Multilayer Physisorption of Argon and Oxygen on Graphite.

Multilayer solid oxygen and argon physisorbed on graphite have been studied using an ellipsometric technique. We could make a clear distinction between a liquid and a solid layer of an oxygen film from the ellipsometric coverage signal, because of the large difference in the areal densities between them as well as anisotropy of a solid layer. We observed the signature of a quasi-liquid layer on the solid film. Near 41 K the top layer of three -or-more-layer films melts continuously. There is a second wetting transition at bulk solid beta -gamma triple point and solid beta-oxygen is found to wet graphite at least down to 37 K. We located the layer critical points of argon for the second through fifth layer near 67 K. There is a reappearance of sharp steps representing first-order transitions in the fourth layer and above over the range of temperature from ~72 K to 77 K. Because the reentrant steps have smaller reduced pressure, the phase boundaries in the mu-T plane shift downward in the intervening region. The slope (partialmu/partialrm T ] _{rm N} suggests disordering of the top layer, then there is another series of layer critical points near 77 K, which probably indicates true roughening at ~78 K.