First Adsorbed Layer of Helium at 4.2° K

An attempt has been made to measure the helium adsorption isotherm on pyrex (Corning 7740) at 4.2° K for pressures above the adsorbed layer ranging from 10 −4 to 10 −12 mm Hg. New ultrahigh vacuum techniques were used for the pressure measurements with geometric adsorbing areas from 10 to 70 cm 2 . Above 10 −9 mm Hg the results give a true measurement. Below 10 −9 mm Hg the results give an upper limit on the pressure above the adsorbed layer. The form of the isotherm indicates that the first adsorbed layer is complete at a pressure Formula: see text. A comparison of the present" results with previous measurements of the helium adsorption isotherm at 4.2° K, which extend down to 10 −6 mm Hg, suggests that all such measurements with solid adsorbents have been made on helium layers thicker than one monolayer. This conclusion supports similar conclusions drawn from heats of adsorption data at higher temperatures. An extrapolation of the results suggests that total gaseous densities approaching those of interstellar space (1.0 × 10 −24 g/cm 3 ) should be possible in the laboratory, with liquid helium trapping.