Our annihilation experiments indicate that positrons bind to all but the smallest molecules, and the binding energies for over 80 molecular species have now been measured. They include alkanes, cycloalkanes, halo-methanes, alcohols, aldehydes, ketones, formates, acetates, nitriles, and aromatics. Binding energies range from a few millielectron volts to ≥ 300 meV. Generally, the measured binding energy increases with molecular polarizability and dipole moment, but molecular geometry can also play a role.
While electrons bind weakly to many molecules, positron binding is a factor of 10 – 100 times stronger. Two contributing factors are that positron-electron interactions are attractive, and if there is a permanent dipole moment, the negative end of the dipole is nearer the periphery of the molecule where the positron can enjoy a lower potential energy. Both effects lead to stronger positron binding.
Beyond dipole moment and polarizability, it is found that p bonds on the molecule also lead to stronger binding. Geometry can also play an important role; and so, for example, chlorine substitution in hydrocarbons leads to deeper binding if the chlorine atom is placed near the center of the molecule. Shown in the figure are the wave functions of n-propyl chloride and isopropyl chloride, whose binding energies are 97 and 113 meV, respectively [Swann, Phys. Rev. A (2021)].Model calculations by Swann and Gribakin and by Tachikawa and colleagues capture many of these effects. A recent ab initio many-body theory of positron binding by Green and collaborators elucidates the large (e.g., 30% or more) contribution to positron binding due to virtual positronium formation.
Positron-molecule interactions: Resonant attachment, annihilation, and bound states, G. Gribakin, J.Young, and C. Surko, Rev. Mod. Phys. 82, 2257 (2010).
Many-body theory of positron binding to polyatomic molecules, J. Hofierka, B. Cunningham, C. M. Rawlins, C. H. Patterson, D. G. Green, preprint (2021)
Effect of chlorination on positron binding to hydrocarbons: experiment and theory, A. R. Swann, et al., Phys. Rev. A 104, 012813 (2021)
Positron-electron correlation-polarization potential model for positron binding in polyatomic molecules, Y. Sugiura, et. al., J. Comp. Chem. 41, 1576 (2020).
Effect of molecular constitution and conformation on positron binding and annihilation in alkanes, A. R. Swann and G. F. Gribakin, J. Chem. Phys. 153, 184311 (2020).
Calculation of positron binding and annihilation in polyatomic molecules, A. R. Swann and G. F. Gribakin, J. Chem. Phys. 149, 244305 (2018).
Interplay between permanent dipole moment and polarizability in positron-molecule binding, J. R. Danielson, et al., Phys. Rev. A 85, (2012).
Interplay between permanent dipole moments and polarizability in positron-molecule binding, J. Danielson et al., Phys. Rev. A 85, 022709 (2012).
Dipole Enhancement of Positron Binding to Molecules, J. Danielson et al., Phys. Rev. Lett., 104, 233201 (2010).
Bound states of the positron with nitrile species with a configuration interaction multi-component molecular orbital approach, M. Tachikawa et al., Phys. Chem. Chem. Phys. 13, 2701 (2011).