Liquid-vapor density profile of helium: An x-ray study.

The average liquid-vapor density profiles {l angle}{rho}({ital z}){r angle} of thick {sup 4}He films adsorbed onto a silicon substrate were measured using x-ray reflectivity. The results are well represented by a 90%-10% interfacial width of 9.2{plus minus}1 A at 1.13 K which extrapolates to a {ital T}=0 K, 90%-10% interfacial width of 7.6+1,{minus}2 A. The sensitivity of the measurement to the width, shape, and asymmetry of the density profile is discussed.