An experimental study of the in-plane distribution of atoms in the liquid–vapor interface of mercury

We report measurements of the in-plane (transverse) structure function, S∥(k), of the liquid–vapor interface of Hg at 298 K, obtained using the method of grazing incidence x-ray diffraction. Our results show that: (i) The transverse structure function of the liquid–vapor interface of Hg is very similar to the bulk liquid structure function, but there are subtle differences between the two functions. (ii) As k→0, S∥(k) appears to diverge as k−2, in agreement with theoretical predictions and computer simulation studies. (iii) The observable S∥(k) for different longitudinal (parallel to the normal) density distributions in the liquid–vapor interface are very nearly the same, the largest differences being for momentum transfers less than that at the first peak of S∥(k). For this reason the measurement of S∥(k) is not a good monitor of the longitudinal density distribution. (iv) The interpretation of the differences in S∥(k) in the small k region (but outside the region where the k−2 divergence overtakes other effects) requires consideration of the effect of the longer ranged part of the effective pair potential and assessment of the relative contributions of the ionic and electronic components of the liquid metal to the compressibility. A comparison of the shape of S∥(k) in the threshold region just before the first peak and that of the same region in a bulk liquid metal leads to the weak conclusion that our data are consistent with the Monte Carlo simulations which predict that the liquid–vapor interface of Hg is stratified for about three atomic layers.