  Aqueous solution of tetrabutylammonium bromide is studied by quasi-elastic
neutron scattering, to give information on the dynamic modes involving the ions
present. Using a careful combination of two techniques, time-of-flight (TOF)
and neutron spin echo (NSE), we de- couple the dynamic information in both the
coherently and incoherently scattered signal from this system. We take
advantage of the different intensity ratio of the two signals, as detected by
each of the techniques, to achieve this decoupling. By using heavy water as the
sol- vent, the tetrabutylammonium cation is the only hydrogen-containing
species in the system and gives rise to a significant incoherent scattered
intensity. The dynamic analysis of the incoherent signal (measured by TOF)
leads to a translational diffusion coefficient of the cation as that is in good
agreement with previous NMR, neutron scattering and tracer diffusion
measurements. The dynamic analysis of the coherent signal observed at
wave-vectors < 0.6 angstrom^(-1) (measured by NSE) leads to a significantly
slower diffusive mode, by a factor of 2. This study explains that apparent
difference and the reasons for that.
