Introduction

In the last decade studies on adsorbed molecules and the monolayers formed by them have been receiving a great deal of interest due to diverse features both of applied and fundamental theoretical significance. On the one hand, adsorption and catalysis are extensively used in microelectronics, semiconductor engineering, chemical industry, and biotechnology, which calls for a deeper insight into adsorbate states. On the other hand, a system of adsorbed molecules offers a manifold of facets to be studied: it can be regarded both as an impurity subsystem with respect to a corresponding solid surface and as a quasi-two-dimensional entity in its own right, with all the particular cooperative properties arising from low dimensionality. It is also noteworthy that in most cases an adsorbed molecule is tightly bound to a surface through a single atom, whereas the other atoms (further from the surface) retain considerable mobility. Symmetric arrangement of substrate atoms often admits of several equivalent positions for the atoms of adsorbed molecules, i.e. several equivalent molecular orientations. As a result, the systems concerned are distinguished by high reorientational activity. Provided that intermolecular lateral interactions forming a molecular ensemble are strong enough to remove the orientational state degeneracy for isolated adsorbed molecules, they give rise to orientational ordering. It is evident that changes in temperature may cause phase transitions between orientationally ordered and disordered phases in such systems…