High-frequency dynamics in the near-surface region studied by inelastic x-ray scattering: the case of liquid indium

Inelastic x-ray scattering in grazing angle geometry provides a novel tool for studying the surface and bulk lattice dynamics in a single experiment by varying the incidence angle around the critical angle of total reflection. At very small incidence angles (below the critical angle), it is possible to study the collective dynamics in a subsurface region of a few nanometres at interatomic length and time scales. An experimental study on liquid indium in the near-surface region is presented here and the results are analysed within a theoretical framework, based on classical hydrodynamics for the height-height fluctuations (capillary waves and non-propagating fluctuations) and generalized hydrodynamics for the bulk density fluctuations. The investigation reveals the presence of capillary waves in the inelastic x-ray spectra as an additional contribution at zero-energy transfer and a modification of the bulk density fluctuation contribution. A longer structural relaxation time and a larger longitudinal viscosity with respect to bulk indium are observed, similarly to related studies in confined liquids.