No 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.

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...

We present x-ray reflectivity and diffuse scattering measurements from the liquid surface of pure potassium. They strongly suggest the existence of atomic layering at the free surface of a pure liquid metal with low surface tension. Prior to this study, layering was observed only for metals like Ga, In, and Hg, the surface tensions of which are five-to sevenfold higher than that of potassium, and hence closer to inducing an ideal ''hard wall'' boundary condition. The experimental result requires quantitative analysis of the contribution to the surface scattering from thermally excited capillary waves. Our measurements confirm the predicted form for the differential cross section for diffuse scattering, d/d⍀ϳ1/q xy 2Ϫ where ϭk B Tq z 2 /2␥, over a range of and q xy that is larger than any previous measurement. The partial measure of the surface structure factor that we obtained agrees with computer simulations and theoretical predictions.

X-ray reflectivity measurements of the binary liquid Ga-Bi alloy reveal a dramatically different surface structure above and below the monotectic temperature Tmono = 222• C. A Gibbs-adsorbed Bi monolayer resides at the surface at both regimes. However, a 30Å thick, Bi-rich wetting film intrudes between the Bi monolayer and the Ga-rich bulk for T > Tmono. The internal structure of the wetting film is determined withÅ resolution, showing a theoretically unexpected concentration gradient and a highly diffuse interface with the bulk phase.

We report an x-ray scattering study of the microscopic structure of the surface of a liquid alkali metal. The bulk liquid structure factor of the eutectic K 67 Na 33 alloy is characteristic of an ideal mixture, and so shares the properties of an elemental liquid alkali metal. Analysis of off-specular diffuse scattering and specular x-ray reflectivity shows that the surface roughness of the K-Na alloy follows simple capillary wave behavior with a surface structure factor indicative of surface-induced layering. Comparison of the low-angle tail of the K 67 Na 33 surface structure factor with the one measured for liquid Ga and In previously suggests that layering is less pronounced in alkali metals. Controlled exposure of the liquid to H 2 and O 2 gas does not affect the surface structure, indicating that oxide and hydride are not stable at the liquid surface under these experimental conditions.The structure of the free surface of a liquid metal ͑LM͒ is fundamentally different from that of a dielectric liquid, due to the strong coupling between conduction electrons and ion cores. 1 At the liquid-vapor interface of a LM, the Coulomb interaction between the free electron Fermi gas and the classical gas of charged ions acts like an effective hard wall and forces the ions into ordered layers parallel to the surface. The existence of surface-induced layering in LM's has been verified unambiguously by experiment for liquid Hg, 2 Ga, 3 and In. 4 Layering is also present in liquid binary alloys, even though it may be suppressed by surface segregation 5 or the formation of surface phases. 6,7 Even though surface layering appears to occur in LM's in general, comparison of the detailed surface structure reveals qualitative differences between different LM's. 4,8 In fact, very little is understood about how the surface-induced layering predicted for an ideal LM is affected by the details of the more complicated electronic structure exhibited by the polyvalent metals studied so far. One important goal therefore is to study the surface structure of those LM's that show the most ideal electronic structure in the bulk phase, characterized by itinerant conduction electrons which are only weakly perturbed by a small ionic pseudopotential. These LM's are referred to as nearly-free-electron type liquid metals. 9 In a previous study we have compared the surface structure of liquid In, which is considered to be a nearly-free-electron type liquid metal, with the surface structure of liquid Ga, which displays a considerable degree of covalency in the bulk. 4 However, even though liquid In has less tendency toward covalent bonding than Ga, it is trivalent, and the most simple models do not adequately describe its electronic structure or metallic properties. 11 The liquid metals closest to nearly-free-electron behavior are the monovalent alkali metals, which have a half-filled s band and an almost spherical Fermi surface. 10 The metallic properties of alkali metals are explained by the most simple solid state models such as the Drude theory...

No abstract

Surface x-ray scattering measurements from several pure liquid metals (Hg, Ga and In) and from three alloys (Ga-Bi, Bi-In, and K-Na) with different heteroatomic chemical interactions in the bulk phase are reviewed. Surface induced layering is found for each elemental liquid metal. The surface structure of the K-Na alloy resembles that of an elemental liquid metal. Surface segregation and a wetting film are found for Ga-Bi. Bi-In displays pair formation at the surface.