A model dielectric function for low and very high momentum transfer

A model dielectric function is derived for TiO2 based on reflection electron energy loss spectroscopy data and photoabsorption cross sections. The model is based on a set of Mermin oscillators. The input data is dominated by excitations at low momentum transfer, i.e. near the optical limit. Surprisingly the dielectric function derived at low momentum transfer describes the Compton profile quite well, while approaches based on Drude oscillators fail dramatically. The link between the dielectric function in the high-momentum transfer limit and a Compton profile is discussed. The underlying reason why the Mermin approach, which is based on a free electron model, is successful in describing the Compton profile is tentatively discussed.