Measurements of positron-annihilation rates on molecules

Iwata, Koji; Greaves, R. G.; Murphy, T. J.; Tinkle, M. D.; Surko, C. M.

doi:10.1103/physreva.51.473

Cited by 128 publications

(172 citation statements)

References 61 publications

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“…Here the measured Z eff values far exceed the number of molecular electrons (e.g., Z ¼ 74 for CCl 4 and 146 for CBr 4 ). The observed rates are much closer to those measured previously with thermal positrons at 293 K, namely Z eff $ 9; 500 and 40 000 for CCl 4 and CBr 4 , respectively [3].…”

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confidence: 70%

“…The binding energies for CCl 4 and CBr 4 , are estimated to be 75 and 125 meV. These values are based on the binding energies of CHCl 3 and CHBr 3 and an empirical formula for " b developed previously (see Eq. (5) in Ref.…”

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confidence: 99%

“…(2), with the contribution of each mode adjusted for best fit, as discussed below. The values of " b in CHCl 3 and CHBr 3 are determined from the downshifts of the VFR peaks in their measured Z eff spectra relative to the energies of the modes, as described by Eq. (1), giving " b ¼ 50 and 130 meV, respectively.…”

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Ubiquitous Nature of Multimode Vibrational Resonances in Positron-Molecule Annihilation

et al. 2012

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Positron annihilation on many molecules occurs via positron capture into vibrational Feshbach resonances, with annihilation rates often further enhanced by energy transfer to vibrational excitations weakly coupled to the positron continuum. Data presented here uncover another scenario in which the positron couples directly to a quasicontinuum of multimode vibrational states. A model that assumes excitation and escape from a statistically complete ensemble of multimode vibrations is presented that reproduces key features of the data. DOI: 10.1103/PhysRevLett.108.093201 PACS numbers: 34.80.Uv, 34.50.Às, 34.80.Lx Positron annihilation on atoms and molecules is important in many areas of science and technology including positron emission tomography in medicine, astrophysics, and studies of material properties. For many molecules, the annihilation rates as a function of incident positron energy " exhibit vibrational Feshbach resonances (VFRs) (e.g., populated via excitation of infrared-active modes) [1]. These resonances occur at specific values of incident positron energy given bywhere ! is the vibrational mode energy and " b is the positron-molecule binding energy. The observation of these VFRs indicates that the molecule can form a bound state with a positron. The magnitudes of these VFRs are often further enhanced by intramolecular vibrational energy redistribution (IVR). A theory has been developed to explain quantitatively VFR-enhanced annihilation rates in selected small molecules [2]. However the details of the IVR enhancements remain sketchy [1]. Described here are new data for positron annihilation resolved as a function of positron energy. The initial motivation was to understand the annihilation spectra in the small molecules CCl 4 and CBr 4 that are known to have very large annihilation rates for thermal positrons at 293 K. For these targets, simple theoretical estimates for VFRs and IVR-enhanced VFRs appeared to be incapable of explaining the magnitudes of the observed rates. In particular, the positron binding energy in CBr 4 is expected to be greater than all of the mode energies. This makes the VFR energies in Eq. (1) negative, thus shutting off the resonant process.Based upon the study described here, we are led to conclude that broad quasicontinuous spectra of multimode vibrations (e.g., combinations and overtones) are responsible for the observed behavior. Further, this mechanism appears to be universal in the sense that it is present, at least at a significant level, in the spectra of most molecules studied to date. Qualitative agreement is obtained between the experimental data and the predictions of a model that assumes positron capture into (and possible detachment from) a statistically complete ensemble of strongly mixed multiquantum vibrational states. We term this mechanism statistical multimode resonant annihilation (SMRA). The more general implications of these results are discussed.The experimental procedures are described in [1]. An energy tunable, trap-based positron beam is used t...

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Ubiquitous Nature of Multimode Vibrational Resonances in Positron-Molecule Annihilation

et al. 2012

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“…The annihilation parameters Z e f f is about 150 for positron temperature 293K (thermal energy ∼ 0.024 eV, momentum ∼ 0.042a −1 o ) which means that at room temperature probability that the positron annihilate is great. The present value of Zeff is less than the measurements of UCL group [29,30] which is 400-450 and the measurements of San Diego group, (Z e f f = 400) [31,32].…”

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confidence: 99%

Variational calculation of low-energy positron life time in Xe atoms

Abdel-Mageed

2010

Braz. J. Phys.

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The least-squares variational method (LSVM) is used for determining trial wavefunctions representing lowenergy positron-xenon elastic scattering, which are used to determine the positron lifetime. The trial function is taken to depend on several adjustable parameters, and is improved iteratively by increasing the number of terms. The bound state wavefunctions are obtained using Hartree-Fock-Slater method. The 2γ-rays annihilation rates of the positron in xenon atoms below Positronuim (Ps) formation threshold are calculated. Polarization potential V pl (r) is applied to enhance annihilation rates and lifetime. The present results of annihilation parameters are consistent with semiempirical, theoretical and experimental results.

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“…One of the outstanding results of positron scattering is the very large annihilation rates for the organic molecules [1]. In a recent Letter, Laricchia and Wilkin [2] postulated that the large values of Z eff were caused by two mechanisms.…”

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confidence: 99%