Plot of frequency versus wave vector . The panel at the top is for the fast wave (; , ) of Figure 1: curve (a) is for the case when there are no heavier ions (, ) while curve (b) depicts the situation when negatively charged oxygen is also added (, , and ). The lower panel is for the slow wave (; , ) of Figure 1: curve (c) is for the case where only the lighter ions are present (, ) and curve (d) depicts the situation when is also added (, , and ). Other relevant parameters are the same as in Figure 1.