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We report quartz crystal microbalance measurements of the adsorption of neon on surfaces of cesium and rubidium at temperatures up to the critical point of neon. In the case of Ne͞Rb there is little adsorption until the temperature approaches 0.97 of T c , where a wetting transition occurs. In the case of Ne͞Cs no adsorption is seen all the way to T T c . Instead our data suggest the presence of a vapor film adjacent to the Cs surface when the sample cell is filled with liquid. This may indicate a crossover from wetting to drying.[S0031-9007 (97)02427-7] PACS numbers: 68.45.Gd, 68.15. + e, 64.60.FrIn 1977 Cahn [1] predicted that wetting transitions should be a general phenomenon in systems which are not wet at low temperature. In 1991 the first observation in a physisorption system of a wetting transition not tied to a bulk phase transition [2], and the associated prewetting transition [3,4], was made in the system 4 He on Cs, following the explicit prediction of Cheng et al. [5]. Taborek and Rutledge [4] determined the wetting temperature T w of 4 He on Cs to be 1.95 K. When the cesium layer deposited on the gold electrode of their quartz crystal microbalance was reduced to a few atomic layers, these authors found a significant reduction of T w [6]. The calculated potential well depth of such a composite substrate is intermediate between that of Cs and Au [7]. This result, and the finding that T w 0.31 K for 4 He͞Rb [8], confirms the expectation that a stronger substrate tends to move T w towards T 0 and a weaker substrate will push T w toward T c , the liquid-vapor critical point of the adsorbate. Cahn [1] showed that, in mean field treatment of a model with short-range interactions, wetting always occurs at some temperature below T c , or alternatively, if the substrate is sufficiently weak, a symmetrical "drying" transition occurs on the liquid side of the coexistence line, in which a vapor layer of diverging thickness is interposed between the substrate and the bulk liquid. More recent theoretical treatments which include long-range interactions have found that wetting and especially drying behavior near T c depends on a subtle interplay between short-range and longrange interactions [9][10][11].One way to quantify the strength of a substrate/ adsorbate pair is to compare the well depth D of the adsorbate molecule-substrate potential with the well depth of the molecule-molecule potential. For 4 He͞Cs the ratio D͞´is 0.40 [12], which may be compared to the value 8.4 for 4 He͞Au [13], for which wetting occurs from T 0. The systems Ne͞Rb and Ne͞Cs are estimated to have ratios D͞´of 0.30 and 0.25, respectively, making them the weakest substrate cases available [13]. Thus they are promising systems for investigation of wetting behavior near T c .In this Letter we report measurements of the adsorption of neon on cesium and rubidium, using the quartz crystal microbalance (QCM) technique [14]. The sensor is a beveled plano-convex AT-cut quartz crystal designed for and used in the third-overtone thickness shear mode...

We report ellipsometric coverage-vapor-pressure isotherm measurements for argon on graphite over the temperature range 65 to 84 K, which resolve up to eight layers. We confirm previous observations that the first-order layering transitions broaden near 67 K. However, in the fourth and higher layers, condensation again becomes sharp and apparently first order in the range between about 73 K and a new series of layer critical points near 77 K. Thus the surface-roughening transition is above 77 K and the behavior near 67 K represents disordering of only the top layer of the film.PACS numbers: 68.55.Jk, 64.70.Dv, 68.45.Gd A face of a crystal is said to exhibit surface melting l if a mobile "quasiliquid" layer exists on the equilibrium surface at a temperature below the bulk melting point T m , and its thickness increases with temperature, diverging as T approaches T m . Surface roughening 2,3 is the unlocking of the crystal surface from a particular lattice plane through the proliferation of steps, resulting in the loss of faceting in thermal equilibrium. 3 This transition is preceded on the low-temperature side by the appearance and proliferation of vacancies in and adatoms on the initially smooth surface plane. As the temperature increases towards the roughening temperature TR, the characteristic size of clusters of adatoms and vacancies diverges, and the clusters gain their own (divergent) adatom and vacancy clusters. Roughening is thus an essentially many-layer phenomenon, but it must be preceded by short-range disordering of the surface layer. Simulations show a strong correlation between disordering of the top layer and onset of a large surface mobility. 4 Therefore, while surface roughening and surface melting are quite distinct many-layer phenomena, their precursors in the top layer may be coupled.The properties of the surfaces of bulk crystals should be reflected in the properties of the surfaces of multilayer wetting films of the same solid on a foreign substrate. 5 " 7 Disordering of the (partially filled) top layer of a film is a transition of the 2D Ising class. A succession of Ising critical temperatures T Ct " is expected for films of different coverages near n-j layers, but always the transition is in the top layer. Using different models, Huse 8 and Nightingale, Saam, and Schick 9 have shown that the sequence T Cyn should converge to the roughening temperature, with TR -T c , n~-(lnn) ~2.Zhu and Dash 7 (ZD) have reported evidence for both surface melting and surface roughening in heat-capacity studies of two-layer to about twenty-layer films of both neon and argon on graphite. The evidence for surface roughening is small heat-capacity peaks near 0.8T m , attributed to disordering of the partially filled top layer. Previous volumetric isotherm measurements for argon on graphite by Gilquin 10 found broadening of the second-