Two-dimensional ordering of chlorine on Ag(100)

Chlorine adsorption on Ag(100) saturates at (1/2) monolayer (ML) in a c(2×2) structure. Desorption occurs above 650 K without disordering. The sticking coefficient remains constant to within 0.05 ML of saturation, consistent with the existence of a highly mobile precursor to adsorption. The c(2×2) structure forms at a critical coverage of 0.394±0.007 ML at room temperature. This is slightly higher than the critical coverage of the hard-square model (0.368 ML), suggesting the influence of further-neighbor interactions. The transition to an ordered structure as a function of coverage is second order and the measured critical exponent β/(1-α) is 0.12±0.04, consistent with the Ising value of (1/8).