Growth ofHe4films on graphite, krypton-plated graphite, magnesium oxide, and Mylar substrates

Adsorption isotherms of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on graphite foam, uncompressed exfoliated graphite, krypton-plated graphite foam, MgO, and Mylar indicate type-II growth, i.e., uniform layer deposition up to a critical thickness, and bulk condensation thereafter. On graphite the critical thickness is about four atomic layers; plating with one monolayer of krypton reduces the critical thickness by more than one layer. The critical thickness on MgO is 1.5 layers, and on Mylar is between three and four layers. None of the vapor pressure isotherms obey the exponential $d^{-3\mathrm{}}$ Frenkel-Halsey-Hill equation within the range of the experiments.