The adsorption of neon on graphitized carbon in the submonolayer and multilayer region between 1.5 and 30°K

Abstract

Heat capacities and heats of adsorption (calorimetrically measured) together with equilibrium pressures are reported for coverages up to three monolayers. The data indicate that neon remains localized up to 30°K in the submonolayer region. Phase transitions of the adsorbed neon were observed in the temperature and coverage range of this investigation. The regions in which these transitions take place are shown in a coverage vs temperature plane. In region II of this plane, a phase transition takes place which results in excess heat capacities in the submonolayer region. In region III, a transition of a different nature takes place which resembles the ’’melting’’ process. The ’’two-dimensional gaslike’’ phase could only appear at temperatures higher than those attained in this work. In region I, the adatoms behave as highly localized oscillators, and those at the high energy sites contribute a major portion to the heat capacity below 3°K. The residual entropy was calculated for the two submonolayer coverages and was found to be equal to zero within the experimental error. The Ne–solid interaction energy is 2990 J/mole at 24°K and 2770 J/mole at 29°K. The integral heat of adsorption at 0°K was calculated in the submonolayer and multilayer region, and the difference between the first and subsequent monolayers is found to be more than 700 J/mole. The integral adatom–adatom interaction energy of adsorbed neon in the submonolayer region at 24°K varies almost linearly with coverage and is approximately equal to 290 J/mole at the completion of the first monolayer. The monolayer capacity of the adsorbent for neon depends on temperature and the coefficient of the thermal expansion of the first monolayer in the xy plane is estimated to be about 0.002 deg−1 in the temperature range 24–29°K. For higher coverages a substantially higher coefficient is observed, and this is attributed to a phase change which takes place in the structure of the film. Finally, the degree of homogeneity of graphitized carbon, Spheron 6, i.e., the ratio of the high energy sites to the planar sites of graphitized carbon, is found to be approximately equal to 0.1.