A photocathode immersed in an insulating liquid might inject electrons. The analysis of the wavelength versus photocurrent curve enables one to evaluate the energy of the injected, quasi-free electron in the liquid, i.e. the bottom of the conduction level V-0 The pressure dependencies of V-0 observed with several liquid hydrocarbons indicate certain discrepancies between experimental data and theoretical expectations. Thermodynamic analysis shows that, contrary to theoretical descriptions, the injection proceeds via a polytrope which runs between the isothermal-isochoric and isoentropic-isobaric curves. This being attributed to local perturbations of temperature and pressure by the injection process, an estimate of the time scale of injection can be given, finding characteristic times to be in the order of 0.01-0.1 ps.