Time-of-flight measurements have been performed on the ionic system Ca0.4K0.6(NO3)1.4 in the equilibrium liquid phase, in order to determine the temperature and wavenumber dependences of the quasielastic line width. The quasielastic neutron scattering has been examined by convoluting the resolution function with three different models, Lorentzian, Lorentzian raised to the power 2/3 (a good approximation for a Kohlrausch line shape with β ≈ 0.7), and two Lorentzians. The results show a line shape incompatible with a single Lorentzian form. The Kohlrausch line width shows a marked deviation from the Dq2 behaviour expected in the incoherent approximation. Analogous deviations arise for the narrower main contribution from the two-Lorentzian model. Both of these models can be interpreted qualitatively in terms of mode-coupling theories, but the experimental results show no clear distinction between them, or even between them and more complex models. Thus no meaningful quantitative comparison to theory can be obtained, an ambiguity which is endemic for studies of glass phenomena.
