The existence of plasmon excitation produced by the interaction of fast charged particles in liquid water is examined critically from the theoretical point of view. By the analysis of the Fano index at different excitation energies and by comparing the real and imaginary parts of the dielectric function for both the liquid and the gas at unit density, we find no convincing evidence for plasmon excitation in the condensed phase of water. A density and a condensation effect on the dielectric function are observed. Implications of this finding for diffusion kinetics are discussed briefly. A procedure for the determination of the real and imaginary parts of the dielectric function from the dipole oscillator strength distribution is presented.