Third sound of helium on a hydrogen substrate

Measurements of third sound of thin helium films on molecular-hydrogen substrates have been made as a function of the measured helium film thickness, both by a time-of-flight technique and in a third-sound resonator. Great care has been devoted to making uniform films of hydrogen, to determine reliable values of the helium-hydrogen van der Waals constant α and the thickness of the helium dead layer, Δ. Areal and gas-loading parameters have been measured to allow an absolute determination. We find a value of 21.3 K ${\mathrm{layer}}^3$ for α and 2.0 for Δ. Comparisons are made to other values in the literature. Our value of the van der Waals constant is in good agreement with a complementary determination by Paalanen and Iye and by Cieslikowski et al., using electron mobility, but is much larger than a value reported by Mochel and a value calculated by Cheng et al. We also determine values for helium on copper; however, this determination depends on the literature value for the van der Waals parameter for helium on glass.