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

The average liquid-vapor density profiles 〈ρ(z)〉 of thick ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ 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±1 Å at 1.13 K which extrapolates to a T=0 K, 90%-10% interfacial width of 7.6+1,-2 Å. The sensitivity of the measurement to the width, shape, and asymmetry of the density profile is discussed.