Positron Annihilation Study of Physisorbed Submonolayers of Argon and Nitrogen.

Positron annihilation spectroscopy has been used to investigate nitrogen and argon films adsorbed on exfoliated graphite. Lifetime and Doppler-broadening spectra have been obtained for different coverages of N_2 on grafoil surface at 77K. Also Doppler-broadening experiments have been performed for grafoil surface having different coverages of physisorbed monolayers of argon at 77K. The lifetime spectra have been resolved in three exponentials and the Doppler-broadening spectra have been analyzed in terms of the shape-parameters. Also the Doppler -broadening spectra have been deconvoluted and resolved in one gaussian and a parabola. The results of lifetime and Doppler-broadening measurements show that lifetime components, their relative intensities, the shape-parameters are sensitive to coverage. These results are discussed in terms of the local density of the surface electrons, positron work function, and roughening transition. In addition, the Doppler-broadening measurements of the melting of two-dimensional argon and nitrogen physisorbed on grafoil have been performed. Results of these experiments show that the S-parameter varies in a manner that is characteristic of the trapping of low energy positrons in lattice defects. From the S-parameter data, the 2D melting transition temperatures of the films have been found to be 51.5 +/- 0.5 K and 45.5 +/- 0.5 K for argon and nitrogen, respectively. In order to understand the behavior of the films as function of temperature, two models have been proposed based on: (1) dislocation theory of two dimensional melting, and (2) surface roughening transition. We discuss the observed changes in the local electron density, electron momentum distribution, and surface defects in terms of these models.