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 Å thick in the temperature range 0.44 to 1.3 K. The 10%/90% interfacial width for temperature $T=0.45\mathrm{}$ K varies from $5.3\pm 0.5$ Å for $36\pm 1.5$ Å thick films to $6.5\pm 0.5$ Å for $125\pm 1.5$ Å thick films. The profile width at zero temperature should not differ significantly from that measured at $T=0.45\mathrm{}$ K. For $T=1.22\mathrm{}$ K, the width is $7.8\pm 1.0$ Å.

• Received 22 February 2000

DOI:https://doi.org/10.1103/PhysRevB.62.9621