpm\fi{}1$ to $176\ifmmode\pm\else\textpm\fi{}1$ ps. Below 150 K the lifetime shows a gradual increase and attains a value of $181\ifmmode\pm\else\textpm\fi{}1$ ps at 35 K, below which the lifetime shows a small decrease. This behavior of the positron lifetime is seen to correlate with earlier measurements [Y. Uwamoto et al., J. Magn. Magn. Mater. 104, 643 (1992)] on the thermal expansion coefficient in CeNiSn, which has been understood in terms of electronic structure changes associated with the system transforming from an independent Kondo regime to a Kondo insulating state, via an intermediate mixed valent regime. Strong support for these electronic structure changes are provided by ab initio positron lifetime calculations which are seen to be in good agreement with the positron experiments.

We present all-electron computations of the three-dimensional (3D) Fermi surfaces (FS's) in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{BiO}}_{3}$ for a number of different compositions based on the self-consistent Korringa-Kohn-Rostoker coherent-potential-approximation approach for incorporating the effects of Ba/K substitution. By assuming a simple cubic structure throughout the composition range, the evolution of the nesting and other features of the FS of the underlying pristine phase is correlated with the onset of various structural transitions with K doping. A parametrized scheme for obtaining an accurate 3D map of the FS in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{BiO}}_{3}$ for an arbitrary doping level is developed. We remark on the puzzling differences between the phase diagrams of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{BiO}}_{3}$ and ${\mathrm{BaPb}}_{x}{\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{O}}_{3}$ by comparing aspects of their electronic structures and those of the end compounds ${\mathrm{BaBiO}}_{3},$ ${\mathrm{KBiO}}_{3},$ and ${\mathrm{BaPbO}}_{3}.$ Our theoretically predicted FS's in the cubic phase are relevant for analyzing high-resolution Compton scattering and positron-annihilation experiments sensitive to the electron momentum density, and are thus amenable to substantial experimental verification.

Electron momentum density and Compton profiles in Lithium along $$, $$, and $$ directions are calculated using Full-Potential Linear Augmented Plane Wave basis within generalized gradient approximation. The profiles have been corrected for correlations with Lam-Platzman formulation using self-consistent charge density. The first and second derivatives of Compton profiles are studied to investigate the Fermi surface breaks. Decent agreement is observed between recent experimental and our calculated values. Our values for the derivatives are found to be in better agreement with experiments than earlier theoretical results. Two-photon momentum density and one- and two-dimensional angular correlation of positron annihilation radiation are also calculated within the same formalism and including the electron-positron enhancement factor., 11 pages, 7 figures TO appear in Physical Review B

Native vacancies in Te-doped ${(5\ifmmode\times\else\texttimes\fi{}10}^{16}{\char21{}5\ifmmode\times\else\texttimes\fi{}10}^{18}{\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})\mathrm{ }\mathrm{GaAs}$ were investigated by means of positron lifetime and Doppler-broadening coincidence spectroscopy. The experimental data were related to theoretical calculations of the positron lifetime and the annihilation momentum distribution. Monovacancies were observed in all Te-doped GaAs samples under study. It will be shown that they can directly be identified to be Ga-vacancy\char21{}${\mathrm{Te}}_{\mathrm{As}}$-donor complexes. These complexes are the dominating type of vacancy defects in the doping range under observation.

Fachbereich Physik der Martin-Luther-Universita¨t Halle-Wittenberg, Experimentelle Physik III, Friedemann-Bach-Platz 6,D-06108 Halle (Saale), Germany~Received 11 August 1998!Isochronal annealing investigations in electron-irradiated 6H-SiC were correlated with temperature-dependent measurements of positron lifetime. It turned out that the positron trapping at temperatures up to 300K was dominated by the trapping in shallow positron traps. These defects were already present in the unirra-diated material and could be attributed to the antisite defects. The annealing of the irradiation-induced mono-vacancies and divacancies took a continuous course up to 1740 K. In the wide-temperature range between 300and 1200 K, different monovacancylike defects annealed and above 1200 K the divacancy disappeared. Theweak increase of the average positron lifetime between 750 and 1200 K detected at measurement temperaturesbelow 300 K must be caused by partial annealing of shallow traps, probably irradiation-induced shallow traps.After the final annealing at 1740 K all of the irradiation-induced defects vanished.@S0163-1829~99!00516-0#I. INTRODUCTION

