Determination of the density profile in the liquid–vapor interface near the triple point

The density profile in the transition region between liquid and vapor Hg has been determined, at room temperature, from a study of the angular dependence of the reflection of Cr Kα x rays. This profile ρ (Z) has a hyperbolic tangent form with width L=5.6±0.5 Å, where L≡−(ρL−ρG)dρ (Z)/dZ−1Z=0, with ρL and ρG the densities of liquid and vapor, respectively. The nature of the profile and the value of L are compared with the corresponding quantities obtained from computer simulations of the surface of liquid Ar. For liquid Hg, L is dominated by a static structural contribution fundamentally traceable to the nonzero wavelength of the electrons and their influence on the effective ion–ion potential. For liquid Ar, L is dominated by the contribution from thermal motion. The currently available theory of the liquid metal surface transition zone predicts a value for L which is too small by a factor of 2. This discrepancy, the role of surface capillary waves in broadening the transition zone, and the possible relationship of the structure of the transition zone to the stability of the liquid are discussed.