Two-dimensional melting of physisorbed submonolayers of argon and nitrogen

Phase transitions in physisorbed ethane investigated by positron-annihilation spectroscopy

“…Following this drop, it rises slowly with increasing coverage. This coverage dependence of the S parameter is qualitatively similar to what has been seen in cases of nitrogen and argon 17,22,23 where it shows a maximum at ϳ0.5. This parameter is sensitive to changes in the ͑1͒ valence vs core electrons contributing to the annihilation probability, ͑2͒ Ps formation, and ͑3͒ zero-point energy of the Ps atom.…”

“…Wang et al 31 Two models have been proposed to explain the coverage dependence of the positron annihilation parameters. 17,22,23 In one of these models, adsorption is viewed as creating defects on the surface that trap positrons and Ps. Since even a small coverage, on the order of 0.1 monolayer, significantly changes the annihilation parameters, adsorption should be affecting the surface state.…”

“…Details of the volumetry equipment used are given elsewhere. 23 The system consisted of a gas manifold system, two beratron absolute pressure gauges with overlapping ranges, a high vacuum system, and a closed-cycle helium Displex Cryostat equipped with a temperature controller with an accuracy of Ϯ0.1 K. Standard baking and flushing techniques were used in order to clean the entire system prior to measurements. A stack of 31.154 g grafoil discs was baked at 1073 K under 10 Ϫ4 Torr for 38 h and subsequently transferred to the sample cell under dry helium atmosphere.…”

“…For temperature-dependent experiments, a given coverage was maintained constant by adjusting the number of the molecules in the ''dead-volume'' of the cell. 23 For example, additional molecules were added to compensate for desorption, especially at TϾ77 K. 24 Since the number of molecules in the dead volume was negligibly smaller than needed for a monolayer, only a small correction was needed. 24 The positron lifetime and Doppler broadening measurements were performed simultaneously as a function of coverage ͑͒ at 108 K and as a function of temperature at fixed coverages of 0.4, 0.9, and 1.5 monolayers.…”

“…14 Positron-annihilation spectroscopy ͑PAS͒, which is a sensitive technique for investigating the electronic properties of condensed matter, 15,16 has also been employed to study certain features of physisorbed gases. [17][18][19][20][21][22][23][24] The ethane-graphite system has been examined by several experimental methods. At low temperatures, it exhibits a very rich phase diagram.…”