The present measurement of positron annihilation lifetimes for the high-${T}_{c}$ superconductor $({\mathrm{Bi}}_{0.92}{\mathrm{Pb}}_{0.17}{)}_{2}{\mathrm{Sr}}_{1.91}{\mathrm{Ca}}_{2.03}{\mathrm{Cu}}_{3.06}{\mathrm{O}}_{10+\ensuremath{\delta}}$ or (Bi,Pb)-2223, as a function of temperature, has been inspired by our recent DBPARL (Doppler-broadened positron annihilation radiation line shape) finding of a step and minima in the region around ${T}_{c}(R=0)=104$ K, in the variation with temperature, of the fraction of low momentum electrons at the annihilation sites. The positron annihilation lifetime in the superconductor probes the number density of electrons at the annihilation sites. The present work observes a reduction of lifetime components in the ${T}_{c}$ region. Significant changes in the lifetime components in other temperature regions have also been observed and discussed.

Single-crystal structure determination of ${[\mathrm{Zn}(\mathrm{ptz})}_{6}{](\mathrm{B}\mathrm{F}}_{4}{)}_{2}$ has been performed at 110 and 190 K in order to investigate the possible existence of a local structural deformation proposed by positron annihilation study of Nagai et al. [Phys. Rev. B 57, 14 119 (1998)]. A large thermal contraction along the $c$ axis with decreasing temperature is observed, while the displacement of the atoms within the $\mathrm{ab}$ plane is very small. This implies that the effect observed in the positron annihilation spectroscopies is caused by interactions involving positronium.

The pickoff component of ortho-positronium decay in liquids is conventionally described through the bubble model. However, the idealized picture of a sharp ``bubble'' radius is unsatisfactory. This leads to a much better description and enables a substantially improved simultaneous fit to the lifetime data and the observed angular correlation of the decay $\ensuremath{\gamma}$'s. We have tried to restrict the discussion to modifications that permit elegant analytic solutions.

We measured the positron-annihilation lifetime in A${\mathrm{l}}_{71.4}$P${\mathrm{d}}_{20.2}$M${\mathrm{n}}_{8.4}$, A${\mathrm{l}}_{71.9}$P${\mathrm{d}}_{19.7}$M${\mathrm{n}}_{8.4}$, A${\mathrm{l}}_{70.2}$P${\mathrm{d}}_{20.3}$M${\mathrm{n}}_{9.5}$, and A${\mathrm{l}}_{69.1}$P${\mathrm{d}}_{21.5}$M${\mathrm{n}}_{9.4}$ polyquasicrystalline icosahedral quasicrystals. As-grown and plastically deformed single quasicrystals of A${\mathrm{l}}_{70.6}$P${\mathrm{d}}_{21.1}$M${\mathrm{n}}_{8.3}$ were also studied. In every sample the lifetime spectrum consists of a single component with the lifetime of $206\ifmmode\pm\else\textpm\fi{}4\mathrm{ps}$. The results indicate that icosahedral quasicrystalline AlPdMn contains a dense distribution of vacancy-type sites. These trapping sites seem to remain unaltered during plastic deformation at high temperature.

The temperature and composition dependence of the formation of thermal vacancies in the intermetallic compound ${\mathrm{Ni}}_{3}\mathrm{Al}$ with the ${L1}_{2}$ structure was studied by means of positron lifetime spectroscopy. High values of the effective vacancy formation enthalpy ${H}_{\mathrm{V}}^{\mathrm{F}}$ increasing monotonically from 1.65$\ifmmode\pm\else\textpm\fi{}$0.08 eV to 2.01$\ifmmode\pm\else\textpm\fi{}$0.08 eV with increasing Ni content from 74.1 to 76.5 at. % are found. A fit of a model based on the interaction of nearest-neighbor atoms to the experimental data demonstrates predominant formation of thermal vacancies on the Ni sublattice and high concentrations of antisite atoms. The results are discussed with respect to recent tracer-diffusion measurements in ${\mathrm{Ni}}_{3}\mathrm{Al}$.

Defects in highly Si-doped GaAs were identified and their concentration determined by combining positron lifetime spectroscopy with scanning tunneling microscopy. We observed with increasing Si-doping concentration an increasing concentration of a deep positron trap identified as S${\mathrm{i}}_{\mathrm{Ga}}$-donor--Ga-vacancy complex. The concentration of shallow positron traps increased with the Si concentration too. The shallow traps are found to be S${\mathrm{i}}_{\mathrm{As}}$ acceptors and Si clusters.

