Photoionization spectrum, spatial distribution function of photo-electrons and evolution of photoionization parameters as a function of excess energy, for some nonpolar dielectric liquids (cyclohexane, 2-2 dimethylbutane, tetramethylsilane and polydimethylsiloxane oils) irradiated by VUV photons

Photocurrents were produced by VUV irradiation, at photon energies from 7 eV to 11 eV, of some pure dielectric liquids. Photoionization spectra are weakly structured. Nevertheless it was possible to detect some inflexions in the derivative spectrum. These characteristic energies were tentatively assigned to Rydberg series. Experimental plots giving the photocurrent as a function of an applied electric field were fitted, using a three-parameter adjustment method, following Onsager theory. The evolution of the deduced parameters (i.e. B1, the penetration length of photons, B3, the thermalization parameter of photo-electrons and the photo-ionization quantum yield) as a function of excess photon energy is examined. Following the Jay-Gerin model and using Monte-Carlo simulations it was concluded that data obtained for VUV injection from the liquid itself and for UV injection from a photocathode match each other very well. Assuming a modified exponential function of thermalization distances mean free paths for momentum exchanges during the thermalization stage are roughly deduced.