Dielectric response of a silicone liquid excited by VUV radiation and electron scattering

The frequency-dependent dielectric properties of liquid silicone diffusion pump oil, Dow-Corning DC705, were determined by two methods. First, the optical properties were redetermined from literature data and from new measurements taken in the vacuum ultraviolet to ~ 25 eV. Second, electron energy loss data were obtained by energy analysis of scattered electrons produced by incident beams of electrons with energies between 35 and 300 eV. A sharp structure near 7 eV appeared in the optical data, was reflected in the electron-scattering data, and was attributed to a π → π^{{black star}} transition in the phenyl groups of the molecule. Broad structure was observed in the optical loss function near 15 eV where conditions for surface collective effects are most closely satisfied. A direct observation of a corresponding peak in the electron energy loss data provides additional support for the existence of surface collective effects. Thus, many-body as well as single particle effects appear to have a role in the properties of this insulating dielectric liquid.