Positronium annihilation spectroscopy has been used to study capillary condensation of nitrogen adsorbed in mesoporous VYCOR glass. The temperature and pressure dependence of the relative change in the 3γ:2γ annihilation ratio are reported. Both isobars and isotherms exhibit features which are associated with a shifted gas-liquid transition. The isotherm data suggest that pore filling occurs via progressive layer formation at the pore surface followed by an abrupt gas-liquid phase transition at some pressure below the bulk saturated vapor pressure. Such a picture is consistent with the theoretical treatment in individual pores.

• Received 20 July 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.3535