ADSORPTION OF WATER ON NACL(100) SURFACES - ROLE OF ATOMIC STEPS

Using an atomic force microscope operated in contact and noncontact mo de, we have studied the structures formed on the (100) cleavage surfac e of NaCl when exposed to water vapor. Above similar to 35% relative h umidity (RH), a uniform layer of water is formed and the surface steps are observed to evolve slowly. At similar to 73% RH, the step structu re becomes unstable and disappears abruptly because of dissolution (de liquescence) of the salt surface. Reversing the process by drying lead s to the reappearance of new, more uniform monatomic steps. At humidit y levels less than 30%, water adsorbs primarily at the step edges.