Introduction

The phenomenon of condensation is one of the most familiar properties of bulk matter and so has attracted the attention of physicists for decades. The basic questions are straightforward to pose but remarkably difficult to answer. In fact, a conceptual revolution was required before truly rapid progress was achieved (Wilson, 1979). As we have indicated earlier (Chapter 5), dimensionality plays a crucial role in this modern theory of phase transitions (Ma, 1976). It then is natural to ask how much (if any) of our common three-dimensional experience and intuition carry over to the two-dimensional problem. Typical questions might be: What is the nature of the adsorbate phase diagram? How does a surface species pass from an ordered crystallographic state to a disordered state? What microscopic mechanisms are involved? How does an overlayer freeze and/or melt? Are any properties unique to two dimensions?

From the thermodynamic point of view, we have learned that clean surface critical phenomena and melting do indeed both differ from their three-dimensional bulk counterparts. We further quantify this notion here and examine the universality hypothesis, which states that only symmetry considerations and (in some cases) the range of adsorbate interactions determine the intrinsic nature of overlayer phase changes on a Langmuir checkerboard. Both static and dynamic issues will receive attention.