Photoconductivity induced by VUV photons in polydimethylsiloxane and polymethylphenylsiloxane oils

The influence of the incident photon energy between 6 and 11.2 eV and of the applied electric field strength between 0 and 32 kV/cm on the photoconductivity of four polydimethylsiloxane (PDMS) oils of the same structure but of different viscosities, and of three polymethylphenylsiloxane oils of different viscosities and structures, is studied. Results show that, in each class of oils, the photocurrent values measured under identical experimental conditions and the photoionization energy threshold values of the different oils are almost the same. These results on oils of very different molecular chain lengths or chemical structures display the importance of the elementary links (CH₃)₂ SiO and (CH₃)(C₆H₅) SiO in the photoionization mechanism of these silicone oils. With regard to the PDMS oils the fitting of experimental I=f(E) curves corresponding to different photon energies with theoretical expressions based on the one-dimensional and the three-dimensional Onsager probabilities enables the calculation of the order of magnitude of their ionization quantum yield phi _i, of the penetration distance of the photons B₁ and of the thermalization distance of the ejected photoelectrons B₃ in these oils.