Positronium formation in molecular media: The effect of the external electric field

Abstract

A model is described for positronium (Ps) formation in molecular liquids and polymers. The developed model explicitly takes into account the electrostatic attraction between a deenergized positron and the terminal positron blob. This interaction is caused by the Debye screening of ${e}^{+}$ by intrablob electrons. The positron distribution function is divided into two parts, one describing the positrons ${e}_{\mathrm{in}}^{+}$ residing within the blob and the other describing the positrons ${e}_{\mathrm{out}}^{+}$ originally thermalized outside the blob. Because of the energy preference for ${e}^{+}$ to reside within the blob, diffusing ${e}_{\mathrm{out}}^{+}$ is partially converted to ${e}_{\mathrm{in}}^{+}$. An external electric field shifts the ${e}_{\mathrm{out}}^{+}$ distribution aside from the center of the blob, reduces its diffusion flux into the blob and decreases the Ps yield. The model is compared with experimental data of the electric field effect on Ps formation in neat liquids, amorphous $\mathrm{Si}{\mathrm{O}}_{2}$, and polymers.