This paper deals with high-resolution neutron scattering studies of the librational mode v 6 (0 → 1) and its higher overtones for NH + 4 in the ordered cubic phase of ammonium bromide, NH 4Br. Experiments were conducted on the time-focused crystal analyser (TFXA) spectrometer of the UK spallation neutron source, ISIS. v 6 (0 → 2) and v 6 (0 → 3) show distinct splittings of ≈ 5 meV, the results are compared with optical data and a theoretical model.