SORPTION STUDIES OF HELIUM AND NEON BY CRYSTALS OF C-60 AND C-70

We present sorption measurements for He-3 and He-4 in the temperature range of 1.5 K to 4.1 K, and for Ne-20 in the temperature range of 22 K to 27 K by crystals of C-60, C-70, and crystals of the mixture of th ese two molecules, 80% C-60, 20% C-70. We analyze these data by taking into account the non-ideality of the gas in equilibrium with the adso rbate. We calculate chemical potentials and isosteric heats. We find t hat there is no obvious evidence of intercalation of helium in these c rystals at low temperatures. Ar higher temperatures there are some ano malies in the helium isotherms, and indication of excess sorption. The isosteric heat shows a minimum in this region which can be interprete d as penetration of the helium into a region of repulsive potential. W e also find that levels of sorption, at the same chemical potential di fference from saturation, are higher for He-4 than for He-3. They are also higher for He-4 on C-70 than for the other crystals. For neon our work is concentrated around the triple point. We find that the isothe rms indicate the formation of liquid or solid films. Below the triple point, and above a few atomic layers, the neon film does not grow unif ormly.