Adsorption of Neon on Graphitized Carbon. I. The Heat Capacity of Adsorbed Neon between 1.5 and 6°K

Abstract

The heat capacity of adsorbed neon on graphitized carbon, Spheron 6, was measured in the temperature range 1.5–30°K for two coverages, namely, 0.5 and 1.5 monolayers. The heat capacity values for both coverages show that the state of the adsorbate is liquidlike at temperatures close to the bp of the adsorbate. As the temperature is lowered, a peak in the heat capacities is observed. The temperature at which this peak is observed depends on the coverage, and is lower at the lower coverage. These peaks suggest that a transition takes place, and that this transition is a `melting' process. The observed low temperature heat capacities depend markedly on coverage, and their values are higher at the lower coverage. This dependence of the heat capacity on coverage can be explained if the localized adatoms are considered as asymmetric three-dimensional oscillators and the two-dimensional component of these oscillators (parallel to the surface) contributes the larger portion to the heat capacity. Entropies were calculated from the observed heat capacities for 0.5 monolayers and are compared with those calculated from the partition function of independent three-dimensional localized oscillators. The frequencies considered for these oscillators are reported. The observed heat capacities for the 1.5 monolayers coverage show that the state of the adsorbate at this coverage differs markedly from that of the bulk state.