The possibility of transverse excitation modes in liquid Ga.

The dynamic structure factor S(Q,ω) of liquid Ga was measured at 100 °C using a high resolution inelastic x-ray scattering (IXS) spectrometer at 3-ID-C/APS. The spectra obtained clearly demonstrate the existence of longitudinal propagating modes at small Q values, like a previous IXS result at 42 °C obtained by Scopigno et al and an inelastic neutron scattering (INS) one at 47 °C obtained by Bove et al, but unlike an INS study at 57 °C by Bermejo et al. The dispersion relation of the excitations deviates positively from the hydrodynamic prediction by about 13%. There are two new findings from this experiment. Firstly, an additional lower energy excitation is necessary to reproduce S(Q,ω) spectra in the Q range beyond 10 nm(-1), in agreement with the result of a first-principles molecular dynamic simulation, which may indicate a transverse acoustic mode in this peculiar liquid metal. Secondly, the quasielastic line comprises a Gaussian contribution at Q values near the first maximum in S(Q), which may indicate the existence of short-lived covalent correlation in liquid Ga with a lifetime of 0.39 ps.