Collective modes in a saturated lithium-ammonia solution as a probe of the response of the low-density homogeneous electron gas

The ion dynamics of a saturated lithium ammonia solution was investigated by inelastic neutron scattering with optimized resolution. The experimental dispersion curve and mode damping, here carefully probed in a wave-vector range extending well below and above 1 A−1, display visible anomalies around 0.8 A−1. We relate the dispersion relation anomaly to the shape of the electron gas dielectric function, which screens the ion-ion potential in a way peculiar of the low electron density of the system. The increase of the damping around 0.8 A−1 also supports the presence of a new decay channel for the collective excitations. An interpretation of these effects, based on the low electron-density properties of the lithium-ammonia solution, is proposed.