Adsorption-isotherm study of monolayer films ofN2on BN

We have measured adsorption isotherms for ${\mathrm{N}}_2$ films on BN for temperatures between 51 and 75 K and coverages in the first layer. Data measured below 65.8 K display two features: a substep and (at slightly higher coverages) a kink in the slope of the isotherm. Isotherms measured at and above 65.8 K have no features present in them. The isotherm and isothermal compressibility data for ${\mathrm{N}}_2$ on BN are compared with very similar corresponding data for Kr, CO, and ${\mathrm{N}}_2$ on graphite; this comparison is used to propose an interpretation for the observed features of ${\mathrm{N}}_2$ on BN. Our results for ${\mathrm{N}}_2$ are also contrasted with data for Kr on BN; the differences observed between these two systems are discussed.