LIQUID-VAPOR DENSITY PROFILE OF HELIUM - AN X-RAY STUDY

The average liquid-vapor density profiles [rho(z)] of thick He-4 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 angstrom at 1.13 K which extrapolates to a T = 0 K, 90%-10% interfacial width of 7.6(-2)+1 angstrom. The sensitivity of the measurement to the width, shape, and asymmetry of the density profile is discussed.