Defect profiles were determined in proton-implanted low-doped ([P]=1\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) n-type silicon layers by performing positron-electron pair momentum-distribution measurements with a slow-positron beam, conventional positron lifetime, and ${\mathrm{e}}^{+}$-${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$ pair momentum-distribution measurements with a $^{22}\mathrm{source}$ and spreading resistance measurements. The dominant positron trap induced by 1.15 and 3.0 MeV proton implantations is the silicon divacancy ${\mathrm{V}}_{2}$. Compared to the values in bulk, the characteristic positron lifetime and the characteristic low- and high-momentum parameters of the ${\mathrm{e}}^{+}$-${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$ pair momentum distribution at the divacancy are ${\mathrm{\ensuremath{\tau}}}_{\mathrm{d}}$=300 ps=1.35${\mathrm{\ensuremath{\tau}}}_{\mathrm{b}}$, ${\mathrm{S}}_{\mathrm{d}}$=1.${052\mathrm{S}}_{\mathrm{b}}$, and ${\mathrm{W}}_{\mathrm{d}}$=0.${78\mathrm{W}}_{\mathrm{b}}$, respectively. The divacancy is observed in the negative charge state ${\mathrm{V}}_{2}^{\mathrm{\ensuremath{-}}}$. The divacancy profile is determined in n-type Si implanted with 1.15-MeV (20 \ensuremath{\mu}m) protons to a dose 1\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and the maximum concentration [${\mathrm{V}}_{2}^{\mathrm{\ensuremath{-}}}$]=4--8\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ is observed at depths 16--18 \ensuremath{\mu}m. The resistivity increases with increasing divacancy concentration. After annealing at 400 \ifmmode^\circ\else\textdegree\fi{}C the spreading resistance measurements reveal a region of shallow hydrogen-related donors at depths 15--21 \ensuremath{\mu}m. The positron annihilation results support the idea that the introduction of shallow donors is due to the formation of hydrogen-vacancy complexes during the annealing.

Positron lifetime measurements on InAs wafers have shown that the positron bulk lifetime in InAs is 246\ifmmode\pm\else\textpm\fi{}2 ps. Most samples exhibit a defect lifetime of 287\ifmmode\pm\else\textpm\fi{}6 ps, which is attributable to monovacancy-impurity complexes with a concentration of 7\ifmmode\pm\else\textpm\fi{}2\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ . Very heavily doped n-type samples exhibit a defect lifetime of 332\char21{}340 ps. The defects in these samples have divacancy character, but change their configuration at low temperatures to a monovacancy-type defect. The concentration of these defects is also close to ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ . Both types of defects are stable for rapid thermal annealing up to 850 \ifmmode^\circ\else\textdegree\fi{}C, and both defects are neutral. It is proposed that the formation of the divacancy-type defects may be correlated with a discrepancy between the carrier concentration and the total dopant concentration.

The bound-state spectra of the positronium hydrides $^{\mathrm{\ensuremath{\infty}}}\mathrm{HPs}$, TPs, DPs, $^{1}\mathrm{HPs}$, and MuPs are considered. The properties of the bound ground S states (L=0) in these systems and the ${\mathrm{Ps}}_{2}$ molecule have been determined by extensive variational calculations. The hyperfine structure of these states is also investigated. The positron annihilation rates have been evaluated for the positronium hydrides, the ${\mathrm{Ps}}_{2}$ molecule, and the ${\mathrm{Ps}}^{\mathrm{\ensuremath{-}}}$ ion and compared. The positron annihilation rates ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{n}\ensuremath{\gamma}}$ (where n\ensuremath{\geqslant}2) in the positronium hydrides are significantly closer to those in the ${\mathrm{Ps}}^{\mathrm{\ensuremath{-}}}$ ion (the three-body system) than in the ${\mathrm{Ps}}_{2}$ molecule.

Vibrational Feshbach resonances are dominant features of positron annihilation for incident positron energies in the range of the molecular vibrations. Studies in relatively small molecules are described that elucidate the role of intramolecular vibrational energy redistribution into near-resonant multimode states, and the subsequent coupling of these modes to the positron continuum, in suppressing or enhancing these resonances. The implications for annihilation in other molecular species, and the necessary ingredients of a more complete theory of resonant positron annihilation, are discussed.

By combining angular correlation results of the positron annihilation quanta with lifetime results it is found that a weak but long-lived (599{plus_minus}10 ps) lifetime component can be correlated with the para-positronium peak in angular correlation measurements when taking into account both pickoff annihilation and conversion of positronium. Conversion has not been considered in earlier works. {copyright} {ital 1996 The American Physical Society.}