Relaxation processes on graphitic surfaces. Part 1.—In the absence and presence of adsorbed 4He on Spheron and Grafoil with increasing temperature

Abstract

An analysis of the time dependence (after-periods) of heat capacities measured for Spheron and Grafoil in the presence and absence of adsorbed ⁴He (and Ne) reveals the existence of slow first-order relaxation processes. For low temperatures these are endothermic but above 19.6 and 11.5 K for Spheron and Grafoil, respectively, they are exothermic. The rate at which these processes occur is determined by thermoelastic stresses associated with relatively small readjustments in the disordered surface geometry. In the absence of adsorbates the dimensional expansion in the c direction and contraction parallel to the surface can lead, respectively, to endothermic (dominant at low temperatures) and exothermic processes. In the presence of adsorbed ⁴He the superimposition of two endothermic processes is attributed to thermal expansion of the superimposed film and of the surface layers. The decrease, with increasing coverage of adsorbate, of the temperature above which the exothermic processes are observed is attributed to the effect of adsorbate on the structure of the surface layers of graphite. The increased deformation in the substrate lattice due to the interaction of the superimposed film results in the longer relaxation times and the higher ‘relaxed heat capacities’ of these processes.