X-ray reflectivity study of temperature-dependent surface layering in liquid Hg

Abstract

We report x-ray reflectivity measurements of liquid mercury between Ϫ36°C and ϩ25°C. The surface structure can be described by a layered density profile convolved with a thermal roughness T . The layering has a spacing of 2.72 Å and an exponential decay length of 5.0 Å. Surprisingly, T is found to increase considerably faster with temperature than the ͱT behavior predicted by capillary wave theory, in contrast with previous measurements on Ga and dielectric liquids. ͓S0163-1829͑98͒52144-3͔The effect of temperature on the surface structure of a liquid is quite different from that of a solid surface. In the liquid, thermal surface waves are excited at all wavelengths from the particle spacing to a long wavelength gravitational cutoff, and produce a surface roughness on the order of the particle spacing. For nonmetallic liquids, these capillary waves broaden the liquid-vapor interface, which is a monotonically decreasing density profile with a width of several Å. 1 Metallic liquids exhibit a more complex surface structure in which the atoms are stratified parallel to the liquid-vapor interface in layers that persist into the bulk for a few atomic diameters. This layering arises from the strongly densitydependent nonlocal interionic potential 2 and is unique to metallic liquids. Nevertheless, the effect of temperature is still expected to be principally in the form of capillary waves, which roughen the surface-normal profile and diminish the layering amplitude. X-ray reflectivity measurements have confirmed the existence of surface layering in liquid Hg, 3 Ga, 4 In, 5 and several alloys. 6,7 Models based on capillary waves as the only mechanism for surface roughening were found to describe the temperature-dependent layering of liquid Ga 8 as well as diffuse scattering measured from the liquid In surface. 5 In previous comparisons of the x-ray reflectivity of liquid Hg and Ga, 9 two important differences were identified. Although reflectivities for both metals exhibit quasi-Bragg peaks indicative of surface layering, the Hg data have a minimum at low-momentum transfer (q z Ϸ0.6 Å Ϫ1 ) not found in Ga ͑see Fig. 2 in Ref. 9͒. To describe this minimum, the model for the surface structure had to be more complex than that of Ga. One successful model incorporated a low density region persisting a few Å into the vapor side of the interface.This feature was taken to be intrinsic to Hg. In addition, the room-temperature layering peak appeared to be broader for Hg than for Ga, leading to the conclusion that in Hg, surface layering decayed over a much shorter length scale.Subsequent measurements yielded variations in this lowq z minimum, prompting us to address the possible effect of impurities at the surface. An important difference between experiments on Hg and Ga is that due to the low vapor pressure, Ga can be measured under ultra-high-vacuum ͑UHV͒ conditions and cleaned in situ by argon sputtering, which is not possible for Hg. In this work, we compare Hg measured in a reducing H 2 atmosphere 10 to measurements taken in a U...

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.…”

“…At low Au concentrations, the main effect of Au alloying is to partially disrupt the layered profile near the surface, compared to the structure of Hg. Model profiles for pure Hg at +25°C (s) and -36°C (---), calculated from eq 12 in studies reported previously, 12 are shown in Figure 7a. The oscillatory profile has a layer spacing d ) 2.72 Å, a decay parameter σ j ) 0.46 Å, and a temperature-dependent roughness σ T , given in Table 1, that determines the amplitude of the layers in the density profile.…”

“…We have fit these measurements to a simple layered density profile having an additional low-density term positioned near the liquid-vapor interface, as described by eq 12. Since this is the parametrization used previously in studies of pure liquid Hg, 12 we can compare the pure and alloyed systems quantitatively. The layer spacing d, the thermal roughness σ T , and the decay parameter σ j are parameters which characterize the layered subsurface structure and are all well determined by the fits.…”

“…The empty chamber is evacuated to 10 -7 Torr and baked out, after which the liquid Hg alloy is dropped from the reservoir through the valve and capillary into a ceramic sample pan within the chamber, as was done previously for measurements of pure Hg. 12 Immediately after opening the valve to the reservoir, the Hg vapor pressure determines the total pressure in the chamber, while the partial pressures of possible contaminants such as water and oxygen remain low. The resulting liquid alloy surfaces were found to be stable for over a week, due to two mechanisms.…”

Ab initio study of intrinsic profiles of liquid metals and their reflectivity

“…Since the original prediction of Rice and coworkers that free liquid surfaces (FLS) of metals would exhibit surface layering in the density pro le (DP) perpendicular to the interface [1], (z), almost a decade had to ellapse until this expectation was realized experimentally for the case of liquid Hg [2,3], through the analysis of the x-ray re ectivity of the FLS, R(q z ), which showed a peak related to the layering. Since then, up to 2009 [4], measurements have been performed for other ve pure liquid metals, namely Ga, In, K, Sn and Bi [5].…”