An x-ray specular reflectivity study of the helium liquid-vapor interface

The helium liquid-vapor interfacial density profile has been measured using the technique of x-ray specular reflectivity. In order to increase the intensity of the measured signal, and to acquire sensitivity to possible interfacial asymmetries, the measurement was performed on thick films of helium adsorbed onto atomically flat silicon substrates. With this geometry the reflection from the adsorbed film was measured. Measurements were performed on films whose thickness varied from 15 angstrom to 220 angstrom, over a range of temperatures from 1.1K to 3.0K. Both polished silicon with a native oxide layer, and silicon with a hydrogen passivated surface were used as substrates. The best results were obtained on thick undersaturated superfluid helium films adsorbed onto hydrogen passivated silicon substrates. No significant variation was seen in the interfacial widths measured at other temperatures between 1.1K and 1.8K. Although a variation of the interfacial width with temperature is expected, this was offset by the variation in film thickness with temperature for undersaturated films. The measurements also indicate that the interface is asymmetric, with the decay of the density into the vapor having a sharper fall-off than the asymptotic approach of the density to the bulk liquid value. The 0K interfacial width could be extrapolated from the finite temperature measurements using a quantized capillary wave theory. The extrapolated 0K interfacial width is 7.6 + 1-2 angstrom. Measurements were also made on thin undersaturated films, saturated films, and normal fluid helium films. For differing reasons these other measurements were less useful for obtaining information about the interface. Saturated films suffered from instabilities in thickness, possibly related to the presence of bulk superfluid liquid helium in the bottom of the cell