X-ray study of the liquid potassium surface: structure and capillary wave excitations

TL;DR

This study uses x-ray techniques to reveal atomic layering and capillary wave excitations on liquid potassium's surface, a phenomenon previously observed only in metals with higher surface tensions.

Contribution

First direct evidence of atomic layering on a low surface tension liquid metal surface, supported by quantitative analysis and agreement with theoretical models.

Findings

Atomic layering observed on liquid potassium surface.

Capillary wave scattering matches theoretical predictions.

Results align with computer simulations and theories.

Abstract

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 5-7 fold 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, $d\sigma /d\Omega \sim 1/q_{xy}^{2-\eta}$ where $\eta = k_BT q_z^2/2\pi \gamma $, over a range of $\eta$ 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.