First Adsorbed Layer of Helium

The discrepancy between the force constants for the adsorption of helium found by Bowers at high saturations using the Frenkel isotherm, and Strauss at medium saturations using the Brunauer-Emmet-Teller theory creates some doubts whether or not the amount $v_m$ derived from the BET isotherm in the saturation range $\frac{P}{P_0}=0.02-0.2\mathrm{}$ represents in the case of helium the amount adsorbed in the first layer. Adsorption measurements at very low saturations show a distinct plateau in the 2°K isotherm at $\frac{P}{P_0}\sim {10}^{-7\mathrm{}}$ for ∼0.5 ${\mathrm{cm}}^3$/${\mathrm{m}}^2$ adsorbed suggesting the completion of the first layer already at these low saturations. At 4.2°K this plateau coalesces with the usual point $B$ of sigmoid isotherms, where $\sim 0.8 {\mathrm{cm}}^3/{\mathrm{m}}^2=v_m$ is adsorbed.