Torsional Oscillator Study of Thin Helium Films on Hydrogen Substrates

We have made torsional oscillator measurements of thin helium films on a Mylar surface preplated with 2 and 5 layers of H₂. The minimum ⁴He coverage needed for superfluidity, or "inert" layer, n₀ on both surfaces is found to be 6 μmole/m². This is equivalent to 1/2 of a monolayer at bulk liquid density. The superfluid transition in coverages above n₀ is similar to that found on bare Mylar, exhibiting the standard Kosterlitz-Thouless characteristics. We find no anomaly in ρ_s or the dissipation in the film that can be identified with a second transition reported in recent third sound experiments.