Positron-lifetime studies of physisorbed nitrogen and argon surfaces

Positron-annihilation lifetimes have been measured in two-dimensional nitrogen and argon adsorbed on Grafoil surfaces as a function of coverage and of temperature. Two positron lifetimes were observed, a short lifetime ≊0.2 nsec which is attributed to annihilation in the bulk of graphite and a long-lived component ≊0.4 nsec which is attributed to annihilation in adsorbates and on the graphite surface. The surface lifetime increases as a function of adsorbate coverage, reaching a maximum near half a monolayer and then decreasing. This lifetime variation with the fraction of coverage is interpreted to be a result of the positron specifically annihilating with the surface defects created by the presence of adsorbates. The positron lifetimes were found to vary with temperature for surfaces with a fixed fraction of monolayer adsorption. The temperatures at which the significant lifetime changes occur were found to correspond to the known solid-liquid phase-transition temperatures for ${\mathrm{N}}_2$ and Ar two-dimensional surfaces. The lifetime-temperature variation was found to be weaker for two-dimensional adsorbates than in three-dimensional solid-liquid phase transitions.