Excitation Energy of Liquid Helium. II

The excitation spectrum of liquid Helium II is re-examined using the formalism de-veloped by Sunakawa et a!. In this formalism, certain · divergent terms appear-a constant term and the terms involving products of two field operators. We show that it is necessary to retain them in order to obtain convergent results in pertubation calculations. Results for the energy spectrum and structure factor are derived in perturbation theory and are thus shown to be convergent in contrast to the earlier observations by Sunakawa et al. to the· contrary. Similar corrections are also derived for the temperature dependent formalism de-veloped recently by Kebukawa. The new expression for the excitation energy is numerically evaluated in such a way as not to depend on model interatomic potentials and is found only to be in fair agreement with experimental result