Lattice spacing and isosteric heat of argon monolayers below 50K

The lattice constants of 2D solid Ar adsorbed on graphite are reported for many combinations of T and vapor pressure P, where 30 < T < 50 K and 10^?7 < P < 10^?4 Torr. Low-energy electron-diffraction patterns of high resolution provide accuracy to ±0.01 Å. Straight lines are fitted through points of constant lattice spacing on a log P versus T^?1 plot; their slopes and intercepts yield isosteric heats of adsorption as well as estimates of the 2D isothermal compressibility of the Ar solid. Comparison is made to calorimetric measurements of isosteric heat, and to a previous LEED compressibility measurement performed isothermally at 42 K.