Sorption studies of helium and neon by crystals of C₆₀ and C₇₀

We present sorption measurements for ³He and ⁴He in the temperature range of 1.5 K to 4.1 K, and for ²⁰Ne in the temperature range of 22 K to 27 K by crystals of C₆₀, C₇₀ and crystals of the mixture of these two molecules, 80% C₆₀, 20% C₇₀. We analyze these data by taking into account the non-ideality of the gas in equilibrium with the adsorbate. We calculate chemical potentials and isosteric heats. We find that there is no obvious evidence of intercalation of helium in these crystals at low temperatures. At higher temperatures there are some anomalies in the helium isotherms, and indication of excess sorption. The isosteric heat shows a minimum in this region which can be interpreted as penetration of the helium into a region of repulsive potential. We also find that levels of sorption, at the same chemical potential difference from saturation, are higher for ⁴He than for ³He. They are also higher for ⁴He on C₇₀ than for the other crystals. For neon our work is concentrated around the triple point. We find that the isotherms indicate the formation of liquid or solid films. Below the triple point, and above a few atomic layers, the neon film does not grow uniformly.