Adsorption of 4He on graphitized carbon in the submonolayer region between 2 and 15°K

Publisher Website
Google Scholar
Abstract
The adsorption of 4He on graphitized carbon (Spheron 6) was studied at submonolayer coverages between 2 and 15°K. Heat capacities, heats of adsorption (measured calorimetrically), and equilibrium pressures are reported. The data indicate that adsorbed helium remains localized up to at least 15°K. A phase transition is observed at temperatures above 9°K for the 22.26 cm3 STP/g adsorbent coverage. Frequencies of three−dimensional asymmetric localized harmonic oscillators which can reproduce the experimental results are given. Excited levels are heavily populated at the temperatures of measurement. The zero point energy of adsorbed helium is approximately 200 J/mole. Another change in the state of adsorbed helium takes place below 7°K to the lowest temperature attained, 2°K. The rate of this change in the state of adsorbed helium follows first order kinetics. It is suggested that this process involves the ’’falling’’ of the adatoms from higher to lower levels as the temperature is lowered. The residual entropies are calculated for two coverages. The residual entropy is attributed to the distribution of helium atoms over the localized surface sites.