Relaxation processes on graphitic surfaces. Part 2.—Adsorption of 4He on Spheron and Grafoil

Abstract

An analysis of the after-periods of the calorimetric heat of adsorption determined for ⁴He on Spheron at temperatures close to 12 K and for ⁴He on Grafoil between 8 and 12 K provides information on the processes by which adsorption takes place with increasing coverage. Within the lower coverage range the exothermic processes which follow the steep rise of temperature occurring when the adatoms come into contact with the adsorbent surface are related to the migration and ‘trapping’ of the adatoms at the ‘high-energy’ sites of these adsorbents. Above a certain coverage, depending on structure (especially of the outermost surface layers of the substrate), the steep rise of temperature is followed by endothermic processes. These processes are related to the rearrangement of the adatoms in different configurations within the films superimposed on the surface of the adsorbent. At higher coverages, during the completion of the first monolayer and the formation of the second, both the relaxation times and the energies involved in the endothermic processes provide information on the processes by which the formation of the superimposed second monolayer takes place.