POSITRON-ANNIHILATION STUDY OF PHASE-TRANSITIONS IN ETHANE PHYSISORBED ON GRAFOIL

Abstract

Adsorption, desorption, and positron-annihilation spectroscopy have been employed to study ethane physisorbed on grafoil. From the adsorption and desorption measurements, the interlayer and intralayer ethane-ethane and ethane-substrate interaction energies have been obtained as 9.6, 13.8, and 175 meV, respectively. Doppler broadening of the annihilation radiation has been measured as a function of coverage at 108 K. The S parameter exhibits a marked change at a coverage of 0.8. Doppler broadening measurements have also been carried out as a function of temperature in the range 10-100 K for a constant coverage of 0.9. We discuss the temperature dependence of the S parameter in terms of an order-disorder phase transition around 39 K, a solid-solid transition associated with the rotation of the ethane molecule about the C-C axis, and a solid-fluid transition.