Interplay between permanent dipole moments and polarizability in positron-molecule binding

Abstract

Energy-resolved studies of positron-molecule collisions exhibit vibrational Feshbach resonances in annihilation, thus providing evidence that positrons can bind to these species. The downshifts of the observed resonances from the positions of the vibrational modes provides a measure of the positron-molecule binding energies, which range from 1 to 300 meV. Reported here are annihilation spectra and binding energies for a wider range of chemical species than studied previously, including aldehydes, ketones, formates, acetates, and nitriles. While the measured binding energies show an approximate correlation with molecular dipole polarizability and permanent dipole moment, other effects are important for dipole moments 2.0 D. For these compounds, it appears that localization of the positron wave function near a portion of the molecule leads to enhanced binding and an increased dependence on both the molecular dipole moment and the electron-positron correlations. The relationship of these results to theoretical calculations is discussed.

Plasma and trap-based techniques for science with positrons

“…A highlight of the positron-atomic physics results was the discovery that the large annihilation rates on molecules imply that molecules can support positron bound states (Barnes et al, 2003(Barnes et al, , 2006Danielson et al, 2010Danielson et al, , 2012Gilbert et al, 2002;Gribakin et al, 2010;. As illustrated in Fig.…”

“…The positron does not penetrate the valence shell due to repulsion from the nuclei, and the electron is kept outside due to Pauli exclusion. From Danielson et al (2012).…”

Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms

“…In Figure 6, we compare the quantum-mechanical cross sections (18) for Cu and CH 3 F with the corresponding semiclassical cross sections (22) for several values of n. It can be seen that the agreement is very close for incident Ps energies 0.01 eV, even for low n, and particularly for s states of Ps. This comparison also shows that for l n the charge-exchange cross section is almost l independent.…”

“…A comparison of the measured cross section with the theoretical results derived in this paper should provide an estimate of the positron binding energy, which could be compared with existing high-quality predictions [12][13][14]. It would also be interesting to apply this method to molecules for which the binding energies are known from the resonant annihilation studies [15,[19][20][21][22]. Unlike positron-molecule annihilation which probes resonant, quasibound states, the molecular analog of reaction (2) should lead to population of the true positron-molecule bound states.…”

“…The downshift of a resonance relative to the vibrational excitation energy provided a measure of * aswann02@qub.ac.uk † d.cassidy@ucl.ac.uk ‡ a.deller@ucl.ac.uk § g.gribakin@qub.ac.uk the positron binding energy. This has enabled positron binding energies to be determined for over seventy molecules [19][20][21][22].…”