Effect of chlorination on positron binding to hydrocarbons: Experiment and theory

Abstract

Measured and calculated positron binding energies are presented for a range of hydrocarbons with up to six carbon atoms (viz., methane, acetylene, ethylene, ethane, propane, butane, and hexane) and their chlorinated counterparts. Both experiment and theory confirm the large effect that the chlorine atoms have on the positron binding energy and the strong sensitivity of the binding energy to the exact position of the chlorine atoms. The experimental binding energies have been obtained by measuring positron resonant annihilation using a trapbased positron beam. The calculations are performed using the previously developed model-correlation-potential method [A. R. Swann and G. F. Gribakin, J. Chem. Phys. 149, 244305 (2018)]. The overall trends are discussed with regard to the molecular polarizability, dipole moment, and geometry. Good agreement between theory and experiment is found, with the exception of the chlorinated ethylenes and chlorinated hexane. Calculations of the electron-positron annihilation rate in the bound state are also presented.

Improving polarization description in positron-molecule scattering: a model potential approach

“…In order to properly address this matter, Swann and Gribakin 28,31 expanded the work that Mitroy and Ivanov have performed for atoms in order to obtain better calculations for bound states and scattering cross sections. Their work used a model potential to describe the correlation and polarization interactions between the incoming positron and the target.…”

Many-body Theory Calculations of Positron Binding to Halogenated Hydrocarbons

“…Trap-based positron beams have enabled resonantannihilation-based measurements of positron binding energies to around 90 molecules over the last two decades (see e.g., [1][2][3][4][5]). Whilst the corresponding model of positron capture into vibrational Feshbach resonances is well established [6][7][8], progress in the accurate calculation of positron binding energies has only been made relatively recently (see e.g., [9][10][11][12][13][14][15][16]). Whilst attempts have been made to relate the observed binding energies to the coarse molecular properties including the dipole moment, polarizability and ionization potential [1,3,15], no universal formula has been established.…”

“…The ansiotropy of the positron-molecule potential, not captured by the coarse molecular properties, is thus important. Recently, Swann and Gribakin applied a modelpolarization-potential method [10] to study binding in a range of chlorinated hydrocarbons, in a joint theory-experimental study with measurements performed by the Surko group [13]. Whilst the model calculations were in excellent agreement with experiment for many of the molecules considered, for planar molecules the calculations substantially overestimated the measured binding energies, with the suggestion that this was due to their model assuming a long-range isotropic positronmolecule interaction [19].…”

“…Whilst the model calculations were in excellent agreement with experiment for many of the molecules considered, for planar molecules the calculations substantially overestimated the measured binding energies, with the suggestion that this was due to their model assuming a long-range isotropic positronmolecule interaction [19]. Indeed, DFT-model calculations performed by Suzuki et al for planar chloroethylenes [14] accounted for the anisotropy approximately and found better agreement with experiment, but in some cases still overestimated the binding energy, e.g., 80 meV DFT vs. 66 ± 10 meV experiment [13] for cis-1,2-dichloroethylene.…”

“…The purpose of the present study is twofold. First, we apply our MBT approach in the fixed-nuclei approximation [20] to study positron binding in the chlorinated hydrocarbons considered in the recent model calculations [13,14] and experiment [13], accounting for the positron-molecule correlations and anisotropic non-local potential ab initio. We find excellent agreement with experiment, and confirm the veracity of the (computationally much cheaper) DFT-based model for the planar molecules.…”