Beyond the Point Ps Approximation

Stepanov, S. V.; Zvezhinskiy, Dmitry; Byakov, V. M.

doi:10.4028/www.scientific.net/msf.733.7

Cited by 18 publications

(25 citation statements)

References 13 publications

(24 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…However, by definition, this assumption gives no information about contact densities. Another family of models consider both Ps constituent particles as independent but confined into the cavity [5,6], or interacting with the material through effective potentials [7], or finally forming a bubble-like state in a dielectric medium with special assumption about work functions [8]. Only the last model can describe the wellknown observation that the contact density is usually found to be well below the vacuum value, while all other models predict an increase.…”

Section: Introductionmentioning

confidence: 99%

Numerical Solution of a Two-Particle Model of Positronium Confined in Small Cavities

2017

View full text Add to dashboard Cite

The positronium atom (Ps) is widely used as a probe to characterize nanoporous and mesoporous materials. Existing theoretical models for describing Ps annihilation rates by pick-off processes generally treat Ps as a point particle confined in a potential well. Hence these models do not justify any change in the internal structure of Ps, which is experimentally accessible by means of the contact density parameter. Recently we formulated a twoparticle model in which only the electron is confined in the cavity, while the positron is moving freely and feels the medium via a positive work function. We present here a numerical treatment of the problem of calculating contact densities and pick-off annihilation rates, by using a variational method. Results are in agreement with experimental data for a large class of materials, and suggest a way to connect these data with pore sizes and positron work functions.

show abstract

Section: Introductionmentioning

confidence: 99%

Numerical Solution of a Two-Particle Model of Positronium Confined in Small Cavities

2017

View full text Add to dashboard Cite

show abstract

“…There are some theoretical descriptions of positronium decay characteristics at various shapes of potential (see e.g. [3]), accounting for the internal structure of Ps atom [4], etc. However, for the sake of simplicity, recommended in the case of application to the material studies, the model of structureless Ps particle in a step-wise (rectangular) potential is commonly accepted.…”

mentioning

confidence: 99%

The size of smallest subnanometric voids estimated by positron annihilation method. Correction to the Tao-Eldrup model

Zgardzińska

2015

Chemical Physics Letters

View full text Add to dashboard Cite

“…A different approach is considered in Ref. [15]: the electron-positron pair bounded by Coulombic interaction forms a bubblelike state in a dielectric medium. A contact density lowering is demonstrated, but the results are not easily applicable to real materials due to special assumptions about work functions.…”

mentioning

confidence: 99%

“…Another independent measurable quantity is the rate of pickoff annihilation λ PO , well studied by many authors [3,6,8,12,15,21]. The annihilation rate of o-Ps in a small cavity is largely dominated by pickoff and can be expressed as…”

mentioning

confidence: 99%

Positronium Confinement in Small Cavities: A Two-Particle Model for the Lowering of Contact Density

2016

View full text Add to dashboard Cite

Positronium (Ps) is widely used as a probe for studying nanometric porosities in condensed matter. Accessible experimental measurements concern annihilation rates by pickoff processes and contact densities (the electron density at the positron position). Existing models for describing Ps properties in small cavities do not justify the lowering of the contact density with respect to that of Ps in vacuum, as found in most materials. We formulate a two-particle model in which only the electron is confined in the cavity, while the positron is moving freely and feels the medium via a positive work function. Our calculation fully explains experimental data for a large class of materials and suggests a way to gain information on pore sizes and positron work functions. DOI: 10.1103/PhysRevLett.116.033401 Positronium (Ps) is a hydrogenlike bound state of an electron and its antiparticle, the positron. Ps can exist in two different states: the singlet state p-Ps, rapidly decaying into 2γ rays with a lifetime of 0.125 ns, and the triplet state o-Ps annihilating in 3γ rays with a lifetime of 142 ns in vacuum [1]. Ps is usually formed by implanting positrons in condensed matter. Many insulators and molecular solids allow Ps formation, sometimes with high efficiency; when generated near the surface (e.g., by using a slow positron beam) Ps can rapidly spread toward the surface and eventually be released into the vacuum [2]. In porous materials, o-Ps is also emitted into cavities, and, mainly in subnanometricsized voids, its lifetime can be notably reduced by pickoff annihilation: the o-Ps positron can annihilate via 2γ decay with an electron of the surrounding cavity walls if a relative singlet state is realized [3]. This property is at the heart of the use of Ps as a probe for studying nanoscale structures in condensed matter. In fact, while 2γ annihilation in vacuum depends on the electron density at the positron position (the contact density [4]), pickoff processes depend on the electron density in the bulk around the cavity.Experimental data obtained with the positron annihilation lifetime spectroscopy technique, also in the presence of a magnetic field (via magnetic quenching), primarily concern pickoff annihilation with lifetimes as low as a few ns [5,6]. The contact density can be determined from fitting lifetime spectra [7]. In molecular solids, this quantity is usually found to be well below the vacuum value. It is of paramount importance to connect these measurable quantities with the properties of Ps in small cavities as a means to obtain information on pore dimensions and other material characteristics.To describe Ps inside small cavities, the most used models are based on the Tao-Eldrup approach [8][9][10][11], which relates pickoff annihilation rates to pore sizes by considering Ps as a single quantum particle. Consequently, these models cannot give information about contact density and are suitable only when cavities are much larger than twice the Ps Bohr radius α 0 ¼ 0.106 nm. Another family of models, more ...

show abstract

Beyond the Point Ps Approximation

Cited by 18 publications

References 13 publications

Numerical Solution of a Two-Particle Model of Positronium Confined in Small Cavities

Numerical Solution of a Two-Particle Model of Positronium Confined in Small Cavities

The size of smallest subnanometric voids estimated by positron annihilation method. Correction to the Tao-Eldrup model

Positronium Confinement in Small Cavities: A Two-Particle Model for the Lowering of Contact Density

Contact Info

Product

Resources

About