Microscopic View on Short-Range Wetting at the Free Surface of the Binary Metallic Liquid Gallium-Bismuth: An X-ray Reflectivity and Square Gradient Theory Study

We present an x-ray reflectivity study of wetting at the free surface of the binary liquid metal gallium-bismuth (Ga-Bi) in the region where the bulk phase separates into Bi-rich and Ga-rich liquid phases. The measurements reveal the evolution of the microscopic structure of wetting films of the Bi-rich, low-surface-tension phase along different paths in the bulk phase diagram. A balance between the surface potential preferring the Bi-rich phase and the gravitational potential which favors the Ga-rich phase at the surface pins the interface of the two demixed liquid metallic phases close to the free surface. This enables us to resolve it on an Angstrom level and to apply a mean-field, square gradient model extended by thermally activated capillary waves as dominant thermal fluctuations. The sole free parameter of the gradient model, i.e. the so-called influence parameter, $\kappa$, is determined from our measurements. Relying on a calculation of the liquid/liquid interfacial tension that makes it possible to distinguish between intrinsic and capillary wave contributions to the interfacial structure we estimate that fluctuations affect the observed short-range, complete wetting phenomena only marginally. A critical wetting transition that should be sensitive to thermal fluctuations seems to be absent in this binary metallic alloy.Comment: RevTex4, twocolumn, 15 pages, 10 figure

Surface Structure of Liquid Metals and the Effect of Capillary Waves: X-ray Studies on Liquid Indium

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 $156^\circ$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 $3.5\pm 0.6$ \AA for In and $5.5\pm 0.4$ \AA 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.Comment: 9 pages, 5 figures; submitted to Phys. Rev.

VARIATION OF F-CENTER CONCENTRATION IN DILUTE K-KCL SOLUTION : COMPARISON OF AN OPTICAL STUDY WITH THERMODYNAMIC PREDICTION

We have measured the concentration of liquid F-centers by optical absorption in KxKCl1-x solutions at 790 °C over a wide range of compositions up to a metal mole fraction x = 0.01. The composition has been accurately controlled by coulometric titration in situ. These results are compared with the predicitions of the F-center concentration as determined from metal activity measurements using a thermodynamic electron defect model. Good agreement is found between these separate data sets for a variation of composition by two orders of magnitude

DIPOLAR ATOMS IN Na-NaX SOLUTIONS ? -AN OPTICAL STUDY-

In a recent quantum molecular dynamics calculation of diluted Na-NaBr solutions Parrinello and coworkers found that excess electrons may localize to form dipolar Na atomic - like states. This is at variance with spectroscopic observations that F-center like states prevail in alkalimetal-alkalihalide solutions. In order to get further insight into this problem we have reinvestigated the optical absorption spectra of dilute alkalimetal-alkalihalide-NaCl solutions as a function of temperature and for various salt compositions. The main result of this study is that indeed in NaCl-rich solutions deviations from the expected F-center characteristics are visible