Surface structure of liquid metals and the effect of capillary waves:  X-ray studies on liquid indium

Abstract

We report x-ray reflectivity (XR) and small angle off-specular diffuse scattering (DS) measurements from the surface of liquid Indium close to its melting point of 156 • C. From the XR measurements we extract the surface structure factor convolved with fluctuations in the height of the liquid surface. We present a model to describe DS that takes into account the surface structure factor, thermally excited capillary waves and the experimental resolution. The experimentally determined DS follows this model with no adjustable parameters, allowing the surface structure factor to be deconvolved from the thermally excited height fluctuations. The resulting local electron density profile displays exponentially decaying surface induced layering similar to that previously reported for Ga and Hg. We compare the details of the local electron density profiles of liquid In, which is a nearly free electron metal, and liquid Ga, which is considerably more covalent and shows directional bonding in the melt. The oscillatory density profiles have comparable amplitudes in both metals, but surface layering decays over a length scale of 3.5 ± 0.6Å for In and 5.5 ± 0.4Å for Ga. Upon controlled exposure to oxygen, no oxide monolayer is formed on the liquid In surface, unlike the passivating film formed on liquid Gallium.

What x rays can tell us about the interfacial profile of water near hydrophobic surfaces

“…X-ray and neutron reflectivity have been extensively used to determine the density profiles at the free surfaces of many liquids [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] including water, 22,25 and also at liquid-solid [26][27][28] and liquidliquid [29][30] interfaces. These probes are sensitive to electron density and scattering length density respectively, and for a known material these are both measures of the local mass density.…”

Competition between Surface Layering and Surface Phase Formation in Dilute Liquid Hg−Au Alloys

“…Recent atomic-resolution surface scattering measurements have revealed the formation of a wide variety of structures in liquid metals and alloys. Elemental Hg, Ga, and In exhibit surface-induced layering, in which atoms are stratified parallel to the liquid-vapor interface, [11][12][13][14][15] a result long predicted by theory. 16 This stratification of ions, with the corresponding oscillatory surface-normal density profile, is shown schematically in Figure 1.…”

“…12 This is in contrast with the surface roughnesses of liquid Ga and In, for which capillary wave theory accounted well for all the measured temperature dependence. 14,15 In the inset of Figure 7 we compare the excess roughness σ T 2 -σ cw 2 of Hg with that of Hg 0.06 atom % Au. We see that the 0.06 atom % alloy has a larger roughness, but the slope of the excess roughness versus T is the same as for pure Hg.…”

“…The most reliable way to produce atomically clean metal surfaces is to keep them under ultra high vacuum (UHV) conditions, and to remove any residual oxide through argon ion sputtering. These UHV techniques, which we found to be very successful for X-ray scattering measurements of Ga, In, and low vapor pressure alloys, 13,15,19 must be modified in the case of Hg and its alloys due to the high vapor pressure of Hg (∼10 -2 Torr at room temperature). For the measurements presented here, each sample was poured into a stainless steel reservoir within an argon-filled glovebox (<5 ppm oxygen, <2 ppm water).…”

“…Detailed discussions of the effect of capillary waves on the scattering cross section are given elsewhere. 15,21 The capillary wave roughness σ cw is a function of the temperature T and the surface tension γ and is given by This expression 31 has been arrived at by integrating over those capillary modes having wavevectors less than q max ) π/a, where a is the atomic diameter, and greater than q res , determined by the instrumental resolution.…”