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 H-2. The minimum He- 4 coverage needed for superfluidity, or ''inert'' layer, n(o) on both surfaces is found to be 6 mu mole/m(2). This is equivalent to 1/2 of a monolayer at bulk liquid density. The superfluid transition in covera ges above n(o) is similar to that found on bare Mylar, exhibiting the standard Kosterlitz-Thouless characteristics. We find no anomaly in rh o(s) or the dissipation in the film that can be identified with a seco nd transition reported in recent third sound experiments.