<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow/><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:math>liquid-vapor interface below 1 K studied using x-ray reflectivity

The free surface of thin films of liquid helium adsorbed on a solid substrate has been studied using x-ray reflectivity. The film thickness and interfacial profile are extracted from the angular dependence of measured interference between signals reflected from the liquid-vapor and liquid-substrate interfaces. Polished silicon wafers, chemically cleaned and passivated, were used as substrates. Results are reported for measurements for ${}^{4}\mathrm{He}$ films 35 to 130 \AA{} thick in the temperature range 0.44 to 1.3 K. The 10%/90% interfacial width for temperature $T=0.45$ K varies from $5.3\ifmmode\pm\else\textpm\fi{}0.5$ \AA{} for $36\ifmmode\pm\else\textpm\fi{}1.5$ \AA{} thick films to $6.5\ifmmode\pm\else\textpm\fi{}0.5$ \AA{} for $125\ifmmode\pm\else\textpm\fi{}1.5$ \AA{} thick films. The profile width at zero temperature should not differ significantly from that measured at $T=0.45$ K. For $T=1.22$ K, the width is $7.8\ifmmode\pm\else\textpm\fi{}1.0$ \AA{}.