Positron annihilation in methane gas

Smith, P.M.; Paul, D. A. L.

doi:10.1139/p70-370

Cited by 38 publications

(23 citation statements)

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“…At low incident energies this interaction may increase the collision cross section σ above the value determined by the geometric size of the atom or molecule, if a virtual (κ < 0) or a shallow bound (κ > 0) s state exists for the positron-atom or positron-molecule system at ε 0 = ±h 2 κ 2 /2m. In this situation the scattering length a = κ −1 and the cross section at zero energy σ = 4πa 2 = 4πκ −2 can be much greater than the size of the atom or molecule [13,26]. This effect can explain the rapid increase and large values of Z eff in Ar, Kr and Xe [14,15].…”

Section: A Direct Annihilationmentioning

confidence: 98%

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Positron annihilation on large molecules

et al. 2000

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Positron annihilation on molecules is known to depend sensitively on molecular structure. For example, in the case of hydrocarbon molecules, modest changes in molecular size produce orders of magnitude changes in the observed annihilation rates. Although this process has been studied for more than three decades, many open questions remain. Experimental studies are described which are designed to test specific features of the annihilation process. Two possible mechanisms of the annihilation are considered theoretically: direct annihilation of the positron with one of the molecular electrons, including possible enhancement of this process when low-lying virtual or bound positron-molecule states are present, and resonant annihilation through positron capture into vibrationally excited states of the positronmolecule complex. The dependence of annihilation rates, λ, on positron temperature, T p , is studied for the first time for molecules, and at low values of T p the dependence follows a power law, λ ∝ T −ξ , with ξ ≈ 0.5. These data are used to test the predictions of direct numerical calculations and theories of the virtual-level enhancement. Partially fluorinated hydrocarbons are studied in order to understand the rapid changes in annihilation rate produced in hydrocarbons as a result of fluorine substitution. These data are compared with the behavior expected due to direct annihilation when there is virtual or bound level enhancement. Measurements of positron annihilation on deuterated hydrocarbons are described which test the dependence of the annihilation on the nature of the molecular vibrations. The relationship of the presently available experimental data for annihilation in molecules to current theories of the annihilation process is discussed.

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Section: A Direct Annihilationmentioning

confidence: 98%

“…Enhancement of annihilation due to the excitation of a single resonance was considered theoretically in [26] and [30]. The possibility of forming such resonances by excitation of the vibrational degrees of freedom of molecules was discussed by Surko et al [5].…”

Section: B Resonant Annihilationmentioning

confidence: 99%

Positron annihilation on large molecules

et al. 2000

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“…Resonant processes have long been suspected to be the origin of anomalously large positron annihilation rates observed in many polyatomic gases [1,2,3,4]. This explanation remained a hypothesis until vibrational resonances were actually observed using a high-resolution trap-based positron beam [5].…”

Section: Introductionmentioning

confidence: 99%

Resonant positron annihilation in ammonia

Gribakin

2010

J. Phys.: Conf. Ser.

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Abstract. Positron annihilation in ammonia is analyzed using the framework of resonant annihilation [G. F. Gribakin and C. M. R. Lee, Phys. Rev. Lett. 97, 193201 (2006)]. In particular, we show that molecular rotations can have a measurable effect on the annihilation rates at room temperatures. Rotation leads to broadening of vibrational Feshbach resonances. Rotations also allow a distinct contribution at low positron energies in the form of a rotational Feshbach resonance. This resonance can enhance the annihilation rate for thermalized roomtemperature positrons. Comparison of theory and experiment shows that overtone and combination vibrations, including those due to inversion doubling, likely play an important role.

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“…In spite of the gross discrepancy between experimental data and naïve view of Z eff , the problem remained poorly understood for decades. Explanations of high molecular Z eff were sought in terms of positron virtual or weakly bound states [23], resonances [24,25], or long-lived vibrationally excited positron-molecule complexes [17]. All of these have now become part of the comprehensive picture that is emerging thanks to a concerted effort from theory [26][27][28] and energy-resolved annihilation measurements [19][20][21][22].…”

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confidence: 99%

Positron annihilation in large polyatomic molecules. The role of vibrational Feshbach resonances and binding

Gribakin

Lee

2008

Eur. Phys. J. D

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Abstract. We analyse the process of rapid positron annihilation in large polyatomic molecules due to positron capture into vibrational Feshbach resonances. Resonant annihilation occurs in molecules which can bind positrons, and we analyse positron binding to alkanes using zero-range potentials. Related questions of spectra of annihilation gamma quanta and molecular fragmentation following annihilation, are discussed briefly.

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Positron annihilation in methane gas

Cited by 38 publications

References 0 publications

Positron annihilation on large molecules

Positron annihilation on large molecules

Resonant positron annihilation in ammonia

Positron annihilation in large polyatomic molecules. The role of vibrational Feshbach resonances and binding

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