Positronium contact density in ferrocene

In a medium where positronium (Ps) is formed, a lowering of the electron density at the positron position (contact density) with respect to a vacuum is generally observed. The effect is usually attributed to the interaction with the medium, which induces Ps polarization. The contact density reduction, explainable in polar media due to Debye forces, can be expected also in non-polar media, where London forces are present. To investigate this subject we measured the Ps contact density in ferrocene, an organometallic compound whose molecule does not have an intrinsic dipole moment; therefore, only induced dipole-induced dipole interactions may be present. Contact density was obtained by means of two different experimental techniques. A lowering by 23% with respect to vacuum value was observed, which allowed us a simple estimation of the order of magnitude of the effective microscopic electric field mimicking the Ps-ferrocene interaction.