Modeling the energy and momentum dependent loss function of the valence shells of liquid water

Within the Born approximation the loss function is the important material property for describing the inelastic interaction of charged particles with the extended electronic subsystem of condensed media. Following the dielectric approach as elaborated by Ritchie and co-workers we determine the loss function of liquid water by a two-step process: (i) an optical energy-loss model is deduced by an analytic representation of the available data at the long wavelength limit, and (ii) the momentum dependence is introduced by simple dispersion models which provide characteristic features of the Bethe surface. By this semi-empirical procedure many-body effects such as polarization, correlation and collective excitations, which are still impractical to compute, are accounted for in a self-consistent manner. Effects on the Bethe surface characteristics of liquid water associated with the choice of the optical-data model and its extension to the momentum plane are explored.