Positron annihilation studies in oxides

Sen, P.; Patro, A.P.

doi:10.1007/bf02711014

Cited by 25 publications

(7 citation statements)

References 13 publications

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“…Measurements of a series of Rydberg transitions indicate that these energy shifts affect only the lower-lying levels, which we attribute to resonant interactions between these Ps levels and overlapping MgO PL absorption bands. Since such UV PL properties are very common in materials that are suitable for Ps confinement, similar shifts can be expected in other useful Ps forming materials as well [81]. The existence of these shifts could affect attempts to employ spectroscopic characterization and control (particularly laser cooling) of confined Ps, which are required for the optimization of experiments involving high-density Ps ensembles.…”

Section: Discussionmentioning

confidence: 79%

Resonant shifts of positronium energy levels in MgO powder

et al. 2020

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S transitions optically driven in positronium atoms while they are inside the open volumes of MgO smoke powder. The observed intervals are larger than the corresponding vacuum excitations, but, surprisingly, the transitions to Rydberg states are less strongly affected, and the energy shifts exhibit no dependence on the principal quantum number n of the final state. We attribute these shifts to resonant interactions between Ps atoms and MgO surfaces, mediated via spectrally overlapping MgO ultra violet (UV) photo-luminescent absorption bands. Since many insulating materials suitable for Ps confinement exhibit similar broadband UV absorption characteristics, the observed phenomena have implications for optical diagnostics and laser cooling schemes of relevance to studies of high-density Ps ensembles in insulating cavities, including the production of a Ps Bose-Einstein condensate.

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Section: Discussionmentioning

confidence: 79%

Resonant shifts of positronium energy levels in MgO powder

et al. 2020

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“…Because the gain of the spectrometer was kept as 0.195 ns/channel, the short-lived components τ 1 and τ 2 in the lifetime spectra were not reliably extracted and will not be discussed further. The search for the fifth component with very long lifetime of the order of 100 ns, usually observed in pure silica gel, − did not yield a good result. A summary of the lifetime data (τ 1 − τ 4 ) is presented in Table .…”

Section: Resultsmentioning

confidence: 98%

Positron Annihilation Studies in Silica Supported Nickel Carbonate Systems

and¹,

Manohar²,

Sudarshan³

et al. 2002

J. Phys. Chem. B

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A number of silica supported nickel carbonate systems were studied by positron annihilation lifetime spectroscopy (PAS) and nitrogen gas adsorption method (BET) to evaluate the pore size distribution in the samples. Results show that the long-lived o-Ps component gives a pore size much smaller than the average pore radius obtained from BET analysis. This indicates that o-Ps represents the pores of smaller size in a sample with wide pore size distribution.

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“…16 In this study it is not clear whether the Ps signal is formed in or while escaping the overlayer. If formed in the overlayer, Ps could also be trapped in the amorphous region exhibiting a behavior similar to positron trapping whereby a pickoff process occurs and the Ps decays by two photons.…”

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confidence: 81%

Observation of the Amorphous-to-Crystalline Surface Transition in Al-AlxOyUsing Slow Positrons

Lynn

1980

Phys. Rev. Lett.

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The amorphous-to-crystalline surface transition of k\ x O y on the Al(lll) surface is observed between 650 and 800 K with different 0 2 exposures by measuring the positronium (Ps) fraction produced by e* impinging on the surface. The data are interpreted in terms of vacancy-type defects in the film or at the metal-metal-oxide interface which as trapping sites for e + or Ps. As the ordering process proceeds to completion the trapping centers anneal out and the Ps fraction increases, showing an irreversible transition. This technique provides a new experimental method to study interfaces. PACS numbers: 68.20.+ t, 64.70.Kb, 78.70.Bj With the advent of efficient monoenergetic positron sources incorporated in ultrahigh-vacuum systems, it is now possible to study positron interactions with well-characterized surfaces. In a recent series of papers 1-5 the behavior of positrons at a number of metal surfaces has been reported. In some metals an implanted positron can (i) be reemitted into the vacuum, 2 (ii) escape with an electron and form positronium (Ps), 1,s or (iii) annihilate in the perfect lattice, in a lattice vacancy, or in the surface state. 4 ' 5 The positron can be thermally activated from this surface state and the depth of the potential well can be deduced. 4 " 6 In all clean metals studied the formation of Ps has been found to be the dominant mechanism for the escape of positrons from a metalvacuum interface.In the present study we have exposed an Al(lll) single-crystal surface to molecular oxygen (0 2 ). The fraction of Ps produced at room temperature increased initially with increasing 0 2 exposures saturating at less than 500 L 0 2 (1 langmuir = 1L = 10~6 Torr sec) and subsequently decreased at larger oxygen exposures. After separate total exposures of 500 L and 10 6 L 0 2 , the surface was examined with low-energy positrons as a function of sample temperature, at various incident positron energies, E. When the temperature at which the transition from amorphous to ordered Al x O y occurs at the surface the Ps fraction increases and is identified as the removal of trapping centers (vacancy-type defects) for positrons or Ps.The experimental details of the apparatus and the method of measurement have been described elsewhere. 3 In this experiment the existence of Ps (bound electron-positron state) is detected by observing changes in the energy spectrum produced by the three-photon emission that occurs upon the decay of orthopositronium ( 3 S 1 ). 7 The incident positron energy was varied by biasing the target at different voltages. The sample was cleaned by repeated cycles of Ar-ion bombardment and annealing (>400 C). Surface characterization was performed by low-energy electron diffraction (LEED) and Auger-electron spectroscopy (AES). Before the sample was exposed to 0 2 , no detectable impurities were observed by AES. The samples were maintained in a vacuum of ~1 xlO" 10 Torr during the experimental run (<8 h) except for those periods of 0 2 exposure. After each run AES spectra were taken again and no d...

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Positron annihilation studies in oxides

Cited by 25 publications

References 13 publications

Resonant shifts of positronium energy levels in MgO powder

Resonant shifts of positronium energy levels in MgO powder

Positron Annihilation Studies in Silica Supported Nickel Carbonate Systems

Observation of the Amorphous-to-Crystalline Surface Transition in Al-AlxOyUsing Slow Positrons

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