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

Gribakin, G. F.; Lee, C.M.R.

doi:10.1140/epjd/e2008-00188-9

Cited by 24 publications

(30 citation statements)

References 68 publications

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“…An example of this is the model wave function for positrons bound to alkanes described in Ref. [17]. For these molecules (with ¼ 0), the positron wave function extends over the entire molecule, and " b is seen to increase $linearly with molecular size.…”

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confidence: 95%

Comparisons of Positron and Electron Binding to Molecules

Danielson

Jones²,

Natisin³

et al. 2012

Phys. Rev. Lett.

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Positron binding to molecules is compared to the analogous electron-molecule bound states. For both, the bound lepton density is diffuse and remains outside the valence shell. Positron binding energies are found to be one to two orders of magnitude larger than those of the negative ions due to two effects: the orientation of the molecular dipole moment allows the positron to approach it more closely and, for positrons, lepton correlations (e.g., via dipole polarizability) contribute more strongly.

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confidence: 95%

Comparisons of Positron and Electron Binding to Molecules

Danielson

Jones²,

Natisin³

et al. 2012

Phys. Rev. Lett.

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“…1, the missing spectral weight appears to be broad in energy and rises at low energies. The SMRA model developed by Gribakin and Lee describes just such a contribution [6]. It assumes that the positron attaches to all energetically allowed multimode vibrational excitations (i.e., not just low-order combinations and overtones, as considered previously [1,4,7]).…”

mentioning

confidence: 99%

“…In the statistically complete limit, and N can be calculated directly from the vibrational mode spectrum [6].…”

mentioning

confidence: 99%

“…The SMRA theory assumes that all multimode vibrations couple with equal strengths to the positron continuum and all such states can act as inelastic escape channels with the same coupling strengths [6]. The fact that < 1 suggests that some class of multimode vibrations has a substantially smaller excitation rate.…”

mentioning

confidence: 99%

“…The theory was originally formulated to explain annihilation spectra of alkanes, C n H 2nþ2 [6]. However, in regions of IVR-enhanced single-mode VFRs, the Z eff in alkanes are larger in magnitude than the SMRA background by factors of 10 to 50.…”

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confidence: 99%

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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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Positron–electron correlation‐polarization potential model for positron binding in polyatomic molecules

et al. 2020

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Positron binding energies (PBEs) of 41 polyatomic molecules were calculated using the positron-electron correlation-polarization potential (CPP) approach and compared with experimentally measured values. In this approach, the short-range positron-electron potential is modeled using the density-functional expression, whereas the long-range potential is approximated by the attractive polarization potential. The positron-electron CPP model based on local-density approximation yields larger PBEs than experimental values; however, the calculated values can be substantially improved by introducing generalized gradient approximation. We also investigated the conformational dependence of PBEs for representative molecules.

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

Cited by 24 publications

References 68 publications

Comparisons of Positron and Electron Binding to Molecules

Comparisons of Positron and Electron Binding to Molecules

Ubiquitous Nature of Multimode Vibrational Resonances in Positron-Molecule Annihilation

Positron–electron correlation‐polarization potential model for positron binding in polyatomic molecules

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