Multilayer Solid Adsorption and the Roughening Transition
- PDF / 506,943 Bytes
- 8 Pages / 418.68 x 637.2 pts Page_size
- 27 Downloads / 190 Views
MULTILAYER SOLID ADSORPTION AND THE ROUGHENING TRANSITION JOHN D. WEEKS Bell Laboratories 600 Mountain Avenue Murray Hill, NJ 07974 USA ABSTRACT We discuss some the physical issues involved with the adsorption of several layers of a solid adsorbate on a strongly attractive substrate. The experimental observation at low temperature of vertical risers (the 'layering transitions") in the plot of coverage versus pressure can be understood using a one-layer 2D lattice-gas model. The behavior at higher temperature is described in terms of a coarse-grained interface hamiltonian which emphasizes the role of long-wavelength interface fluctuations. A simple variational theory predicts that the coverage in the nth layer is a continuous function of pressure above a critical temperature Tcn which depends weakly on n and has as its large n limit the bulk roughening temperature TR.
INTRODUCTION There are many interesting problems involved with the physical adsorption of atoms on a homogeneous substrate. Recently this field has been the focus of much experimental and theoretical work, and rapid progress has been made [1]. In this talk we concentrate only on examining the ("class I') physical adsorption of atoms on a strongly attractive substrate after several adsorbate layers have already been deposited. (The most interesting behavior occurs when the adsorbed phase is a solid, though the variational theory described below applies also to liquid adsorbates.) This allows us to avoid several complications associated with the nature of the epitaxy in the first few layers, arising from the details of the short-rangedadsorbate-substrate interaction. This can produce quite different behavior for the first few layers, as the adsorbate wets the substrate and, e.g., tries to accommodate itself to a possibly different lattice spacing favored by the substrate periodicity. If several solid layers can be adsorbed, however, it is reasonable to assume that the lattice constant for atoms in the further layers approaches that of the "natural" spacing implied by the adsorbate-adsorbate interaction, and any possible defects induced by substrate mismatch in the first few layers can be ignored. Even then, however, the substrate continues to play an important role in the adsorption process because of the long-ranged van der Waals interaction. Treating the substrate as a semi-infinite continuum (a good approximation far from the substrate), the individual atomic r-6 attractions integrate out to a very long-ranged interaction un = -c/n 3 between the substrate and an adsorbate atom in the nth layer above the substrate [2]. It is the nature of adsorbate solid-fluid (or liquid-vapor) coexistence as influenced by this external field from the substrate that concerns us here [3]. The simplifications made in our treatment of the substrate interactions for the first few layers must be kept in mind in comparison with experiments. EXPERIMENTAL RESULTS Typical of the experiments we wish to examine tire those of Thomy, Duval and coworkers [4,5]. They measured the ad
Data Loading...
